Two inhibitors of store operated Ca2+ entry suppress excitation contraction coupling in frog skeletal muscle

Abstract

Two drugs, 2-APB and SKF-96365, commonly used to block Store Operated Ca(2+) Entry (SOCE) were found to have inhibitory effects at different levels of the Excitation Contraction Coupling (ECC) process in frog skeletal muscle fibers. Treatment with either drug suppressed Ca(2+) release from the Sarcoplasmic Reticulum, but this effect was not due to inhibition of SOCE as it occurred in Ca(2+)-free conditions. 2-APB applied extracellularly at 100 microM, the usual concentration to suppress SOCE, reversibly reduced the charge movement elicited by pulses in the range between -45 and -35 mV from 7.99 +/- 0.73 nC/microF (N = 17) before drug application to 6.27 +/- 0.68 nC/microF in the presence of 2-APB. This effect was mostly on the delayed Q(gamma) component. In fibers treated with the SERCA ATPase inhibitor CPA the Q(gamma) component disappeared, under this condition the application of 2-APB did not suppress the remaining charge movement. Thus the effect of 2-APB on charge movement currents seemed to be secondary to the suppression of Ca(2+) release, likely occurring directly on the release channels. No significant suppression of ECC was observed for concentration below 20 muM. 2-APB also inhibited the L-type Ca(2+) current (20 +/- 4%, N = 8). On the other hand SKF-96365 had a direct effect on the voltage sensor promoting its voltage dependent inactivation. Applied at 20 muM, a typical concentration used for inhibiting SOCE, to fibers held at -80 mV inhibited the charge moved in response to pulses ranging -45 to -30 mV from 7.95 +/- 2.59 nC/microF to 3.48 +/- 0.9 nC/microF (N = 12). A parallel reduction of Ca(2+) release was observed. Wash out was drastically increased by hyperpolarization of the holding potential to -100 mV. SKF-96365 also inhibited the L-type Ca(2+) current (41 +/- 8%, N = 4) and increased its rate of inactivation.