Contractile regulation by overexpressed ETA requires intact T tubules in adult rat ventricular myocytes

Endothelin (ET)-1 regulates the contractility and growth of the heart by binding G protein-coupled receptors of the ET type A receptor (ETA)/ET type B (ETB) receptor family. ETA, the predominant ET-1 receptor subtype in myocardium, is thought to localize preferentially within cardiac T tubules, but the consequences of mislocalization are not fully understood. Here we examined the effects of the overexpression of ETA in conjunction with T-tubule loss in cultured adult rat ventricular myocytes. In adult myocytes cultured for 3 to 4 days, the normally robust positive inotropic effect (PIE) of ET-1 was lost in parallel with T-tubule degeneration and a decline in ETA protein levels. In these T tubule-compromised myocytes, an overexpression of ETA using an adenoviral vector did not rescue the responsiveness to ET-1, despite the robust expression in the surface sarcolemma. The inclusion of the actin polymerization inhibitor cytochalasin D (CD) during culture prevented gross morphological changes including a loss of T tubules and a rounding of intercalated discs, but CD alone did not rescue the responsiveness to ET-1 or prevent ETA downregulation. The rescue of a normal PIE in 3- to 4-day cultured myocytes required both an increased expression of ETA and intact T tubules (preserved with CD). Therefore, the activation of ETA localized in T tubules was associated with a strong PIE, whereas the activation of ETA in surface sarcolemma was not. The results provide insight into the pathological cardiac conditions in which ETA is upregulated and T-tubule morphology is altered.