Ro. Arellano et R. Miledi, OSMO-DEPENDENT CL- CURRENTS ACTIVATED BY CYCLIC-AMP IN FOLLICLE-ENCLOSED XENOPUS-OOCYTES, Proceedings - Royal Society. Biological Sciences, 258(1353), 1994, pp. 229-235
The role of adenosine 3',5'-cyclic monophosphate (cAMP) in generating
the osmo-dependent slow inward membrane currents (S-in) elicited by ac
tivation 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 curre
nts, and in low concentrations it potentiated the S-in currents elicit
ed by FSH but not those elicited by ACh. Moreover, intra-oocyte inject
ions of cAMP elicited similar slow inward currents (cAMP-S-in) that: (
i) were carried mainly by chloride ions; (ii) were abolished by defoll
iculating the oocytes; and (iii) were dependent on the osmolarity of t
he external medium. Compared with the Ca2+-dependent chloride channels
that are located in the oocyte membrane; the cAMP-activated S-in chan
nels were less permeable to I- and Br-, and their current-voltage rela
tion did not rectify strongly at negative potentials. Generation of cA
MP-S-in desensitized the FSH-S-in currents, but did not have effects o
n both the S-in and the fast chloride current (F-in) specifically elic
ited by ACh. Furthermore, follicular phospholipase C activation throug
h stimulation of angiotensin II (AII) receptors failed to generate the
current responses elicited by ACh. We conclude that cAMP acts as a po
tent second messenger in generating the osmo-dependent Cl- currents el
icited by FSH but not those elicited by ACh. The mechanisms underlying
the ACh responses remain unknown. The osmo-dependent chloride channel
s activated by cAMP may play a role in the control of volume of the fo
llicular cells-oocyte complex.