FUNCTIONAL-ROLE OF FOLLICULAR CELLS IN THE GENERATION OF OSMOLARITY-DEPENDENT CL- CURRENTS IN XENOPUS FOLLICLES

1. Osmolarity-dependent (osmo-dependent) ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using the two-microelectrode voltage-clamp technique, combined with intra-oocyte pressure injection of sucrose or polyethylene glycols (PEGs). 2. Intra-oocyte injections of sucrose or PEG (3-25 nmol) generated inward membrane currents (follicles held at -60 mV) associated with an increase in membrane conductance. These currents were carried mainly by chloride ions (I-Cl(osm)), and were strongly attenuated by increasing the tonicity of the external medium, or by external application of La3+ (0.1-1 mM). 3. The ability to generate I-Cl(osm) depended on the molecular weight of the injected PEG. Injections of PEG 200 or 300 generated I-Cl(osm) in 95% of the follicles tested, PEG 600 generated comparable currents in only 20% of the follicles, while similar injections of PEG 1000 did not elicit I-Cl(osm). 4. Octanol (1-1.5 mM), a gap junction channel blocker, reversibly inhibited 50-90% of the I-Cl(osm) generated by injections of sucrose or PEG 300. Moreover, sucrose or PEG injections did not elicit I-Cl(osm) in defolliculated oocytes. 5. It is concluded that an increase in the internal osmolarity of the follicular cells activates a mechanism, probably involving cellular swelling, which leads to the opening of I-Cl(osm) channels most probably located in the follicular cell membrane.