S100A1 increases the gain of excitation–contraction coupling in isolated rabbit ventricular cardiomyocytes

The effect of S100A1 protein on cardiac excitation–contraction (E–C) coupling was studied using recombinant human S100A1 protein (0.01–10 μM) introduced into single rabbit ventricular cardiomyocytes via a patch pipette. Voltage clamp experiments (20 °C) indicated that 0.1 μM S100A1 increased Ca2+ transient amplitude by ~ 41% but higher or lower S100A1 concentrations had no significant effect. L-type Ca2+ current amplitude or Ca2+ efflux rates via the Na+/Ca2+ exchanger (NCX) were unaffected. The rate of Ca2+ uptake associated with the SR Ca2+-ATPase (SERCA2a) was increased by ~ 22% with 0.1 μM S100A1, but not at other S100A1 concentrations. Based on the intracellular Ca2+ and INCX signals in response to 10 mM caffeine, no significant change in SR Ca2+ content was observed with S100A1 (0.01–10 μM). Therefore, 0.1 μM S100A1 appeared to increase the fractional Ca2+ release from the SR. This result was confirmed by measurements of Ca2+ transient amplitude at a range of SR Ca2+ contents. The hyperbolic relationship between these two parameters was shifted to the left by 0.1 μM S100A1. [3H]-ryanodine binding studies indicated that S100A1 increased ryanodine receptor (RyR) activity at 0.1 and 0.3 μM Ca2. As with the effects on E–C coupling, 0.1 μM S100A1 produced the largest effect. Co-immunoprecipitation studies on a range of Ca2+-handling proteins support the selective interaction of S100A1 on SERCA2a and RyR. In summary, S100A1 had a stimulatory action on RyR2 and SERCA2a in rabbit cardiomyocytes. Under the conditions of this study, the net effect of this dual action is to enhance the Ca2+ transient amplitude without significantly affecting the SR Ca2+ content.