Submitted on January 4, 2008
Revised on February 21, 2008
Accepted on March 6, 2008
Contractile regulation by overexpressed ETA requires intact T-tubules in adult rat ventricular myocytes
Ka Young Chung1, Misuk Kang1, and Jeffery W Walker2*
1 University of Wisconsin-Madison
2 University of Wisconsin
* To whom correspondence should be addressed. E-mail: jwalker{at}physioloy.wisc.edu.
Endothelin-1 (ET-1) regulates contractility and growth of the heart by binding G-protein-coupled receptors (GPCRs) of the ETA/ETB 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 overexpression of ETA in conjunction with T-tubule loss in cultured adult rat ventricular myocytes. In adult myocytes cultured for 3-4 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 level. In these T-tubule compromised myocytes, overexpression of ETA using an adenoviral vector did not rescue responsiveness to ET-1, despite robust expression in the surface sarcolemma. Inclusion of the actin polymerization inhibitor cytochalasin D (CD) during culture prevented gross morphological changes including loss of T-tubules and rounding of intercalated discs, but CD alone did not rescue responsiveness to ET-1 nor prevent ETA downregulation. Rescue of a normal PIE in 3-4 day cultured myocytes required both increased expression of ETA and intact T-tubules (preserved with CD). Therefore, activation of ETA localized in T-tubules was associated with a strong PIE, whereas activation of ETA in surface sarcolemma was not. The results provide insight into pathological cardiac conditions in which ETA is upregulated and T-tubules morphology is altered.
