The role of the renal medulla in blood pressure control

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.

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.

Decreased cytosolic calcium and prostaglandin synthesis in prehypertensive rats

The capacity of cultured renal medullary interstitial cells derived from Dahl salt-sensitive and salt-resistant rats to synthesize prostaglandin E2 (PGE2) was compared. Basal and arginine vasopressin (AVP)-induced PGE2 production by interstitial cells from salt-resistant rats was fourfold to fivefold higher than corresponding values of those from the salt-sensitive rats. Similarly, basal and AVP-responsive release of [3H]arachidonate were twofold higher by interstitial cells from salt-resistant compared with salt-sensitive rats. Differences in PGE2 production were abolished by the calcium inophore A23187 or the addition of exogenous arachidonate. The latter findings suggested a role for altered availability of endogenous arachidonate, possibly mediated by reduced calcium-responsive lipase activity. Basal and AVP-induced increases in cytosolic free calcium concentration, assessed by the aequorin method, were significantly lower in interstitial cells from salt-sensitive versus salt-resistant rats, further supporting a possible role for altered cellular calcium homeostasis. Studies of the potential contribution of various phospholipases and of triglyceride lipase to the release of arachidonate for PGE2 synthesis in interstitial cells implicated phospholipase A2 activity as a major pathway. When assessed in vitro in cell cytosolic fractions at identical calcium concentration, phospholipase A2 activity was lower in interstitial cells from salt-sensitive versus salt-resistant rats. Thus, both reduced cytosolic free calcium and phospholipase A2 activity of interstitial cells from salt-sensitive rats may contribute to the diminished capacity of these cells to liberate endogenous arachidonate for PGE2 synthesis.