Magnesium and hydrogen ions inhibit sarcoplasmic reticulum function in cardiac muscle

We investigated the direct effects of magnesium and hydrogen ions (Mg2+ and H+) on cardiac sarcoplasmic reticulum (SR) functions using saponin-treated ferret ventricular muscles. Ca2+ uptake function and the Ca(2+)-induced Ca2+ release (CICR) mechanism were examined. For the analysis of Ca2+ uptake, the SR was loaded with Ca2+ under various conditions and the Ca2+ content of SR was estimated by releasing Ca2+ with 50 mM caffeine. To determine CICR, following Ca2+ loading of the SR, solutions with various free Ca2+ concentrations ([Ca2+]) were applied to the preparation to release Ca2+, and the Ca2+ remaining in the SR was estimated. The amount of the released Ca2+ was measured with the fluorescent indicator fluo-3. Mg2+ (10mM) and H+ (pH 6.6) inhibited both Ca2+ uptake and CICR. The rate of Ca2+ accumulation was decreased but the steady-state Ca2+ content of the SR was not significantly altered in the loading solutions with a high [Mg2+] or low pH. CICR at around 1 microM [Ca2+] was clearly inhibited by a moderately low pH 6.6 but Mg2+ (0.6-4mM) did not significantly inhibit CICR except for at a very high [Mg2+] (10 mM). At the high [Ca2+] which shows maximal CICR, the Ca2+ release was not inhibited by either Mg2+ or H+. It is suggested that the elevation of intracellular H+ more profoundly influences the Ca2+ regulation mechanisms of the SR compared to the elevation of intracellular [Mg2+] under ischemic conditions.