Decreased intracellular potassium levels underlie increased progesterone synthesis during ovarian follicular atresia

More than 99% of ovarian follicles are lost by a degenerative process known as atresia, a phenomenon characterized by apoptosis of granulosa cells. Un iquely, dying granulosa cells also greatly increase their progesterone bios ynthesis while reducing estrogen production. Recent studies have documented a dramatic decrease in intracellular K+ concentration during apoptosis tha t plays an important role in regulating apoptotic enzymes. However, it is u nclear whether this ionic change affects related processes such as the chan ge in steroidogenesis in dying granulosa cells, To explore this question, g ranulosa cells were cultured in hypotonic medium, which initially swells th e cells, The cells respond by extruding K+, which we have documented by flu orescence spectrophotometry. The K+ efflux osmotically draws water out the cell, returning it to a near normal volume (as measured by flow cytometry). The result is a cell of normal size with a decreased intracellular K+ conc entration. FSH stimulation of these cells caused an increase in progesteron e biosynthesis. This response was enhanced at higher doses of FSH, although basal progesterone production was not affected, suggesting that K+ levels may affect the gonadotropin-signaling pathway. No increase in steroidogenic acute regulatory or cholesterol side-chain cleavage cytochrome P450 mRNA w as detected, although cAMP production was enhanced. These results suggest t hat the loss of intracellular K+ by apoptotic granulosa cells greatly facil itates FSH-stimulated progesterone production.