Mediators of the hypotensive response to increased renal perfusion in rabbits.

Abstract

We have previously shown that increasing the renal perfusion pressure by using an extracorporeal circuit in anesthetized rabbits resulted in a progressive fall in systemic arterial pressure. Prior ablation of the renal medulla with 2-bromoethylamine abolished the hypotensive response. In the present study, we investigated whether vasodilator prostanoids or platelet activating factor (PAF), both known to be produced in the renal medulla, were responsible for the hypotensive response to increased renal perfusion pressure. Anesthetized animals were treated with indomethacin (5 mg/kg+0.5 mg/kg per hour), the PAF antagonist WEB 2086 (0.5 mg/kg+0.5 mg/kg per hour), enalaprilat (2 mg/kg+10 /ig/kg per hour), or all three agents. In response to acute elevation of renal artery pressure to 170 mm Hg, systemic mean arterial pressure fell at 0.76±0.17, 0.59±0.08, and 0.76±0.17 mm Hg/min in the indomethacin, WEB 2086, and enalapril groups, respectively. These responses were not significantly different from the rate of 1.00±0.21 mm Hg/min in a control group that received vehicle infusion alone. Renal blood flow and the diuretic and natriuretic responses were also similar in all groups. Thus, increased renal perfusion pressure resulted in a progressive fall in systemic arterial pressure that was not mediated by PAF, prostaglandins, or suppression of renin release and angiotensin II production. T here is increasing evidence that vasodepressor substances from the renal medulla, particularly the yet to be identified "medullipin," 1 are involved in the circulatory responses to arterial hypertension. The strongest evidence comes from studies of the "normalization" of blood pressure after removal of renal artery clips in chronic Goldblatt hypertensive rats. Prior medullectomy of the kidney in these rats with 2-OH-bromoethylamine retards the rate at which arterial pressure returns to normal.2 There is also evidence for the involvement of renal medullary substances in the responses to acute rises in arterial pressure; we have shown recently that the renal medulla releases hypotensive factors in response to acutely increased renal perfusion pressure in dogs and rabbits, 3 and Karlstrom and colleagues 4 have reported similar results in rats. The nature of the hypotensive substance in these latter experiments remains to be elucidated, but several vasoactive substances are known to be produced in the renal medulla, including platelet activating factor (PAF) 5 and prostaglandins 6 ' 7 as well as the putative vasodepressor hormone medullipin.1 The latter is a neutral lipid of unknown chemical structure, 1 and the importance of Received June 19, 1992; accepted in revised form October 7, 1992. this substance in the regulation of blood pressure in vivo is still to be established.The present study was aimed at clarifying the factors responsible for the hypotensive response to acute elevation of renal perfusion pressure. As in our previous study, 3 we perfused the left kidneys of rabbits in situ by using an extracorporeal circuit that allowed...

Effects of Renal Medullary and Intravenous Norepinephrine on Renal Antihypertensive Function

“…However, as has been argued previously, autoregulation in this model is seen as an increase in renal vascular resistance in response to increased RAP, but its effect on RBF is limited by the fixed rate of the pump and high resistance of the vena caval limb. 3 …”

“…However, given our previous finding that medullary interstitial infusion of [Phe, 2 Ile, 3 Orn 8 ]vasopressin reduces MBF and attenuates the depressor response to increased RAP, 2 a role for the medullary microvasculature seems worthy of further investigation. To this end, future studies should replicate this experimental paradigm with other pharmacological agents that might selectively decrease and increase MBF.…”

“…The depressor response to increased RAP appears to be largely independent of the reduced activity of the renin-angiotensin system, in view of the fact that it is little affected by the blockade of angiotensin-converting enzyme. 3 It also appears to be largely independent of the associated diuresis and natriuresis, in view of the fact that hemoconcentration is not observed. [2][3][4] There is, however, clear evidence for a role of the renal medulla, inasmuch as the depressor response is abolished by chemical medullectomy.…”

“…3 It also appears to be largely independent of the associated diuresis and natriuresis, in view of the fact that hemoconcentration is not observed. [2][3][4] There is, however, clear evidence for a role of the renal medulla, inasmuch as the depressor response is abolished by chemical medullectomy. 4 We recently found that this depressor response to increased RAP was blunted by medullary interstitial infusion of [Phe 2 ,Ile 3 ,Orn 8 ]vasopressin (V 1 -agonist), a treatment that selectively reduces MBF, 2 indicating a possible role of MBF in the release of this putative hormone.…”

Interactions Between Furosemide and the Renal Sympathetic Nerves

“…Since prostaglandin-mediated vasodilation is endothelium-independent, Gerkens and coworked formed the hypothesis that furosemide stimulates renal prostaglandin synthesis which in turn causes release of a renomedullary antihypertensive hormone to the circulation (Gerkens 1987). The final renomedullary hormone is not yet defined, but circumstantial evidence suggest that a renomedullary lipid (medullipin I) is released from the renal medulla in response to increased renal perfusion pressure, and that the active hepatic metabolite (medullipin 11) has vasodilatory, sympathoinhibitory, and diuretic-natriuretic actions (Muirhead et al 1983;Karlstrom et al 1988Karlstrom et al & 1989Christy et al 1993;Thomas et al 1994). Whether renomedullary lipids have actions on capacitance vessels and the pulmonary circulation is unknown.…”