Effects of thapsigargin in normal and pretreated with ryanodine guinea pig cardiomyocytes
Lewartowski, B.; Rózycka, M.; Janiak, R.
American Journal of Physiology 266(5 Pt 2): H1829-H1839
1994
ISSN/ISBN: 0002-9513 PMID: 8203582 Document Number: 429388
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 times 10-7 M) inhibited the rapid phase of Ca-2+ transient, increased time to peak intracellular Ca-2+ concentration ((Ca-2+)-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 t 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 (Ca-2+)-i and/or transient contracture. TG did not affect the amplitude or time course of Ca-2+ current (I-Ca) or the current-voltage relation. We propose that Ca2+ transients and contractions in the cells treated with TG were initiated by sarcolemmal Ca-2+ influx. Ry (1.0 mu-M) initiated similar changes in the time course of Ca-2+ 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 Ca-2+ was also depleted by Ry. TG superfused over the cells pretreated with Ry increased the amplitude of Ca-2+ transients and respective contractions to the pre-Ry level. TG did not affect the I-Ca in the cells pretreated with Ry nor did it change configuration of action potentials to increase the Ca-2+ influx. We propose that the effect of Ry on amplitude of Ca-2+ transients anf contractions results from the trapping of a fraction of sarcolemmal Ca-2+ influx by the SR and its rapid release into subsarcolemmal space. From there it is extruded out of the cell by Na+-Ca-2+ exchange before ever reaching the contractile system.