The Calcium‐Induced and Sodium‐Induced Effluxes of Calcium from Heart Mitochondria

Abstract

Addition of Na + , Li + , Ca 2+ or Sr 2+ induces efflux of the intramitochondrial 45 Ca 2+ from rat heart mitochondria. These reactions are inhibited by La 3+ but not by ruthenium red. The ability of La 3+ to inhibit has been exploited to develop an inhibitor‐stop technique to study the effluxes of 45 Ca 2+ induced by Ca 2+ and other cations. The Ca 2+ ‐induced efflux of 45 Ca 2+ shows Michaelis‐Menten kinetics ( K m , 13 μM) and is attributable to a Ca 2+ ‐Ca 2+ exchange across the inner mitochondrial membrane with a stoichiometry of 1:1. The Ca 2+ ‐Ca 2+ exchange and the Na + ‐induced efflux of Ca 2+ show similar sensitivities to inhibition by La 3+ . Moreover, the Ca 2+ ‐Ca 2+ exchange is inhibited by Na + . The Na + ‐dependent efflux of Ca 2+ is stimulated by mitochondrial energisation without changing the affinity of the system for Na + or Ca 2+ . In contrast, the Ca 2+ ‐Ca 2+ exchange is not influenced by the mitochondrial energy state. The data agree with the existence of an exchange‐diffusion carrier in the inner membrane able to catalyse an exchange between intramitochondrial Ca 2+ and extramitochondrial Ca 2+ , Sr 2+ , Na + and Li + . The energy‐dependence of the exchanges is discussed.