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 sol ution on excitation-contraction coupling in rat ventricular myocytes were s tudied. After short exposure, the action potential duration at 90% repolari zation (APD(90)), the intracellular Ca2+ concentration ([Ca2+](i)) transien t amplitude, and contraction increased, whereas the L-type Ca2+ current (I- Ca,I- L) amplitude decreased. Fractional sarcoplasmic reticulum (SR) Ca2+ r elease 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 ab breviation of APD(90) was partially reversed by 50 mu M DIDS, which is cons istent with the participation of Cl- current activated by swelling. After 1 0-min exposure to hyposmotic solution in cells labeled with di-8-aminonapht hylethenylpyridinium, t-tubule patterning remained intact, suggesting the l oss 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 fractiona l 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 ca n be accounted for by shortening of the action potential duration and a fal l in the I-Ca,I- L amplitude.