Endothelin, receptor subtype mediates endothelin-induced contractility in left ventricular cardiomyocytes isolated from rabbit myocardium

Endothelin (ET)-1 is a potent positive inotropic agent, the effects of which are mediated by increases in cytosolic Ca2+ in the myocardium. The object of this study was to examine 1) the influence of ETA and ETB receptor subtypes, and 2) the role of the phospholipase C (PLC) pathway in mediating ET-1-induced contraction. Left ventricular cardiomyocytes were isolated from the hearts of New Zealand White rabbits (2-2.5 kg) by the use of Langendorff perfusion with collagenase. Cardiomyocyte function was examined during unloaded, electrically stimulated (0.5 Hz) contractions with a video-edge detection system. ET-1 increased cell shortening with greater potency than ET-3: mean EC50 values were 1.1 x 10(-11) and 2.6 x 10(-10) M, respectively. With the same order of potency, ET-1 and ET-3 increased (P < .05) velocity of cell shortening. The ETA receptor-selective antagonist ABT-627 shifted the ET-1-induced cell shortening response curve to the right with a pA(2) value of 10.3. The ETB receptor-selective antagonist A-192621 (10(-8)-10(-7) M) did not alter the concentration-response of ET-1. Moreover, the ETB receptor-selective agonist sarafotoxin 6c did not have any effect on cell shortening over the concentration range of 10(-11) to 10(-7) M. ET-1 in the presence of the PLC inhibitor U-73122 did not alter the contractile amplitude. However, ET-1 in the presence of the protein kinase C inhibitor bisindolylmalemide increased cell shortening. These findings indicate that 1) the ETA receptor subtype, and not the ETB receptor subtype, mediates the positive inotropic effect of ET-1, acid 2) the response of ET-1 is mediated by a PLC pathway, but not through protein kinase C, in ventricular cardiomyocytes isolated from rabbit myocardium.