Loading of calcium and strontium into the sarcoplasmic reticulum in rat ventricular muscle

Previous work suggests that strontium ions (Sr2+) are less effective than c alcium ions (Ca2+) at supporting excitation-contraction (EC) coupling in ca rdiac muscle. We therefore tested whether this was due to differences in th e uptake and release of Ca2+ and Sr2+ by the sarcoplasmic reticulum (SR) of rat ventricular trabeculae and myocytes at 22-24 degrees C, In permeabiliz ed trabeculae, isometric contractions activated by exposure to Ca2+ and Sr2 +-containing solutions produced similar maximal force. but were four limes more sensitive to Ca2+ than to Sr2+. The rate of loading and maximal SK cap acity for caffeine-releasable Ca2+ and Sr2+ were similar. In isolated, volt age-clamped ventricular myocytes, the SR content was measured as Na+-Ca2+ e xchange current during caffeine-induced SR cation releases, The SR Ca2+ loa d reached a steady maximum during a train of voltage clamp depolarizations. A similar maximal Sr2+ load was not observed, suggesting that the SR capac ity for Sr2+ exceeds that for Ca2+ Therefore, the relative inability of Sr2 + to support cardiac EC coupling appears not to be due to failure of the SR to sequester Sr2+, Instead, increases in cytosolic [Sr2+] seem to poorly a ctivate Sr2+ release from the SR. (C) 2000 Academic Press.