On the relative roles of Ca2+ and Mg2+ in regulating the endogenous K+/H+ exchanger of rat liver mitochondria.

1. A23187 and ionomycin cause the release of Ca2+, Mg2+, and K+ from the mitochondrial matrix. The electroneutral K+ efflux does not reflect direct transport by these ionophores but, rather, results from release of the endogenous K+/H+ exchanger from inhibition by divalent cations. 2. A23187 and ionomycin differ in their affinities for CA2+ and Mg2+, having relative affinity ratios (Ca2+/Mg2+) in respiring mitochondria of 3 and 35, respectively. 3. This difference in Ca2+/Mg2+ affinities was exploited in dose-response studies designed to determine which cation exerts primary control over K+/H+ exchange activity. The results of such studies demonstrate that removal of Ca2+ has no effect on K+ efflux, and that K+ efflux follows Mg2+ efflux whether induced by ionomycin or A23187. 4. K+ steady states, in which K+ uniport equals K+/H+ antiport, were induced by adding low levels of valinomycin to respiring mitochondria. The addition of Ca2+ perturbed the steady state in the direction of increased K+/H+ exchange. 5. Addition of Ca2+ to respiring mitochondria resulted in electroneutral K+ efflux which was strongly affected by the anion composition of the medium. We propose that Ca2+ turns on the K+/H+ exchanger indirectly, through a decrease in matrix [Mg2+] secondary to swelling and uptake of chelating anions. This phenomenon may be of physiological importance as a temporary protection against excessive swelling due to Ca2+ uptake. 6. These studies support the contention that Mg2+, not Ca2+, is the inhibitory cation responsible for the physiological regulation of mitochondrial K+/H+ exchange.