Abstract

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 ET_A and ET_B 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 × 10⁻¹¹ and 2.6 × 10⁻¹⁰ M, respectively. With the same order of potency, ET-1 and ET-3 increased (P < .05) velocity of cell shortening. The ET_A receptor-selective antagonist ABT-627 shifted the ET-1-induced cell shortening response curve to the right with a pA₂ value of 10.3. The ET_B receptor-selective antagonist A-192621 (10⁻⁸-10⁻⁷ M) did not alter the concentration-response of ET-1. Moreover, the ET_Breceptor-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 ET_A receptor subtype, and not the ET_B receptor subtype, mediates the positive inotropic effect of ET-1, and 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.