Correction of enhanced Na⁺-H⁺ exchange of rat small intestinal brush-border membranes in streptozotocin-induced diabetes by insulin or 1,25-dihydroxycholecalciferol

Diabetes was induced in rats by administration of a single i.p. injection of streptozotocin (50 mg/kg body wt). After 7 d, diabetic rats were further treated with insulin or 1,25-dihydroxycholecalciferol (1,25(OH)₂D₃] for an additional 5-7 d. Control, diabetic, diabetic + insulin, and diabetic + 1,25(OH)₂D₃ rats were then killed, their proximal small intestines were removed, and villus-tip epithelial cells were isolated and used to prepare brush-border membrane vesicles. Preparations from each of these groups were then analyzed and compared with respect to their amiloride-sensitive, electroneutral Na⁺-H⁺ exchange activity, using ²²Na uptake as well as acridine orange techniques. The results of these experiments demonstrated that (a) H⁺ gradient-dependent ²²Na uptake as well as Na⁺ gradient-dependent transmembrane H⁺ fluxes were significantly increased in diabetic vesicles compared to their control counterparts, (b) kinetic studies demonstrated that this enhanced ²²Na uptake in diabetes was a result of increased maximal velocity (V_max) of this exchanger with no change in apparent affinity (K_m) for Na⁺, (c) serum levels of 1,25(OH)₂D₃ were significantly lower in diabetic animals compared with their control counterparts; and (d) insulin or 1,25(OH)₂D₃ treatment restored the V_max alterations to control values, without any significant changes in K_m, concomitant with significantly increasing the serum levels of 1,25(OH)₂D₃ in diabetic animals. These results indicate that Na⁺-H⁺ activity is significantly increased in proximal small intestinal luminal membranes of streptozotocin-induced diabetic rats. Moreover, alterations in the serum levels of 1,25(OH)₂D₃ may, at least in part, explain this enhanced antiporter activity and its correction by insulin.