Cyclic AMP stimulation of calcium efflux from kidney, liver, and heart mitochondria

Summary

The effect of cyclic AMP on subcellular calcium turnover was studied in isolated kidney, liver and heart mitochondria. The calcium concentration of the incubating medium was determined by fluorometric methods after its separation by millipore filtration. Liver and kidney mitochondria take up calcium in exchange for H⁺ and lower the medium calcium to 1 to 40×10⁻⁶ m in less than 2 min. Cyclic AMP produces an instantaneous release of calcium from mitochondria and a rise in the steady-state calcium concentration of the medium. A new medium calcium level of 0.7 to 3×10⁻⁴ m is achieved in less than 3 sec and is proportional to cyclic AMP concentrations between 10⁻⁷ and 3×10⁻⁶ m. Cyclic AMP is inactive above 5×10⁻⁶ m and below 10⁻⁷ m. Cyclic IMP, 5′ AMP, dibutyryl cAMP are inactive at any concentration. Cyclic GMP is active at 10⁻⁵ m and competitively inhibits cyclic AMP action. The same staedy-state calcium level is reached from higher or, lower calcium concentrations, i.e. whether cyclic AMP is added before or after the addition of calcium to the mitochondrial suspension. At low calcium or phosphate concentrations, the calcium released by cyclic AMP is immediately reaccumulated by the mitochondria is less than 2 min with a further release of H⁺. This “pulse” can be repeated by sequential additions of cyclic AMP. The transient or sustained response to cyclic AMP depends on the medium calcium x phosphate product and presumably on the presence or absence of calcium phosphate precipitate inside the mitochondria. These results support the hypothesis that cyclic AMP regulates cytoplasmic calcium by controlling the mitochondrial calcium efflux rate. This mechanism may be involved in the regulation of calcium transport and in some hormonal effects mediated by cyclic AMP.