Ca2+ clearance mechanisms in isolated rat adrenal chromaffin cells

1. Intracellular Ca2+ clearance mechanisms were studied in rat adrenal chromaffin cells, by measuring slow tail currents through small-conductance Ca2+-activated K+ channels and using indo-1 photometry following depolarization-induced Ca2+ loading. 2. Following several-hundred millisecond depolarizations, [Ca2+](i) decayed in three phases. An initial fast decay was followed by a long-lasting, low plateau, then [Ca2+](i) returned to the resting level slowly. 3. Replacement of external Na+ moderately slowed [Ca2+](i) decay, indicating a contribution of plasma membrane Na+-Ca2+ exchange. 4. Raising external pH or application of extracellular Eosin or La3+ prolonged slow tail currents, indicating a contribution of plasma membrane Na+-Ca2+ exchange. 5. Ca2+-induced Ca2+ release from caffeine-sensitive stores occurred during depolarization. 6. Inhibition of endoplasmic reticulum Ca2+-ATPase had little effect on Ca2+ clearance. 7. Slow tail currents and [Ca2+](i) decay following 0.2-2 s depolarizations were much prolonged by mitochondrial inhibition with carbonyl cyanide m-chlorophenylhydrazone (CCCP) or Ruthenium Red, which abolished the initial rapid decay and plateau of [Ca2+](i). 8. In conclusion, mitochondrial Ca2+ uptake plays a major role in Ca2+ clearance by rapidly and reversibly sequestering Ca2+ during depolarization-evoked Ca2+ loads.