Inhibin production in vitro by granulosa cells from Booroola ewes which were either homozygous or non-carriers of a fecundity gene influencing their ovulation rate

Summary. The production of inhibin by granulosa cells was studied in vitro using cells from follicles of various sizes and health. Follicles were recovered on Days 10–13 of the oestrous cycle, from Booroola × Romney ewes which were homozygous (FF) carriers or non-carriers (++) of the fecundity (F) gene. Inhibin was measured using a bioassay based on the suppression of follicle-stimulating hormone (FSH) output by cultured pituitary cells from ovariectomized Romney ewes and, in some instances, for comparative purposes, by radioimmunoassay also. Geometric mean inhibin production by granulosa cells from nonatretic follicles increased with increasing follicle diameter, during the first 24 h of culture, for both genotypes. The geometric mean production of inhibin by cells from nonatretic 3–4·5 mm diameter FF follicles (the largest follicles found in FF ewes), was significantly higher (P < 0·05) than that by cells from non-atretic 3–4·5 mm diameter ++ follicles, but similar to that of cells from non-atretic ⩾5 mm diameter ++ follicles. The production of oestradiol-17β by cells cultured in the presence of testosterone (1 μg/ml followed a pattern similar to cellular inhibin production. There was a positive linear correlation between inhibin and oestradiol-17β production during the first 24 h of culture, for both genotypes. In addition to acting as a substrate for oestradiol-17β synthesis, testosterone generally had a slight, stimulatory effect on inhibin production. Irrespective of follicle size, or genotype, no detectable amounts of inhibin were produced by granulosa cells from atretic follicles during the first 24 h of culture, or by cells from nonatretic or atretic follicles during the second 24 h of culture. These studies show that the highest mean amounts of inhibin produced by granulosa cells in vitro are similar for both genotypes. Moreover, they are achieved with cells from the largest nonatretic follicles in both ++ (i.e. ⩾5 mm diameter) and FF (i.e. 3–4·5 mm diameter) ewes.