cause an increase in the HCO 3 Ϫ concentration of oviductal liquid at midcycle is believed to markedly enhance fertility, we have studied active secretion of HCO 3 Ϫ across highly differentiated cultures of monkey oviductal epithelium. Cultured cell sheets were mounted in Ussing chambers and bathed in medium containing 25 mM HCO 3 Ϫ . Purinergic agents potently stimulated short-circuit current (Isc) with an initial transient response declining within ϳ2 min to a sustained response. The potency

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... equence of ATP Ϸ UTP Ͼ ADP ϾϾ AMP suggested that the I sc response was mediated mainly by P2Y2 receptors. Acetazolamide, an inhibitor of carbonic anhydrase, had little or no effect on baseline I sc or the transient response to ATP but abolished the sustained response to ATP. Similar results were obtained on sheets of native epithelium. In pH-stat experiments, the abluminal medium of cell cultures was bathed in HCO 3 Ϫ -CO2 medium, and the pH of the unbuffered luminal medium was maintained at ϳ7.4 by addition of strong acid or base. ATP stimulated base secretion, and this was inhibited by acetazolamide. Furthermore, these changes in secretion of base were in good quantitative agreement with the I sc responses. When phenol red (an estrogen) was removed from the culture medium, ATP-dependent HCO 3 Ϫ secretion was markedly reduced but could be restored by treatment with estradiol. Estrogens also markedly increased ciliation of the cultures. These results suggest that the midcycle increase in the HCO 3 Ϫ concentration of oviductal liquid may be mediated by the effects of estradiol on purinergic pathways or on ATP secretion. carbonic anhydrase; short-circuit current; estradiol; adenosine 5Јtriphosphate THE OVIDUCTAL EPITHELIUM regulates the composition of the oviductal liquid, thereby creating a favorable environment for key reproductive events to take place. Among the most important components of oviductal liquid is HCO 3 Ϫ