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Analysis of fluid dynamics in perfused glomeruli of the hagfish eptatretus stouti (Lockington)

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The structure of the hagfish mesonephros is the simplest of any known vertebrate: it consists of 60 or so large glomeruli that drain individually into one of two paired ureters (archinephric ducts). The ureters subserve many of the same functions as nephron tubules, which are absent. The volume of the primary urine produced by the glomeruli is not reduced by distal water reabsorption (Munz and McFarland, 1964; Rall and Burger, 1967; Stolte and Eisenbach, 1973; Alt et al. 1981). Measurements made in glomerular capillaries of lightly anaesthetised hagfish indicate that the requirements for pressure filtration demanded by Starling’s hypothesis (Starling, 1896) are not met. Average hydrostatic pressures (PGC) were no more than half the colloid osmotic pressure (COP) of the blood plasma; furthermore, primary urine collected by direct cannulation of Bowman’s spaces had a negligible COP (Riegel, 1986b). Despite a close structural resemblance between the glomeruli of the hagfish mesonephric kidney (Heath-Eves and McMillan, 1974; Kühn et al. 1975; Albrecht et al. 1978; Brown, 1988) and the glomeruli of other vertebrates, there appears to be a fundamental difference in function (Riegel, 1986b, 1998, in preparation).

Riegel (1998) demonstrated that in the capillary tuft of perfused glomeruli there exist vessels that exhibit marked differences in their responses to pressure. ‘Low’-pressure glomerular vessels (LPGVs) responded relatively slowly to changes in perfusion pressure (Pperf), and in them the maximum PGC reached only approximately 0.4 kPa. ‘High’-pressure glomerular vessels (HPGVs) responded relatively rapidly to changes in Pperf; pressures within HPGVs could rise to values well in excess of 0.4 kPa. The discovery of the HPGV raised the possibility that under some circumstances pressure-filtration could occur in the hagfish kidney. This study reports the results of a detailed investigation of the characteristics of the HPGV and the LPGV.

Materials and methods

Specimens of a Pacific hagfish, Eptatretus stouti (Lockington), were studied at the Marine Laboratory of the University of California, Davis, in Bodega Bay, California, USA. Experiments were carried out on anaesthetised animals weighing between 56.5 and 124 g. Most Printed in Great Britain © The Company of Biologists Limited 1998

JEB1540

The capillary tuft of glomeruli of the hagfish mesonephros contains both ‘low’-pressure and ‘high’-pressure glomerular vessels (LPGVs and HPGVs). The existence of the HPGV raised the possibility that pressure filtration could occur in the hagfish kidney when the blood pressure was sufficiently high. Therefore, measurements of glomerular capillary pressure were made in HPGVs and LPGVs whilst single glomeruli were perfused with hagfish Ringer’s solution that contained the colloid Ficoll 70. Calculations of the effective colloid osmotic pressure in perfused capillaries were made; these showed that hydrostatic pressures within the HPGV were inadequate to effect pressure filtration except at high rates of perfusion. However, high rates of perfusion provoked perfusion pressures that exceeded the highest values measured in the renal blood supply of lightly anaesthetised hagfish. It was concluded that some process other than pressure filtration must account for formation of the primary urine by hagfish glomeruli.

The proportion of the perfusate that became urine, the single glomerulus filtration fraction (SGFF), bore a strong positive relationship to the vascular resistance of perfused glomeruli. Both the SGFF and the vascular resistance were inversely related to the rate of perfusion except when that rate was very high. From these two observations it was concluded that at least two flow pathways exist in hagfish glomeruli: one that has a high vascular resistance and that contributes to the elaboration of the urine, and one that has a low vascular resistance and does not contribute to urine formation. The possible anatomical location of the various flow pathways through hagfish glomeruli and how they may function are discussed.

Key words: myxinoid, kidney, Eptatretus stouti, perfused glomerulus, fluid dynamics, glomerular filtration rate, glomerular-capillary pressure.

Summary

Introduction

ANALYSIS OF FLUID DYNAMICS IN PERFUSED GLOMERULI OF THE HAGFISH

EPTATRETUS STOUTI (LOCKINGTON)

J. A. RIEGEL*

Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK

*e-mail: jar1004@hermes.cam.ac.uk

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of the methods used have been described previously (Riegel, 1978, 1986a–c), but for clarity a brief summary is provided here. Anaesthesia was induced by placing an animal in 2 l of chilled (5 °C) sea water that contained a quantity of MS 222 (0.262–0.522 mg g−1body mass l−1) that varied inversely with the body mass. The anaesthetised animal was then transferred to a platform immersed in a water bath through which was circulated chilled sea water containing a sustaining dose of MS 222 which was one-third the induction dose.

The body cavity of the animal was opened by making a midventral incision, and a glomerulus with its associated vasculature was exposed by removing the overlying peritoneum. All branches of a segmental artery were ligated except the renal artery serving the glomerulus chosen for study. Deep anaesthesia was induced by irrigating through the animal’s nostril 100 ml of sea water that contained a concentration of MS 222 of either 1 g l−1(animals below 100 g body mass) or 1.4 g l−1 (animals above 100 g body mass). Immediately thereafter a microcannula was tied into the segmental artery just laterad to the branching of the renal artery; perfusion was begun at once.

The single glomerulus filtration rate (SGFR) was measured by noting the rate of advance of urine into an oil-filled catheter located in a segment of the ureter adjacent to the perfused glomerulus. The ureter segment was isolated with ligatures placed well anterior and well posterior to the glomerulus to avoid interference with the postglomerular blood supply. The equality of the SGFR determined in this way with that determined by direct cannulation of the Bowman’s space has been established in previous studies (Riegel, 1986b,c). One glomerulus was used in each animal, and animals were not permitted to recover conciousness at the end of the experiments.

Glomeruli were perfused through dual-channel micro-cannulae (Fig. 1) which were constructed in such a way that one of two perfusion channels could be selected by switching at a remote location. Remote switching eliminated the risk of displacing a pressure-sensing electrode by movement in close proximity to a perfused glomerulus. Perfusion fluid consisted of hagfish Ringer’s solution (Riegel, 1978) to which an amount of Ficoll 70 was added to give a COP of 1.4 kPa (=Ficoll Ringer). A formula devised by Gamble (1983) was utilised to calculate the amount of Ficoll 70 to be added to the Ringer’s solution to yield the required COP. The COPs of Ficoll Ringer and collected urine samples were measured directly using methods described by Riegel (1986a).

The perfusion pump used consisted of two glass syringes whose plungers were driven by a stepping motor. The speed of the perfusion-pump motor was controlled electronically: a coarse control (CC) approximately doubled the rate of delivery between approximately 1.4 and approximately 22µl min−1 when switched between five calibrated positions. A continuously variable (uncalibrated) fine control (FC) changed the rate of delivery from the full value of a CC setting to approximately one-eighth of that setting.

Pressures in the perfusion lines (Pperf) were monitored by

blood-pressure transducers. Pressures in individual vessels of the glomerular vasculature (PGC) were measured using a servo-nulling pressure microtransducer (Riegel, 1986b). Output from pressure-measuring devices was recorded on an oscillographic chart recorder.

The LPGVs and the HPGVs were differentiated using the following test. After penetrating a blood vessel with a pressure-sensing electrode, the Pperf was changed briefly from a low value to a high value by varying the speed of the perfusion pump. The PGC in a LPGV could be increased only to approximately 0.4 kPa, whilst the PGCin a HPGV increased to values well in excess of 0.4 kPa.

Two series of perfusions were made: (1) experiments in which glomeruli were perfused with Ficoll Ringer alone in both control and experimental perfusion channels. Measurements of the PGCmade in the HPGVs and the LPGVs during these perfusions form the basis of most of the results presented here. (2) Experiments in which Ficoll Ringer was present in the control perfusion channel and Ficoll Ringer plus 100µmol l−1of the vasodilator papaverine were present in the experimental channel.

The following parameters were calculated from the measured data: the single glomerulus filtration fraction (SGFF=SGFR/perfusion rate), the vascular resistance (=Pperf/perfusion rate), the reflection coefficient (Staverman, 1951) of Ficoll 70 (σ=1−Cu/Cp) and the effective colloid osmotic pressure (Michel, 1997) (COPeff=COPperfusate−COPurine). In calculations of the reflection coefficient, Cu and Cp were the concentrations (g %) of colloid in urine and perfusate. The formula of Gamble (1983) was used to convert COP to concentration.

0.6 mm×38 mm syringe needle

1 ml tuberculin syringe 1.4 mm×25 mm tube

Epoxy seal

Epoxy seal

[image:2.609.362.516.72.182.2]

Glass tip

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Results Ficoll Ringer perfusions

Ten glomeruli were perfused with Ficoll Ringer at rates that varied from approximately 1.4 to approximately 10µl min−1;

the Pperf, the PGC and the SGFR were measured

simultaneously. The COP of urine accumulated during eight of the perfusions was measured at the end of the experiments.

Fig. 2A illustrates the characteristic increase in Pperf and

decrease in the vascular resistance observed when hagfish glomeruli were perfused with Ficoll Ringer at rates that were changed incrementally. As shown in Fig. 2B, the SGFF fell as the perfusion rate was increased from 1.4 to 5.4µl min−1,

but when the perfusion rate was increased to 10µl min−1, the

SGFF rose. In Fig. 2C, values of the SGFF and the vascular resistance are identified with respect to the perfusion rates that produced them. The regression line of best fit for all of the data was a third-order polynomial which showed that the SGFF and the vascular resistance were positively related except when the perfusion rate was 10µl min−1. A linear

regression line plotted through the data for perfusion rates between 1.4 and 5.4µl min−1 (broken line, Fig. 2C) had a

highly significant positive slope (P<0.001, N=92). The significance of the difference between perfusions made at 10µl min−1 and 1.4–5.4µl min−1 will be analysed in the

Discussion.

Hydrostatic pressures measured in vessels of perfused glomeruli

Low-pressure glomerular vessels

The LPGVs were found within the interior of the glomerular vascular tuft. Fig. 3A shows part of an oscillographic recording of pressures measured within a LPGV. At t=0, the perfusion rate was approximately 1.4µl min−1(CC=4; FC=minimum), which resulted in a P

perf

of approximately 0.5 kPa and a PGC of approximately

0.17 kPa. Near t=100 s, the perfusion rate was increased using the fine control (FC+), and the Pperf increased to

approximately 0.78 kPa with no change in the PGC. At

t=150 s, the perfusion rate was again increased using the FC

causing the Pperfto increase to approximately 1 kPa, with no

change in the PGC. Near t=200 s, the FC was set to maximum

(perfusion rate approximately 10µl min−1); the P perf

increased to approximately 1.5 kPa and the PGC increased

abruptly to approximately 0.41 kPa. Between t=400 s and 450 s, the FC was set to minimum (perfusion rate approximately 1.4µl min−1); the P

perf decreased to

approximately 0.38 kPa, whilst the PGC decreased to

approximately 0.18 kPa.

Measurements of the SGFR were made near t=0 (SGFR=11.9 nl min−1), near t=350 s (SGFR=129 nl min−1) and

between t=500 s and t=550 s (SGFR=54.6 nl min−1).

All values of the PGCand the Pperfmeasured in four LPGVs

are plotted in Fig. 3B. The vertical broken line represents the highest pressure measured in the dorsal aorta of E. stouti (Riegel, 1986b; J. A. Riegel, unpublished data). The horizontal

broken line indicates the mean COPeff calculated for all the

experiments in which glomeruli were perfused with Ficoll Ringer alone (see Materials and methods).

High-pressure glomerular vessels

The HPGVs were found at the periphery of the glomerular vascular tuft. Fig. 4A shows part of an oscillographic recording illustrating pressure responses characteristic of the HPGV. At t=0, the glomerulus was perfused at approximately 5.4µl min−1 (CC=3, FC=maximum). The P

perf averaged

approximately 1.12 kPa and the PGCaveraged approximately

0.26 kPa. Near t=50 s, the CC was switched to 4 and the FC was set to minimum (perfusion rate approximately 1.4µl min−1). Following this change, the P

perf decreased to

approximately 0.46 kPa; the PGC fell initially to a low of

approximately 0.17 kPa, after which it increased abruptly to approximately 0.39 kPa. Near t=100 s, the perfusion rate was increased using the FC; this caused the Pperf to increase to

approximately 0.64 kPa, but the PGC appeared to be little

affected. Near t=150 s, the perfusion rate was again increased using the FC. The Pperf rose to approximately 0.78 kPa; the

PGC showed a shallow decrease. Just after t=200 s, the

perfusion rate was again increased using the FC. The Pperf

increased to approximately 1.05 kPa; the PGC decreased to

0.26 kPa. Between t=300 s and t=350 s, the FC was set to maximum, increasing the perfusion rate to approximately 10µl min−1; the P

perfincreased to approximately 1.5 kPa and

remained at that level. The PGC increased abruptly by

approximately 30 % and continued to increase at a slower rate over a period of a few minutes, reaching a maximum value of 0.98 kPa. At that point, the FC was set to minimum (perfusion rate 1.4µl min−1); the P

perfdecreased to 0.51 kPa and the PGC

decreased to 0.04 kPa.

Between t=100 s and t=150 s, the SGFR was 34.5 nl min−1;

near t=500 s, the SGFR had risen to 271 nl min−1. Just after

t=600 s, it was observed that fluid in the ureter catheter had

retreated into the ureter segment, probably indicating that the glomerular vascular tuft had collapsed.

All values of the PGCmeasured in HPGVs of six perfused

glomeruli are plotted in Fig. 4B. The positive relationship between the Pperfand the PGCwas much more marked in the

HPGVs than was the case for the LPGVs. The slopes of the regression lines of Figs 3B and 4B are highly significantly different (d=5.09, P<0.001).

Maximum values of the PGCin the LPGVs and the HPGVs

Fig. 5 presents values of the PGC produced by rapidly

increasing the perfusion rate during manoeuvres designed to identify the HPGVs (open circles) and the LPGVs (filled circles). The measurements were made during (control) periods when Ficoll Ringer was being perfused through the glomeruli. Maximum values of the PGCare plotted against maximum values

of the Pperf measured simultaneously. There was a clear

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Pperf

Polynomial function (Pperf) Power function (Resistance) Resistance

0 0.5 1.0 1.5 2.0 2.5

Pperf

(kPa)

0 2 4 6 8 10

y=0.0042x30.0715x2+0.3929x+0.3475

y=0.662x−0.7187

r2=0.7725

r2=0.3479

0 0.5 1.0 1.5 2.0 2.5 3.0

Resistance (kPa min

µ

l

-1)

A

1.4 µl min-1

10 µl min-1

5.4 µl min-1

2.7 µl min-1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Vascular resistance (kPa min µl-1)

y=−0.6027x3+0.867x2−0.3082x+0.0433

r2=0.3964

C

0 0.02 0.04 0.06 0.08 0.10 0.12

SGFF

0 0.02 0.04 0.06 0.08 0.10 0.12

SGFF

0 2 4 6 8 10

Perfusion rate (µl min-1)

y=0.001x2−0.0135x+0.0519

r2=0.301

B

Fig. 2. (A) Relationship between the perfusion pressure (Pperf), the vascular resistance and the perfusion rate whilst glomeruli were perfused

with Ficoll Ringer at rates that varied from 1.4 to 10µl min−1. (B) Variation in the single glomerulus filtration fraction (SGFF) with perfusion

[image:4.609.113.488.67.668.2]
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LPGVs, 32 HPGVs) were made in 47 glomeruli during perfusion at maximum pressures that varied from 0.85 to 3.1 kPa.

The effect of papaverine

To compensate for variation between individual perfusions, individual values of the Pperf, the SGFR and the vascular

resistance were converted to a percentage of the average value during the control period of each perfusion and then averaged. As shown in Table 1, papaverine significantly reduced the

Pperf, the SGFR and the vascular resistance of perfused

glomeruli.

Theσof Ficoll 70 and the COPeff

The average COP of the urine produced by eight glomeruli perfused with Ficoll Ringer was 0.40±0.30 kPa. The calculated concentrations of colloid (Gamble, 1983) were 0.65 g % and 1.95 g %, respectively, for urine and perfusion fluid. From these values, a σvalue of 0.67 (i.e. 1−0.65/1.95) for Ficoll 70

was calculated. A COPeffof 0.67 was calculated from the data [i.e. 0.67(1.4−0.4)].

Discussion

The argument against pressure filtration

For pressure filtration to occur, the PGC must exceed the COPeff. The COPeffof 0.67 is indicated in Figs 3B and 4B by the horizontal broken line. Pressures in the LPGVs were never adequate to effect pressure filtration (Fig. 3B). Pressures in the HPGVs were adequate to effect pressure filtration in approximately 20 % of the measurements (Fig. 4B), all but one of which were made whilst glomeruli were being perfused at 10µl min−1, a rate that produced values of the Pperf that exceeded the maximum pressures measured in the renal blood supply of E. stouti (vertical broken line, Figs 3B, 4B; Riegel, 1986b,c; J. A. Riegel, unpublished data).

It is likely that the high values of the SGFF relative to the

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0 0.5 1.0 1.5 2.0 2.5

Pperf (kPa) b=0.117±0.026, P<0.001

PGC

(kPa)

B

1.4 µl min-1 10 µl min-1 1.4 µl min-1

0

FC+

FC+ FC+ 0.5

1.0 1.5

Pperf

(kPa)

0 50 100 150 200 250 300 350 400 450 500 550

Time (s)

0 0.5 1.0

PGC

(kPa)

A

FC−

Fig. 3. (A) A portion of an oscillographic recording made during perfusion of a glomerulus with Ficoll Ringer at varying rates: the upper trace is the the perfusion pressure (Pperf); the lower trace is the

pressure (PGC) in a low-pressure

glomerular capillary (LPGV). The horizontal arrow with a vertical bar at the top of the figure indicates where the coarse control of the perfusion pump was changed; horizontal arrows with vertical bars marked FC+ or FC− indicate where the fine control of the perfusion pump was changed. (B) All values of the PGC

recorded in the LPGVs during perfusion of four glomeruli with Ficoll Ringer. The regression line (y=0117x+0.136) was fitted to the data using methods outlined by Bailey (1976). The maximum pressure measured in the dorsal aorta of lightly anaesthetised specimens of Eptatretus

stouti is indicated by the vertical broken

line. The horizontal broken line indicates the pressure that must be exceeded by the

[image:5.609.213.563.320.732.2]
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low values of the vascular resistance shown in Fig. 2C for glomeruli perfused at 10µl min−1 were due to pressure

filtration. Nevertheless, for the majority of the time during the six perfusions summarised in Fig. 4B, the PGCof HPGVs was

not adequate to overcome the COPeff; despite this, the

production of the primary urine was continuous.

Fluid dynamics of perfused hagfish glomeruli

It has been observed consistently (Stolte and Eisenbach,

1973; Riegel, 1978, 1986c, 1998, present study) that the SGFR of perfused hagfish glomeruli bears a positive relationship to the perfusion rate. Except at high rates of perfusion, this is unlikely to be caused by changes in the PGC(see above). As

shown in Fig. 2A,B, when the perfusion rate was increased from 1.4 to 5.4µl min−1, there was a progressive decrease in

both the vascular resistance and the SGFF; an obvious explanation of this is that there are at least two flow pathways through hagfish glomeruli which are of unequal vascular

b=0.498±0.104, P<0.001

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0 0.5 1.0 1.5 2.0 2.5

Pperf (kPa)

PGC

(kPa)

B

1.4 µl min-1

5.4 µl min-1 10 µ

l min-1 1.4 µl min-1

0

FC+

FC+ FC+

FC+ 0.5

1.0 1.5

Pperf

(kPa)

0 50 100 150 200 250 300 350 400 450 500 550 600

Time (s)

0 0.5 1.0

PGC

(kPa)

A

FC−

FC−

Fig. 4. (A) A portion of an oscillographic recording made during perfusion of a glomerulus with Ficoll Ringer at varying rates: the upper trace is the perfusion pressure (Pperf); the lower trace is the

pressure (PGC) in a high-pressure

glomerular capillary (HPGV). Horizontal arrows with a vertical bar at the top of the figure indicate where the coarse control of the perfusion pump was changed; horizontal arrows with vertical bars marked FC+ or FC− indicate where the fine control of the perfusion pump was changed. (B) All values of the PGC

recorded in HPGVs during perfusion of six glomeruli with Ficoll Ringer. The regression line (y=0.498x−0.208) was fitted to the data using methods outlined by Bailey (1976). The maximum pressure measured in the dorsal aorta of lightly anaesthetised specimens of Eptatretus

stouti is indicated by the vertical broken

line. The horizontal broken line indicates the pressure that must be exceeded by the

[image:6.609.57.561.99.154.2]

PGC to overcome the effective colloid osmotic pressure of the perfusate.

Table 1. Perfusion pressure (Pperf), single glomerulus filtration rate (SGFR) and estimated vascular resistance during perfusion of five glomeruli with either Ficoll Ringer (control) or Ficoll Ringer +100 µmol l1papaverine (experimental)

Perfusion rate Pperf SGFR Resistance

(% Control Av.) (% Control Av.) (% Control Av.) (% Control Av.)

Control (N=27) 100 100±8.54 100±30.2 100±9.33

Experimental (N=18) 100 86.2±10.8 57.2±25.0 85.8±10.2

Values (mean ±S.D.) are expressed as a percentage of the average value during the period of control perfusion (% Control Av.).

[image:6.609.205.558.347.734.2]
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resistance and contribute unequally to urine formation. Over this range of perfusion rates, the SGFF and the vascular resistance were significantly positively related (Fig. 2C), suggesting that the major part of the urine was formed by vessels in the high-resistance flow pathway.

The location of vascular resistances that control flow in hagfish glomeruli

Flow through the vertebrate glomerulus is controlled by resistances both afferent and efferent to the capillary tuft; generally these resistances are considered to be comprised of the afferent and efferent arterioles (e.g. Hura and Stein, 1992). In hagfish glomeruli, the situation may be more complicated.

Anatomical evidence

In her study of corrosion casts of the glomeruli of Myxine

glutinosa, Brown (1988) demonstrated the presence of

vessels that branch off the renal artery just after its entry into the Bowman’s capsule; these vessels shunt the glomerular capillaries. Shunt vessels were found in only a minority (approximately 28 %) of the large number of glomeruli examined, although it was possible that the low incidence was due to failure of the shunt vessels to fill with casting material. Were shunt vessels to exist in Eptatretus stouti, it is possible that they could represent a low-resistance pathway bypassing the glomerular capillary tuft; they would have been perfused when the renal artery was perfused in the present study.

Albrecht et al. (1978) have described constrictions, suggestive of sphincters, at sites where peripheral vessels joined more deeply lying vessels in glomeruli of Myxine glutinosa. The similar location of the peripheral vessels of Albrecht et al. (1978) and the HPGVs described here raises the possibility that they are the same. Finally, it has been a consistent observation that hagfish glomeruli may have more than one efferent arteriole (Heath-Eves and McMillan, 1974; Albrecht et al. 1978; Brown, 1988; J. A. Riegel, unpublished observations).

Functional evidence

The existence of a resistance afferent to vascular tufts of hagfish glomeruli is supported by the data of Figs 3B and 4B. In neither the HPGVs nor the LPGVs did the PGCincrease in

direct proportion to the increase in the Pperf. Low values of the

PGC in the LPGVs and, especially, the shallow slope

(0.117±0.026) of the regression line relating PGC to Pperf

(Fig. 3B) indicate that there may be a further resistance imposed on flow in the LPGVs. The location of this resistance is problematical, but if the peripheral vessels described by Albrecht et al. (1978) are the same as the HPGVs described here, the constrictions that they contain could act as a resistance to flow.

Speculations

Analysis of fluid flow through hagfish glomeruli must account for the consistent observation (Riegel, 1986c, 1998, in preparation; Fig. 2C) that at low to intermediate perfusion rates the SGFF is positively related to the vascular resistance. The resistance involved is most likely to be a resistance efferent to the glomerular capillaries or the resistance of the glomerular capillaries themselves or perhaps both. Even under conditions of a deficit in the net filtration pressure, the SGFR is dependent upon the perfusion rate. Changes in the perfusion rate must provoke changes in a parameter other than PGC; the most likely

candidate is the area of the urine-forming epithelium.

It is possible that flow through the glomerular capillary tuft is controlled by both variable and fixed vascular resistances located on the efferent side of the glomerulus. The variable resistance may control flow from peripheral vessels (HPGVs) to the postglomerular circulation, whilst the fixed resistance may control flow from the deep vessels (LPGVs) to the postglomerular circulation. This arrangement might permit flow through the glomerular capillary tuft to be controlled entirely by alterations in the patency of the variable resistance. When the variable efferent resistance is patent, much of the flow could shunt the interior of the capillary tuft; when the variable efferent resistance is fully or partially closed, flow

0 0.2 0.4 0.6 0.8 1.0 1.2

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

HPGV LPGV

Maximum Pperf (kPa)

Maximum

PGC

[image:7.609.268.561.77.275.2]

(kPa)

Fig. 5. Maximum values of the hydrostatic pressure (PGC) recorded in capillaries of 47 glomeruli perfused

with Ficoll Ringer at rates that produced perfusion pressures (Pperf) of between approximately 0.85 and

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could be forced into the interior of the capillary tuft. The constrictions described by Albrecht et al. (1978) might be involved in this scheme; even if the patency of their opening was not controllable, they might impose a resistance on flow between the peripheral vessels and interior vessels of the capillary tuft. The low slope of the regression line of Fig. 3B might be attributable to this.

That there may be separate postglomerular drainage of HPGVs and LPGVs is supported by the fact that pressures characteristic of both kinds of vessels are found in the postglomerular circulation of Eptatretus stouti (Riegel, 1986b,c).

Control of vasomotor tone in perfused hagfish glomeruli

Chemicals that affect vasomotor tone affected flow in hagfish glomeruli. The vasodilator papaverine reduced the SGFR, the Pperf and the vascular resistance of perfused

glomeruli (Table 1). The vascoconstrictors adrenaline and noradrenaline elevated the SGFR, the Pperf and the vascular

resistance (Fels et al. 1987; J. A. Riegel, in preparation). In conclusion, it seems clear that the hagfish mesonephric glomerulus normally does not utilise pressure filtration to form the primary urine. Future work (J. A. Riegel, in preparation) will present evidence that fluid secretion underlies that process.

It is a pleasure to thank the Director, Dr James S. Clegg, and the staff of the Bodega Marine Laboratory for their help. Professor Charles Michel and, especially, Dr Simon Maddrell have been most generous in helping me to formalise ideas. I thank them both warmly.

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Figure

Fig. 1. Diagram of a dual microcannula (scale only approximate)which was constructed as follows
Fig. 2. (A) Relationship between the perfusion pressure (Pusing methods outlined by Bailey (1976); the calculated regression coefficient with Ficoll Ringer at rates that varied from 1.4 to 10rate
Fig. 3. (A) A portion of an oscillographicrecording made during perfusion of athe fine control of the perfusion pump waschanged
Table 1. Perfusion pressure (Pperf), single glomerulus filtration rate (SGFR) and estimated vascular resistance during perfusionof five glomeruli with either Ficoll Ringer (control) or Ficoll Ringer +100 µmol l−1 papaverine (experimental)
+2

References

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