Effect of diltiazem on the release of calcium from the canine fragmented cardiac sarcoplasmic reticulum

Fragmented sarcoplasmic reticulum fraction (SR) was prepared from the ventricle of canine heart, and the effect of diltiazem on its Ca2+ binding and Ca2+ release was examined by centrifugation and filtration methods using 45Ca. Cardiac SR bound 45-55 nmoles/mg of Ca ions in the presence of Mg-ATP. Diltiazem in concentrations up to 10-4 M had little effect on the Ca2+ binding of SR. The membrane of cardiac SR was “depolarized” by either changing propionate to chloride (anionic) or potassium to Tris (hydroxymethyl) aminomethane (Tris) (cationic). About 12% of the maximum Ca2+ bound to the S R was released by cationic “depolarization”, but no Ca2+ was released by anionic “depolarization”. The Ca2+ release induced by the cationic “depolarization” was inhibited by diltiazem, and the inhibitory effect of diltiazem on the Ca2+ release was dependent on the incubation time. Incubation of the SR with 3X10-6 M diltiazem for 30 sec almost completely inhibited the release of Ca2+, while incubation with 3x10-7 M diltiazem incompletely inhibited the Ca2+ release. About 20% of the maximum Ca2+ bound to the SR was released by the addition of 5.1 mM caffeine. The Ca2+ release induced by caffeine was inhibited by increasing the concentration of MgCI2 from 5 to 10 mM, but was not inhibited by 10 mM procaine. An increase of ATP concentration accelerated the time course of the caffeine-induced release of Ca2+ from the SR and subsequent rebinding of Ca2+. Diltiazem up to 10-5 M had no effect on the caffeine-induced Ca2+ release. These findings clearly indicate that diltiazem inhibits the “depolarization”-induced Ca2+ release from cardiac sarcoplasmic reticulum.