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

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.

Effects of renal medullary and intravenous norepinephrine on renal antihypertensive function

Increasing renal arterial pressure activates at least 3 antihypertensive mechanisms: reduced renin release, pressure natriuresis, and release of a putative renal medullary depressor hormone. To examine the role of renal medullary perfusion in these mechanisms, we tested the effects of the infusion of norepinephrine, either infusion into the renal medullary interstitium or intravenous infusion, on responses to increased renal arterial pressure in pentobarbital-anesthetized rabbits. We used an extracorporeal circuit, which allows renal arterial pressure to be set to any level above or below systemic arterial pressure. With renal arterial pressure initially set at 65 mm Hg, intravenous and medullary interstitial norepinephrine (300 ng. kg(-1). min(-1)) similarly increased mean arterial pressure (by 12% to 17% of baseline) and reduced total renal blood flow (by 16% to 17%) and cortical perfusion (by 13% to 19%), but only medullary norepinephrine reduced medullary perfusion (by 28%). When renal arterial pressure was increased to approximately 160 mm Hg, in steps of approximately 65 mm Hg, urine output and sodium excretion increased exponentially, and plasma renin activity and mean arterial pressure fell. Medullary interstitial but not intravenous norepinephrine attenuated the increased diuresis and natriuresis and the depressor response to increased renal arterial pressure. This suggests that norepinephrine can act within the renal medulla to inhibit these renal antihypertensive mechanisms, perhaps by reducing medullary perfusion. These observations support the concept that medullary perfusion plays a critical role in the long-term control of arterial pressure by its influence on pressure diuresis/natriuresis mechanisms and also by affecting the release of the putative renal medullary depressor hormone.

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.

Evidence for a renomedullary vasodepressor hormone

1. Recent physiological experiments have established that increasing the perfusion pressure of the kidney causes the release of vasodepressor substance from the renal medulla. 2. The substance is not a platelet activating factor, a prostaglandin or nitric oxide and the vasodepressor response to increased renal perfusion is not due simply to inhibition of renin release. 3. The mechanisms by which the renomedullary vasodepressor substance lowers arterial pressure remain to be determined. Sympathoinhibition may account for part of the response, but the hypotension still occurs in autonomic ganglion blocked animals. 4. The source of substance appears to be the renomedullary interstitial cells, though the control of the production and release of the substance remain to be determined. 5. The substance may be a lipid but it is yet to be fully isolated and identified. 6. The threshold for release of the substance appears to be close to normal resting arterial blood pressure. 7. Despite strong evidence that the renal medulla releases a vasodepressor hormone in response to increased renal perfusion pressure, much is still to be determined regarding the physiology of this hormone and its involvement in the aetiology of hypertension.

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.