EFFECT OF BRADYKININ ON ISOLATED MESENTERIC-ARTERIES OF THE RAT

Bradykinin is a potent vasodilator peptide; however, its half-life in vivo is very short because of various plasma and tissue peptidases that hydrolyze bradykinin to inactive fragments. We studied the role of kininase II (angiotensin converting enzyme) and neutral endopeptidase 24.11 (enkephalinase) in the catabolism of bradykinin in vascular tissue by determining the effect of inhibitors of kininase II (captopril) and of endopeptidase 24.11 (phosphoramidon) on the action of bradykinin on rat isolated mesenteric arteries. Because bradykinin may induce prostaglandin formation and release, we also studied the effect of a cyclooxygenase inhibitor, indomethacin, on the action of bradykinin. The mesenteric bed was isolated from rats (250-300 g) with rats under ether anesthesia and was perfused with Krebs' solution (4 ml/min) containing phenylephrine (0.5-1.0-mu-g/ml) to produce a mean perfusion pressure of 120-130 mm Hg. Bradykinin (2.5-40.0 ng), injected as a bolus, produced a dose-dependent decrease in perfusion pressure. In the presence of indomethacin (1.0-mu-ml), the amplitude of the vasodilator responses to bradykinin was not significantly affected, although the duration of the responses was increased approximately two to four times. In the presence of captopril (1.0-mu-g/ml), bradykinin elicited either a vasodilator or a biphasic effect. The vasodilator effect was greatly potentiated by captopril, whereas the duration of the response was unchanged when compared with control experiments. When present, the pressor responses were also dose related. In the presence of indomethacin plus captopril, bradykinin produced only a fall in perfusion pressure that lasted five to six times longer than without any treatment. Phosphoramidon (1-200 ng/ml) did not affect the responses to bradykinin. We conclude that bradykinin induces vasodilation of rat isolated mesenteric arteries followed by a vasoconstrictor effect due to prostaglandin release. The contribution of neutral endopeptidase 24.11 to bradykinin inactivation seems to be negligible, whereas kininase II plays an important role in the metabolism of bradykinin in vascular tissue.