Effect of in vitro metabolic acidosis on luminal Na+/H+ exchange and basolateral Na+:HCO3- cotransport in rabbit kidney proximal tubules.

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Abstract
The aim of this study was to evaluate the role of the kidney in mediating the signals involved in adaptive changes in luminal Na+/H+ exchange and basolateral Na+:HCO3- cotransport systems in metabolic acidosis. Proximal tubular suspensions were prepared from rabbit kidney cortex and incubated in acidic (A) or control (C) media (pH 6.9 vs 7.4, 5% CO2) for 2 h. Brush border membrane (BBM) and basolateral membrane (BLM) vesicles were isolated from the tubular suspensions and studied for the activity of Na+/H+ exchange and Na+:HCO3- cotransport. Influx of 1 mM 22Na at 10 s (pH6 7.5, pH(i) 6.0) into BBM vesicles was 68% higher in group A compared to group C. The increment in Na+ influx in the group A was amiloride sensitive, suggesting that Na+/H+ exchange was responsible for the observed differences. Kinetic analysis of Na+ influx showed a Km of 8.1 mM in C vs 9.2 in A and Vmax of 31 nmol/mg protein per min in group C vs 57 in A. Influx of 1 mM 22Na at 10 s (pH0 7.5, pH(i) 6.0, 20% CO2, 80% N2) into BLM vesicles was 83% higher in the group A compared to C. The HCO3-dependent increment in 22Na uptake in group A was 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid sensitive, suggesting that Na+:HCO3- cotransport accounted for the observed differences. Kinetic analysis of Na+ influx showed a Km of 11.4 mM in C vs 13.6 in A and Vmax of 35 nmol/mg protein per min in C vs 64 in A. The presence of cyclohexamide during incubation in A medium had no effect on the increments in 22Na uptake in group A. We conclude that the adaptive increase in luminal Na+/H+ exchange and basolateral Na+:HCO3- cotransport systems in metabolic acidosis is acute and mediated via direct signal(s) at the level of renal tubule.