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Adenosine Triphosphate Induces Inhibition of Na⁺ Absorption in Mouse Endometrial Epithelium: A Ca²⁺-Dependent Mechanism¹

^a Epithelial Cell Biology Research Centre, Department of Physiology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, Hong Kong

ABSTRACT

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 µM). Apical addition of ATP (10 µM) 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 µM), 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 P₂ receptor antagonist, reactive blue (100 µM), indicating the involvement of a P₂ receptor in mediating the ATP response. ATP and uridine 5'-diphosphate (UDP; 100 µM), 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 µM), whereas the Ca²⁺ chelators, EGTA 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 µM), 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.

FOOTNOTES

First decision: 15 May 2000.

¹ Supported by the Strategic Research Programme and Direct Grant of the Chinese University of Hong Kong.

² Correspondence. FAX: 825 2603 5022; hsiaocchan{at}cuhk.edu.hk

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