Circulating concentrations of inhibin-related proteins during the ovulatory cycle of the domestic fowl (Gallus domesticus) and after induced cessation of egg laying

Circulating inhibin A, inhibin B, activin A, total immunoreactive inhibin a -subunit (ir-a inhibin), LH, FSH and progesterone concentrations were measu red throughout the normal ovulatory cycle and after cessation of egg laying induced by feed restriction to investigate the potential involvement of in hibins and activins in the ovulatory cycle of the domestic hen. Plasma inhi bin A varied significantly (P < 0.05) during the ovulatory cycle; the conce ntration was highest at the preovulatory LH surge and reached a nadir 10 h later, at about the time the F, follicle makes the transition to become the new F, follicle. Plasma FSH concentrations did not change significantly th roughout the cycle and showed no correlation with inhibin A. Total ir-a inh ibin concentrations were much higher than those of inhibin A at all stages of the ovulatory cycle and showed no correlation with inhibin A or FSH. Pla sma concentrations of inhibin B and of activin A were below the detection L imit of the assays in all plasma samples analysed. in the feed restriction study, plasma inhibin A and total ir-alpha inhibin showed little change unt il the last day of oviposition (day 0) after which they fell significantly (P < 0.05) and remained low to the end of the experiment (approximately 70- 78% decrease relative to day -4). Conversely, plasma FSH increased after ce ssation of laying and was significantly higher (P < 0.05) from day 3 to the end of the study (approximately 50% increase on day 6 relative to day -4). Plasma FSH values were negatively correlated with inhibin A (r = -0.39; P < 0.005) and total ir-a inhibin (r = -0.36; P < 0.005). Plasma LH and proge sterone also decreased (P < 0.05) during feed restriction. The decrease in LH preceded the terminal oviposition and the associated fall in inhibin A b y 2 days; there was a positive correlation between LH and inhibin A (r = 0. 35; P < 0.005). Taken together these findings support (i) a role for LH in promoting inhibin A secretion by preovulatory follicles and (ii) an endocri ne role for inhibin A secreted by preovulatory follicles in the maintenance of tonic FSH secretion in laying hens.