Key result
Thapsigargin inhibits rapid Ca2+ transients and reduces total amplitude in guinea pig cardiomyocytes.
Thapsigargin and ryanodine alter calcium transients and contractions in guinea pig ventricular myocytes, suggesting sarcolemmal calcium influx initiates contractions when sarcoplasmic reticulum calcium is depleted.
Does not support clinical Ca2+ modulation strategies; hypothesis-generating for SR-independent contraction mechanisms in rodent cardiomyocytes.
We compared the effects of thapsigargin (TG), a selective blocker of Ca(2+)-adenosinetriphosphatase of sarcoplasmic reticulum (SR), and ryanodine (Ry) in the single isolated myocytes of guinea pig ventricular myocardium loaded with indo 1 acetoxymethyl ester (AM). TG (2 x 10(-7) M) inhibited the rapid phase of Ca2+ transient, increased time to peak intracellular Ca2+ concentration ([Ca2+]i) from 158 +/- 12 to 391 +/- 60 ms and decreased the total amplitude of the transient to 89 +/- 4% of the pre-TG control. Time to peak of contractions increased from 350 +/- 47 to 410 +/- 37 ms and total duration from 666 +/- 62 to 850 +/- 198 ms. Total amplitude of contractions was hardly affected. In the cells not loaded with indo 1-AM TG decreased the amplitude of contractions to 71 +/- 3% of control. When the effects of TG were fully developed, the cells ceased to respond to 1 s of superfusion with 15.0 mM caffeine with transient elevation of [Ca2+]i and/or transient contracture. TG did not affect the amplitude or time course of Ca2+ current (ICa) or the current-voltage relation. We propose that Ca2+ transients and contractions in the cells treated with TG were initiated by sarcolemmal Ca2+ influx. Ry (1.0 microM) initiated similar changes in the time course of Ca2+ transients and contractions as TG; however, total amplitude of the transients and contractions was reduced to 78 +/- 5 and 55 +/- 7% of the control, respectively. The SR Ca2+ was also depleted by Ry. TG superfused over the cells pretreated with Ry increased the amplitude of Ca2+ transients and respective contractions to the pre-Ry level. TG did not affect the ICa in the cells pretreated with Ry nor did it change configuration of action potentials to increase the Ca2+ influx. We propose that the effect of Ry on amplitude of Ca2+ transients and contractions results from the trapping of a fraction of sarcolemmal Ca2+ influx by the SR and its rapid release into subsarcolemmal space. From there it is extruded out of the cell by Na(+)-Ca2+ exchange before ever reaching the contractile system.
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Lewartowski et al. (1994) studied Normal and pretreated with ryanodine guinea pig cardiomyocytes. Thapsigargin vs. Control / Ryanodine was evaluated on Ca2+ transients and contractions. Thapsigargin inhibited the rapid phase of Ca2+ transient, increased time to peak intracellular Ca2+ concentration, and decreased the total amplitude of the transient in guinea pig cardiomyocytes.