Redistribution of cortical blood flow during renal vasodilatation in dogs

[Hydrodynamic and biochemical adaptation to perfusion pressure profile in isolated hypothermic renal perfusion]

OBJECTIVES

To evaluate hydrodynamic, biochemical and pathologic consequences of positive forced pressure on hypothermic isolated renal perfusions in comparison to softly progressive administration of perfusion solution.

MATERIAL AND METHOD

16 hypothermic experimental pig renal perfusion o 180 min completed under constant flow conditions. A vacuum computer-controlled pump was used. A comparison was performed of the results obtained by two groups of organs (forced perfusion pressure -I.B.- and soft perfusion pressure -I.S.).

RESULTS

Even though perfusion flor did not offer significant differences among both groups at the end of the procedure, I.B. group showed medium perfusion pressure and renal vascular resistance values significantly higher. Nitric oxide metabolites concentration was also greater in I.B. group. Finally, these kidneys showed more tubular cell aedema and glomerular shrinkage in comparison to I.S. group.

CONCLUSIONS

Sharp and forced hypothermic perfusion of the isolated kidney don't increase flow perfusion values. Inversely, hydrodynamic and pathologic results are worst in comparison to soft and progressive organ perfusion.

Effect of furosemide on local and zonal glomerular filtration rate in the rat kidney

Furosemide has been reported to produce disproportional changes in blood flow in cortical zones and to inhibit tubuloglomerular feedback (TGF), suggesting that furosemide might alter the intracortical distribution of glomerular filtrate. We have tested this hypothesis by a new method for measuring local and total glomerular filtration rate (GFR) based on proximal tubular accumulation of the basic polypeptide aprotinin (mol wt 6513). Local GFR was calculated in tissue samples dissected from outer cortex (OC), inner cortex (IC) and the corticomedullary border zone (CM) from the plasma clearances of two aprotinin tracers injected i.v. before and after a 3 min i.v. infusion of 25 mg kg-1 furosemide. The mean of five samples from each region was used to determine zonal GFR. Isotonic saline was infused at a rate corresponding to urine flow. Furosemide reduced whole kidney GFR from 1.17 to 1.00 mL min-1 and gave a similar reduction of renal artery blood flow. Urine flow increased from 0.6 to 17% of GFR. Haematocrit (approximately 0.48) and plasma protein concentration (approximately 55 mg mL-1) were maintained while the arterial blood pressure tended to decline (118 +/- 5 mmHg to 108 +/- 6 mmHg, P < 0.05). GFR in OC, IC and CM (1.58, 1.18, 0.42 mL min-1 g-1) fell to 87, 88 and 88% of control after furosemide infusion respectively. The furosemide/control ratio for each sample showed a coefficient of variation of about 3%. We conclude that furosemide produced a modest GFR reduction that was uniform throughout the renal cortex. The homogenous GFR response suggests a similar TGF constriction tone in preglomerular vessels of deep and superficial nephrons.

Intrarenal vascular sites of action of adenosine and glucagon

Our purpose was to localize the intrarenal vascular sites of action of adenosine and glucagon. Renal blood flow (RBF) and glomerular filtration rate (GFR) were measured in anesthetized dogs, and renal perfusion pressure (RPP) was varied by an adjustable aortic clamp. At normal RPP, RBF was increased by all agents. In contrast, GFR was increased by glucagon, decreased by adenosine and unchanged by acetylcholine (ACh) or adenosine plus glucagon. The increases in RBF by glucagon occurred only at RPPs within the autoregulatory pressure range, and renal autoregulatory capability was attenuated during the infusion of glucagon. In contrast, adenosine increased RBF at RPPs both within and below the autoregulatory pressure range, and the autoregulatory capability was not perceptibly impaired. Superimposition of glucagon to adenosine caused further vasodilation, and the autoregulatory efficiency was completely attenuated. There was no difference between the RPP-RBF or RPP-GFR relations obtained during infusion of adenosine plus glucagon and ACh, which dilates both the afferent and efferent arterioles. It is generally accepted that afferent arteriolar resistance attains a minimum value at RPP near the lower limit of the autoregulatory range. Thus, our data indicate that glucagon and adenosine preferentially dilate the afferent arteriole and the efferent arteriole, respectively.

Indomethacin and the gastric mucosal blood flow changes of sepsis

Recent evidence suggests that sepsis results in increased gastric mucosal blood flow (GMBF). To investigate the possible role of prostaglandins in mediating this response, the GMBF was measured in the fundus, corpus, and antrum of pig stomachs with and without pretreatment with indomethacin, an inhibitor of prostaglandin synthesis, before and after the induction of bacteremia. The studies were done in 22 piglets (seven sepsis controls, seven indomethacin controls and eight experimental [indomethacin pretreated sepsis] ). Sepsis was produced in piglets by bolus intravenous injection of 10(9) live Escherichia coli followed by an infusion of 10(9) E. coli/hr. Cardiac output (C.O.) was measured by thermodilution. GMBF was measured by microsphere trapping. Following sacrifice, hyperemia was noted in the sepsis group but not in the other groups. GMBF was determined by standard techniques (expressed as ml/min/100 gm tissue). There were significant (p less than 0.05) increases in gastric mucosal blood flow to the fundus (+47%), corpus (+50%), and antrum (+101%) at 15 minutes following the onset of E. coli infusion. At 135 minutes, the increase was only significant in the antrum. GMBF, however, did not change in the indomethacin control or indomethacin pretreated sepsis groups. These data demonstrate GMBF in the stomach following sepsis. The changes were not present in the indomethacin control or in the indomethacin pretreated sepsis groups. Since indomethacin is an inhibitor of prostaglandin synthesis, the results suggest that the GMBF may be a prostaglandin mediated response.

Gastric mucosal blood flow and Escherichia coli bacteremia

The specific changes in gastric blood flow during sepsis are controversial. Previous investigations of intragastric blood flow using endotoxin models revealed decreased total gastric blood flow and decreased gastric mucosal blood flow. Endotoxin models are now thought to be inadequate due to the accompanying depression of the systemic circulation. More recently, increased gastric blood flow has been demonstrated in a septic hindlimb model. To further elucidate the changes in intragastric blood flow in sepsis, the gastric mucosal and nonmucosal blood flow were measured in the antrum and corpus of pig stomachs before and after the onset of sepsis. Increased gastric mucosal blood flow was demonstrated in both the antrum and corpus 15 minutes after the onset of E. coli bacteremia. By 75 minutes postonset, the changes were not significant, which implied partial recovery. Clinically, gastric mucosal ulcers may accompany the multiple organ system failure of sepsis. These data suggest that mucosal ischemia is not an etiologic factor as previously postulated from experiments with endotoxin.

Effects of diuretics on inner medullary hemodynamics in the dog

Although the hemodynamic effects of diuretics have been studied extensively, their effects on inner medullary blood flow remain unknown. In the present study, renal hemodynamics, including papillary plasma flow measured by the albumin accumulation technique, and associated alterations in papillary tissue solute content were determined in anesthetized, hydropenic dogs and during euvolemic diuresis induced by furosemide (3 mg/kg plus 2 mg/kg per hr, iv), ethacrynic acid (3 mg/kg plus 2 mg/kg per hr, iv) or chlorothiazide (10 mg/kg plus 10 mg/kg per hr, iv). Renal blood flow increased significantly after furosemide and ethacrynic acid and decreased significantly after chlorothiazide. Sixty minutes after diuretic administration, papillary plasma flow was 10.8 +/- 1.0 (mean +/- SE) in six furosemide- and 11.3 +/- 2.6 ml/min per 100 g in six ethacrynic acid-treated dogs, both significantly lower than in eight normal or eight chlorothiazide-treated dogs [26.4 +/- 2.6 and 26.7 +/- 2.7 ml/min per 100 g, respectively (P less than 0.01)]. A similarly low papillary plasma flow was also noted 10 minutes after diuretic administration in five furosemide and four ethacrynic acid dogs (13.6 +/- 2.3 and 13.4 +/- 1.8 ml/min per 100 g, respectively). In furosemide and ethacrynic acid dogs, papillary osmolality and sodium content were significantly lower than those in normal or chlorothiazide dogs. In normal and chlorothiazide dogs, papillary sodium content was similar, with a significantly reduced papillary osmolality in the latter. At the time papillary plasma flow was measured, extracellular fluid volume was similar among the four groups of dogs; however, plasma renin activity increased significantly in furosemide and ethacrynic acid dogs (P less than 0.01) and remained unchanged in normal and chlorothiazide dogs. Furthermore, papillary plasma flow was restored to normal (25.3 +/- 3.9 ml/min per 100 g) in five dogs in which furosemide was infused during angiotensin II blockage with saralasin, despite a similar diuresis and natriuresis as the other furosemide group. These data demonstrate that after administration of furosemide, ethacrynic acid and chlorothiazide, regulation of papillary plasma flow is independent of renal blood flow, and suggest that angiotensin II may play a role in the reduced papillary plasma flow in furosemide and ethacrynic acid dogs.

Distribution of blood flow in the dog kidney. II. Saturation rates of inert diffusible tracers versus uptake of 15 mu microspheres during vasodilation and vasoconstriction

While 15 mu microspheres (Ms) in principle provide a measure of glomerular flow, uptake rate of inert diffusible tracers indicates "effective" or "nutrient" flow, i.e. essentially postglomerular capillary flow. Paired measurements of glomerular and postglomerular flow were made in tissue samples from outer, middle and inner cortex (C1, C2, C3) and medullary zones. After a control Ms injection, renal vascular conductance (RVC) was altered in one kidney whereupon a second Ms injection was made, immediately followed by infusion of the 125I-iodoantipyrine and tritiated water. RVC was increased maximally by i.a. infusion of acetylcholine with and without reduced renal arterial pressure, whereas moderate vasodilation was produced by lowering renal arterial or rising ureteral pressure. RVC was reduced by i.a. or i.v. infusion of angiotensin II. Within a mean RVC range of 50 to 180% of control the fractional distribution of zonal postglomerular flow remained unaltered, in agreement with previous results obtained from local H2 gas desaturation rate. Glomerular flow was about 20% higher in C1, equal in C2 and 40% lower in C3 as compared to postglomerular flow in control kidneys. This disparity nearly disappeared during maximal vasodilation and tended to increase during vasoconstriction. The results might suggest a variable net postglomerular effective flow in radial direction through the renal cortex. Alternatively, the fractional redistribution of Ms might reflect a variable degree of Ms skimming at the afferent arteriolar inlets along the interlobular arteries.

The effects of Escherichia coli bacteremia on in vitro perfused kidneys

The renal response to sepsis results in increased renal blood flow, decreased renal vascular resistance, polyuric renal failure and a change in intracortical renal blood distribution. Prior reports used whole animal preparations, where the effects of sepsis on other organs may have led to systemic vasoactive changes altering the experimental model. To elucidate the direct effect of gram negative bacteremia on the kidney, intracortical hemodynamics and urinary flow were investigated using isolated canine kidneys perfused with a nonpulsatile pump oxygenator primed with modified dog plasma. Bacteremia was produced by 2.5 x 10(11) live Escherichia coli organisms infused directly into the perfusate. Intracortical blood flow distribution was measured by radioactive microsphere trapping using 15 microns diameter plastic radioactive microspheres. Urine flow increased 157% (p less than 0.05) following E. coli bacteremia while intracortical blood distribution was unchanged. The polyuric renal failure of sepsis is therefore, a direct result of bacteremia and not secondary to a change in intracortical flow as previously reported. The changes in intracortical blood distribution reported previously in sepsis using intact animal models probably reflect the renal response to release of vasoactive compounds originating in other organs rather than an intrinsic renal reaction to bacteremia.

Renal blood flow and intrarenal distribution of blood flow after decapsulation in the postischemic kidney

Previous reports suggest the value of renal decapsulation in the prevention of renal failure after acute ischemia. It has been suggested that this response is due to a release of "compartmental" pressure resulting in increased blood flow to the decapsulated kidney. Ten dogs were evaluated following 90 minutes of renal ischemia created by occlusion of the suprarenal aorta. Each animal underwent random unilateral decapsulation, with the contralateral kidney acting as control. Labeled 15 micron microspheres (Se85 and Ce141) were injected into the left ventricle at 15 minutes and one hour following decapsulation in six dogs. In the remaining animals the injection was carried out at 15 minutes and 48 hours. No difference in renal blood flow was found between decapsulated and control kidneys in either group. Similarly, using sectioned kidneys no difference in intrarenal distribution of blood flow was found. These data suggest that the effects caused by decapsulation are not due to hemodynamic alterations.

Renal cortical blood flow distribution measured by hydrogen clearance during dopamine and acetylcholine infusion. Effect of electrode thickness and position in cortex

Blood flow distribution in the renal cortex was investigated in control and during i.a. infusion of dopamine (DA) and acetylcholine (Ach) in dogs. Local blood flow in outer cortex (OCF) and in inner cortex (ICF) was measured by platinum electrodes detecting hydrogen washout rate in tissue. Mean cortical blood flow measured by hydrogen washout rate in the renal vein (CFV) was compared with renal arterial blood flow (RAF) measured by electromagnetic flowmeter. With electrodes of 0.05-0.2 mm diameter control blood flow rates in outer and inner cortex were 4.57 +/- (S.D.) 1.73 ml/min.g, and 4.35 +/- 0.57 ml/min.g, which is higher than found using 0.2-0.5 mm electrodes in this and previous studies. OCF and ICF increased proportionally during intraarterial infusion of DA or Ach. The increase in local blood flow per unit volume was about 20% less than the increase in RAF, most likely due to an increase in renal volume and a reduced vasodilatory response in the surrounding of some electrodes. CFV rose almost to the same degree as RAF, showing a diffusion equilibrium for hydrogen gas even at maximal flow rate. During vasoconstriction induced by high doses of DA, OCF and ICF fell proportionately. Thus, equal vascular responses in outer and inner cortex were observed during both vasodilator and vasoconstrictor infusion. This indicates that changes in sodium excretion with renal blood flow may not be associated with a redistribution of cortical peritubular blood flow.

Diuretic action of metolazone in dogs

Metolazone, the sulfonamide diuretic was investigated to determine the sites of action. We used a radioactive microsphere, clearance and stop-flow method in anesthetized dogs. Urine flow and urinary excretion of sodium and potassium were increased at 5--60 min when metolazone was given intravenously at doses of 0.2--5.0 mg/kg, while total renal blood flow, distribution of cortical renal blood flow and GFR did not change. The urinary excretion rate of sodium to potassium (Na/K) increased from 5.69 +/- 0.82 to 8.07 +/- 0.76 in a dose of 1.0 mg/kg, i.v. Osmolar clearance and free water reabsorption increased almost proportionally, indicating that metolazone has little effect on the medullary portion of the ascending limb of Henle and may have a proximal site of action. In stop-flow experiments, a significantly raised U/PNa/U/Pcreatinine was observed at the dip situated distally to the ascending limb of Henle. These findings indicate that the diuretic action of metolazone may be due to the inhibition of sodium reabsorption in the distal nephron segments, in addition to the absence of modification of the cortical regional blood flow.

Renal function and intrarenal hemodynamics in acutely hypoxic and hypercapnic rats

On the basis of microsphere distribution, inert gas washout, and standard clearance data, the effects of acute hypoxia and hypercapnia on the kidney were studied in anesthetized, mechanically ventilated rats. Moderate hypoxia (mean PO2, 48 mm Hg) did not significantly change diuresis, GFR, and tubular sodium rejection. Due to a decrease in renal vascular resistance (R) from 40.1 to 31.8 mm Hg ml-1 min, mean renal blood flow stayed constant in spite of a significant drop in mean arterial blood pressure. Hypoxic changes in R were not accompanied by significant changes in intrarenal distribution of blood flow (IDBF). In severe hypoxia (PO2 less than 45 mm Hg) with oliguria and marked arterial hypotension, R was the lowest of all groups (28.8 mm Hg ml-1 min). Hypercapnia did not significantly change the renal excretory parameters, although an increase in R (without change in IDBF), together with a decrease in MAP caused a marked drop in mean renal blood flow. From these studies we conclude: 1) in the anestheized rat, acute hypoxia caused significant changes in intrarenal hemodynamics without changes in excretory function, 2) hypoxic renal vasodilation persists even in severe hypotension with oliguria and anuria, 3) in acute hypoxia and hypercapnia, changes in renal blood flow and renal vascular resistance are not accompanied by significant changes in IDBF.

Renal circulation after cardiac arrest. Angiography and microangiography

Renal circulation following 3 to 14 min of cardiac arrest was investigated in 21 mongrel dogs by angiography, microangiography and histology. Arrest longer than 7 min caused variable degrees of abnormal angiographic findings after resuscitation: (1) segmental perfusion defects and cortical areas without nephrographic effect, (2) loss of demarcation of cortex and medulla, (3) decreased nephrography, (4) faint and delayed filling of the renal veins, (5) diffuse segmental spasm of the interlobar arteries with delayed emptying. Microangiography demonstrated non-filling of glomeruli in segmental areas of renal cortex, diffuse malperfusion and vasoconstriction as well as obliteration of afferent arterioles due to red blood cell aggregation. No tendency toward recovery within the 5-hour resuscitation period.

The effect of haemorrhage on renal blood flow and intrarenal flow distribution

Bleeding of dogs to constant arterial pressure of 90 mm/g and 60 mmHg respectively decreased renal blood flow in proportion to pressure reduction. There was no evidence of autoregulation of renal blood flow or of selective renal vasoconstriction. With the radioactive microsphere technique a moderate shift of blood flow from the outer to the inner cortex was observed. In the renal 133Xe wash-out curves exponent 1(attributed to the elimination of the indicator from the cortex) was more reduced than exponents 2 and 3 (medulla). These findings may not indicate a redistribution of renal blood flow through resistance changes in specific parts of the renal vasculature but may represent the consequences of focal cortical ischaemia, most prominent in the outer cortex.

Pharmacology of the renal circulation

The circulatory response of the kidney to drugs is conditioned by a variety of factors, such as basal vascular tone, dietary sodium and structural changes in the renal vasculature which accompany aging and disease. In addition, any drug which affects systemic arterial pressure will activate renal autoregulatory processes, which are superimposed upon the direct effects of the drug on the renal circulation. Renal autoregulation in addition to passive pressure effects probably accounts for the relative constancy of renal blood flow during the administration of renal vasodilators such as nitroprusside, diazoxide and minoxidil. Renal vasodilators which have minor effects on systemic arterial pressure, such as dopamine and glucagon, increase renal blood flow. These effects have been employed clinically in low cardiac output states. A variety of drugs affect the renal circulation by modifying the effects of endogenous vasoactive substances. The mechanisms of action include: receptor blockade; ex, adrenergic and Ag II-mediated vasoconstriction: enhanced production by the administration of biochemical precursors; ex, arachidonic acid and I-dopa: inhibition of endogenous production; ex, prostaglandin synthetase inhibitors: and inhibition of breakdown of endogenous substances; ex, converting enzyme inhibition. The effect of each of these interventions will depend in part upon the rate of endogenous production of the relevant vasoactive material. The administration of diuretics affects renal blood flow in individually distinctive ways, the mechanisms of which have been only partially elucidated.

The effect of indomethacin, 6-hydroxydopamine, saralasin, and hemorrhage on renal hemodynamics

This report describes the response of the renal circulation to prolonged hemorrhagic hypotension and reinfusion of blood, and to the effect of a variety of drugs (saralasin, indomethacin, and 6-hydroxydopamine, 6-OH-DA). Plastic microspheres were used to measure blood flow perfusing the entire kidney and also the outer cortex, inner cortex, and medulla of the kidney. Cardiac output was determined with a Doppler flow probe, and total and regional flows were calculated. Redistribution of blood flow from outer cortex to inner cortex and medulla occurred during hemorrhage and after administration of saralasin and 6-hydroxy-dopamine, while indomethacin did not alter intrarenal flow distribution. Total renal flow increased after reinfusion of blood and saralasin, but decreased after indomethacin. It did not change after 6-hydroxydopamine. The results demonstrate that changes in total and intrarenal flow occur independently and are probably due to different mechanisms.

Effects of acetylcholine on the regional distribution of microspheres in the skeletal muscle of the cat

The present experiments were performed to study the question whether intraarterially infused acetylcholine changes in local tissue perfusion of the skeletal muscle. In experiments on cats the triceps surae muscles were isolated. While acetylcholine (0.5 mug/kg X min) was infused intraarterially on one side, the opposite side was taken as control. 4 minutes after the start of the infusion non-radioactive microspheres of 15 mum diameter were injected into the ascending aorta. Subsequently the muscles were freeze-sectioned into 1.0 mm thick slices and cleared by sodium hydroxide and glycerine. The concentration of microspheres per mm3 was measured in representative cross sections of the muscle. Higher concentrations of microspheres were observed in the proximal than in the distal slices of the acetylcholine-treated side. The distribution of microspheres was found to be different in the segments as well: higher values were found in the central than in peripheral segments. The untreated side showed a qualitatively similar state. The differences in the distribution pattern, however, were smaller than in the acetylcholine-treated muscles. It is concluded that the increased inhomogeneity of the regional perfusion observed in this study during vasodilation corresponds to the results reporting a changed nutritive tissue perfusion during an acetylcholine treatment.

Effect of steric restriction on the intracortical distribution of microspheres in the dog kidney

The effect on the intracortical distribution of microspheres and radioactivity caused by steric hindrance of the free movement of spheres into afferent arterioles are described by two mathematical models. The results are compared with corresponding experimental data obtained in six kidneys from normotensive dogs. The first model (A) assumes that spheres are distributed as blood flow, regardless of their size, except for those having diameters greater than that of an afferent arteriole and which do not enter this vessel. The second model (B) includes the Ferry correction. The experimental data show that the percent recovery of spheres with diameters of 20-25 mum was significantly greater in the outer cortex and significantly less in the juxtamedullary cortex than recovery of the smaller spheres, and that the distribution of spheres with diameters of 10 mum to about 17 mum seems uninfluenced by the sphere size. The experimental results we have obtained fit best with model A. We found that according to both models steric restriction is a factor of major importance in relation to the intracortical distribution of spheres, and the analysis shows that the blood flow in the inner part of the renal cortex is grossly underestimated by the method of isotope labeled microspheres when diameters of 15 +/- 5 mum are used in the dog. Furthermore we found that dilation of the afferent arterioles will change the steric hindrance so that a redistribution of spheres and radioactivity may occur without any redistribution of blood flow. It is suggested that the results interpreted as redistribution of blood flow can be explained as due to altered steric hindrance alone, i.e., as a methodological artifact.

Renal cortical blood redistribution after bumetanide related to heterogenicity of cortical prostaglandin metabolism in dogs

Bumetanide is shown to increase renal blood flow and to augment the proportion of the cortical blood flow to middle cortex. This redistribution still takes place even when renal blood flow is maintained constant by renal artery clamping. Indomethacin pretreatment inhibits the increase of renal blood flow as well as the cortical blood redistribution. In vitro examinations of canine kidney tissue slices suggest that outer cortex and papillar are sites of prostaglandin synthesis. No differences in prostaglandin E degradation are observed within the cortex. This suggests a relative autonomy for prostaglandins in the outer cortex, whilst inner cortical areas are dependent on medullary/papillary prostaglandin E supply. The renal hemodynamic effect of bumetanide is therefore thought to be a result of a stimulation of mainly medullary/papillary prostaglandin synthesis.

[The influence of the renin-angiotensin system on autoregulation of renal blood flow and intrarenal hemodynamics (author's transl)]

The discussion of renal hemodynamics has to include three phenomena: the autoregulation of renal blood flow, the existence of different intrarenal perfusion compartments, and the intrarenal renin-angiotensin-system. The autoregulation of renal blood flow is characterized by the constancy of renal blood flow inspite of a wide range of renal perfusion pressure (70-220 mm Hg). This is due to the ability of the kidney to adjust vascular resistance to pressure changes. Renal perfusion is not homogenous but consists of different perfusion compartments which may be distinguished according to characteristic anatomy, function and innervation. Autoregulation of renal blood flow and distribution of intrarenal perfusion to different compartments are interdependent and are shown to be both under the control of an intrarenal "local" renin-angiotensin system.

Autoregulation of renal circulation in mannitol-loaded dogs

Renal haemodynamics and autoregulatory behaviour of the renal vessels have been investigated in mannitol-loaded dogs both for free-flow and for stop-flow conditions. Arterial pressure has been decreased by aortic clamping. For free-flow conditions: (i) relative constancy of RBF and GFR are well preserved in the entire autoregulatory pressure range, i.e., over 80 mm Hg arterial pressure; (ii) decreased GFR is due to enchanced intratubular pressure; (iii) autoregulation is connected with a sharp rise in afferent resistance, accompained by a passive decrease in efferent resistance. For stop-flow conditions: (i) enchancement of total vascular resistance is due to an increase in the passive resistance of the postglomerular vessels; (ii) afferent resistance drops to minimal values as casused by the relaxation of the corresponding arterioles; (iii) autoregualation is abolished: pressure-flow relations are linear over the entire arterial pressure range examined.

A possible sympathetic cholinergic mechanism in the renal reflex elicited by stimulation of abdominal viscera in the dog

1. Isolated blood-bathed tissues which were superfused with renal venous blood during various manipulations of the visceral organs of dogs showed no detectable pharmacologically active substances, except a noradrenaline-like substance which was liberated under some conditions of manipulation. 2. An acetylcholine-like substance was liberated into renal venous blood during experimental manipulation of visceral organs in dogs during infusion of physostigmine; the acetylcholine activity was detected with the dorsal muscle of the leech. 3. The amount of liberated acetylcholine-like substance depended on the strength of visceral stimulation as judged by the changes in the hepatic portal venous pressure. Strong reflex stimulation produced the release not only of acetylcholine but also of noradrenaline. 4. Physostigmine augmented and prolonged the reflexly induced renal vasodilator and diuretic responses, and these were abolished by atropine. It was suggested that the reflex involves a cholinergic nervous mechanism. 5. In some dogs there was a high output of acetylcholine from the kidney during visceral stimulation, and this was accompanied by a decrease in urine flow. The reason was not known. 6. Prostaglandin E1 administered into the renal artery produced similar vasodilator and diuretic responses to acetylcholine, but the onset of the effects were slower and they persisted longer than those produced by acetylcholine.

The effect of ischemia on renal blood flow in the dog

Renal blood flow (RBF) and the distribution of cortical blood flow (microspheres) were measured in the dog after 90 min of total unilateral renal ischemia. RBF was 21% greater than control 2 min after release of the renal artery occlusion, and returned toward control 60 min later. At 2 min after release there was a small but significant increment in deep cortical blood flow which reverted to control by 60 min. When renal artery occlusion was maintained for 180 min, return of blood flow was blunted at 2 min after release of the occlusion, but was not significantly different from control within 10 min after release. Clearance rates of inulin and para-aminohippurate (Cin and Cpah) were 81 and 82% below control after release of occlusion. These data demonstrate that in the dog there is prompt and complete return of blood flow to or above control levels after complete renal artery occlusion. There was no evidence for the "no-reflow" phenomenon.

Ergometrine and apomorphine as selective antagonists of dopamine in the canine renal vasculature

1 Increases in renal blood flow were produced by intra-arterial injections of dopamine (5-50 mug) in anaesthetized dogs pretreated with alpha- and beta-adrenoceptor antagonists.2 Intra-arterial administration of ergometrine (0.5 mg) or apomorphine (1 mg) produced a depression of the renal dilator responses to dopamine, without affecting renal dilatations in response to intra-arterial acetylcholine (0.1-1 mug) or histamine (2-50 mug).3 The depression of dopamine responses lasted 10-15 min, and was greater with ergometrine than with apomorphine.4 It is concluded that both ergometrine and apomorphine can be used as specific blocking agents at vascular dopamine receptors. Ergometrine is the preferred drug for this purpose.

Studies of the mechanism of oliguria in a model of unilateral acute renal failure

To further evaluate the mechanism of the oliguria of acute renal failure, a model was utilized in which intense and prolonged vasoconstriction produced the unilateral cessation of urine flow. The radioactive microsphere method was used to measure total and regional blood flow before and after the intrarenal infusion of norepinephrine, 0.75 mug/kg/min, for 2 h in the dog. In the control kidney, renal blood flow increased 32% 48 h after norepinephrine in association with a fall in the fractional distribution of flow to the outer cortex. In the experimental kidney, total renal blood flow fell from 190 ml/min before norepinephrine to 116 ml/min at 48 h (P < 0.025) with a uniform reduction in cortical blood flow. After the administration of 10% body wt Ringer's solution, there was a marked redistribution of flow to inner cortical nephrons in both the control and experimental kidney. In addition, there was a marked increase in total blood flow in both kidneys. On the experimental side, flow rose to 235 ml/min, a value greater than in either the control period (P < 0.05) or at 48 h after norepinephrine (P < 0.001). However, in spite of this marked increase in blood flow, there was essentially no urine flow from the experimental kidney. In separate studies, the animals were prepared for micropuncture. In all studies, the surface tubules were collapsed, and there was no evidence of tubular obstruction or leakage of filtrate. Over 99% of the 15-muM spheres were extracted in one pass through the experimental kidney. An analysis of the forces affecting filtration suggested that an alteration in the ultrafiltration coefficient may be responsible, at least in part, for the anuria in this model. In this regard, transmission and scanning electron microscopy revealed a marked abnormality in the epithelial structure of the glomerulus. It is suggested that a decrease in glomerular capillary permeability may be present in this model of acute renal failure.

Postnatal changes in renal glomerular blood flow distribution in puppies

The intrarenal distribution of radionuclide microspheres injected into the thoracic aorta was used to examine glomerular blood flow distribution (GBFD) in 26 healthy, unanesthetized puppies, ranging in age from 5 h to 42 days, and in 5 adult dogs. For analysis, the cortex was divided into four equally thick zones designated zone I (subcapsular) to zone IV (juxtamedullary). During the first 36 h of life, the highest flow rate was in zone II, which received 35.5+/-2.0%/g, compared with 26.8+/-1.4% to zone I, 23.7+/-1.4% to zone III, and 13.4+/-1.4% to zone IV. At age 6 wk, zone I had the highest rate of perfusion (48.6+/-2.1%, compared with 28.8+/-1.4% in zone II, 15.8+/-0.8%, in zone III, and 6.8+/-0.6% in zone IV). The 6-wk old animals resembled the adult animals, except for relatively greater perfusion per gram of zone I in the former group. Changes in relative GBFD did not correlate with those in arterial pressure or peripheral hematocrit. The distribution of glomeruli among the four zones of the cortex followed its own pattern of development. At birth and at 6 wk, the greatest density of glomeruli was in zone I (50.6+/-5.4 and 42.7+/-3.9%/g respectively, as compared with 24.1+/-2.9% in adults); in adults zone II contained the greatest density (39.1+/-1.6%). At birth the relative perfusion of glomeruli in zone I was only one-fifth that of glomeruli in zone IV, with intermediate values in zones II and III. By 6 wk of age, increased perfusion of the outer cortical glomeruli resulted in rates of flow in the four zones that did not differ significantly from each other. Relative perfusion in zone I continued to increase, so that in the adult animals perfusion in that zone was significantly greater than in the three deeper zones. These data demonstrate the marked hemodynamic changes that take place within the kidney during the first few weeks of life. The relatively greater blood flow of the most deeply situated nephrons in the early postnatal period suggests ascendancy of this population of nephrons and may have important functional implications.

Redistribution of renal intracortical blood flow during dopamine infusion in dogs

We determined the effect of intravenous and renal intra-arterial infusion of dopamine on the distribution of intracortical blood flow in kidneys of anesthetized dogs. Total renal and renal intracortical blood flows in dogs receiving dopamine intravenously were quantified by the radioactive microsphere technique with reference sampling. In dogs receiving dopamine by renal intra-arterial infusion, total renal and renal intracortical blood flows were determined from radioactive microsphere and electromagnetic flowmeter data. Tissue perfusion rates for the total kidney, the renal cortex, the renal outer cortex, and the renal inner cortex increased following either intravenous or direct renal intra-arterial infusion of dopamine. Dopamine infusion by either method caused a relative redistribution of renal blood flow from the outer twothirds to the inner one-third of the renal cortex. During direct dopamine infusion into the renal artery, no significant changes in renal hemodynamics occurred in the contralateral kidney. No changes in arterial blood pressure occurred during dopamine infusion by either method. These observations imply that the change in the fractional distribution of renal intracortical blood flow following dopamine infusion is not dependent on a systemic mode of action. This pattern of flow redistribution suggests that the intrarenal dopamine-specific receptor may be in higher number in the inner cortex or that the redistribution of cortical flow after dopamine infusion may reflect differing initial physiological states of the inner and outer cortical receptors for dopamine.

The action of bombesin on the kidney of the anaesthetized dog

1. In the anaesthetized dog bombesin had a potent antidiuretic effect, and sometimes arrested urine flow completely. Threshold doses, by i.v. infusion, were of the order of 0.5-1 (ng/kg)/minute. Antidiuresis was the result of a reduction in glomerular filtration rate provoked by a fall in intraglomerular hydrostatic pressure. This, in its turn, was due to afferent vasoconstriction.2. The spasmogenic effect of bombesin on the smooth muscle of the afferent arterioles was directly demonstrated by the radioactive microspheres technique and indirectly by the (85)Kr washout method and by [(3)H]-p-aminohippurate clearance. The vascular compartment most sensitive to bombesin was that of the outer cortical zone, especially in its external half.3. Filtration fraction decreased under the influence of bombesin, indicating that the effect of the polypeptide on postglomerular arterioles was, if present, only of minor importance.4. At high infusion rates (above 6 (ng/kg)/min), bombesin produced a decrease in [(3)H]-p-aminohippurate extraction. The effect of the polypeptide on fractional distal delivery of sodium varied with the dose: at moderate infusion rates it decreased, at high infusion rates it increased. The total glucose appearing in urine following a glucose load was sharply reduced by bombesin. However, the glomerular filtration rate/maximum tubular glucose transport ratio did not show any appreciable change.5. Afferent vasoconstriction produced by bombesin was accompanied by an intense activation of the renin-angiotensin system, as shown by a conspicuous increase in renin secretion, followed by increases in renin activity and angiotensin II concentration in arterial blood. When bombesin was infused into one renal artery only the infused kidney showed afferent vasoconstriction and increased renin secretion. The time-course of renin secretion produced by bombesin depended upon the rate of infusion of the polypeptide. At low rates an increased renin secretion was observed throughout the infusion period, at high rates two peaks of renin secretion could be seen, one at the beginning of the infusion, the other soon after the infusion had finished.6. The mechanism of action of bombesin is discussed and the interest of the polypeptide as a possible hormonal regulator of the circulation and function of the kidney is pointed out.

Mechanism of the redistribution of renal cortical blood flow during hemorrhagic hypotension in the dog

Studies were performed to define the mechanisms involved in the redistribution of renal cortical blood flow to inner cortical nephrons which occurs during hemorrhagic hypotension in the dog. The radioactive microsphere method was utilized to measure regional blood flow in the renal cortex. Renal nerve stimulation decreased renal blood flow 40% but had no effect on the fractional distribution of cortical blood flow. Pretreatment with phenoxybenzamine, phentolamine, propranolol, or atropine did not alter the redistribution of cortical flow during hemorrhage. A reduction in renal perfusion pressure by aortic constriction caused a qualitatively similar alteration in regional blood flow distribution as occurred during hemorrhage. When perfusion pressure was kept constant in one kidney by aortic constriction followed by hemorrhage, no redistribution occurred in the kidney with a constant perfusion pressure while the contralateral kidney with the normal perfusion pressure before hemorrhage had a marked increase in the fractional distribution of cortical flow to inner cortical nephrons. Additionally, retransfusion had no effect on the fractional distribution of flow in the kidney in which perfusion pressure was maintained at the same level as during hemorrhage while in the contralateral kidney in which pressure increased to normal there was a redistribution of flow to outer cortical nephrons. These studies indicate that the redistribution of renal cortical blood flow which occurs during hemorrhage is not related to changes in adrenergic activity but rather to the intrarenal alterations which attend a diminution in perfusion pressure.

Release of a prostaglandin E-like substance from canine kidney by bradykinin

Renal vasodilation produced by two dissimilar vasodepressor polypeptides, bradykinin and eledoisin, was correlated with changes in renal venous concentrations of substances having the properties of prostaglandins of the E and F series in anesthetized dogs. Samples of renal venous blood were extracted for acidic lipids, and the prostaglandin E and prostaglandin F zones of the chromatographed extracts were eluted and assayed in vitro for prostaglandins of the E and F series by a parallel bioassay system (sensitivity 0.015 ng/ml blood). During the first 2 minutes of infusion, bradykinin increased the concentration of a prostaglandin E-like substance in renal venous blood from a mean control level of 0.16 ng/ml to 1.05 ng/ml ( P <0.01); this increase occurred simultaneously with the greatest increase in renal blood flow to 432 ml/min from a control value of 282 ml/min. After 12 minutes of bradykinin infusion, the concentration of the prostaglandin E-like substance had decreased to 0.30 ng/ml, and renal blood flow had fallen to 398 ml/min. In contrast, eledoisin infused in equidilator doses did not increase the concentration of the prostaglandin E-like substance. The concentration of prostaglandin F-like substances was not affected by either polypeptide. A transient increase in urine flow occurred during the first 2 minutes of bradykinin infusion only. These results suggest that a prostaglandin E-like substance participates in the renal vasodilator and the diuretic responses to bradykinin.