THAPSIGARGIN INHIBITS CA2+ UPTAKE, AND CA2+ DEPLETES SARCOPLASMIC-RETICULUM IN INTACT CARDIAC MYOCYTES

We examined the effects of thapsigargin on Ca2+ accumulation by the sarcoplasmic reticulum (SR) and on electrically stimulated beats in single adult rat ventricular myocytes loaded with indo 1 and bathed in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer containing 1 mM Ca2+ at 23-degrees-C. The SR Ca2+ content was assessed from the magnitude of intracellular Ca2+ (Ca(i)2+) transients and contractions elicited by rapid, brief applications of caffeine. After 20-30 min of exposure to 200 nM thapsigargin, the caffeine-dependent Ca(i)2+ transients were abolished or markedly diminished (by 89 +/- 4%). The postrest potentiation of the Ca(i)2+ transient and contraction, typical for rat myocardium, was abolished. Thapsigargin did not significantly change resting Ca2+ but diminished the amplitude of the steady-state Ca2+ transients by 73%, prolonged the time to peak by 24%, and prolonged the half-time (t1/2) of the Ca(i)2+ transient decline by 42%. Progressive SR Ca2+ depletion by thapsigargin was strongly related (r = -0.78) to the prolongation of the t1/2 of relaxation of the steady-state Ca(i)2+ transients, suggesting that the thapsigargin-dependent SR Ca2+ depletion results from an inhibition of the SR Ca2+ uptake. This interpretation was corroborated by comparison of the effects of thapsigargin with those of ryanodine (100 nM), which depletes SR of Ca2+ by accelerating the SR Ca2+ efflux but does not inhibit the SR Ca2+ pump. During rapid pacing (5 Hz), which raises Ca(i)2+ and thus Ca2+ available for SR uptake, the caffeine-dependent SR Ca2+ release was restored in ryanodine-treated cells but not in the presence of thapsigargin. Thus, in contrast to ryanodine, thapsigargin prevents SR Ca2+ uptake, consistent with an inhibition of the SR Ca2+ pump.