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Role of Sarcoplasmic Reticulum and Mitochondria in Ca 2+ Removal in Airway Myocytes

Biophysical Journal, ISSN: 0006-3495, Vol: 86, Issue: 4, Page: 2583-2595
2004
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Article Description

The aim of this study was to use both a theoretical and experimental approach to determine the influence of the sarco-endoplasmic Ca 2+ -ATPase (SERCA) activity and mitochondria Ca 2+ uptake on Ca 2+ homeostasis in airway myocytes. Experimental studies were performed on myocytes freshly isolated from rat trachea. [Ca 2+ ] i was measured by microspectrofluorimetry using indo-1. Stimulation by caffeine for 30 s induced a concentration-graded response characterized by a transient peak followed by a progressive decay to a plateau phase. The decay phase was accelerated for 1-s stimulation, indicating ryanodine receptor closure. In Na 2+ -Ca 2+ -free medium containing 0.5 mM La 3+, the [Ca 2+ ] i response pattern was not modified, indicating no involvement of transplasmalemmal Ca 2+ fluxes. The mathematical model describing the mechanism of Ca 2+ handling upon RyR stimulation predicts that after Ca 2+ release from the sarcoplasmic reticulum, the Ca 2+ is first sequestrated by cytosolic proteins and mitochondria, and pumped back into the sarcoplasmic reticulum after a time delay. Experimentally, we showed that the [Ca 2+ ] i decay after Ca 2+ increase was not altered by the SERCA inhibitor cyclopiazonic acid, but was slightly but significantly modified by the mitochondria uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone. The experimental and theoretical results indicate that, although Ca 2+ pumping back by SERCA is active, it is not primarily involved in [Ca 2+ ] i decrease that is due, in part, to mitochondrial Ca 2+ uptake.

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