Inhibition of SERCA pumps induces desynchronized RyR activation in overloaded internal Ca²⁺ stores in smooth muscle cells

Abstract

We have previously shown that rapid inhibition of sarcoplasmic reticulum (SR) ATPase (SERCA pumps) decreases the amplitude and rate of rise (synchronization) of caffeine induced-Ca(2+) release without producing a reduction of free luminal SR Ca(2+) level in smooth muscle cells (Gómez-Viquez L, Guerrero-Serna G, García U, Guerrero-Hernández A. Biophys J 85: 370-380, 2003). Our aim was to investigate the role of luminal SR Ca(2+) content in the communication between ryanodine receptors (RyRs) and SERCA pumps. To this end, we studied the effect of SERCA pump inhibition on RyR-mediated Ca(2+) release in smooth muscle cells with overloaded SR Ca(2+) stores. Under this condition, the amplitude of RyR-mediated Ca(2+) release was not affected but the rate of rise was still decreased. In addition, the caffeine-induced Ca(2+)-dependent K(+) outward currents revealed individual events, suggesting that SERCA pump inhibition reduces the coordinated activation of RyRs. Collectively, our results indicate that SERCA pumps facilitate the activation of RyRs by a mechanism that does not involve the regulation of SR Ca(2+) content. Importantly, SERCA pumps and RyRs colocalize in smooth muscle cells, suggesting a possible local communication between these two proteins.