Effect of pH on glutamate efflux from rat kidney mitochondria

To provide further insight into the relationship between glutamate transport and metabolism in rat kidney mitochondria, the effect of pH on [U-14C]-glutamate efflux from [U-14C]glutamate-loaded mitochondria was tested. Glutamate efflux, measured in rotenone-inhibited energized mitochondria at 28 degrees C, was first order with respect to matrix glutamate. Glutamate efflux was tested at different medium pH values ranging from 6.4 to 7.9. Efflux was fastest at pH 7.9 with a rate constant of 0.988 min-1 and decreased progressively at lower pH values such that the rate constant at pH 6.4 was 0.359 min-1. A highly significant correlation between pH and the rate constant of glutamate efflux was observed. When medium pH was held constant at 7.0, matrix pH (range pH 7.2-8.4) did not affect the rate of glutamate efflux. The reduction in glutamate efflux at low pH provides an explanation for the increased concentration of glutamate observed in the mitochondrial matrix space of isolated rat kidney mitochondria incubated at acid medium pH. The elevated glutamate concentration in turn contributes to the accelerated oxidative deamination of glutamate by glutamate dehydrogenase at acid pH. Moreover, these findings suggest that the reduction in renal cortex glutamate levels observed in acute acidosis in vivo may be explained by a decreased rate of glutamate transport from mitochondria to cytosol coupled with an augmented rate of glutamate oxidation in the matrix.