Effects of diltiazem or verapamil on calcium uptake and release from chicken skeletal muscle sarcoplasmic reticulum

Abstract

The purpose of this study was to determine the effects of 2 Ca2+channel blockers, verapamil and diltiazem, on calcium loading (active Ca2+uptake) and the following Ca2+release induced by silver ion (Ag+) and Ca2+from the membrane of heavy sarcoplasmic reticulum (SR) of chicken skeletal muscle. A fluorescent probe technique was employed to determine the calcium movement through the SR. Pretreatment of the medium with diltiazem and verapamil resulted in a significant decrease in the active Ca2+uptake, with IC50of about 290 µmol/L for verapamil and 260 µmol/L for diltiazem. Inhibition of Ca2+uptake was not due to the development of a substantial drug-dependent leak of Ca2+from the SR. It might, in part, have been mediated by a direct inhibitory effect of these drugs on the Ca2+ATPase activity of the SR Ca2+pump. We confirmed that Ca2+channel blockers, administered after SR Ca2+loading and before induction of Ca2+release, caused a dose-dependent inhibition of both Ca2+- and Ag+-induced Ca2+release rate. Moreover, if Ca2+channel blockers were administered prior to SR Ca2+loading, in spite of Ca2+uptake inhibition the same reduction in Ca2+- and Ag+-induced Ca2+release rate was seen. We showed that the inhibition of Ag+-induced Ca2+release by L-channel blockers is more sensitive than Ca2+-induced Ca2+release inhibition, so the IC50for Ag+- and Ca2+-induced Ca2+release was about 100 and 310 µmol/L for verapamil and 79 and 330 µmol/L for diltiazem, respectively. Our results support the evidence that Ca2+channel blockers affect muscle microsome of chicken skeletal muscle by 2 independent mechanisms: first, reduction of Ca2+uptake rate and Ca2+-ATPase activity inhibition, and second, inhibition of both Ag+- and Ca2+-induced Ca2+release by Ca2+release channels. These findings confirm the direct effect of Ca2+channel blockers on calcium release channels. Our results suggest that even if the SR is incompletely preloaded with Ca2+because of inhibition of Ca2+uptake by verapamil and diltiazem, no impairment in Ca2+release occurs. Key words: calcium, sarcoplasmic reticulum, diltiazem, verapamil, chicken, skeletal muscle.