The effects of temperature upon calcium exchange in intact cultured cardiac myocytes

Cardiac myocyte Ca transport systems, such as sarcoplasmic reticulum Ca-ATPase and sarcolemmal Na/Ca exchange (Na/Ca), are critically dependent on temperature. The purpose of this work was to study the effect of temperature on cellular Ca compartmentation and its exchange characteristics in intact functional neonatal cultured myocytes. The Na/Ca mediated Ca exchange (CaNa/Ca)--including its sarcoplasmic reticulum (SR) and sarcolemmal (SL) contributions, a slow exchange component related to mitochondrial Ca and the La displaceable Ca pool were studied combining isotopic and gas-dissection techniques for membrane isolation. The major findings of this study are: (i) The amount of Ca exchanged through CaNa/Ca is clearly dependent on temperature (Q10 approximately 1.6) in the range studied (17-37 degrees C); (ii) the addition of 1 microM nifedipine does not modify the temperature dependence of CaNa/Ca; (iii) the sarcolemmal bound fraction contributing to CaNa/Ca is not changed by temperature; (iv) the increase in CaNa/Ca with temperature is explained by an increment in the contribution of SR-Ca to CaNa/Ca; (v) a fraction of SR which does not exchange via CaNa/Ca at low temperatures can be released and mobilized by caffeine-this caffeine sensitive fraction is reduced as temperature is increased and is no longer measurable as a separate entity at 37 degrees C; (vi) if we consider (iv) and (v) together, SR content would be temperature dependent with a Q10 of approximately 1.5; (vii) a La displaceable pool, which represents over 66% of the total exchangeable Ca, increases in the range of 22-33 degrees C with a Q10 of 1.25 which is consistent with a pool distribution of 70% SL-bound and 30% SR-derived [Post J.A., Langer G.A. Cellular origin of the rapidly exchangeable calcium pool in the cultured neonatal rat heart cell. Cell Calcium 1992; 13: 627-634]; and (viii) the rate constant for the mitochondrial Ca component increases by 60% from 22 degrees C to 37 degrees C, but Ca content in this organelle is not modified over this temperature range.