Mitochondrial Ca2+ uptake is important over low [Ca2+]i range in arterial smooth muscle

Mitochondrial Ca(2+) uptake is usually thought to occur only when intracellular Ca(2+) concentration (Ca(2+)](i)) is high. We investigated whether mitochondrial Ca(2+) removal participates in shaping Ca(2+)](i) signals in arterial smooth muscle over a low Ca(2+)](i) range. Ca(2+)](i) was measured using fura 2-loaded, voltage-clamped cells from rat femoral arteries. Both diazoxide and carbonyl cyanide m-chlorophenylhydrazone (CCCP) depolarized the mitochondria. Diazoxide application increased resting Ca(2+)](i), suggesting that Ca(2+) is sequestered in mitochondria. Over a low Ca(2+)](i) range, diazoxide and CCCP slowed Ca(2+) removal rate, determined after a brief depolarization. When Ca(2+)](i) was measured during sustained depolarization to -30 mV, CCCP application increased Ca(2+)](i). When Ca(2+) transients were repeatedly evoked by caffeine applications, CCCP application elevated resting Ca(2+)](i). Caffeine-induced Ca(2+) transients were compared before and after CCCP application using the half decay time, or time required to reduce increase in Ca(2+)](i) by 50% (t((1/2))). CCCP treatment significantly increased t((1/2)). These results suggest that Ca(2+) removal to mitochondria in arterial smooth muscle cells may be important at a low Ca(2+)](i).