Properties of Ca²⁺ handling in sarcoplasmic reticulum of saponin-treated SERCA2a transgenic mouse myocardium

Abstract

We investigated whether the increase in Ca²⁺ uptake rate of sarcoplasmic reticulum (SR) would influence Ca²⁺ content and Ca²⁺ leakage in SR. To investigate SR functions at different Ca²⁺ uptake rates, we used transgenic mouse myocardium overexpressed SR-Ca²⁺ ATPase (SERCA2a-TG). We evaluated Ca²⁺ uptake rate, Ca²⁺ release and Ca²⁺ leakage using saponin-treated thin trabeculae obtained from the TG and non-TG mice hearts. Each SR function was estimated by measuring the remaining Ca²⁺ in SR which was released by caffeine (50 mM) after SR was loaded with Ca²⁺ at various concentrations (pCa 8-5.6) for different periods (5-120 sec). The released Ca²⁺ was measured with fluo-3. In saponin-treated preparation, the Ca²⁺ handling proteins (SERCA2a, phospholamban and ryanodine receptor) were well preserved, confirmed with Western immunoblotting. The structures of transverse tubules and SR were intact, confirmed by electron microscopy. At short Ca²⁺ loading time, Ca²⁺ content was larger in TG myocardium (113.8%) than that in non-TG myocardium. The maximal Ca²⁺ content and Ca²⁺ leakage were similar in both myocardium. In conclusion, the modulation of Ca²⁺ uptake rate would influence Ca²⁺ handling in each heartbeat but does not alter the maximal Ca²⁺ content and Ca²⁺ leakage at a steady state. [J Physiol Sci. 2007;57 Suppl:S208]