Importance of Sympathetic Innervation in the Positive Inotropic Effects of Bradykinin and Ramiprilat

Isolated rat left atria or right ventricular strips were electrically stimulated at a constant frequency. The amplitude of twitch contractions, thus elicited, rose as a function of stimulation intensity because of increases in the evoked release of sympathetic catecholamines. Bradykinin had no effect on contractile force in preparations paced at a minimal intensity (threshold). By contrast, bradykinin (1 nmol/L to 1 μmol/L) markedly increased twitch contractile force when the preparations were paced at a high intensity (two to three times threshold). The EC50 for the positive inotropic action of bradykinin averaged 42 nmol/L. Ramiprilat (1 μmol/L), an angiotensin I-converting enzyme/kininase II inhibitor, shifted the EC50 for bradykinin to ≈2 nmol/L. Ramiprilat (1 μmol/L) per se also produced a modest positive inotropic effect. The effects of bradykinin and/or ramiprilat were inhibited by HOE 140 (300 μmol/L), a bradykinin β2-receptor antagonist. Propranolol (1 μmol/L), a β-adrenoceptor blocker, abolished the effects of bradykinin. After the destruction of sympathetic nerve endings by use of 6-hydroxy-dopamine, bradykinin no longer exerted a positive inotropic action. Cocaine (10 μg/mL), an inhibitor of catecholamine reuptake, potentiated the effect of bradykinin. Bradykinin did not affect the positive inotropic response to tyramine (10 μmol/L), whereas cocaine blocked it. Furthermore, bradykinin did not modify the dose-response curves for added norepi-nephrine. ω-Conotoxin (100 nmol/L) inhibited the positive inotropic effect of intensified stimulation and bradykinin potentiation. Bradykinin is suggested to facilitate the evoked release of sympathetic catecholamines and thereby cause a positive inotropic effect.