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J Appl Physiol 87: 2143-2150, 1999;
8750-7587/99 $5.00
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Vol. 87, Issue 6, 2143-2150, December 1999

In situ SR function in postinfarction myocytes

Xue-Qian Zhang1, Yuk-Chow Ng2, Russell L. Moore3, Timothy I. Musch4, and Joseph Y. Cheung1,5

Departments of 1 Medicine, 2 Pharmacology, and 5 Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey, Pennsylvania 17033; 3 Department of Kinesiology, University of Colorado, Boulder, Colorado 80309; and 4 Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas 66506

Previous studies have shown lower systolic intracellular Ca2+ concentrations ([Ca2+]i) and reduced sarcoplasmic reticulum (SR)-releasable Ca2+ contents in myocytes isolated from rat hearts 3 wk after moderate myocardial infarction (MI). Ca2+ entry via L-type Ca2+ channels was normal, but that via reverse Na+/Ca2+ exchange was depressed in 3-wk MI myocytes. To elucidate mechanisms of reduced SR Ca2+ contents in MI myocytes, we measured SR Ca2+ uptake and SR Ca2+ leak in situ, i.e., in intact cardiac myocytes. For sham and MI myocytes, we first demonstrated that caffeine application to release SR Ca2+ and inhibit SR Ca2+ uptake resulted in a 10-fold prolongation of half-time (t1/2) of [Ca2+]i transient decline compared with that measured during a normal twitch. These observations indicate that early decline of the [Ca2+]i transient during a twitch in rat myocytes was primarily mediated by SR Ca2+-ATPase and that the t1/2 of [Ca2+]i decline is a measure of SR Ca2+ uptake in situ. At 5.0 mM extracellular Ca2+, systolic [Ca2+]i was significantly (P <=  0.05) lower (337 ± 11 and 416 ± 18 nM in MI and sham, respectively) and t1/2 of [Ca2+]i decline was significantly longer (0.306 ± 0.014 and 0.258 ± 0.014 s in MI and sham, respectively) in MI myocytes. The 19% prolongation of t1/2 of [Ca2+ ]i decline was associated with a 23% reduction in SR Ca2+-ATPase expression (detected by immunoblotting) in MI myocytes. SR Ca2+ leak was measured by a novel electrophysiological technique that did not require assigning empirical constants for intracellular Ca2+ buffering. SR Ca2+ leak rate was not different between sham and MI myocytes: the time constants of SR Ca2+ loss after thapsigargin were 290 and 268 s, respectively. We conclude that, independent of decreased SR filling by Ca2+ influx, the lower SR Ca2+ content in MI myocytes was due to reduced SR Ca2+ uptake and SR Ca2+-ATPase expression, but not to enhanced SR Ca2+ leak.

sarcoplasmic reticulum calcium uptake; sarcoplasmic reticulum calcium leak; sarco(endo)plasmic reticulum calcium adenosinetriphosphatase; excitation-contraction coupling; fura 2; patch clamp; quantitative fluorescence microscopy; cardiac hypertrophy; caffeine-induced calcium release


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