Contractile regulation by overexpressed E[T.sub.A] requires intact T tubules in adult rat ventricular myocytes

Endothelin (ET)-I regulates the contractility and growth of the heart by binding G protein-coupled receptors of the ET type A receptor (E[T.sub.A])/ET type B (E[T.sub.B]) receptor family. E[T.sub.A], 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 q to ET-1, despite the robust expression in the surface sarcolemma. The 1 inclusion of the actin polymerization inhibitor cytochalasin D (CD) j during culture prevented gross morphological changes including a 1 loss of T tubules and a rounding of intercalated discs, but CD alone did not rescue the responsiveness to ET-1 or prevent ETA down regulation. The rescue of a normal PIE in 3- to 4-day cultured myocytes T 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 q ETA in surface sarcolemma was not. The results provide insight into i the pathological cardiac conditions in which ETA is upregulated and 2 T-tubule morphology is altered. cytochalasin D; inotropism; endothelin type A receptor; heart failure 1