The regulation of epithelial Na+/H+ exchangers (NHEs) by hyposmolality is poorly understood. In the renal medullary thick ascending limb (MTAL), transepithelial bicarbonate (HCO3 -) absorption is mediated by apical membrane Na+/H+ exchange, attributable to NHE3. In the present study we examined the effects of hyposmolality on apical Na+/H+ exchange activity and HCO3 - absorption in the MTAL of the rat. In MTAL perfused in vitro with 25 mM HCO3 - solutions, decreasing osmolality in the lumen and bath by removal of either mannitol or sodium chloride significantly increased HCO3 - absorption. The responses to lumen addition of the inhibitors ethylisopropyl amiloride, amiloride, or HOE 694 are consistent with hyposmotic stimulation of apical NHE3 activity and provide no evidence for a role for apical NHE2 in HCO3 - absorption. Hyposmolality increased apical Na+/H+ exchange activity over the phi range 6.5-7.5 due to an increase in V(max). Pretreatment with either tyrosine kinase inhibitors or with the tyrosine phosphatase inhibitor molybdate completely blocked stimulation of HCO3 - absorption by hyposmolality. These results demonstrate that hyposmolality increases HCO3 - absorption in the MTAL through a novel stimulation of apical membrane Na+/H+ exchange and provide the first evidence that NHE3 is regulated by hyposmotic stress. Stimulation of apical Na+/H+ exchange activity in renal cells by a decrease in osmolality may contribute to such pathophysiological processes as urine acidification by diuretics, diuretic resistance, and renal sodium retention in edematous states.
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