Ca2+ dependency of Na+ transport by rabbit renal brush border membrane

Intracellular Ca2+ has been suggested to play an important role in the regulation of epithelial Na+ transport. Previous studies showed that preincubation of toad urinary bladder, a tight epithelium, in Ca2+-free medium enhanced Na+ uptake by the subsequently isolated apical membrane vesicles, suggesting the downregulation of Na+ entry across the apical membrane by intracellular Ca2+. In the present study, we have examined the effect of Ca2+-free preincubation on apical membrane Na+ transport in a leaky epithelium, i.e., brush border membrane (BBM) of rabbit renal proximal tubule. In contrast to toad urinary bladder, it was found that BBM vesicles derived from proximal tubules incubated in 1 mM Ca2+ medium exhibited higher Na+ uptake than those derived from proximal tubules incubated in Ca2+-free EGTA medium. Such effect of Ca2+ in the preincubation medium was temperature dependent and could not be replaced by another divalent cation. Ba2+ (1 mM). Ca2+ in the preincubation medium did not affect Na+-dependent BBM glucose uptake, and its effect on BBM Na+ uptake was pH gradient dependent and amiloride (10(-3) M) sensitive, suggesting the involvement of Na+/H+ antiport system. Addition of verapamil (10(-4) M) to 1 mM Ca2+ preincubation medium abolished while ionomycin (10(-6) M) potentiated the effect of Ca2+ in the preincubation medium is likely to be mediated by Ca2+-dependent cellular pathways and not due to a direct effect of extracellular Ca2+ on BBM.