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. Uniquely, dying granulosa cells also greatly increase their progesterone biosynthesis while reducing estrogen production. Recent studies have documented a dramatic decrease in intracellular K+ concentration during apoptosis that plays an important role in regulating apoptotic enzymes. However, it is unclear whether this ionic change affects related processes such as the change in steroidogenesis in dying granulosa cells. To explore this question, granulosa cells were cultured in hypotonic medium, which initially swells the cells. The cells respond by extruding K+, which we have documented by fluorescence 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+ concentration. FSH stimulation of these cells caused an increase in progesterone 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 was detected, although cAMP production was enhanced. These results suggest that the loss of intracellular K+ by apoptotic granulosa cells greatly facilitates FSH-stimulated progesterone production.
Gross, SA; Newton, JM; Hughes, FM
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