AJP - Heart and Circulatory Physiology, Vol 265, Issue 2 517-H522, Copyright © 1993 by American Physiological Society

ARTICLES

Thapsigargin inhibits Ca2+ uptake, and Ca2+ depletes sarcoplasmic reticulum in intact cardiac myocytes

A. M. Janczewski and E. G. Lakatta
Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224.

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+ (Ca2+i) transients and contractions elicited by rapid, brief applications of caffeine. After 20-30 min of exposure to 200 nM thapsigargin, the caffeine-dependent Ca2+i transients were abolished or markedly diminished (by 89 +/- 4%). The postrest potentiation of the Ca2+i transient and contraction, typical for rat myocardium, was abolished. Thapsigargin did not significantly change resting Ca2+i but diminished the amplitude of the steady-state Ca2+i transients by 73%, prolonged the time to peak by 24%, and prolonged the half-time (t1/2) of the Ca2+i 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 Ca2+i 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 Ca2+i 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.(ABSTRACT TRUNCATED AT 250 WORDS)

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