Mitochondrial free calcium regulation during sarcoplasmic reticulum calcium release in rat cardiac myocytes.
Cardiac mitochondria can take up Ca(2+), competing with Ca(2+) transporters like the sarcoplasmic reticulum (SR) Ca(2+)-ATPase. Rapid mitochondrial [Ca(2+)] transients have been reported to be synchronized with normal cytosolic [Ca(2+)](i) transients. However, most intra-mitochondrial free [Ca(2+)] ([Ca(2+)](mito)) measurements have been uncalibrated, and potentially contaminated by non-mitochondrial signals. Here we measured calibrated [Ca(2+)](mito) in single rat myocytes using the ratiometric Ca(2+) indicator fura-2 AM and plasmalemmal permeabilization by saponin (to eliminate cytosolic fura-2). The steady-state [Ca(2+)](mito) dependence on [Ca(2+)](i) (with 5 mM EGTA) was sigmoid with [Ca(2+)](mito)<[Ca(2+)](i) for [Ca(2+)](i) below 475 nM. With low [EGTA] (50 microM) and 150 nM [Ca(2+)](i) (+/-15 mM Na(+)) cyclical spontaneous SR Ca(2+) release occurred (5-15/min). Changes in [Ca(2+)](mito) during individual [Ca(2+)](i) transients were small ( approximately 2-10 nM/beat), but integrated gradually to steady-state. Inhibition SR Ca(2+) handling by thapsigargin, 2 mM tetracaine or 10 mM caffeine all stopped the progressive rise in [Ca(2+)](mito) and spontaneous Ca(2+) transients (confirming that SR Ca(2+) releases caused the [Ca(2+)](mito) rise). Confocal imaging of local [Ca(2+)](mito) (using rhod-2) showed that [Ca(2+)](mito) rose rapidly with a delay after SR Ca(2+) release (with amplitude up to 10 nM), but declined much more slowly than [Ca(2+)](i) (time constant 2.8+/-0.7 s vs. 0.19+/-0.06 s). Total Ca(2+) uptake for larger [Ca(2+)](mito) transients was approximately 0.5 micromol/L cytosol (assuming 100:1 mitochondrial Ca(2+) buffering), consistent with prior indirect estimates from [Ca(2+)](i) measurements, and corresponds to approximately 1% of the SR Ca(2+) uptake during a normal Ca(2+) transient. Thus small phasic [Ca(2+)](mito) transients and gradually integrating [Ca(2+)](mito) signals occur during repeating [Ca(2+)](i) transients.
Andrienko
Tatyana N
TN
Department of Pharmacology, University of California Davis, Davis, CA 95616-8636, USA.
Picht
Eckard
E
Bers
Donald M
DM
D000779:Anesthetics, Local / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008856:Microscopy, Fluorescence; D008929:Mitochondria, Heart / Q000378:metabolism*; D032383:Myocytes, Cardiac / Q000378:metabolism*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013748:Tetracaine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D000779:Anesthetics, Local; D004791:Enzyme Inhibitors; D010726:Phosphodiesterase Inhibitors; D013748:Tetracaine; D002110:Caffeine; D019284:Thapsigargin; D002118:Calcium