Sorcin inhibits calcium release and modulates excitation-contraction coupling in the heart

Activation of Ca2+ release channels/ryanodine receptors (RyR) by the inward Ca2+ current (ICa) gives rise to Ca2+-induced Ca2+ release (CICR), the amplifying Ca2+ signaling mechanism that triggers contraction of the heart. CICR, in theory, is a high-gain, self-regenerating process, but an unidentified mechanism stabilizes it in vivo. We reported previously (Lokuta, A. J., Meyers, M. B., Sander, P. R., Fishman, G. I., and Valdivia, H. H. (1997) J. Biol. Chem. 272, 25333-25338) that sorcin, a 22-kDa Ca2+-binding protein, binds to cardiac RyRs with high affinity and completely inhibits channel activity. Here we show that sorcin significantly inhibits both the spontaneous activity of RyRs in quiescent cells (visualized as Ca2+ sparks) and the ICa-triggered activity of RyRs that gives rise to [Ca2+]i transients. Because sorcin decreased the amplitude of the [Ca2+]i transient without affecting the amplitude or kinetics of ICa, the overall effect of sorcin was to reduce the "gain" of excitation-contraction coupling. Immunocytochemical staining shows that sorcin localizes to the dyadic space of ventricular cardiac myocytes. Ca2+ induces conformational changes and promotes translocation of sorcin between soluble and membranous compartments, but the [Ca2+] required for the latter process (ED50 = approximately 200 microM) appears to be reached only within the dyadic space. Rapid injection of 5 microM sorcin onto the cytosolic face of RyRs reconstituted in lipid bilayers resulted in complete inhibition of channel activity in < or = 20 ms. Thus, sorcin is a potent inhibitor of both spontaneous and ICa-triggered RyR activity and is kinetically capable of playing a role in terminating the positive feedback loop of CICR.