Regional renal haemodynamics of angiotensin II infusion under prostaglandin, kinin or converting enzyme inhibition in the Wistar rat

AIMS

Renal medullary blood flow is important in blood pressure regulation and is surprisingly unaffected by the vasoconstrictor action of angiotensin II (Ang II). This study tested if the effect of Ang II on the renal papillary circulation is modulated by bradykinins, prostaglandins or NO (NO). In anaesthetised Wistar rats, total renal blood flow (RBF) was measured, as was cortical (CBF) and papillary (PBF) blood flow, using the laser-Doppler technique, in responses to Ang II (30 ng kg(-1) min(-1)) alone and after ACE inhibition (enalapril) or bradykinin/prostaglandin synthesis inhibition (ketoprofen, aprotinin). PBF was also measured after blockade of NO formation with or without pretreatment with an Ang II receptor antagonist (losartan).

MAJOR FINDINGS

(i) PBF did not change in response to Ang II infusion but MAP increased (+ 10%) and RBF and CBF decreased. (ii) Treatment with aprotinin and ketoprofen left MAP, RBF and CBF unchanged but decreased PBF. Ang II did not decrease PBF further but a significant increase in MAP was seen. (iii) Enalapril treatment left PBF unchanged but decreased MAP and increased RBF and CBF. When Ang II was infused PBF and MAP increased markedly. (iv) L-NAME reduced PBF independently of losartan treatment.

PRINCIPAL CONCLUSION

Bradykinin and prostaglandins do not appear to cause the lack of renal papillary vasoconstriction to Ang II. However, the increase in PBF to Ang II seen after enalapril treatment suggests that enalapril treatment, possibly via its effects on kinin breakdown and subsequent NO formation, might affect the sensitivity of renal papillary autoregulation. This may be an important aspect of the blood pressure lowering effect of ACE inhibitors.

Effects on regional renal blood flow when unclipping a two-kidney, one-clip hypertensive Wistar rat during renal nerve stimulation

Blood pressure (BP) is rapidly normalized when removing the obstruction from the renal artery of a two-kidney, one-clip renovascular hypertensive rat (unclipping). This study tested whether efferent renal nerve stimulation (ERNS) of the unclipped kidney affects this drop in BP or the associated changes in diuresis-natriuresis and regional renal blood flow. Three groups of anesthetized renovascular hypertensive Wistar rats were studied: 1) W(C) (time control); 2) W(UC) (unclipped after 30 min); and 3) W(UC+NS) (unclipped after 30 min, with ERNS at 5 Hz for 2 h). Renal excretion and regional hemodynamics (laser Doppler) were monitored in the unclipped kidney. Medullary and cortical blood perfusion increased by 84% and 95%, respectively, in W(UC) 30 min after unclipping (P < .001) but only with 8% and 9%, respectively, in W(UC+NS) (P = NS). Unclipping induced a marked increase in diuresis-natriuresis that was largely unaffected by ERNS. In W(UC) and W(UC+NS) BP returned to normotensive levels within 4 h. However, during the first 30 min, average BP decreased significantly less in W(UC+NS) (9%, 20 mm Hg) than in W(UC) (16%, 35 mm Hg) (P < .05). ERNS at 5 Hz effectively prevented the increase in medullary blood perfusion but did not affect the fall in blood pressure or the pressure diuretic/natriuretic response seen after unclipping. The results suggest that both the reduction in BP and the pressure-induced increase in diuresis/natriuresis seen when unclipping the 2K,1C renovascular hypertensive rat occurs largely independently of ERNS and an increase in medullary blood perfusion.

Renal medullary blood flow and renal medullary antihypertensive mechanisms

It has long been recognised that the kidneys take part in blood pressure control via both their exocrine and endocrine functions. An endocrine antihypertensive function of the renal medulla has been proposed. The renal medullary depressor substances ("medullipins"), are released in response to increased renal perfusion pressure. It has been suggested that the release of "medullipin" is controlled via changes in renal medullary blood flow. Recent observations also suggest that renal medullary blood flow is involved in the control of the pressure/natriuretic-diuretic action of the kidney. In this review we outline a unified hypothesis for blood pressure control via a combination of the plasma volume regulating pressure-natriuresis mechanism and the powerful antihypertensive actions of the "medullipins" (i.e. vasodilatation, inhibition of sympathetic drive and a diuretic action). It is hypothesised that the activity of both these systems are under control by renal medullary blood flow.

Impaired glomerular permselectivity for albumin in chemically medullectomized WKY rats

Chemical renal medullectomy with 2-bromo-ethylamine hydrobromide (BEA) has been used to study the importance of the renal medulla in blood pressure regulation. However, conclusive evidence as to whether BEA treatment affects the glomerular barrier is lacking. In the present study, the effects of BEA upon glomerular permselectivity for albumin were studied using isolated kidneys (IPK) perfused at a low temperature (8 degrees C) to inhibit tubular reabsorption of proteins. Sixteen WKY rats (WB) received an i.v. injection of BEA (150 mg kg-1) while 10 rats served as controls (WC). Volume balance, urinary osmolality and creatinine clearance (GFR) were measured in metabolic cages. Acute paired experiments (n = 9) were performed 5-7 weeks after BEA. The rats were anaesthetized and the total in vivo albumin excretion was recorded. The kidneys were then isolated and perfused for measurements of inulin clearance (GFR) and fractional albumin clearance without tubular reabsorption of protein. The nine BEA treated rats showed polyuria and hypoosmotic urine. In vivo GFR was lower in the BEA treated groups when measured with creatinine clearance (459 +/- 22 vs. 213 +/- 41 microL min-1 100 g-1 body wt, P < 0.001), while GFR was not significantly changed in the IPK (WC = 135 +/- 27, WB = 92 +/- 14 microL min-1 100 g-1 body wt, n.s.) when perfused at identical pressures. The fractional albumin clearance was increased three times in the BEA group (WB = 9.6 +/- 3.4/1000, P < 0.05). Moreover, albumin excretion in vivo was similar in the two groups despite low GFR in the BEA group. We conclude that BEA treatment affects glomerular permselectivity for albumin.

Effect of nitric oxide and renal nerves on renomedullary haemodynamics in SHR and Wistar rats, studied with laser Doppler technique

The threshold for activation of the humoral renal antihypertensive system, presumably residing in the renomedullary interstitial cells (RIC), is substantially reset upwards in the spontaneously hypertensive rat (SHR). Depressor reactions, normally elicited by an increased renal perfusion pressure, can be inhibited either by high frequency renal nerve stimulation or blockade of nitric oxide synthesis, i.e. manoeuvres decreasing renal blood flow at this high perfusion pressure. The present study was designed to explore the effects on regional renal haemodynamics of blocking NO synthesis with N-omega-nitro-L-arginine (L-NNA) in chloralose anaesthetized SHR and Wistar rats. Mean arterial blood pressure (MAP), heart rate (HR), renal blood flow (RBF), cortical blood perfusion (CBP) and papillary blood perfusion (PBP) were measured in renally innervated and denervated SHR (Si n = 8, Sd n = 8) and in Wistar rats (Wi n = 10, Wd n = 10). An innervated non-treated Wistar group served as control (Ci n = 12). The laser Doppler technique was used to record CBP and PBP. MAP increased in all groups receiving L-NNA while HR, RBF and CBP simultaneously decreased. The relative decreases in RBF were more marked into the two SHR groups than in the corresponding Wistar groups. After L-NNA PBP also decreased in all four groups despite the increased MAP and more so in the Si group; Wi -19 +/- 8 (P < 0.05), Wd -17 +/- 6 (P = 0.07), Si -50 +/- 9 (P < 0.01) and Sd -25 +/- 9% (P < 0.05). We conclude that NO is important for maintaining PBP especially in SHR. The more marked decrease in PBP in the innervated SHR suggests a NO/renal nerve interaction in the control of renomedullary blood flow in SHR. This finding may be of importance for the regulation of the humoral renal depressor mechanism.

Efferent renal nerve stimulation inhibits the antihypertensive function of the rat renal medulla when studied in a cross-circulation model

The aim of this study was to investigate the effects of renal nerve stimulation on the humoral renal antihypertensive system. An isolated kidney (IK) was perfused at normal or high arterial pressures from a normotensive assay rat by means of a perfusion pump. Perfusion pressure (PP) to the IK was 90 mmHg for a control period of 30 min. In three of five experimental groups PP was then increased to 175 mmHg. In two of the groups the renal nerves were stimulated at 2 (P-175(2Hz)) or 5 Hz (P-175(5Hz)) for 60 min. The remaining group served as a control (P-175C). In two groups IK pressure was maintained at 90 mmHg with 5 Hz nerve stimulation (P-90(5Hz) or without nerve stimulation (P-90C). MAP of the assay rat decreased by 22 and 27% (P < 0.001) in the P-175C and P-175(2Hz) groups, respectively during the 60 min period of nerve stimulation, but remained stable in P-175(5Hz). Renal blood flow increased in the IK when PP was increased in P-175C, but did not change significantly in P-175(2Hz) or P-175(5Hz). Blood pressure remained constant in the assay rat when the IK was perfused at 90 mmHg. The renal excretory functions of the IK decreased in a frequency dependent manner by 2 and 5 Hz renal nerve stimulation compared with P-175C. We conclude that 5 Hz renal nerve stimulation inhibits the pressure dependent release of humoral depressor substances from an IK perfused at 175 mmHg, whereas this is not seen when stimulating at 2 Hz. It is suggested that hte release of antihypertensive substances from the renal medulla requires an increased renomedullary blood flow.

Efferent renal sympathetic nerve stimulation in vivo. Effects on regional renal haemodynamics in the Wistar rat, studied by laser-Doppler technique

Intrarenal blood flow regulation probably affects long-term blood pressure homeostasis. We have previously shown that 5 Hz renal sympathetic stimulation inhibits a humoral renal depressor mechanism, otherwise activated when increasing perfusion pressure to an isolated kidney in a cross-circulation set-up. This inhibition was suggested to occur as a result of a reduction of renomedullary blood flow. Little is known about nervous blood flow regulation within the medulla. Therefore in this study, total renal (RBF), cortical (CBF) and papillary (PBF) blood flows were separately measured by ultrasonic and laser-Doppler techniques in Wistar rats during graded renal sympathetic stimulations. Periods of 15 min stimulation at 0.5, 2 and 5 Hz were performed in random order. RBF decreased at 0.5 Hz by 1%, at 2 Hz by 16% (P < 0.001) and at 5 Hz by 49% (P < 0.001). In a similar fashion (r = 0.73, P < 0.001), CBF decreased by 1%, 10% (P < 0.001) and 37% (P < 0.001), respectively. By contrast, PBF increased by 2% at 0.5 Hz and 4% at 2 Hz, while it decreased at 5 Hz, by 4% (P < 0.05, compared with 2 Hz). It seems therefore, that superficial renocortical and total renal blood flows are closely regulated by renal sympathetic nerves with increasing vasoconstriction at higher frequencies, while medullary blood flow, on the other hand, seems to be under strong local control, tending to offset neurogenic flow restrictions.

Renal and haemodynamic effects of nitric oxide blockade in a Wistar assay rat during high pressure cross-circulation of an isolated denervated kidney

Blockade of NO synthesis with N-omega-nitro-L-arginine (L-NNA) inhibits the vasodepressor response seen in intact Wistar assay rats in which isolated kidneys perfused via an extracorporeal circuit are perfused at high pressure. This study explores the renal and haemodynamic changes associated with this inhibition. Isolated kidneys (IK) were perfused at high pressure (175 mmHg) by a pump in series with intact Wistar assay rats in which blood pressure (BP), haemodynamics and renal function were studied. Nitric oxide (NO) synthesis was blocked by L-NNA (2.5 mg kg-1) in 13 experiments (175NO) while 14 control experiments (175C) were performed. IK was perfused at 90 mmHg in seven experiments (90C). The BP drop in the 175C assay rat was blocked by L-NNA in 175NO (P < 0.01). However, when the blockade was reversed with L-arginine infusion (20 mg kg-1 min-1) BP declined also in 175NO. Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) fell dramatically after L-NNA in both the assay rat and in IK despite a high perfusion pressure. The marked increase in filtration fraction (FF) after L-NNA suggests a dominating postglomerular vasoconstriction. The natriuretic response in IK to 175 mmHg was also markedly blunted by L-NNA. We conclude that NO blockade inhibits the renomedullary depressor mechanism probably by restricting renal blood flow, and also blunts the pressure induced natriuretic response as a result of a reduced sodium filtration. Finally, the autoregulation of whole kidney blood flow seems to be more efficient although set at a higher level of vasoconstriction.

Influence of the renal medulla and early treatment with enalapril upon the development of hypertension in young spontaneously hypertensive rats

OBJECTIVE

To investigate the role of the renal medulla in early hypertension in spontaneously hypertensive rats (SHR), and to explore whether the attenuated increase of pressure induced by enalapril treatment is affected by chemical medullectomy.

DESIGN

Forty-four male SHR were studied from 5 to 18 weeks of age: 22 remained intact; 22 were medullectomized at 5.5 weeks of age with 2-bromoethylamine hydrobromide; 11 of each of these two groups were treated with enalapril from 6 to 12 weeks of age. Blood pressure, heart rate and body weight were recorded intermittently, and at 18 weeks renal function was also analysed.

RESULTS

The results indicate a protective effect of the renal medulla against severe pressure rises in SHR, although even when enalapril also lowered blood pressure in medullectomized SHR, persistent improvements of glomerular filtration rate and renal flow conductance occurred only in intact SHR. Furthermore, after enalapril treatment ended blood pressure rose to higher levels in medullectomized SHR, despite greater sodium-water losses.

CONCLUSION

The renal medulla seems to exert a protective role both during and after enalapril treatment.

Is the humoral renal antihypertensive activity of the spontaneously hypertensive rat (SHR) reset to the high blood pressure?

The kidneys have a humoral antihypertensive system, located in the renal medulla and presumably antagonizing the pro-hypertensive renin-angiotensin system. Medullipin I and II and maybe platelet activating factor (PAF), seem to be the mediators of this system, known to be activated after reversal of renovascular hypertension or when the perfusion pressure to a normotensive kidney is suddenly elevated. The present study was undertaken to investigate whether this system is functioning also in the spontaneously hypertensive rat (SHR), and if it is then reset in proportion to the increased mean arterial pressure (MAP). Isolated kidneys from spontaneously hypertensive rats and from Wistar Kyoto rats (WKY) were cross-perfused in vivo from anaesthetized intact Wistar Kyoto rat 'donors'. After 30 min of perfusion at 100 mmHg the perfusion pressure to the isolated kidneys were, for 60 min, either kept unaltered at 100 mmHg or, for the Wistar Kyoto rat kidneys, increased to 150-200 mmHg and, for the spontaneously hypertensive rat kidneys, raised to 200 or 250 mmHg. The results show that the humoral antihypertensive system is present also in spontaneously hypertensive rat kidneys, but is here reset upwards to or even beyond the elevated MAP level. Furthermore, all mean arterial pressure reductions caused by high-pressure perfusion of Wistar Kyoto and spontaneously hypertensive rat kidneys were accompanied by reductions in heart rate (HR) in the 'donors', in agreement with previous observations after reversing renal hypertension and after i.v. medullipin I injection. In fact, in spontaneously hypertensive rat kidneys, the 'incretory' depressor mechanism appears to be more markedly reset upwards than is the 'excretory' depressor mechanism inherent in pressure diuresis with consequent salt-volume elimination. In conclusion spontaneously hypertensive rats, like Wistar Kyoto rats and Wistar rats, have a humoral antihypertensive system in the kidneys, but it is reset upwards even beyond the elevated mean arterial pressure level in spontaneously hypertensive rats. The combination of a depressor response and reduced heart rate in the 'donors' renders further evidence that the medullipins are the principal, though probably not the only, humoral antihypertensive factors released from the cross-circulated kidneys.

Do angiotensin converting enzyme inhibitors lower blood pressure in the rat partly via the humorally mediated antihypertensive system of the renal medulla?

It has been suggested that there is a negative-feedback interaction between the humoral renomedullary antihypertensive system and angiotensin II. If so, the acute blood pressure-lowering effects of angiotensin converting enzyme (ACE) inhibitors might be due, in part, to an increased secretion of renomedullary depressor substances. Groups of anaesthetized Wistar-Kyoto rats (WKY) with an intact or chemically destroyed renal medulla received either saline or the ACE inhibitor enalapril, and mean arterial pressure (MAP), heart rate and renal function were measured. MAP was clearly decreased after enalapril administration in the WKY controls with an intact renal medulla, but was not changed in the medullectomized group. In one WKY control group, where the prostaglandin and kallikrein-kinin systems had also been acutely blocked, the MAP reduction after enalapril was even more marked than in the intact controls. Thus, the acute blood pressure-lowering effect of enalapril is clearly dependent on an intact renal medulla, further suggesting that the renomedullary antihypertensive system is important to normal blood pressure homeostasis.

Renal and circulatory effects of medullipin I, as studied in the in-vivo cross-circulated isolated kidney and intact Wistar-Kyoto (WKY) rat

The renal medulla harbours powerful humoral antihypertensive mechanisms, as earlier explored in unclipping experiments on renal hypertensive rats or in normotensive isolated kidneys cross-circulated at increased perfusion pressures from 'donor rats', in which renal function also seemed to be affected. Injection of the renomedullary factor medullipin I (Med I; formerly ANRL) mimics these haemodynamic responses, and Med I seems to be one of the most important mediators of the depressor effects. The present study was performed to analyse further the haemodynamic and, particularly, the renal effects of Med I, using anaesthetized intact WKY rats and constant-pressure perfused (90 mmHg) isolated WKY kidneys, cross-circulated by these intact 'donor' rats. Mean arterial pressure (MAP), heart rate (HR) and renal function were followed for one 30-min period before and two 30-min periods after injection of 1 mg Med I (M; n = 7) or an equal volume of saline as control (C; n = 13). In the intact 'donor' WKY, MAP and HR remained largely constant in C during the three periods, being 126 +/- 5, 125 +/- 5, and 120 +/- 5 mmHg, while MAP fell in the M group after Med I, from 121 +/- 5 to 107 +/- 7 and 107 +/- 5 mmHg (P less than 0.05), and also HR tended to decrease in M. Renal resistance (RR) fell while renal plasma flow (RPF) and glomerular filtration rate (GFR) increased significantly (P less than 0.05) after Med I in the M donor rats despite their MAP reduction. However, in the constant-pressure perfused, cross-circulated kidneys the RR, RPF and GFR changes were clearly more pronounced (P less than 0.01) and also diuresis, natriuresis, osmolar excretion and osmolar clearance increased significantly after Med I (P less than 0.01). In conclusion, the present results support the view that Med I not only has important and long-lasting depressor effects but also affects renal function in important ways, inducing vasodilatation and increasing GFR, RPF, diuresis and sodium-osmolar excretion.