OSMO-DEPENDENT CL- CURRENTS ACTIVATED BY CYCLIC-AMP IN FOLLICLE-ENCLOSED XENOPUS-OOCYTES

The role of adenosine 3',5'-cyclic monophosphate (cAMP) in generating the osmo-dependent slow inward membrane currents (S-in) elicited by activation of follicle stimulating hormone (FSH) or acetylcholine (ACh) receptors was studied in voltage-clamped, follicle-enclosed oocytes of Xenopus laevis (follicles). Forskolin (FSK) also generated S-in currents, and in low concentrations it potentiated the S-in currents elicited by FSH but not those elicited by ACh. Moreover, intra-oocyte injections of cAMP elicited similar slow inward currents (cAMP-S-in) that: (i) were carried mainly by chloride ions; (ii) were abolished by defolliculating the oocytes; and (iii) were dependent on the osmolarity of the external medium. Compared with the Ca2+-dependent chloride channels that are located in the oocyte membrane; the cAMP-activated S-in channels were less permeable to I- and Br-, and their current-voltage relation did not rectify strongly at negative potentials. Generation of cAMP-S-in desensitized the FSH-S-in currents, but did not have effects on both the S-in and the fast chloride current (F-in) specifically elicited by ACh. Furthermore, follicular phospholipase C activation through stimulation of angiotensin II (AII) receptors failed to generate the current responses elicited by ACh. We conclude that cAMP acts as a potent second messenger in generating the osmo-dependent Cl- currents elicited by FSH but not those elicited by ACh. The mechanisms underlying the ACh responses remain unknown. The osmo-dependent chloride channels activated by cAMP may play a role in the control of volume of the follicular cells-oocyte complex.