Effect of chronic potassium loading on
potassium secretion by the pars recta or
descending limb of the juxtamedullary nephron
in the rat.
C A Battilana, … , P A Johnston, R L Jamison
J Clin Invest. 1978;62(5):1093-1103. https://doi.org/10.1172/JCI109215.
Recently we demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullary nephron. The purpose of this present investigation is to study the effect of a chronic high-potassium intake on this phenomenon. Fractional reabsorption of water and sodium by the juxtamedullary proximal nephron was decreased when compared to that in normal hydropenic rats. There was a striking increase in the fraction of filtered potassium at the end of the juxtamedullary descending limb from 94+/11% to 180+/18%, which was principally a result of enhanced potassium secretion. When the concentration of potassium in the collecting tubule fluid of potassium-loaded rats was reduced after the administration of amiloride, a sharp fall was observed in the amount of potassium which reached the end of the descending limb (64+/8%). A direct correlation was observed between the fraction of filtered potassium at the descending limb and the potassium concentration in the final urine (P less than 0.001). The findings suggest that potassium, like urea, normally undergoes medullary recycling, which is enhanced by chronic potassium loading.
Research Article
Effect
of Chronic Potassium Loading on Potassium
Secretion by the Pars
Recta or Descending
Limb of
the
Juxtamedullary Nephron in the Rat
CARLOS A. BATTILANA, DENNIS C. DOBYAN, FRANK B. LACY, JAHAR BHATTACHARYA,
PAUL A. JOHNSTON, and REX L. JAMISON, Division of Nephrology, Statnford University School of Medicine, Stanford, Californtia 94305
A B S T R A C T Recently we demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullary nephron. The purpose of this present investigation is to study theeffect of a chronic high-potassium intake on this phenomenon. Fractional reabsorptionof water and sodium by the juxtamedullary proximal nephron was decreased when compared to that in normal hydropenic rats. There was a striking increase inthe fractionof filteredpotassium at the end ofthejuxtamedullary descending limb from 94+11%
to 180+18%, whichwasprincipallyaresult of enhanced potassium secretion. When the concentration of potas-sium inthecollectingtubule fluidof potassium-loaded rats wasreduced afterthe administration ofamiloride, a sharp fall was observed in the amount of potassium which reached the end of thedescending limb (64+8%). A direct correlation was observed between the frac-tion of filtered potassium at the end ofthe descend-ing limb and the potassium concentration in the final urine (P<0.001). Thefindings suggestthatpotassium, like urea, normally undergoes medullary recycling, which is enhanced by chronic potassium loading.
Portionsof this work were presented to the Ninth American SocietyofNephrology, 1976; to the Tenth American Society ofNephrology, 1977; andtothe Westem Society for Clinical Research, 1977. Clin. Res. 25: 165A. (Abstr.)
Dr. Johnston was a Fellow of the Bay Area Heart
As-sociation. His present address is: Division of Nephrology, Departmentof Medicine, Boston University School of Medi-cine, Boston, Mass.02215. Dr.Battilana was a Fellow of the National Kidney Foundation. His present addres is: Avda Javier Prado 3220, San Borja, Lima, Peru. Dr. Bhattacharya
was aFellow of the Dean's Fellowship Fund, Stanford versity. His present address is: Department of Medicine, Uni-versityof California School of Medicine, San Francisco, Calif.
94122. Dr. Dobyan is a recipient of U. S. Public Health Service Research Fellowship 1 F32-AM 05741-01. During a portion of this study, Dr. Jamison was aFellow of the John SimonGuggenheim Foundation.
Received for publication 19 October 1977 and in revised
form 10July1978.
INTRODUCTION
We recently reported that potassium is normally
se-creted by the pars recta or by the descending limb of thejuxtamedullary nephron (1). ("Juxtamedullary" isused here to meannephrons with long loops of Henle.) The purpose of this present investigation is to study the effect of a chronic high-potassium intake on this phenomenon because potassium loading augments secretion ofpotassium elsewhere in the renal tubule (2, 3). Theresults demonstrate that potassiumsecretion
by the juxtamedullary nephron is substantially
in-creased.Furthermore,averygoodcorrelationwasfound between the fraction of filtered potassium which
re-mained at the end ofthejuxtamedullary descending limbin the exposed left renal papillaand the urinary fractional excretion ofpotassium from the right (un-exposed) kidney. To distinguish cause and effect in
this correlation, amiloride was administeredto potas-sium-loaded rats. Amiloride inhibition of potassium secretioninthedistalandcollectingtubule downstream wasassociated with asharp fallinthe fraction of filtered potassium remaining upstream in the juxtamedullary descending limb. To account for these findings, we
propose that in the renal medulla, potassium is reab-sorbed by the collecting tubule and secreted into the juxtamedullary descending limb orpars recta; that is, potassium, like urea, normally undergoes medullary recycling,aprocess which is enhancedby chronic po-tassium loading.
METHODS
52 young Munich-Wistar rats of either sex which weighed
80-125gwere dividedintofive groups:
Group1:Potassium-loaded rats,rnicropuncture of Hentle's loop andvasa recta. 13 rats werepotassium-loaded as fol-lows:seven rats weregivenaliquiddiet withahighpotassium
content (0.38 mmol KCI/ml) for at least 7 days before the
dayof micropuncture.Six rats werefedahigh-potassium pellet
diet (2 mmolKCI/g)(ICNPharmaceuticals,Inc.,LifeSciences
1093 J. Clin. Invest. X) The American Societyfor Clinical Investigation, Inc., 0021-9738i78/1101-1093 $1.00
Group, Cleveland, Ohio) for thesamelength of time.In
pre-liminary studies,sixratswereplacedinindividual metabolic cages to assess theeffectiveness of potassium loadingas de-termined by urinary excretion of potassium per24 h. Three rats were fed the liquid diet and three rats the solid diet. Theaverage daily potassium intake was -20 meq with either diet.
The day of micropuncture, the animalswere anesthetized
with Inactin(BykGulden-LombergChemische FabrikgmbH, Konstanz, WestGermany) (100mg/kgbodywt) andthe left renal papilla exposed by methods previously described (4).
Urinefromtheunexposedright kidney was collected through
abladder catheter. The animalswereinfusedat 0.03ml/min
with 0.9% saline which contained inulin at aconcentration sufficient to maintain the plasma inulin concentration
be-tween80and 100 mg/100 ml.Samples were obtained by micro-puncture of the descending limb near the bend of Henle's loopand from adjacent vasa recta. The collection technique forloopfluid and vasa recta plasma, and the criteria
estab-lishedbefore beginning theexperimentsfor acceptable
collec-tions wereidenticaltothosepreviously described (5). Group2:Normalhydropeniccontrolrat. 18
Munich-Wis-tar rats werefed regularratchow (0.13 mmol KCI/g). Urinary
andplasma data andelectrolyte concentrations in loopfluid andplasmafrom vasa recta forgroup2 rats werepreviously reported (5). Micropuncture experiments were performed withaprotocolsimilartothat forgroup 1.
Group3:Chronically water-loadedrats. Sevenrats were
placed in individual metabolic cages, fed regular ratchow, and given 10% dextrose in waterto drink until their daily
urinevolumes equalled those ofpotassium-loadedrats.
Micro-puncture experimentswereperformedwith aprotocol similar
to that for group 1 except that no vasa rectasamples were
obtained.
Group4:Potassium-loaded rats, micropunctureof
collect-ingduct. Seven rats werefed a highpotassium diet forat
least 7 days. Samples were obtained from the base and tip ofa collecting duct of the left kidney with the dorsal ex-posureof the papilla recently describedbyStein etal. (6)
in-stead oftheusualventralapproach.("Base" referstotheregion
of the exposed papilla adjacenttothe mainbody of the kid-ney.) Heavier rats were selected for this group to ensure a maximum length of exposed papilla.
Group5:Amiloride-infused, potassium-loaded rats, loop of Henleandcollectingductmicropuncture. Seven rats were
prepared accordingtothe protocolused in group 4.45min beforemicropuncture began, aloadingdose of amiloride (1 mg/kgbodywt) wasadministered intravenously, followed bya
maintenancedose(0.4mg/kgbody wt/min) ofamiloridewhich wasdissolvedinnormal saline and infusedat aconstant rate
of0.03 ml/min. Sampleswere obtained fromthe end ofthe
descending limb and from the base and tip ofa collecting tubule.
The osmolality and concentrations of sodium, potassium, and inulin of loop fluid, vasarecta plasma, urine from the
right kidney, and femoral arterial plasma were determined by methods previously described(7).
Values invasarectaplasma were correctedforplasma water, assumingavasa rectaplasmaproteinconcentrationof 7 g/100 ml (5,8).Because theelectrolyte concentrations in loopfluid
and vasa rectaplasmaare in part determinedbytheosmolality
of themedullary interstitium, theelectrolyte composition of
loop fluid from control rats and of vasa rectasamples from
both control andpotassium-loaded ratswasnormalized with respect to the mean osmolality of loop fluid in potassium-loadedratsaccordingto thefollowing equation (5):
E=Osm E
Osm2 2
where E1 =correctedelectrolyteconcentrationofloop fluid
orvasarectaplasma;E2=uncorrected electrolyte concentra-tion ofloop fluid or vasa recta plasma; Osm,=mean loop
fluid osmolality in potassium-loaded rats; and Osm2 =
os-molalityof the correspondingE2 sample.
Statistical analysis betweengroups wasdone with Student's
ttestfor comparingtwoindependentmeans.Thedifference
inelectrolyteconcentrationsbetweenloop fluid andvasarecta
plasmawasanalyzedwith Student's t test forpaired
determina-tions. A regressionanalysiswasperformed by theleastsquares' method(9).
RESULTS
Comparisonof potassium-loadedrats(group 1) with normally-fedrats (group2). In rats fed the liquidor
solid high-potassium diet,asteady-state of
high-potas-siumintake andexcretion wasreached well before the 7th day. Thepotassiumoutput was21+1.7 meq(liquid diet) and 19+0.7meq(solid diet) the 6th day, and
uri-nary flow was63+8.5ml/24 h and 58+6.9 ml/24 h,
re-spectively. The high urinary flow rates are presumed
aresult of the increasedosmotic excretoryload of
po-tassium chloride.
Some parameters of arterial blood and right kidney functionarepresentedinTables Iand II. Therewere no significant differences in mean blood pressure,
hematocrit, and plasma sodiumconcentrationbetween potassium-loaded and control rats. The mean plasma
potassium concentration inpotassium-loaded rats was
higher than thatinnormal hydropenicrats.Glomerular filtrationrate (GFR),1 urinaryosmolality, and urine-to-systemic plasma(U/P) inulin inpotassium-loaded rats were lower than the respectivevalues in controlrats.
Thecompositionofloop fluid and fractional delivery
of solute are shown in Tables III and IV. The tubule
fluid-to-systemic plasma (TF/P) inulin in potassium-loaded rats was lower than that in control rats. There were no significantdifferences in TF/P osmolality or TF/P sodium. Although the mean TF/P potassium in potassium-loaded rats was higher than that in control rats,the differencewasnotstatistically significant. The ([TF/P potassium]:[TF/P sodium]) ratio in
potassium-loaded rats, however, significantly exceeded that in control rats, which indicates that fractional delivery
of potassium relative to thatofsodium was higherin
potassium-loaded rats. Thepercentage of filtered load of potassium([TF/Ppotassium]:[TF/Pinulin])x 100,in
potassium-loaded rats, 180+18%, was almost double
that in normal rats. The value is significantly higher than 100% (P<0.001),whichis
unequivocal
evidence for potassium secretion (i.e., net potassium addition) by the juxtamedullary nephronproximal to thehairpinturn.
Acomparisonof electrolyte concentrations in Henle's
1Abbreviations
used in this paper: GFR,glomerular filtra-tion rate;TF/P, tubule fluid-to-systemic plasma;U/P,TABLE I
Values forArterialBloodantdRightKidney Functiotn
Chronic
potassium loading
Chronic andamiloride
potassium- infusion;
Chronic loaded- collecting duct
potassium- Normlal Chronic collecting duct andloop
loaded hydropenic water-loaded micropuncture micropuncture (group 1) (group 2) (group 3) (group4) (group 5)
rn 13 18 7 7 7
Arterial blood(plasmiia)
Blood pressure, lltulhg 93±2 92+2 97+4 95+1 89+4
Hematocrit 40± 1 40+ 1 44±1 42+ 1 39+2
Sodium,meqlliter 150+1 149±1 149+1 148+2 145+2* Potassium, tueqlliter 5.3±0.2 4.4±0.14 4.9±0.2 5.3±0.1 8.2+0.5§
Right kidney function
Urineflov,,ullnini/igkidnieiy ct 6.0±0.6 4.5+0.3 4.8±0.4 10.5+0.9§ 9.8+1.34
GFR,pullrrliilngkidntey wt 826±37 1,120+40§ 1,095+8111 1,314+106T 810+50 Osmolality, tuostnrollkg H20 1,101+23 1,720+74§ 1,505±102§ 1,352+82 1,199+29
U/Pinulin 153±18 252+23§ 229±15* 129+12 89±+9
FE sodium, % 0.2±0.0 0.2±0.1 0.1±0.0 0.1±0.0 2.8+0.3§
FE potassium, % 38±5 22+2 19±3* 61+4T 19+2*
Weights
Bodv wt, g 93±4 93+2 106+6"1 121+4§ 112+8" Kidney wt
Left, g 0.51±0.03 0.43±0.00 0.49±0.03 0.73±0.06§ 0.65±0.05*
Right, g 0.51+0.04 0.41±0.02 0.50+0.03 0.72±0.05§ 0.63±0.06"1
Values aremeanis±SE. FE,fractional excretion (percent of filtered load). In this table, data from groups 2, 3, 4, anid5 are compared with corresponding datain group 1. There werealso some significant differences among theother groups, but for the sake of clarity concerning theprincipalfindings,thesedifferencesare notindicated
in this table. A complete tabulation follows in Table II. Footnote symbols indicate P values significantly
dlifferent compared tothecorrespondinigvalues ingroup1. * P<0.025.
4 P<0.01.
§P<0.001.
"P<0.05.
¶ P < 0.005.
TABLE II
Statistical Cotuiparisoit (PValues) bettceeti Groups: ValuesforArterial Blood andRight KidnteyFuniction
Ivs. 2 1 vs.3 1 vs.4 1 vs. 5 2vs. 3 2vs. 4 2vs.5 3vs. 4 3 vs. 5 4vs. 5
Bloodlpressure NS NS NS NS NS NS NS NS NS <0.02
Hemiiatocrit NS NS NS NS <0.025 <0.05 NS NS <0.05 NS
PlasmaNa NS NS NS <0.025 NS NS <0.025 NS NS NS
PlasmiiaK <0.01 NS NS <0.001 NS <0.01 <0.001 NS <0.001 <0.001
Urinie flow NS NS <0.001 <0.01 NS <0.001 <0.001 <0.001 <0.005 NS
GFR <0.001 <0.05 <0.005 NS NS <0.05 <0.001 NS <0.025 <0.005 Osmolality <0.001 <0.001 NS <0.05 NS <0.01 <0.001 NS <0.025 NS
U/Piniulin <0.001 <0.025 NS <0.025 NS <0.001 <0.001 <0.001 <0.001 <0.02
FE Na NS NS NS <0.001 NS NS <0.001 NS <0.001 <0.001 FE K <0.005 <0.025 <0.005 <0.025 NS <0.001 NS <0.001 NS <0.001
Bodvwt NS <0.05 <0.001 <0.05 <0.005 <0.001 <0.005 NS NS NS
Leftkidney wvt NS NS <0.001 <0.05 <0.005 <0.001 <0.001 <0.005 <0.05 NS
Right kidney wt NS NS <0.001 <0.025 <0.025 <0.001 <0.001 <0.005 <0.05 NS FE,fractionial excretion (percent offiltered load).
TABLE III
Compositionl ofFluidfromEndof Descending Limb
Chronicpotassium- Normalhydropenic Chronic water-loaded Chronic potassium-loaded,
loaded (group1) (group2) (group 3) amilorideinfusion (group5)
TF/Pinulin 3.9+0.2 6.7+0.5* 6.9+0.6* 6.9±1.24
TF/Posmolality 2.6±0.2 3.0±0.1 2.5±0.2 2.5±0.1
TF/Psodium 2.1±0.2 2.3±0.1 1.9±0.2 1.9±0.1
TF/Ppotassium 7.1±0.7 5.8±0.5 4.2±0.8§ 3.8±0.34
TF/P potassium/TF/P sodium 3.6±0.4 2.5±0.1" 2.2+0.3$ 2.0±0.2§
Fraction of filtered loadremainingat end ofdescendinglimb
Totalsolute 69±4 46±3* 42±7"1
45±9§
Sodium 57±4 39±3t 31±t6t 33±6"1
Potassium 180+18 94±11* 66+14* 64±8*
Valuesaremeans± SE.Inthis table, data from groups 2,3, and5are compared with corresponding datain group 1.There werealso significant differences among the other groups, but for the sakeofclarity theyare not
indicated in this table. A complete tabulation follows in Table IV. Footnote symbols indicate P values significantly different comparedtothe corresponding values in group 1.
* P<0.001.
4P<0.005. § P<0.025.
"p<0.01. ¶P<0.05.
loop fluid with thatofvasa recta plasma is presented in Table V. The uncorrected and corrected potassium concentration of
loop
fluid and the correctedvasa rectaplasma potassium concentration in potassium-loaded rats were higher than the respective values in loop fluid orvasa rectaplasma of control rats (Table V).
Comparison
of potassium-loaded
rats(group
1)with water-loaded rats (group 3). Group 3 animals were used to exclude nonspecific effects as a result ofthe copious urinary flow induced by the large osmotic load of potassium chloride. As shown in Tables I and II,themeanarterialbloodpressure,hematocrit, plasma sodium, and potassium concentrations inchronically
water-loaded rats did not differ significantly from
cor-responding valuesforpotassium-loadedrats.TheGFR,
U/Pinulin,and urinary osmolality in chronically water-loaded rats were greater than the respective values in potassium-loaded rats. Urinary excretion of potas-sium was less in chronically water-loaded rats than in
potassium-loaded rats.
The TF/P inulin of loop fluid in chronically water-loaded rats exceeded that in potassium-loaded rats (Tables III and IV). No significant differences in TF/P sodium and TF/P osmolality were observed between the two groups, but the TF/P potassium in chronically water-loaded rats was lower than that in potassium-loaded rats. The percentage of filtered sodium reach-ingthe endofthejuxtamedullary descending limbwas less in chronically water-loadedrats than inthe potas-sium-loadedgroup.The moststriking finding,however,
TABLE IV
StatisticalComparison (PValues) between Groups: Composition of Fluid
from End of Descending Limb
1vs. 2 1vs. 3 1vs. 5 2vs.3 2 vs. 5 3vs. 5
TF/P inulin <0.001 <0.001 <0.005 NS NS NS
TF/Posmolality NS NS NS NS <0.05 NS
TF/P sodium NS NS NS <0.025 <0.01 NS
TF/Ppotassium NS <0.025 <0.005 NS <0.05 NS
TF/P potassium/TF/P sodium <0.01 <0.05 <0.025 NS NS NS
Fractionof filtered soluteremaining <0.001 <0.01 <0.025 NS NS NS
Fractionof filteredsodiumremaining <0.005 <0.005 <0.01 NS NS NS
TABLE V
Sodium a(ndPotassiumConcentrations inLoop Fluid andVasa Recta Plasma
Group 1 Group 2
Chronic Normal- Normal
potassium-loaded ized* hydropenic Normalized*
it 9 18
Descending limb fluid,meqlliter
Sodium 332±31 332±31 344±12 303+9
Potassium 36+5 36±5 26+2§ 23+2"
Osmolality, mosmollkgH20 818+66 938+37
Vasa rectaplasma, meqlkgH20t
Sodium 316+33 309+33 304+12 265+11
Potassium 43+6 46+7 36+2 32+2§
Osmolality, mosmollkgH20 866±44 936+34
Values are means-SE.
* Samples were normalized toisosmolality with group 1 loopfluid osmolality by calcu-lationsdescribedin Methods.
IConversion to meq/kg H20 assuming a vasa recta plasma protein concentration of 7
g/lOOml(5).
Footnote symbols indicate P values significantly different from corresponding values in group1.
§ P <0.05. 1"P < 0.005.
is that the percentage of filtered potassium remaining
atthe end of thedescendinglimbin
chronically
water-loaded rats was one-third thatofpotassium-loadedrats.
Group 4 (potassium-loaded rats, micropuncture of collecting duct). These rats were studied in an
at-tempt to determine the sourceof the potassium secreted into the pars recta or descending limb (Discussion). The direction and magnitude ofnet transtubular
po-tassium movement across thepapillary collecting duct
was assessed bycomparing samples ofcollecting duct fluid obtained at the base of the papilla with those obtained at the tip. To insure maximum exposure of papilla,andthereforemaximumlength of
papillary
col-lecting duct accessible to micropuncture, heavier ratswere selected and a dorsal approach through a flank incision (6) was employed, rather than the standard ventralapproach throughamidlineincision.(Thedorsal aspect of theexposed papilla is longerthanits ventral aspect.) The blood pressure, hematocrit, and
plasma
sodium and potassium concentrationsingroups 1and4
were similar (Tables I and II). GFR was greater;
uri-naryosmolality and urinaryexcretionofpotassiumwere
slightly higher as well.
Analyses ofcollecting tubule fluid are summarized in Table VI. There was a significant rise in TF/P os-molality and TF/P inulin; the latter reflects net water reabsorption equivalent to 0.9% ofGFR.Net
reabsorp-tion of total solute and sodium also occurred. These values are essentially similar to those reported pre-viously from this laboratory for normal hydropenicrats
(4). The fraction of filtered potassium in collecting
tubule fluid at the baseof the exposed papilla (68%) was not significantly different from that excreted (71%); i.e.,
onthe average, therewas nodetectablenettranstubular movementof potassium across the exposed segment of papillary collecting tubuleinthepotassium-loaded rats.
Group 5(potassium-loaded rats, infusion of amilo-ride). These experiments represent another approach
to determining the source of potassium secreted by the pars recta or by the descending limb of the jux-tamedullary nephron (Discussion). Before administra-tion ofamiloride, the meanplasmapotassium
concen-tration(not shown)was 5.4 meq/liter, whichis similar
to that in groups 1 and 4; after amiloride, plasma
po-tassiumreachedhigh levels (TablesIand II). U/P inulin
was significantly lower in this group than in any of the otherfourgroups.Fractional excretion of potassium was strikingly reduced while that of sodium was
in-creased substantially.
The micropuncture data (Tables III and IV) pre-sumably reflect the diuretic-induced
rpduction
in extra-cellular fluid volume. Thus thehighermean TF/P inulin and reduced fraction offiltered sodium which remain suggest increased salt and water reabsorption in the juxtamedullary proximal tubule. The TF/P potassium and TF/P potassium-to-TF/P sodium ratios were sig-nificantly reduced. The fraction of the filtered load of potassium remaining (64+8%) was sharply decreasedto one-third of thatin potassium-loaded rats.
As shown in Table VI, when the values at the tip of the collecting duct are compared with the values
TABLE VI
Micropunctureof the Collecting Duct
Chronicpotassium-loaded Chronicpotassium-loaded,amiloride (group 4) (n=7) infusion (group 5) (n=7)
Base Tip P Base Tip P
TF/Pinulin 47+6 77+9 <0.001 47±8 71±12 <0.025 TF/Posmolality 2.2+0.1 3.1±0.1 <0.001 2.1±0.1 2.4±0.2 NS TF/Psodium 0.60±0.0 0.33±0.1 <0.05 1.5±0.1* 1.3±0.1* NS TF/P potassium 28±3 51±3 <0.001 7.3±2.0* 6.1±1.0* NS
Fractionof filtered load remaining, %
Solute 5.3+0.7 4.1±0.6 <0.05 5.6±1.3 3.8±0.5 NS Sodium 1.5±0.2 0.5±0.2 <0.001 4.2±1.21 2.2±0.4§ NS
Potassium 68±12 71±8 NS 16±3* 10±2* NS
Water 2.3±0.2 1.4±0.4 <0.001 2.9±0.9 1.7±0.3 NS
Values are means±SE. The mean distance between base and tip was 1.6±0.0 mm in group 4 and 1.7±0.0 mm in group 5. P values in the separate columns refer to paired comparisons
between baseand tip. Footnote symbols indicate P values significantly different compared tothecorresponding values in group 4.
* P<0.001.
t P <0.025.
§P<0.005.
except for the rise in TF/P inulin. When the values
in group 5 are compared with corresponding values
ingroup 4, the TF/P sodium at the base and tip in group 5 are higher, and the fraction of the filtered load of sodium which remains atthe base and tip in group 5 is also higher. Insharpcontrast, the fraction of filtered load ofpotassium remainingatthebaseand tipingroup 5 (16 and 10%, respectively), is much lower than in
group 4 (68 and 71%, respectively).
Correlation offraction of filtered potassium in end descending limb
fluid
with urinary potassium. The fraction of filteredpotassium at the endofthe descend-ing limb in groups 1-3 was plotted as a function of fractionalexcretion ofpotassium inthe urine fromthe right kidney. The results, shown in Fig. 1, reveal a very good correlation (y= 18.9+3.7x; r=0.81; P<0.001). When the results of theamiloride-treatedrats
(group5)arecombined withgroups 1-3,asimilar
cor-relationisobserved (Fig. 1). Amiloride, like furosemide (1), decreased the fraction of filtered potassium inloop fluid, but, unlike furosemide, reduced urinary
excre-tion ofpotassium (Fig. 1). A findingcommon to both amiloride- and furosemide-treated rats, however, is a
significantly lower urinary potassium concentration.
Therefore, the fraction of potassium remainingin loop fluid(dependent variable) was plottedagainst urinary potassiumconcentration (independent variable)in the
two diuretic groups as well as in groups 1-3 (Fig 2). Again agood correlationisobserved, butincontrastin
Fig. 1, the results from the furosemide-treatedrats are
0 0
x
z
z
IL. I-0.
La.
-z zx
Ua.
0
IL
0
0
4i
A
3001200
-100o
A& A a
A
0
do eo 0
a 1
a
A AA
A A
A
A
0 10 30 50 70 90
URINARY FRACTIONAL EXCRETION OF POTASSIUM (%)
FIGURE 1 The fraction of filtered potassium remaining in
fluid at the end of the juxtamedullary descending limb,
([TF/P,K]/[TF/P, inulin]) x 100,as afunction of urinary
frac-tion excretion ofpotassium. Symbols as follows: open
tri-angles, potassium-loaded rats (group 1); open circles,
nor-mally-fed rats (group 2); open squares, water-loaded rats
(group 3); closed circles, potassium-loaded rats given amiloride (group 5); and closed triangles, normally-fed rats
given furosemide (published previously, [1]). Data from
groups 1-3, open symbols, demonstrate a clear correlation
(y = 18.9+ 3.7x;r =0.81;P<0.001). Datafrom group5 are
indistinguishablefrom those of groups1-3. Incontrast, while furosemide decreased the fraction of filtered potassium
0 0 x z -J z -i LL &-y a.
L-i
I-w-lJ
z w Li. 0 a. 0 0 -J 300 -A A6 200 -100 a 0 A 0 A 0 A A4 hA &A AA^
0* °0 0
* 00o a0
0 00 . 0 aP0 100 0 300
URINARY POTASSIUM CONCENTRATION
meq/I iter
FIGURE 2 The fraction of filtered potassium remaining in
fluid at the end of the juxtamedullary descending limb,
([TF/P,K]/[TF/P,inulin]) x 100, as a function of urinary potas-siumconcentration. Symbols are the same as those used in Fig. 1.Thereisaclearcorrelation (y=42.4 +0.34x; r=0.50;
P<0.001).
indistinguishable from those of the other groups (y =42.4 + 0.34x; r =0.50; P<0.001).
DISCUSSION
The complicatedcourse by which potassium entering thekidney inarterial blood reaches the final urinehas been the subject of several recent reviews (2, 3, 10) and a symposium (11). This present study suggests a
newpathway:potassium is reabsorbed from collecting tubule fluid and reenters thejuxtamedullary nephron
in either its pars recta or descending limb segment. Medullary recyclingof potassium appears to be present under normalconditions andisaugmentedby chronic potassium loading.
Havingpreviously demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullarynephron (1),we studied ratsfed a high-potassium dietto assess itseffects on this phenomenon.
Two unavoidable experimental limitations are
ap-parent: (a) the use ofanesthesia and (b) the inacces-sibility of most nephrons and most of the collecting tubule system to micropuncture. Unanesthetized rats ingesting a high-potassium diet excreted 20 meq po-tassium/100 g body wt perday, which represents _2.5 times theirestimated filtered load of potassium. (Assum-ing the GFR was 1 ml/100 g body wt per min [4], the calculated filtered load of potassium in the
unanes-thetized potassium-loaded rats= 7.6meq/day. If aver-agetotalexcretion is20meq/day,excretion is 2.6 times
the filtered load.) When anesthetized, similarly fed rats excreted only 38-61% of their filtered load of potas-sium(TablesIand II),results similar to those reported by others (12). At least two factors impair potassium excretion during anesthesia: low urinary flow and de-creased extracellular fluid volume. Urinary flow from the right kidney during anesthesia averaged 6 ul/min per g kidney wt, or about 15% of the urinary flow in
unanesthetized potassium-loaded rats. Because tubule fluid flow rate, particularly in the distal nephron,
in-fluences net potassium secretion (2, 3, 10-13),the anti-diureticstateundoubtedly reducedurinary excretionof potassium.Thesecondfactor to compromise potassium excretion, recently elucidated by Maddox et al. (14), and confirmed by us (15), is the combination of anes-thesiaand surgery necessary to prepare rats for micro-puncture, which may reduce intravascular and extra-cellular fluid volume by as much as 20%. As a con-sequence, proximal sodium reabsorption will be enhanced (16) and distal potassium secretion di-minished (2, 3, 10, 11, 17, 18).
Despite these limitations, a new finding clearly emerges from this study: during chronic
high-potas-siumintake, fractional delivery ofpotassium to the jux-tamedullary end-descending limb is increased sharply from 94% to 180%, which reflects an increase in po-tassium secretion by the pars recta or by the descend-inglimb (TablesIIIand IV). The datafromthis present study, shown in Fig. 1, reveal a very good correlation between the fraction of filtered potassium at the end of the descending limb and urinary fractional
excre-tion of potassium. Assuming that it is not fortuitous, the correlation implies a causal relationship between the twovariables, but does not distinguish cause and effect. Either the increased urinary excretion of po-tassiumisthe result oftheincreased potassium remain-ingin descending limb fluidor it isthe cause thereof. These twointerpretations areillustrated in Fig. 3. The first hypothesis (Fig. 3A) may be stated as follows: A mechanismexists whereby potassium in the interstitium of the renal medulla is secreted intothe pars recta or
the descending limb of the juxtamedullary nephron, which increases the mass flow of potassium that reaches the hairpin turn. Somewhere in the ascending limb, potassiumisreabsorbed and then secreted directly into collectingtubule fluid,therebyaugmenting overall
B
I OUTER
_STRIPE_
_________ INNER
---
-STRIPE-j
CORTEX
OUTER ZONE
INNER
ZONE
MEDULLA
FIGURE 3 Two hypotheses for potassium movement in the renal medulla. In both A and B,
aI juxtamedullary nephroni is depicted on the right side of a single collecting tubule with its branches. According to hypothesis A, potassium supplied to the medullary interstitium from vasa rectaplasma is secretedlinto either the pars recta or the descending limb of the juxtamedullary
nephron, thereby increasinigthe flow of potassium reaching the hairpin turn. Potassium is
re-absorbed either hy the thin or thick ascending limb and then secreted directly into collecting
tul)ule fluid, therebyaugmenting overall urinaryexcretion ofpotassium.2According tohypothesis B, in contrast,potassiumil isreabsorbed by the collectingtubuleand secreted into the pars recta
orthe descenidinglimb of the juxtamedullary nephron.2 For further discussion, please see text.
nary excretion ofpotassium.2 According tothe second hypothesis (Fig. 3B), the collecting tubule hasafinite permeability to potassium, or actively reabsorbs
po-2HypothesisAimplies transfer of
potassium
from the vas-culature tothe pars recta or to thedescending limbby way of the interstitium, and then transfer from the ascendinglimb to the collecting duct through the interstitium. This
neecl not necessarily be the same interstitium; one may be
spatially separatefromthe other(19).Still anotherpossibility
not depicted in Fig. 3A is that potassiumii secreted intothe parsrecta orthedescending limbfrom the vasculature is not
reabsorbedby theascending limband,instead, is delivered
dlirectly tothecollectingtubuleviathejuxtamedullarydistal tubule (Discussion).
The illustration ofhypothesisB(Fig.3B)(transferof
potas-siuimi fromiicollecting ducttodescenc(linig limb)does not
indi-cate what happens to potassium in the intervening
inter-stitiuimi or its fate onice it is addedl to the pars recta or the
dlescend(linglimb.HypothesisBimpliesthatmostof the
potas-sium reachingthemedullaryinterstitiumiifrom the collecting
tubule is trapped there by countercurrent exchange in vasa
recta and then secreted into the pars recta orthedescending limb. Hypothesis Balso imnplies thatsome of the potassium
tassium.Asmorepotassium isdeliveredtothecollecting tubule,more potassium is reabsorbed by thatsegment
and then secreted into the pars recta or into the de-scending limb.2
To distinguish between these two hypotheses, the
experiments described in groups 4 and 5 were
per-formed.Ifpotassiumsecretedintothepars recta orthe descending limb ofthe juxtamedullary nephron is a sourceofpotassiumforsecretionintocollecting tubule fluid (Fig. 3A),an increase inpotassium remaining in
thecollectingtubule fluid between base andtipof the exposed papilla (i.e., net potassium addition) should be detected. If the alternative hypothesis (Fig. 3B) is correct, net removal of potassium by the collecting tubule in the exposed papilla should be observed. As
shown in Table IV,however, neither addition nor
re-secretedl into the pars recta or the descending limb is not reabsorbed by the ascending limb andisdelivereddirectlyto
the collecting tubule via the juxtamedullary distal tubule (Discussion).
A
absorptionwasconsistently demonstrable.This
incon-sistencyhasbeen
previously reported
by
ourlaboratory
(4) and by Diezi et al. (20) and may be a result ofan
insufficient length of collecting tubule accessible to
micropuncture in the exposed papillary tip.
Itappeared thatthekey region ofthe collecting tubule inwhicheither secretion orreabsorptionoccurred must lie upstream in the inaccessible portionofthe collect-ingtubule system, i.e.,somewhere inthe cortex,outer
medulla, or inaccessible region of the inner medulla. (Weinclude the corticalcollecting tubule here for
com-pleteness,althoughitseemsunlikelythat potassium is
reabsorbed inthis segmentnormally [21].)Therefore, the experiments ingroup 5 were
performed,
basedonthe rationale that ifthecollecting tubule is the source ofpotassium destined for secretion in the pars recta
or in the descending limb (Fig. 3B), then
reducing
themassflowofpotassiuminthecollecting tubule should decrease potassiumavailableforsecretion intothe jux-tamedullary nephron, thereby causing the fraction of potassiumwhichremains atthe end of the
descending
limbtofall. Onthe other
hand,
ifloop
fluidis a sourceofpotassiumsecreted
by
thecollecting
tubule (Fig. 3A), then reducing the amount ofpotassium reaching the collecting tubule shouldnotdecrease the filtered load ofpotassium attheend-descending limb,
because po-tassium secretion upstream intheparsrecta orthe de-scending limb should notbe diminished by a reduc-tion in potassium flow downstream in the collecting tubule.Amiloride was selected forthe purpose of reducing potassium flowinthecollecting tubule.Itwasassumed that the drug has no directeffect on potassium secre-tion by the pars recta or the descending limb, or on potassium
reabsorption
by the proximal tubule, inac-cord with in vitro (21) and in vivo (22, 23)
studies,
exceptthatby Deetjen (24) who reportedarisein end-proximalTF/Pinulin associatedwith amarked decline
in GFR, which he attributedto intratubular precipita-tion of amiloride in the distal nephron. Although we
cannotexclude intratubular blockade,thefactthat GFR in amiloride-treated potassium-loaded rats (group 5)
was the same as in potassium-loaded rats not given thedrug (group 1)indicatesthatitcouldnothave been extensive. In addition, in potassium-loaded rats, the passage time ofan intravenous bolus ofa dye, FDC Blue, whenmeasured fromglomerulitoend-accessible proximal tubules and from glomerulitoloopsto Henle
in the exposed renal papilla, did not increase after amiloride administration (unpublished observations). Finally, the decrease in the fraction of sodium (24%) and water(14%) remainingatthe end of the descend-ing limbintheamiloride-treatedratsrepresents only a small portion ofthe 116% fall inpotassiumremaining
atthe same site (Tables III and IV).
As shown in Table I, amiloride had the expected natriuretic effect andprofound inhibition ofpotassium excretion in chronically potassium-loaded rats. The sameeffects wereobservedinthecollecting tubule of the exposed kidney (Table VI), and agree well with the recent report of Stein et al. (25). With these re-sults in mind, the findings presented in Table III for the amiloride-treated rats clearly distinguish between the two hypotheses in Fig. 3. The fraction of filtered potassium at the end-descending limb in potassium-loaded animalswasreduced nearlythreefold to 64 +8%, as predicted by hypothesis "B".
Ifhypothesis B is correct, and if potassium reabsorp-tionby thecollecting tubule isinfluencedbythe
trans-tubular differenceinpotassium concentration between collectingtubule urine andmedullaryinterstitium, then one might expect to find a correlation between the fraction of filtered potassium atthe end ofdescending limb and urinary potassium concentration. Datainthe present experimentsplus thoseobserved after adminis-tration of furosemide(1),areplottedinFig. 2 and dem-onstrate a good correlation.3 (Furosemide-induced changes in medullary blood flow might also alter po-tassiumaddition tothepars recta or to thedescending limb).
In summary, the hypothesis which accounts forthe findings in all five groups and the relationships de-picted in Figs. 1 and2isthatpotassium isreabsorbed by the collectingtubulesomewherebetweenitsorigin inthe cortex andthe terminal papillary segment (pre-sumably in the medullary collecting tubules), and re-gains entry into thejuxtamedullary nephron by secre-tion eitherinto the pars recta or the descending limb ofthejuxtamedullary nephron (Fig. 3B).
Role of medullarypotassiumrecycling. The physio-logicalimplications ofpotassiumrecyclingare unclear partly since what happens to potassium between hairpin turn and juxtamedullary distal tubule is un-known. Because the electrochemical gradientappears unfavorable for potassium reabsorption from the thin ascending limb (TableV) and(5, 26,27), andbecause
a recent study by Burg and Bourdeau (28) suggests potassium secretion may predominate in the initial portionof the cortical thick-ascending limb,someofthe potassium addedtothejuxtamedullaryparsrecta orthe descending limb may conceivably reach the collect-ing tubule.
The suggestionthatpotassiumundergoesmedullary recycling is notintended to exclude potassium
secre-3In our previous paper (1), we alsoreported that benzol-amide caused an increase in the fraction of filtered potassium remaining at the endofthejuxtamedullary descendinglimb.
Thesedata are not shown in Figs. 1 or 2 because theeffectof
benzolamide on potassium reabsorption in the proximal
tubule is uncertain.
tion by the collecting tubule, for which there is
con-siderable evidence (2, 12, 13, 18, 25,29-34). The
vari-ability betweenpotassium remaininginthe late distal tubule fluid and thatintheurine(12, 13, 18, 24, 33, 34), andeven between base and tip of the exposed papil-lary collecting tubule, however, is well known.
Re-versal ofnet potassium transport as onedescends from the cortical tothemedullary collecting tubule may ac-count for this variability-secretion predominating
in the cortical segment,reabsorption inthe medullary
segment.The key factor which determines the overall result may be the fluid flow rate (12, 13, 35). Under
circumstances ofhighurinary potassium excretionand low urinary flow, the potassium concentration in col-lecting tubule fluid would reach very high levels in
the absence ofpotassiumreabsorption by the medullary collecting tubule,aspointedoutby Reinecketal.(35). Further investigations are necessary to assess the importance of this phenomenon.
ACKNOWLE DGM ENTS
Theauthorsaregratefulto Ms. Mary Fisher andMr.Michael Leach for their technical assistance, to Ms.Margaret Hamil-ton for her secretarial assistance, and to Doctors Gerhard Giebisch, Roy Maffly,Bryan Myers, andMichaelWeinerfor
criticizing the manuscript. We are also indebted to Dr.
Clement A. Stone fromthe Merck Institute forTherapeutic
Research, West Point, Pa.,forageneroussupply of amiloride,
and to Doctors Frank Epstein and David Maddox for their assistance inthe preparation ofthe different diets.
Thisstudy was supported byaresearch grantfromthe
Na-tional Institutes of Health (AM 18077) and byagrant-in-aid from theAmerican Heart Association(72821),with funds con-tributed in partby the Bay Area Heart Association.
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