DELTA-PH INDUCED CALCIUM FLUXES IN RAT-LIVER MITOCHONDRIA

.DELTA.pH-driven Ca2+ fluxes in rotenone-treated rat liver mitochondria were studied. The H+ concentration gradient (acidic inside) required to drive Ca2+ influx is obtained from the K+ concentration gradient through a H+/K+ exchange. The rate of H+/K+ exchange is very low in native mitochondria and is markedly enhanced by the electroneutral ionophore nigericin. the rate of Ca2+ influx depends on the rate of generation of .DELTA.pH; it is negligible in native mitochondria and fast after nigericin. The process of .DELTA.pH-driven Ca2+ influx shows 3 basic features: the rate of Ca2+ influx is markedly increased by the addition of uncouplers; the rate of Ca2+ influx is inhibited by ruthenium red with the same sensitivity as the processes driven by the H+ pump or K+ diffusion; the electroneutral H+/Ca2+ ionophore A23187 restores Ca2+ influx in mitochondria inhibited by ruthenium red; both rate and extent of .DELTA.pH-driven Ca2+ influx are inhibited by the addition of weak bases but not of weak acids. The process of .DELTA.pH-driven Ca2+ influx is accompanied by H+ efflux. A correlation also exists between generation of .DELTA.pH (acidic inside) due to H+/K+ exchange and utilization of .DELTA.pH due to H+/Ca2+ exchange. The process of .DELTA.pH-driven, A23187-catalyzed, H+/Ca2+ exchange is not proceeded by rapid H+/K+ exchange in the absence of nigericin. After termination of the phase of Ca2+ influx a phase of Ca2+ efflux ensues, Ca2+ efflux is not coupled to H+ or K+ reentry and is completely inhibited by ruthenium red. Ca2+ efflux is not observed when Ca2+ transport is catalyzed by A23187 in mitochondria inhibited by ruthenium red. A23187 abolishes Ca2+ efflux also when Ca2+ influx occurs in the absence of ruthenium red. The .delta.pH-driven Ca2+ influx and the subsequent Ca2+ efflux may take place through the native carrier as electrical uniport.