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

Sympatho-renal interactions in the determination of arterial pressure: role in hypertension

Renal mechanisms and the sympathetic nervous system contribute to the development of arterial hypertension. Renal transplantation experiments in spontaneously hypertensive rats (SHRs) were performed to investigate how the sympathetic nervous system and the kidneys interact during the development and maintenance of hypertension. Our findings indicate that the rise in arterial pressure that occurs after transplantation of a kidney from a SHR into normotensive recipients is not mediated by elevations in sympathetic activity. However, chronic reductions in sympathetic tone reduce the rise in arterial pressure which can be induced by SHR renal grafts in normotensive recipients. Untreated SHRs transplanted with a kidney from sympathectomized donors have lower arterial pressure and reduced sodium sensitivity of arterial pressure compared to SHRs transplanted with a kidney from hydralazine-treated donors. It is concluded that chronic non-adapting changes in sympathetic activity modulate the degree to which renal mechanisms can cause hypertension in SHRs. Severe reduction in sympathetic tone during early ontogeny causes long-term changes in renal function that mitigate hypertension development in SHRs even when the extrarenal neuro-hormonal environment is restored.

Mechanisms underlying the antihypertensive functions of the renal medulla

There is good evidence that the renal medulla plays a pivotal role in long-term regulation of blood pressure. 'Renal medullary' blood pressure regulating systems have been postulated to involve both exocrine (pressure natriuresis/diuresis) and endocrine [renal medullary depressor hormone (RMDH)] functions. However, recent studies indicate that pressure diuresis/natriuresis dominates the antihypertensive renal response to increased renal perfusion pressure, suggesting little physiological role for a putative RMDH in compensatory responses to acutely increased blood pressure. The medullary circulation appears to play a key role in mediating pressure diuresis, although the precise mechanisms involved remain controversial. Counter-regulatory vasodilator mechanisms (e.g. nitric oxide), at least partly mediated through cross-talk between the vasculature and the tubular epithelium, protect the medullary circulation from the vasoconstrictor effects of hormonal factors such as angiotensin II. These mechanisms also appear to contribute to compensatory responses to increased salt intake in salt-resistant individuals. Failure of these mechanisms predisposes the organism towards the development of hypertension, appears to underlie the development of some forms of experimental hypertension, and may even contribute to the pathogenesis of essential hypertension.

Analysis of arterial pressure regulating systems in renal post-transplantation hypertension

OBJECTIVE

To investigate if blood volume expansion, increased sodium retention, changes in neurohumoral arterial pressure control, or altered extrarenal resistance vessel function contribute to the development of renal post-transplantation hypertension.

METHODS

F1-hybrids (F1H) obtained from crossing spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats received either an SHR or an F1H kidney graft. Groups consisted of 8-12 animals and were investigated between days 1 and 14 after renal transplantation in three sets of experiments including arterial pressure recordings, plasma volume measurements, metabolic studies, and small vessel myography.

RESULTS

Two days after completion of bilateral nephrectomy, arterial pressure was elevated by 15-20 mmHg in recipients of an SHR kidney, compared with syngeneically transplanted controls. There was no evidence for increased sodium and fluid retention during the early development of renal post-transplantation hypertension despite a 35% reduced creatinine clearance in recipients of an SHR kidney. The plasma renin-angiotensin-aldosterone system was similarly suppressed in both recipients of an SHR kidney and controls. The arterial pressure response to ganglionic blockade did not differ between groups and there was no evidence for changes in extrarenal resistance vessel function, which could be involved in the genesis of this form of hypertension.

CONCLUSIONS

None of the investigated mechanisms was altered in a way that might help to explain the rapid and consistent development of hypertension in recipients of an SHR kidney. We conclude that post-transplantation hypertension in recipients of an SHR kidney is due to mechanisms other than those investigated in the present study.

Long-term arterial pressure in spontaneously hypertensive rats is set by the kidney

OBJECTIVES

We investigated whether arterial pressure in spontaneously hypertensive rats (SHR) can be normalized by a kidney graft from normotensive histocompatible donors. In addition, the effect of differential genetic predisposition to hypertension of recipients of an SHR kidney on the development of post-transplantation hypertension was studied.

METHODS

SHR were transplanted with a kidney from congenic rats (BB.1K) homozygous for a 2 cM segment of SHR chromosome 20, including the major histocompatibility complex class Ia and class II genes. BB.1K and F1 hybrids (F1H, SHR x Wistar-Kyoto rats) were transplanted with an SHR kidney and the development of renal post-transplantation hypertension was monitored.

RESULTS

Thirty days after renal transplantation, mean arterial pressure (MAP) was 116 +/- 4 mmHg in SHR with a BB.1K kidney (n = 8) versus 168 +/- 2 mmHg in sham-operated SHR (n = 10); P < 0.001. Cumulative renal sodium balance (mmol/100 g body weight) over 21 days after bilateral nephrectomy was 6.8 +/- 0.6 in SHR with a BB.1K kidney versus 10.8 +/- 1.6 in sham-operated SHR (P < 0.05). Within 60 days of transplantation, MAP increased in BB.1K and in F1H transplanted with an SHR kidney (n = 7 per group) by 38 +/- 5 mmHg and 43 +/- 8 mmHg, respectively.

CONCLUSIONS

In SHR, arterial pressure can be normalized by a kidney graft from normotensive donors. The genetic predisposition of the recipients to hypertension does not modify the rate and the extent of the arterial pressure rise induced by an SHR kidney graft.

Renomedullary interstitial cell lipid droplet content is increased in spontaneously hypertensive rats and by low salt diet

OBJECTIVE

To compare the volumes of renomedullary interstitial cell (RMIC) lipid droplets (putative source of vasodepressor substance) in spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats on high and low salt diets as an indication of whether the renomedullary vasodepressor system of the SHR is defective.

METHODS

Ten-week-old male SHR and WKY rats received a low (0.05% w/w) or high salt (5.0%) diet for 21 days. Conscious mean arterial pressure (MAP) was measured and the renal papilla perfusion fixed with a high osmolarity fixative. Using electron microscopic stereological techniques, the volume density of lipid in RMIC (VVLipid,RMIC) and the total volumes of lipid (VLipid) and RMIC (VRMIC) in papilla were measured.

RESULTS

MAP of SHR (high 155 +/- 3 mmHg; low 151 +/- 3 mmHg) was significantly greater than WKY rats (high 126 +/- 2 mmHg; low 129 +/- 2 mmHg; P< 0.001), however salt diet had no significant effect on MAP. The VLipid of rats on the low salt diet was approximately 2.5 times greater than in rats on the high salt diet (P < 0.01). SHR had significantly greater VLipid than WKY rats irrespective of salt diet (P< 0.05; SHR-low 0.245 +/- 0.031 mm3, SHR-high 0.093 +/- 0.007 mm3; WKY-low 0.126 +/- 0.032 mm3, WKY-high 0.051 +/- 0.020 mm3). Similar differences were seen for VVLipid,RMIC, however VRMIC was not different between rat strains or salt diet groups.

CONCLUSIONS

SHR and WKY rats responded similarly to the altered salt diets, and SHR demonstrated greater volumes of stored RMIC lipid droplets irrespective of the level of salt intake. These results indicate that SHR hypertension is not due to a deficiency in the amount of lipid droplets, the putative source of the renomedullary vasodepressor substance and that the renomedullary vasodepressor system of the SHR is capable of responding normally to the physiological stimulus of altered salt intake.

Effects of intrarenal infusion of 17-octadecynoic acid on renal antihypertensive mechanisms in anesthetized rabbits

To characterize the role of cytochrome P450 metabolism of fatty acids in the renal response to increased renal perfusion pressure, we tested the effects of renal arterial infusion of 17-octadecynoic acid (17-ODYA, 450 nmol/min) on renal and systemic hemodynamic, and renal excretory responses to step-wise increases in renal perfusion pressure (RPP) in anesthetized rabbits, using an extracorporeal circuit for renal autoperfusion. Inhibition of cytochrome P450-dependent fatty acid metabolism was estimated by comparing the metabolism of arachidonic acid in microsomes prepared from the kidneys of control and 17-ODYA-treated animals. Step-wise increases in RPP decreased mean arterial pressure, which previous studies have indicated is attributable to the release of a depressor hormone from the renal medulla. Elevations in RPP also increased renal blood flow and glomerular filtration rate, and the absolute and fractional excretions of urine and sodium. Intrarenal infusion of 17-ODYA reduced the metabolism of arachidonic acid to 20-hydroxyeicosatetraenoic acid by 41%, but it did not significantly influence the responses to increased renal perfusion pressure. We conclude that either the responses elicited by increased renal perfusion pressure in anesthetized rabbits do not depend on cytochrome P450-dependent fatty acid metabolism, or that cytochrome P450 activity must be inhibited by more than was achieved in the present study (41%), before functional effects on the response to increased renal perfusion pressure are observed.

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.

Role of the native kidney in experimental post-transplantation hypertension

In experimental renal transplantation studies using several animal models of primary hypertension, including stroke-prone spontaneously hypertensive rats (SHRSP) and their normotensive Wistar-Kyoto controls (WKY), single transplanted kidneys from genetically hypertensive but not normotensive donors elicited post-transplantation hypertension in bilaterally nephrectomized genetically normotensive recipients. The underlying mechanisms are presently unclear. The present study was designed to investigate the effects of a remaining native kidney on post-transplantation blood pressure, plasma renin activity and plasma angiotensin II concentration in (WKYxSHRSP) F1 hybrid recipients of a WKY or SHRSP kidney. The presence of a native kidney markedly reduced, but did not prevent, post-transplantation hypertension in recipients of an SHRSP kidney. WKY kidney grafts did not significantly alter blood pressure in bilaterally or unilaterally nephrectomized recipients. Plasma renin activity was lower in bilaterally than in unilaterally nephrectomized recipients, regardless of the source of the graft. The plasma angiotensin II concentration was similar in all groups. Renal graft function as assessed by 99mtechnetium-mercaptoacetyltriglycine scintigraphy was well preserved. These data suggest that post-transplantation hypertension in recipients of an SHRSP kidney may be partly due to the failure of the graft to eliminate a hypertensinogenic substance or to produce a blood pressure lowering agent.

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.

Secretion of medullipin I by isolated kidneys perfused under elevated pressure

1. Medullipin I (Med I) is a hormone extracted from renal papillae and its renomedullary interstitial cells (RIC). Med I is stimulated by elevation of the renal artery perfusion pressure. 2. When isolated normal rat kidneys were perfused either with oxygenated blood or with 5% albumin bubbled with O2 at elevated perfusion pressures, Med I appeared to be secreted into the renal venous effluent (RVE). Addition of Tween 20, treatment of the assay rat with SKF 525A, inhibitor of cytochrome P-450 and removal of the liver from the systemic circulation prevented vasodepression of both the RVE and extracted Med I. The lipid in the RVE gave the same dose-response as extracted Med I. 3. Lowering the renal artery perfusion pressure below normal inhibited the secretion of Med I. As the perfusion pressure was elevated Med I secretion appeared to increase. 4. Previous observations and the present study support the view that the renin-angiotensin system and the Medullipin system are double feedback systems involved in blood pressure control.