Inhibition of calcium-induced calcium release from purified cardiac sarcoplasmic reticulum vesicles

A variety of reagents (local anesthetics, phenothiazines, ruthenium red, ryanodine, dicyclohexylcarbodiimide, R 24571) inhibit Ca2+-induced Ca2+ release from purified canine cardiac sarcoplasmic reticulum (SR). Most of these compounds also increase the rate of net Ca2+ uptake by cardiac SR while moderately inhibiting Ca2+-dependent ATP hydrolysis, and together these two effects produce increased coupling ratios for ATP-dependent Ca2+ transport (Ca2+/ATP approximately equal to 2) compared to controls (Ca2+/ATP approximately equal to 1). We conclude that Ca2+ efflux normally occurs during net Ca2+ uptake by isolated cardiac SR vesicles and that this phenomenon is responsible for the low coupling ratios generally observed for cardiac SR preparations. Blockers of sarcolemmal Ca2+ channels (nitrendipine, diltiazem, methoxyverapamil, dantrolene), at concentrations much greater than those effective for sarcolemmal Ca2+ fluxes, do not affect either Ca2+ uptake or Ca2+ release by cardiac SR. Furthermore, the effects of local anesthetics and phenothiazines on Ca2+ release from cardiac SR are different from those previously reported for skeletal muscle SR. These results indicate that the Ca2+ release “channels” in cardiac SR are distinct from Ca2+ channels present in cardiac sarcolemma or in skeletal muscle SR. The common hydrophobic nature but structural dissimilarity of various inhibitors of Ca2+ release from cardiac SR suggest that in some cases a hydrophobic interaction with the membrane might be involved in blocking Ca2+ efflux. On the other hand, half-maximal inhibition by 80 nM ruthenium red is suggestive of a specific ionic interaction with some component of the Ca2+ efflux pathway.