Molecular mechanisms of Ca^<2+> uptake by sarcolipin in cardiac sarcoplasmic reticulum.

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The aim of the study was to explore the role of Ca^<2+> handling-related protein, sarcolipin (SLN) for the regulation of intracellular Ca^<2+> concentration. Sarcoplasmic reticulum (SR)(SERCA2a) is regulated by phospholamban (PLB) and SLN which couple with SERCA2a. Both non-phosphorylated-PLB and -SLN decrease the Ca^<2+> uptake rate of SR. We overexpressed SLN in ventricular muscles of mouse (SLN-TG) and investigated how SERCA2a is influenced by SLN. We measured the Ca^<2+> transients and contraction in intact left ventricular papillary muscles of mouse using the aequorin method. Also, we used saponin-treated thin trabeculae to investigate Ca^<2+> uptake, Ca^<2+> release and Ca^<2+> leakage. In twitch contraction, Hz, the peaks of the Ca^<2+> transient and contraction in SLN-TG were lower than those of the control. The rate of decline of the Ca^<2+> transient and tension was slower than that of the control. In the skinned fiber, Ca^<2+> was loaded in the SR in the presence of ATP and Ca^<2+> (pCa 6.2) for various periods, and Ca^<2+> in the SR was released by caffeine. We measured the released Ca^<2+> with fluo-3. The rate of Ca^<2+> uptake in SLN-TG was slower when the Ca^<2+> loading time was short but no difference was observed when the loading time was longer. The maximal Ca^<2+> content of SR at a steady state in both TG and non-TG cardiac muscles did not differ. Ca^<2+>-induced Ca^<2+> release (CICR) in SLN-TG did not differ from that in non-TG. Ca^<2+> leakage was estimated by measuring the leaked Ca in the solution without ATP and Ca^<2+> after Ca^<2+> loading. Ca^<2+> leakage in SLN-TG was not different from that in non-TG. Thus, SLN decreases the Ca^<2+> uptake in SR and influences intracellular Ca^<2+> concentration. The role of SLN is significant in beat-to-beat contraction, but SLN does not play a significant role at a steady state.