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 Ca2+-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 m 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+(10 m ) 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 1μm [Ca2+] was clearly inhibited by a moderately low pH 6.6 but Mg2+(0.6–4m ) did not significantly inhibit CICR except for at a very high [Mg2+] (10 m ). 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.