PLASMA-CONCENTRATIONS OF IMMUNOREACTIVE INHIBIN AND GONADOTROPINS FOLLOWING REMOVAL OF OVARIAN FOLLICLES IN THE DOMESTIC HEN

In the domestic hen, the preovulatory follicles are arranged in a hier archy. The largest (F1) follicle ovulates first, the second largest (F 2) follicle ovulates approximately 26 h later, and so on. The nature o f this regulation is not known. Inhibin has been implicated in the fee dback regulation of FSH in mammals, and bioactive as well as immunorea ctive inhibin has been demonstrated in chickens. The present experimen ts were conducted with two aims: 1) to determine the plasma level of i mmunoreactive inhibin in response to removal of various numbers of pre ovulatory follicles and 2) to assess the gonadotropin response to such follicle removal. In experiment 1, the F1 (n = 4), F1 + F2 (n = 5), F 1 + F2 + F3 (n = 5), and F1 + F2 + F3 + F4 (n = 4) follicles were remo ved surgically. Sham-operated animals served as controls. Blood sample s were taken at time 0 (prior to surgery) and at 6, 24, and 48 h after surgery. In experiment 2, animals were sham-operated (n = 5) or the F 1-F4 (n = 4) or F1-F4 plus small yellow follicles (SYF; n = 5) were re moved. Blood samples were removed at time 0 and at 3, 6, and 24 h and at daily intervals until Day 10 after surgery. In both experiments, pl asma immunoreactive inhibin was significantly lowered (p < 0.001) and plasma FSH was acutely increased (p < 0.05) by follicle removal. Plasm a LH was decreased at some time points after follicle removal. Time to resumption of lay after follicle removal was decreased relative to th e expected interval (6.8 days/9 follicles) in the group with the F1-F4 + SYF removed in experiment 2, suggesting that follicular growth was stimulated. These two experiments demonstrated suppression of immunore active inhibin with increasing numbers of follicles removed. The endoc rine relevance of inhibin in the hen is suggested by the specific effe ct of follicle removal and inhibin decline on plasma FSH level.