Temperature dependent contribution of Ca2+ transporters to relaxation in cardiac myocytes: important role of sarcolemmal Ca2+-ATPase

Abstract

Activities of Ca(2+) -ATPase of sarcoplasmic reticulum (SERCA) and Na(+)/Ca(2+) exchanger (NCX) involved in cellular Ca(2+) turnover greatly change in hypertrophied and failing hearts. Unfortunately, contribution of these proteins as well as of the sarcolemmal Ca(2+)-ATPase (PMCA) to cellular Ca(2+) turnover has been investigated almost exclusively at room temperature. PMCA is of particular interest since it may affect activity of calcineurin and nNOS. Therefore the objective of this study was to reinvestigate contribution of SERCA, NCX and PMCA to cell relaxation and the effect of PMCA on cell contraction at 37 degrees C. Myocytes isolated from the ventricles of guinea pig and rat hearts and incubated with Indo-1 were field stimulated at the rate of 60/min. Contribution of SERCA, NCX and PMCA was calculated from the rate constants of the decaying components of electrically stimulated Ca(2+) transients or of the transients initiated by caffeine dissolved in normal Tyrode or in 0Na, 0Ca Tyrode. Increase in temperature from 24 to 37 degrees C increased the relative contribution of NCX from 6.1% to 7.5% in rat and from 21.3 to 51.9% in guinea pig at the expense of SERCA. The contribution of the PMCA to relaxation in both species increased upon rise in temperature from 24% to 37 degrees C from negligible values to 3.7%. In both species amplitude of Ca(2+) transients was at 24 degrees C nearly twice as high as at 37 degrees C. It was nearly doubled by carboxyeosine (CE), a PMCA blocker at 37 degrees C but was hardly affected at 24 degrees C. The effects of CE were concentration-dependent and conformed with the degree of inhibition of activity of PMCA.

Conclusions: PMCA plays an important role in regulation of myocardial contraction despite its small contribution to relaxation. In guinea pig but not in rat relative contribution of SERCA and NCX to relaxation is highly temperature dependent.