Adenosine triphosphate induces inhibition of Na(+) absorption in mouse endometrial epithelium: a Ca(2+)-dependent mechanism

The present study investigated the inhibitory effect of extracellular ATP on Na(+) absorption and the possible underlying mechanism in cultured mouse endometrial epithelium using the short-circuit current (I(SC)) technique. The cultured epithelia exhibited a Na(+)-dependent basal current that could be predominately blocked by the epithelial Na(+) channel (ENaC) blocker, amiloride (10 microM). Apical addition of ATP (10 microM) induced a reduction in basal I(SC). However, in the presence of amiloride or when apical Na(+) was removed, the ATP-induced reduction was abolished and an increase in the I(SC) was observed with kinetic characteristics similar to those reported previously for the ATP-induced Cl(-) secretion, indicating that ATP could induce both Cl(-) secretion and inhibition of Na(+) absorption. Further reduction in I(SC) after ATP challenge could be obtained with forskolin (10 microM), which indicates that different inhibitory mechanisms are involved. The ATP-induced inhibition of Na(+) absorption, but not that induced by forskolin, could be abolished by the P(2) receptor antagonist, reactive blue (100 microM), indicating the involvement of a P(2) receptor in mediating the ATP response. ATP and uridine 5'-diphosphate (UDP; 100 microM), a relatively selective agonist for the pyrimidinoceptor, induced separate I(SC) reduction, and distinct I(SC) increases in the presence of amiloride, regardless of the order of drug administration, indicating the involvement of two receptor populations. The ATP-induced inhibition of Na(+) absorption was mimicked by the Ca(2+) ionophore, ionomycin (1 microM), whereas the Ca(2+) chelators, EGTA and BAPTA-AM, abolished the ATP-induced, but not the forskolin-induced, inhibition of Na(+) absorption, suggesting the involvement of a Ca(2+)-dependent pathway. In the presence of the Cl(-) channel blocker, DIDS (100 microM), both inhibitory and stimulatory responses to ATP were abolished, suggesting the involvement of a Ca(2+)-activated Cl(-) channels (CaCCs) in mediating both ATP responses. The ATP-induced as well as the forskolin-induced reduction in I(SC) was not observed when Cl(-) was removed from the bathing solution, indicating that Cl(-) permeation is important for the inhibition of Na(+) absorption. The results suggest the presence of a Ca(2+)-dependent ENaC-inhibiting mechanism involving CaCC in mouse endometrial epithelial cells. Thus, extracellular nucleotides may play an important role in the fine-tuning of the uterine fluid microenvironment by regulating both Cl(-) secretion and Na(+) absorption across the endometrium.