Cytosolic Ca transients in endothelium-dependent relaxation of pig coronary artery, and effects of captopril

To determine the mechanism of endothelium-dependent relaxation by bradykinin, we simultaneously measured changes in cytosolic calcium concentrations (Ca²⁺]_i) and force of fura-2-loaded strips of porcine coronary artery. We also examined effects of captopril, an angiotensin converting enzyme inhibitor, on bradykinin-induced relaxation. Bradykinin, in a concentration-dependent manner (10^?10 to 10^?7 M), decreased both Ca²⁺]_i and force to resting levels, during 10^?5 M prostaglandin F₂-induced contractions, only when endothelium was intact. Treatment with 10^?5 M captopril enhanced the bradykinin-induced decreases in Ca²⁺]_i and force and shifted the concentration-response bradykinin induced a greater relaxation than that expected from the reduction in Ca²⁺]_i. Captopril had no effects on the relationship between reduction in Ca²⁺]_i and relaxation induced by bradykinin. Bradykinin relaxes porcine coronary artery in an endothelium-dependent manner, by decreasing Ca²⁺]_i and also by controlling the Ca²⁺ sensitivity of the contractile apparatus of smooth muscle. Captopril enhanced the bradykinin-induced relaxation, with no apparent direct effect on Ca²⁺ sensitivity of the contractile apparatus.