In situ SR function in postinfarction myocytes

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Abstract
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 ( t½) of [Ca2+]itransient decline compared with that measured during a normal twitch. These observations indicate that early decline of the [Ca2+]itransient during a twitch in rat myocytes was primarily mediated by SR Ca2+-ATPase and that the t½of [Ca2+]idecline is a measure of SR Ca2+uptake in situ. At 5.0 mM extracellular Ca2+, systolic [Ca2+]iwas significantly ( P ≤ 0.05) lower (337 ± 11 and 416 ± 18 nM in MI and sham, respectively) and t½of [Ca2+]idecline 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 t½of [Ca2+]idecline 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.