Effects of diltiazem or verapamil on calcium uptake and release from chicken skeletal muscle sarcoplasmic reticulum.
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 IC50 of about 290 micromol/L for verapamil and 260 micromol/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 IC50 for Ag+- and Ca2+-induced Ca2+ release was about 100 and 310 micromol/L for verapamil and 79 and 330 micromol/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.
Paydar
Mehrak Javadi
MJ
Department of Pharmacology, School of Medicine, Tehran University of Medical Sciences, Iran.
Pousti
Abbas
A
Farsam
Hasan
H
Amanlou
Massoud
M
Mehr
Shahram Ejtemaei
SE
Dehpour
Ahmad Reza
AR
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002645:Chickens; D004110:Diltiazem / Q000494:pharmacology*; D066298:In Vitro Techniques; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D012835:Silver Nitrate / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology*
D002121:Calcium Channel Blockers; D012835:Silver Nitrate; D014700:Verapamil; D000252:Calcium-Transporting ATPases; D004110:Diltiazem; D002118:Calcium