Biphasic effects of hyposmotic challenge on excitation-contraction coupling in rat ventricular myocytes

The effects of short (1 min) and long (7-10 min) exposure to hyposmotic solution on excitation-contraction coupling in rat ventricular myocytes were studied. After short exposure, the action potential duration at 90% repolarization (APD(90)), the intracellular Ca2+ concentration ([Ca2+](i)) transient amplitude, and contraction increased, whereas the L-type Ca2+ current (I-Ca,I- L) amplitude decreased. Fractional sarcoplasmic reticulum (SR) Ca2+ release increased but SR Ca2+ load did not. After a long exposure, I-Ca,I- L, APD(90), [Ca2+](i) transient amplitude, and contraction decreased. The abbreviation of APD(90) was partially reversed by 50 mu M DIDS, which is consistent with the participation of Cl- current activated by swelling. After 10-min exposure to hyposmotic solution in cells labeled with di-8-aminonaphthylethenylpyridinium, t-tubule patterning remained intact, suggesting the loss of de-t-tubulation was not responsible for the fall in I-Ca,I- L. After long exposure, Ca2+ load of the SR was not increased, and swelling had no effect on the site-specific phosphorylation of phospholamban, but fractional SR Ca2+ release was depressed. The initial positive inotropic response to hyposmotic challenge may be accounted for by enhanced coupling between Ca2+ entry and release. The negative inotropic effect of prolonged exposure can be accounted for by shortening of the action potential duration and a fall in the I-Ca,I- L amplitude.