ANALYTICAL AND EXPERIMENTAL STUDIES ON THE RELATIONSHIP BETWEEN NA+, K+, AND WATER-UPTAKE DURING VOLUME INCREASES ASSOCIATED WITH FUNDULUS OOCYTE MATURATION INVITRO

Oocytes of marine and estuarine teleosts often undergo pronounced volume increases during the maturation phase of development that precedes ovulation and fertilization. To examine the physiological correlates of these volume increases, prematuration follicles of the saltmarsh teleost, Fundulus heteroclitus, were cultured in vitro with a maturation-inducing steroid (17-alpha-hydroxy-20-beta-dihydroprogesterone). Mean follicle volume rose significantly (75%) during a 40-h incubation period. Similar to the situation previously found in vivo, uptake of water by the maturing follicle was responsible for this volume increase in vitro, with the water content increasing from 62% to 78% of the total follicle mass. The follicle contents of two probable osmotic effectors - Na+ and K+ - also rose, the increase in K+ being twice that of Na+. The influx of K+ even exceeded water uptake, resulting in a net increase in the concentration of this cation. It thus appears that the influx of these cations, in particular K+, is a major cause of the uptake of osmotically obligated water and subsequent volume increase experienced by maturing F. heteroclitus follicles. In a search for operant mechanisms, it was found that follicle hydration, but not maturation, was strictly dependent on external K+ in a concentration-dependent manner. The mechanism by which K+ accumulates in the follicle was insensitive to ouabain, so that a typical Na+,K+-ATPase mechanism does not appear to be involved. The ability of external K+ to promote follicle hydration was gradually lost during the maturation process as the oocyte dissociated from the surrounding granulosa cells in preparation for ovulation. Removal of all associated somatic cells prior to maturation prevented subsequent steroid-initiated hydration but not maturation. The results suggest that K+ may be translocated from surrounding granulosa cells to the oocyte via gap junctions during maturation.