Mammalian myocardial cells have extremely high density of specific high-affinity binding sites for endothelin (ET) isopeptides. ET-1 and ET-3 elicited a positive inotropic effect (PIE) in association with a negative lusitropic effect, essentially with identical efficacies and potencies in the isolated rabbit papillary muscle, but with different concentration-dependent characteristics. In isolated rabbit papillary muscle the PIE of ET-1 was mediated by ETA2 subtype that is less sensitive to BQ-123 and FR139317, whereas the PIE of ET-3 was mediated by ETA1 subtype that is sensitive to these ETA antagonists. We found that the potency for the PIE of ET-1 in single rabbit ventricular cardiac myocytes was approximately hundred fold higher than that in isolated papillary muscles. ET-1, therefore, may not be able to reach its site of action or it may release endogenous substances that antagonize the PIE of ET-1 in intact ventricular myocardium. The former, however, appears to be unlikely since the potency of ET-3, structurally closely related to ET-1, elicited a PIE with equivalent potencies in single myocytes and papillary muscles. The latter possibility is under investigation: treatment of papillary muscles with 1% triton-X 100 that has been reported to give selective damage to endothelial cells did not affect the potency of ET-1 to an extent sufficient to explain the difference. Neither L-NAME, indomethacine, AF-DX 116 BS, 8-phenyltheophylline nor treatment with pertussis toxin affected significantly the potency of ET-1 in rabbit papillary muscles, up to now all interventions examined having failed to increase the potency of ET-1 in papillary muscles. It is noteworthy that the selective ETA receptor antagonists are able to inhibit the PIE of ET-1 in single myocytes with high affinity. These findings indicate that the unique pharmacological characteristics of ET-1 mediated PIE in the isolated rabbit papillary muscle are mainly due to the interaction of cellular components, involving various types of cell in intact ventricular myocardium, which are lost in single cardiac myocytes.
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