Na-H exchange was studied using electron probe analysis and microfluorescent pH analysis of individual cells, in 3-day primary cultures of rat proximal tubule cells (RPTC) obtained from 40- to 50-day-old Sprague-Dawley rats. After Na-K pump inhibition, the initial rate of net Na influx was inhibited 87% by 1 mM amiloride. K influx, an estimate of Na-K pump activity, was increased approximately three times in cells containing high Na (0.114 mM K X mM P-1 X min-1) compared with control cells containing low Na (0.038 mM K X mM P-1 X min-1). Single cell measurements of RPTC loaded with the cytoplasmic pH indicator 5- (and -6) carboxy-4',5'-dimethylfluorescein indicated that there was reversible intracellular acidification in the absence of external Na or in the presence of amiloride. When intracellular acidification was induced by the addition and subsequent removal of NH4Cl, recovery of intracellular pH was inhibited in the absence of external Na or in the presence of amiloride. Using a similar protocol, it was found that after intracellular acidification, the rate of Na influx increased at least 5.9 times, and intracellular Na content was increased 3.15 +/- 0.64 times at 60 s. There was an initial 50% inhibition of Na-K pump activity within the first 60 s compared with control (nonacidified) RPTC, secondarily followed by an increase in Na-K pump activity. Amiloride (0.5 mM) inhibited the acidification-induced increase in Na influx, and persistent acidification led to a persistent inhibition of Na-K pump activity compared with control. In summary, these results demonstrate that Na-H exchange mediates the majority of net Na influx into RPTC under our basal conditions and is necessary for maintenance of intracellular pH homeostasis. In RPTC, Na-H exchange is further activated by intracellular acidification, leading to a net increase in intracellular Na content, which secondarily stimulates Na-K pump activity. The initial inhibition of Na-K pump activity may be due to a direct effect of intracellular acidification.