Renal Blood Flow in Acute Cerebral Ischemia
in Spontaneously Hypertensive Rats:
Effects of a- and /3-Adrenergic BlockadeHiroshi Yao, Seizo Sadoshima, Osamu Shiokawa,
Kenichiro Fujii, and Masatoshi Fujishima
The influences of acute cerebral ischemia on renal hemodynamics were examined in spontaneously hypertensive rats in which cerebral ischemia was induced by bilateral carotid artery occlusion. Renal and cerebral blood flow were measured with a hydrogen clearance technique. Either phenoxybenza-mine (0.5 mg/kg body wt) or propranolol (2 mg/kg) was given i.v. immediately after ischemia was induced to examine the drugs' effects on cerebral and renal hemodynamics. One hour after ischemia, cerebral blood flow was markedly reduced to 5, 3, and almost 0% of the preischemic value in the untreated, phenoxybenzamine-treated, and propranolol-treated rats, respectively. In contrast, renal blood flow at that time was decreased to 65, 88, and 67%, respectively. The calculated renal vascular resistance was similarly increased to 151 % In the untreated and 136% in the propranolol-treated rats, but decreased to 82% in the phenoxybenzamine-treated rats. The present results indicate that in acute cerebral ischemia renal blood flow was considerably decreased with concomitant increased renal vascular resistance, and that such reduction in renal blood flow was minimized by a-adrenergic blockade but not by /3-blockade. It is concluded that activation of the a-adrenergic system in acute cerebral ischemia causes renal vasoconstriction. (Stroke 1987;18:629-633)
It is known that cerebrovascular disease not only leads to severe derangements of the cerebral cir-culation and metabolism, but also to undesirable dysfunctions of various extracranial organs. Cardiac arrhythmias, electrocardiographic changes, and myo-cardial necrosis are not uncommon,1"3 acute gastric erosion or ulcer sometimes causes massive hemor-rhage,4-3 and pulmonary congestion and edema lead to respiratory distress.67 Increased activity of the sympa-thetic nervous system is explained to be, in part, re-sponsible for these extracranial changes.
Renal blood flow (RBF) is mainly regulated by in-trinsic (autoregulation) and exin-trinsic factors (auto-nomic nervous system and humoral elements),8"10 al-though the renal hemodynamics in acute cerebral ischemia are not fully understood. In this study, we measured renal cortical blood flow in spontaneously hypertensive rats (SHR) in which acute cerebral ische-mia was produced by bilateral carotid occlusion (BCO). The purposes of this study were to observe whether cerebral ischemia affects the regulation of RBF and, if so, to examine whether sympathetic nerve activity influences its regulation.
Materials and Methods
Twenty-six male SHR aged 5-6 months, weighing 240-360 g, were used in this study. The rats were fed stock chow diet (Oriental Co., Japan) and tap water ad libitum. Under amobarbital anesthesia (100 mg/kg
From the Second Department of Internal Medicine, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Address for reprints: Hiroshi Yao, MD, Second Department of Internal Medicine, Faculty of Medicine, Kyushu University, Mai-dashi 3-1-1, Higashi-lcu, Fukuoka, Japan 812.
Received August 14, 1986; accepted January 13, 1987.
body wt i.p.), both femoral arteries were cannulated, one for continuous recording of systemic arterial blood pressure and heart rate, and the other for anaerobic sampling of blood for pH, Poj, and Pco? determina-tions. A femoral vein was also cannulated for infusion of drugs. Both common carotid arteries were exposed through a ventral midline incision in the neck, separat-ed from the vagosympathetic trunks carefully, and loosely encircled with sutures for later ligation. The rectal temperature was maintained close to 37° C by a heat lamp, and the rats breathed room air spontaneous-ly throughout the experiment.
A hydrogen clearance technique was chosen for ce-rebral cortical blood flow (CBF) and RBF determina-tions.1112 Details of CBF measurement were described previously.13 Briefly, the rat's head was fixed in a head holder, and a small burr hole was made on the skull 2 mm lateral to the bregma. A Teflon-coated platinum electrode, 200 fim in diameter, with a 1-mm portion at its tip uncoated and plated with platinum black, was placed stereotactically in the cerebral parietal cortex, 2 mm from the surface of the brain. The reference Ag-AgCl electrode was inserted under the skin. RBF was determined simultaneously in the same rat. After a longitudinal skin incision was made in the loin, the right kidney was exposed through the muscle layers. A fine platinum electrode, 80 /im in diameter, with a 1-mm portion at its tip uncoated, was put in the renal cortex, 2 mm from the surface of the kidney, through the renal capsule manually.
Hydrogen gas, 10% in room air, was administered under spontaneous breathing. At least 2 baseline CBF and RBF were measured at intervals of about 10 min-utes. Then both carotid arteries were ligated tightly, and CBF and RBF were determined 5, 15, 30, and 60
630 Stroke Vol 18, No 3, May-June 1987 Table 1. Arterial Add-Base Parameters, Mean Arterial Blood Pressure, Heart Rate, Cerebral Blood Flow, and Renal Blood Flow Before and 60 Minutes After Bilateral Carotid Occlusion
Pcoj (mm Hg) P02 (mm Hg) pH MABP (mm Hg) HR (beats/min) CBF (ml/100 g/min) RBF (ml/100 g/min) Untreated Before 35.8+1.1 83.3 ±6.0 7.45 ±0.01 198 ± 3 371 ±10 49.9 ±3.1 275 ±25 After 16.7±1.7 84.3±4.2 7.58±0.02 186±7 448±17 2.4±2.4 180±21$ Phenoxybenzamine Before 33.0±1.0 92.0±8.6 7.45 ±0.01 189±6 372±11 48.4±5.8 249 ±39 After 14.4±2.0 88.6±5.4 7.63 ±0.04 135 ±14* 402 ±21 1.5 + 1.5 218 + 31 Propranolol Before 34.8 ±1.4 84.8±3.7 7.45 ±0.01 194 + 6 365 ± 7 51.7 + 5.4 240 ±35 After 21.1+2.9 8O.0±1.2 7.53±0.04 174 ±15 356±12t 0 160 ±22$ Distal occlusion 71 = 6 Before After 35.3 + 1.1 26.0±3.6 72.8±2.8 69.7±2.5 7.47±0.01 7.56±0.03 173 + 10 186+15 377 ±12 442 ±21 — — 252±30 174±30$ Values are mean ± SEM. Phenoxybenzamine
MABP, mean arterial blood pressure; HR, heart •p < 0.02, tp < 0.01 vs. untreated, $p < 0.
(0.5 rag/kg) and propranolol (2 mg/kg) were given i.v. 1 minute after carotid occlusion, rate; CBF, cerebral blood flow; RBF, renal blood flow.
01 vs. before carotid occlusion.
minutes after BCO. To examine the effect of sympa-thetic activation on RBF during cerebral ischemia, phenoxybenzamine (0.5 mg/kg, 6 rats) or propranolol (2 mg/kg, 6 rats) was adminstered i.v. 1 minute after BCO. After completing the experiment, the rats were killed with an i.v. injection of overdose amobarbital, and their kidneys were macroscopically examined. When either improper placement of or macroscopic tissue damage caused by inserting the electrodes was found, the data were excluded from the present results.
Common carotid occlusion reduces the arterial pres-sure at the carotid sinus, and the resulting enhanced stimulation of the peripheral baroreceptor results in enhanced sympathetic activity, which might reduce RBF. To avoid the effects of common carotid occlu-sion on baroreflex function and RBF, cerebral ische-mia was induced in 6 rats by occlusion of the bilateral internal and external carotid arteries distal to the carot-id sinus (distal occlusion) using microclips under an operating microscope. In these rats, RBF was mea-sured before and 5,15, 30, and 60 minutes after distal occlusion.
The differences in mean arterial blood pressure (MABP), heart rate (HR), CBF, and RBF between the untreated and the other 3 groups were analyzed using the unpaired t test with Bonferroni correction based on one-way analysis of variance, and RBF before and 60 minutes after BCO, by a paired t test.
Table 1 summarizes the mean values for arterial acid-base parameters, MABP, HR, CBF, and RBF before and 60 minutes after BCO. There were no dif-ferences in physiologic parameters between untreated and the other 3 groups of rats before BCO. After BCO, the rats in all groups began to hyperventilate, resulting in decreased Pco^ and increased pH. In the untreated and propranolol-treated groups, MABP rose to 231 and 225 mm Hg 5 minutes after BCO, followed by a gradual fall to 186 and 174 mm Hg, respectively, at 60 minutes after BCO (Figure 1). In the phenoxybenza-mine-treated group, MABP rose less markedly to 206 mm Hg at 5 minutes and fell below the preischemic
level, to 135 mm Hg at 60 minutes after BCO. In the distal occlusion group, MABP did not rise until 15 minutes after BCO, then rose to the same level as the untreated or propranolol-treated groups 30 and 60 min-utes after BCO. HR rose gradually after occlusion in the untreated and phenoxybenzamine-treated groups, but remained unchanged in the propranolol-treated group throughout the experiment.
Five minutes after BCO, CBF in both the phenoxy-benzamine- and propranolol-treated groups was great-ly lowered, to < 10% of the resting value, while in the untreated group CBF was reduced to 34% (Figure 2). At 60 minutes after BCO, CBF was further reduced to
MABP(mmHg) 200 150 0 5 15 30 60(min)
FIGURE 1. Changes in mean arterial blood pressure (MABP) after bilateral common carotid or distal occlusion in untreated (9), phenoxybenzamine-treated (A), propranolol-treated (O), and distal occlusion (O) rats. *p<0.05, ** p<0.02, *** p<0.01 vs. untreated rats. Bars represent SEM.
0 5 15 30 60 (mm)
FIGURE 2. Percent changes in cerebral blood flow after bi-lateral common carotid occlusion in untreated (%), phenoxy-benzamine-treated (A), and propranolol-treated O rats. *p<0.05, **p<0.02vs. untreated rats. Bars represent SEM.
5,3, and almost 0% of the resting value in untreated, phenoxybenzamine-, and propranolol-treated rats, re-spectively, but these differences were not significant. RBF 60 minutes after BCO decreased significantly from 275 ± 25 to 180 ± 21 ml/100 g/min in untreated rats, from 240 ± 35 to 160 ± 22 ml/100 g/min in pro-pranolol-treated rats, and from 252 ± 30 to 172 ± 30 ml/100 g/min in distal occlusion rats, while the de-crease in RBF in phenoxybenzamine-treated rats was not statistically significant (Table 1). Figure 3 illus-trates the percent changes in RBF after BCO. RBF in untreated and propranolol-treated rats showed a rela-tively steep reduction during the first 15 minutes and remained as low as 65-67% of the resting values. In the distal occlusion rats, RBF was less dramatically decreased at 30 minutes after carotid occlusion (not significant), and decreased to almost the same value as the untreated and propranolol-treated groups at 60 minutes after occlusion. In contrast, RBF in rats treat-ed with phenoxybenzamine was less dramatically de-creased, to 88% at 60 minutes after BCO. The changes in renal vascular resistance (RVR), calculated as MABP/RBF, were also markedly different between the phenoxybenzamine-treated and the other 3 groups (Figure 4). After an initial rise in RVR, the former group showed a gradual reduction to 82% of the base-line RVR at 60 minutes after BCO, whereas in the latter 3 groups RVR was sustained as high as 136-168% of baseline values.
The dysfunction of extracranial organs associated with stroke has been investigated clinically. Cardiac arrhythmias with giant negative T are often observed in patients with subarachnoid hemorrhage, focal or sometimes transmural necrosis of the myocardium is reported on postmortem examination,1 neurogenic
pul-RBF 100 90 80 70 60 n
A\ \ \ \ 1 t 0 5 15 30 60 (min)
FIGURE 3. Percent changes in renal blood flow (RBF) after bilateral common carotid or distal occlusion in untreated (%), phenoxybenzamine-treated (A), propranolol-treated (O), and distal occlusion (Q) rats. RBF in phenoxybenzamine-treated rats is better preserved than in untreated rats. 1r p = 0.05 vs. untreated rats. Bars represent SEM.
RVR 180 160 HO 120 100 80 n
f • '
• I !• 0 5 15 30 60 (mm)
FIGURE 4. Percent changes in renal vascular resistance (RVR) after bilateral common or distal occlusion in untreated (%), phenoxybenzamine-treated (A), propranolol-treated fO), and distal occlusion (Q) rats. RVR in phenoxybenzamine-treat-ed rats was significantly decreasphenoxybenzamine-treat-ed comparphenoxybenzamine-treat-ed with untreatphenoxybenzamine-treat-ed rats. * p<0.05, ** p<0.02, *** p<0.01 vs. untreated rats. Bars represent SEM.
632 Stroke Vol 18, No 3, May-June 1987 monary edema with hemorrhage causes respiratory
distress with low P02 and is often misdiagnosed as bronchopneumonia,67 gastric lesions are frequent and the mortality is high especially in patients with acute ulcer, multiple erosions, and petechiae.4-5 All these extracranial lesions are considered to result from the increased sympathetic outflow in acute stroke, al-though less attention has been paid to the derangement of RBF and its mechanism.
We have previously reported that bilateral carotid artery occlusion in SHR reduces supratentorial CBF to < 10 ml/100 g/min13 and develops large cerebral in-farctions identical to those in humans histologically14 and biochemically.13 In the present study, CBF to the cerebral cortex 60 minutes after BCO was 2.4 ml/100 g/min in control SHR and 1.5 or actually 0 in phenoxy-benzamine- or propranolol-treated rats, indicating that the cerebrum was exposed to severe ischemic insult in our model. Under such conditions, RBF was reduced to 72% of the resting value 30 minutes after BCO and to 65% at 60 minutes. RBF and RVR in the distal occlusion group were essentially the same as in the untreated group, indicating that the effect of the baro-reflex on RBF during cerebral ischemia was minimal in our BCO model and that the reduction in RBF was primarily caused by cerebral ischemia. Although the effect of hypocapnia on RBF was not examined in this study, the previous examination by Norman et al16 indicated that variations in PcOj of < 20 mm Hg have little effect on RBF. We assume, therefore, that the PcOj levels in this study may not affect the renal vascu-lar responses during cerebral ischemia.
The kidney constitutes only 0.5% of body weight although it receives 20% of the cardiac output. It is widely accepted that two major factors contribute to regulate RBF: autoregulation and neurogenic or other humoral factors. The autoregulatory range in normo-tension is 70-160 mm Hg MABP. In hypernormo-tension, the range may be shifted to a higher blood pressure. Since RBF was decreased despite raised MABP in this study, the reduced RBF during cerebral ischemia could not be explained simply by disturbance of RBF autoregula-tion but rather requires some other mechanism.
In a resting, stable condition, there is little sympa-thetic vasoconstrictor tone in the kidneys.10 In severe hypoxia, hypercapnia,816 hemorrhagic shock, or exer-cise,1718 however, a neurogenic mechanism causes renal vasoconstriction and reduces RBF. Furthermore, noradrenaline infusion or renal nerve stimulation decreases RBF, indicating that neurogenic vasocon-striction could exceed autoregulatory control of the myogenic mechanism in some conditions.19 During swimming, RBF in dogs was reduced to about 62% of the resting value, although the dogs' blood pressure was increased 43%.1718 Phenoxybenzamine abolished such RBF reduction, indicating that enhanced sympa-thetic activity induced by stress constricts renal blood vessels, increases RVR, and reduces RBF.
In the acute stage of stroke, circulating catechola-mines, as an index of sympathetic activity, are also reported to be elevated. Myers et al3 found that plasma
norepinephrine concentration in patients with cerebral infarction was significantly increased to 433.2 ±33.9 pg/ml in contrast to 282.1 ±19.8 pg/ml in control subjects, and suggested that the high plasma norepi-nephrine could produce the cardiac abnormalities in stroke patients. To examine the possibility of neural influences on RBF in acute cerebral ischemia, a- or /3-adrenergic blocking agents were infused after carotid occlusion in this study. Compared with the untreated group, /3-blockade with propranolol had little effect on changes in RBF and RVR, while a-blockade with phenoxybenzamine significantly minimized such changes, indicating that a-adrenergic activation plays an important or essential role in renal vasoconstriction in acute cerebral ischemia. However, whether such sympathetic activation is mediated by increases in renal sympathetic nerve discharge or in circulating catecholamines is not clear from the present study. Further study is needed to clarify the relation of renal hemodynamics to kidney function in acute stroke.
It is concluded that in acute cerebral ischemia in SHR, RBF is decreased with a concomitant increase in RVR, which is mediated by a-adrenergic activity.
We thank Dr. Akira Takeshita and Dr. Hideki Higa-shi for their valuable advice during the course of this study.
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KEY WORDS • renal • phenoxybenzamine • propranolol
blood flow • cerebral ischemia spontaneously hypertensive rats •