Prostaglandins mediate the vasodilatory effect of mannitol in the hypoperfused rat kidney

We have previously reported that mannitol strikingly increases blood flow to rat kidneys hypoperfused at 35-40mm Hg. This vasodilator effect is not due to volume expansion or alterations in plasma osmolality. We have tested the hypothesis that the vasodilatory effect of mannitol in the ischemic rat kidney is mediated by one of the vasoactive renal hormone systems: renin-angiotension, kallikrein-kinin, or prostaglandins. Rats were infused with 5% mannitol in 0.9% saline to 3-5% of body weight. In agreement with our previous studies, RBF increased 1.3 +/- 0.1 ml/min despite maintenance of perfusion pressure at 35-40 mm Hg. The cyclooxygenase inhibitors, meclofenamate and indomethacin had no effect on renal blood flow (RBF) in hypoperfused kidneys. However, in rats pretreated with these inhibitors, expansion with mannitol increased RBF by only 0.37 +/- 0.02 ml/min, 28% of the response in the untreated group (p less than 0.001). Infusion of prostacyclin (PGI2) into the renal artery during reduced perfusion resulted in an increase in RBF of 1.0 +/- 0.1 ml/min. Subsequent expansion with mannitol increased RBF by only 0.5 +/- 0.1 ml/min more, less than one-half of the effect of mannitol in a concurrent group of rats not treated with PGI2. Unlike PGI2 prostaglandin E2 had only a minimal vasodilator effect during hyperperfusion. Imidazole, an inhibitor of thromboxane synthesis, did not alter RBF or renal vascular resistance during hypoperfusion. Treatment of rats during hypoperfusion. with the angiotensin-converting enzyme (kininase II) inhibitor teprotide increased RBF by 1.1 +/- 0.3 ml/min. However, teprotide did not alter the vascular response to mannitol: RBF increased 1.2 +/- 0.1 ml/min more when mannitol was infused into teprotide-treated rats. The renal vascular response to mannitol was not altered by treatment with aprotinin, an inhibitor of the kallikrein-kinin system. Aprotinin was ineffective whether given before or after the vascular response to mannitol was established. We conclude that the vasodilator response to mannitol in the ischemic rat kidney is mediated in large part by increased prostaglandin (PGI2) activity. The failure of converting enzyme inhibition and aprotinin to block the vasodilator response to mannitol is evidence against a role for the renin-angiotension or kallikreinkinin systems in mediating the vasodilator response.