Mitochondrial free calcium regulation during sarcoplasmic reticulum calcium release in rat cardiac myocytes

Cardiac mitochondria can take up Ca2+, competing with Ca2+ transporters like the sarcoplasmic reticulum (SR) Ca2+-ATPase. Rapid mitochondrial [Ca2+] transients have been reported to be synchronized with normal cytosolic [Ca2+]i transients. However, most intra-mitochondrial free [Ca2+] ([Ca2+]mito) measurements have been uncalibrated, and potentially contaminated by non-mitochondrial signals. Here we measured calibrated [Ca2+]mito in single rat myocytes using the ratiometric Ca2+ indicator fura-2 AM and plasmalemmal permeabilization by saponin (to eliminate cytosolic fura-2). The steady-state [Ca2+]mito dependence on [Ca2+]i (with 5 mM EGTA) was sigmoid with [Ca2+]mito < [Ca2+]i for [Ca2+]i below 475 nM. With low [EGTA] (50 μM) and 150 nM [Ca2+]i (± 15 mM Na+) cyclical spontaneous SR Ca2+ release occurred (5–15/min). Changes in [Ca2+]mito during individual [Ca2+]i transients were small (∼ 2–10 nM/beat), but integrated gradually to steady-state. Inhibition SR Ca2+ handling by thapsigargin, 2 mM tetracaine or 10 mM caffeine all stopped the progressive rise in [Ca2+]mito and spontaneous Ca2+ transients (confirming that SR Ca2+ releases caused the [Ca2+]mito rise). Confocal imaging of local [Ca2+]mito (using rhod-2) showed that [Ca2+]mito rose rapidly with a delay after SR Ca2+ release (with amplitude up to 10 nM), but declined much more slowly than [Ca2+]i (time constant 2.8 ± 0.7 s vs. 0.19 ± 0.06 s). Total Ca2+ uptake for larger [Ca2+]mito transients was ∼ 0.5 μmol/L cytosol (assuming 100:1 mitochondrial Ca2+ buffering), consistent with prior indirect estimates from [Ca2+]i measurements, and corresponds to ∼ 1% of the SR Ca2+ uptake during a normal Ca2+ transient. Thus small phasic [Ca2+]mito transients and gradually integrating [Ca2+]mito signals occur during repeating [Ca2+]i transients.