Involvement of β 2 ‐adrenoceptors in the regional haemodynamic responses to bradykinin in conscious rats

1 Bradykinin can release neuronal calcitonin gene‐related peptide (CGRP) and adrenal medullary catecholamines, both of which could contribute to its cardiovascular effects in vivo. Therefore, in the main experiment, regional haemodynamic responses to bolus injections of bradykinin (3 nmol kg −1 , i.v.) were assessed in the same chronically‐instrumented, conscious, Long Evans rats in the absence and in the presence of human α‐CGRP [8–37] or ICI 118551, antagonists of CGRP 1 ‐receptors and β 2 ‐adrenoceptors, respectively. The selected doses of these antagonists caused specific inhibition of responses mediated by exogenous human α‐CGRP and β 2 ‐adrenoceptor agonists, respectively. 2 Bradykinin administered alone as an i.v. bolus had a slight pressor effect accompanied by a marked tachycardia. There were early (at about 30 s) increases in flow and conductance in the mesenteric vascular bed, and delayed (at about 90 s), but qualitatively similar, changes in the hindquarters vascular bed. There were only slight increases in flow and conductance in the renal vascular bed. 3 Human α‐CGRP [8–37] had no statistically significant effects on the responses to bolus doses of bradykinin. However, in the presence of ICI 118551, the pressor effect of bradykinin was significantly enhanced while its tachycardic effect was significantly suppressed. The hindquarters vasodilator effect of bradykinin was converted to a vasoconstriction and there was a slight renal vasoconstriction, but the mesenteric vasodilator effect of bradykinin was unchanged by ICI 118551. 4 In subsidiary experiments, in other animals, it was found that infusion of bradykinin (36 nmol kg −1 min −1 ) elicited a pattern of haemodynamic responses similar to that seen with bolus injections and, as in the latter case, the hindquarters hyperaemic vasodilatation was inhibited by ICI 118551. In the presence of mecamylamine (at a dose sufficient to block reflex heart rate responses to rises or falls in arterial blood pressure) bolus injection or infusion of bradykinin still elicited increases in renal, mesenteric and hindquarters blood flow. However, in additional experiments in adrenal demedullated rats ( n = 4) the hindquarters hyperaemic effect of bradykinin was absent, although the mesenteric hyperaemic effect remained. 5 The results indicate that the increase in hindquarters blood flow following administration of bradykinin in vivo is largely due to activation of β 2 ‐adrenoceptors by catecholamines released subsequent to direct stimulation of the adrenal medulla by the peptide. However, the bradykinin‐induced increase in mesenteric blood flow does not depend on this mechanism.