Seasonal Variation in Porcine Granulosa Cell Responsiveness to Stimulation of Progesterone Secretion in Vitro1

Ten years of data were analyzed for an effect of season on basal and stimulated progesterone secretion by porcine granulosa cells in vitro. Basal progesterone secretion by granulosa cells from large preovulatory follicles exhibited a marked depression during the summer months, as compared to the rest of the year. The ability of granulosa cells from small (1-2 mm) or medium (3-5 mm) follicles to respond to stimulation of progesterone secretion by either follicle-stimulating hormone (FSH) or follicular fluid was also markedly depressed during the summer months. These observations on decreased steroidogenic capacity of porcine granulosa cells in vitro during the summer parallel observations of decreased reproductive function of pigs in vivo. They suggest that the decreased reproductive capacity of pigs in summer may be due, in part, to a decreased capacity for follicular development within the ovary as a result of an altered neuroendocrine signal rather than due only to an altered neuroendocrine signal.