Ontogeny of the Na(+)-H+ exchanger in rat ileal basolateral membrane vesicles

This study was designed to examine the activity of the Na(+)-H+ exchanger across the basolateral membranes of the ileal enterocyte and its developmental pattern. The function of the Na(+)-H+ exchanger was studied using a well validated basolateral membrane vesicle technique. Na+ uptake represented transport into the vesicle rather than binding as validated by initial rate studies. Na+ uptake represented an electroneutral process as shown by studies in which negative membrane potential was induced by the ionophore valinomycin. Various outwardly directed pH gradients significantly stimulated Na+ uptake compared with no pH gradient conditions at all age groups. However, the magnitude of stimulation was significantly different between the age groups with more marked stimulation of amiloride-sensitive Na+ uptake occurring in adolescent rats as compared to weanling or suckling rats. The amiloride sensitivity of the pH stimulated Na+ uptake was investigated using [Amiloride] = 10(-2)-10(-5) M at pHi/pHo = 5.2/7.5. At 10(-2) M amiloride concentration, Na+ uptake was inhibited by 80%, 70%, 77%, in the basolateral membranes of adolescent, weanling and suckling rats, respectively. Dixon plot analysis in both adolescent and weanling rats was consistent with two amiloride binding sites, a low affinity system and a high affinity system. In the suckling rat, on the other hand, the data supported a single high affinity binding site. Kinetic studies revealed a Km for amiloride-sensitive Na+ uptake of 12.6 +/- 6.6, 10.2 +/- 1.77, 9.46 and Vmax of 4.83 +/- 1.22, 4.47 +/- 0.36 and 8.08 +/- 1.92 n.mol.mg.protein-1.7 s-1 in suckling, weanling and adolescent rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)