SEASONAL AND ACUTE HEAT-STRESS EFFECTS ON STEROID-PRODUCTION BY DOMINANT FOLLICLES IN COWS

The present study concerned the seasonal and acute effects of heat str ess on steroid concentrations in follicular fluid and on steroid produ ction by granulosa and theca interna cells, in bovine dominant follicl es. Three groups of cows were studied: summer (n = 5), autumn (n = 5) and winter (n = 9) cows. During the winter season, another group of co ws was acutely heat-stressed from days 3 through 5 of the estrous cycl e (n = 5). On day 7 of the estrous cycle, follicular fluid from first- wave dominant follicles was aspirated, and dispersed granulosa and the ca cells from each seasonal group were incubated for 18 h at normother mic (37.5 degrees C) or high (40.5 degrees C) temperatures. Cells were incubated in media only or in media containing testosterone (300 ng m l(-1), for granulosa cells) or forskolin (4 mu g ml(-1), for theca cel ls). In follicular fluid the 17 beta-estradiol concentration was high (P < 0.05) in winter and low in autumn and summer; the androstenedione concentration was high in summer (P < 0.05), low in autumn, and inter mediate in winter. During the winter season, acute in vivo heat stress increased follicular fluid androstenedione and decreased estradiol to levels comparable with those prevailing in summer, Basal and forskoli n-stimulated androstenedione production by theca cells was higher (P < 0.05) in the winter group than in the summer and autumn groups, and a lso higher than in the cows that were heat-stressed during winter, whi ch suggests that theca cell function is susceptible to chronic (summer ), short-term (winter) and delayed (autumn) heat stresses. In vitro in cubation at high temperature (40.5 degrees C) reduced the high, forsko lin-stimulated androstenedione production in winter (P < 0.05). Estrad iol production by granulosa cells was high in winter and autumn, and l ow in summer (P < 0.05). Acute heat stress in winter did not alter est radiol production relative to winter controls, whereas a high incubati on temperature (40.5 degrees C) reduced (P < 0.05) estradiol productio n only in the autumn, when the highest production rate was recorded. T he results indicate a differential effect of heat stress on the functi ons of granulosa and theca cells. Both concurrent and delayed effects of heat stress on the steroidogenic capacity of ovarian follicles in c attle are presented. (C) 1997 Elsevier Science B.V.