Chronic hypercapnia enhances Vmax of Na-H antiporter of renal brush-border membranes

Abstract

Chronic hypercapnia is associated with increased proximal HCO3 reabsorption that is thought to be mediated by a Na-H antiporter. We hypothesized that chronic hypercapnia would be associated either with increased Vmax or with decreased Km of the Na-H antiporter. To test this hypothesis we made rabbits hypercapnic for 48 h by exposure to 10% CO2. In both control and hypercapnic animals, cortical luminal membranes were enriched over the homogenate 16-fold in alkaline phosphatase and 10-fold in maltase activity. The kinetic activity of the Na-H antiporter was measured by the dissipation of the quenching of acridine orange by addition of different Na concentrations. Chronic hypercapnic rabbits had significantly higher Vmax of the Na-H antiporter of luminal membranes than controls (593 +/- 81 vs. 252 +/- 40 arbitrary fluorescence units X min-1 X 300 micrograms protein-1, P less than 0.01). The Km, however, was not different between control and hypercapnic rabbits. 22Na uptake in presence of an outwardly directed pH gradient was significantly higher in vesicles from hypercapnic rabbits than controls. Amiloride inhibited the Na-H antiporter (as assessed by acridine orange quenching or 22Na uptake) to the same degree in membranes from both control and hypercapnic rabbits, suggesting that the increase in Vmax is mediated by the electroneutral component of the Na-H antiporter. In addition, under voltage clamp conditions by K and valinomycin the Vmax was still increased in membranes from hypercapnic animals, again suggesting that the increase in Vmax is mediated by the electroneutral component of the Na-H antiporter. The uptake of D-[3H]glucose by luminal membranes was not different between control and hypercapnic rabbits, indicating a specific enhancement of the Na-H antiporter. Acute hypercapnia (4 h) failed to increase the Vmax of the Na-H antiporter despite comparable increase in PCO2. Thus chronic hypercapnia, but not acute hypercapnia, induces a selective and specific increase in the Vmax of Na-H antiporter, and this may mediate the adaptation to chronic hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)