Mesenteric perfusion pressure was measured in the in situ blood-perfused mesentery of anaesthetized rats. Increases in perfusion pressure were produced by mesenteric periarterial electrical stimulation at 3, 6 and 10 Hz before and after the administration of frusemide 5 mg/kg intravenously (i.v.) or vehicle. Loss of volume due to diuresis was prevented by replacement with intravenous saline. Frusemide did not cause any changes in blood pressure or baseline perfusion pressure. Responses to electrical stimulation were inhibited by frusemide (P less than 0.05) but unchanged by vehicle administration. Acute bilateral nephrectomy or treatment with indomethacin (2 mg/kg i.v.) prevented the inhibitory effect of frusemide on responses to sympathetic nerve stimulation. Responses to sympathetic nerve stimulation were potentiated by an infusion of angiotensin II (12 ng/min) into the mesenteric artery. This infusion did not alter either blood pressure or baseline perfusion pressure. Administration of frusemide 5 mg/kg i.v. attenuated the potentiating effect of angiotensin II on vasoconstrictor responses to electrical nerve stimulation. Frusemide may lead to the release of a prostanoid or prostanoid precursor which inhibits vascular constrictor responses.

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In rat isolated perfused tail arteries, dose-response curves were established for the vasopressor effects of phenylephrine (alpha 1-adrenoceptor agonist), clonidine (alpha 1- and alpha 2-adrenoceptor agonist), clonidine in the presence of 10(-7) mol/l prazosin (alpha 2-agonist), and BHT-920 (alpha 2-agonist). The ED50 values were: phenylephrine 1.85 X 10(-10) mol; clonidine 6.3 X 10(-10) mol; clonidine + prazosin 3.2 X 10(-6) mol; BHT-920 6.1 X 10(-6) mol. The arterial reactivity to BHT-920 was stable only after 4-5 h of perfusion. Responses to BHT-920 were not antagonized by yohimbine (alpha 2-adrenoceptor antagonist) but were antagonized by low concentrations of prazosin (alpha 1-adrenoceptor antagonist). These data constitute conflicting evidence regarding the existence of alpha 2-adrenoceptors in rat tail arteries. The data are consistent with the proposal that there are two recognition sites on alpha 1-adrenoceptors; phenylephrine and BHT-920 may stimulate different sites on alpha 1-adrenoceptors.

Blood was withdrawn at a constant rate from the cannulated carotid artery of an anaesthetized rat and perfused an ex vivo segment of tail artery cannulated at both ends and contained in an organ bath. Blood returned to the rat via a cannulated jugular vein. The tail artery was constricted and perfusion pressure increased by peri-arterial stimulation at 5 Hz for 5 s every 2 min. Intravenous frusemide (5 mg/kg) decreased the stimulation responses of the tail artery. Diuresis-induced volume losses after frusemide were circumvented by a urinary bladder-intravenous shunt. Frusemide-induced reduction of tail artery vasoconstrictor responses was not seen in nephrectomized rats nor in rats pretreated with indomethacin or saralasin. Indomethacin did not change responses already reduced by frusemide. Exogenous arachidonate or angiotensin II infused into the blood perfusing the tail artery did not alter stimulation responses. We conclude that intravenous frusemide administration to a rat reduces sympathetic vasoconstrictor responses of the ex vivo blood perfused tail artery segment by a diuresis-independent but prostaglandin and angiotensin II dependent release of another hormone from the kidney.

1. Segments of the tail artery of the rat were cannulated at both ends and mounted in an organ bath filled with Krebs solution. 2. Using an extracorporeal circuit, blood was pumped at a constant 2 ml/min from the carotid artery of anaesthetized rats to perfuse the segment of tail artery and returned to the donor rat via the jugular vein. 3. Peri-arterial electrical stimulation of the ex vivo blood perfused tail artery at 5 Hz produced vasoconstriction and an increase in perfusion pressure. 4. The intravenous administration of frusemide 5 mg/kg to the donor rat resulted in an inhibition of the vasoconstrictor responses of the perfused tail artery segment. Diuresis-induced losses of volume and frusemide were prevented by a urinary bladder-venous shunt. 5. Removal of the endothelium from the tail artery segment, by perfusion with dry gas for 4 min, prevented the vasoconstrictor-inhibitory effect of frusemide administration. Removal of the endothelium was confirmed histologically and by the absence of a vasodilator response to acetylcholine. 6. On the basis of these and previous results it is concluded that parenteral frusemide administration releases an unidentified but non-prostanoid hormone from the kidney which produces an endothelium-dependent inhibition of sympathetic vasoconstriction.