Abstract
Contractures initiated by 1 s superfusion of single myocytes of guinea-pig heart with 10 mM caffeine were used as a relative index of the SR Ca 2+ content. Thapsigargin (Tg) in concentration 2 × 10 -7M completely blocked Ca 2+ uptake during electrical stimulation by the SR from which Ca 2+ has been previously depleted by caffeine. Tg did not affect the SR Ca 2+ content in the resting myocytes and did not block release of the SR Ca 2+ during electrically stimulated contractions (ESCs). It is concluded that in guinea-pig myocytes Tg affects SR Ca 2+ by selective blocking the SR Ca 2+ uptake. The amplitude of steady state ESCs dropped to 68±5.4% (S.D., n=20) of that of the pre-Tg control. Time to peak contraction increased from 454±82.4 ms to 820±157.4 ms and time of relaxation increased from 368±90.8 ms to 474±87 ms after the SR Ca 2+ has been depleted by Tg. Rest decay of contractions, post-extrasystolic potentiation and post-rest potentiation were inhibited.
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