Adenosine triphosphate induces inhibition of Na+ absorption in mouse endometrial epithelium:: A Ca2+-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 muM). Apical addition of ATP (10 muM) 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,, 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 muM), 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 muM), indicating the involvement of a P-2 receptor in mediating the ATP response. ATP and uridine 5'-diphosphate (UDP; 100 muM), 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 Ca2+ ionophore, ionomycin (1 muM), whereas the Ca2+ chelators, ECTA and BAPTA-AM, abolished the ATP-induced, but not the forskolin-induced, inhibition of Na+ absorption, suggesting the involvement of a Ca2+-dependent pathway. In the presence of the Cl- channel blocker, DIDS (100 muM), both inhibitory and stimulatory responses to ATP were abolished, suggesting the involvement of a Ca2+-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 Ca2+-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.