Paracrine effects of endocardial endothelial cells on myocyte contraction mediated via endothelin

Endocardial endothelium is reported to modulate myocardial contraction by releasing diffusible factors, but the nature of the agent(s) responsible is unknown. In the present study we investigated the potential role of endothelin in these effects. Cultured sheep endocardial endothelial cells were found to express endothelin-1 mRNA and to release endothelin-1 into superfusing solution. This superfusate induced positive inotropic effects in isolated rat cardiac myocytes, associated with an increase in the cytosolic Ca2+ transient. Similar positive inotropic effects were induced by vascular endothelial cell superfusate as well as by synthesized endothelin-1, administered at concentrations similar to those present in the superfusate. Incubation of endocardial endothelial cell superfusate with endothelin-1-specific antiserum reduced the free endothelin-1 concentration to undetectable levels and abolished both the positive inotropic effect and the rise in cytosolic Ca2+. These findings indicate that endocardial endothelial cells may modulate myocardial contraction in part through the release of endothelin-1 and suggest that endocardial as well as vascular endothelium could exert potent paracrine effects on myocardium.