Effect of acute immunoneutralization of inhibin in ewes during the late luteal phase of the oestrous cycle on ovarian hormone secretion and follicular development during the subsequent follicular phase.

Ewes with ovarian autotransplants received either inhibin antiserum (10 ml i.v.; n = 6) or sheep serum (10 ml i.v.; n = 5) on day 10 of the luteal phase with additional daily injections (1 ml i.v.) from 48 h after the initial injection until the end of blood sampling, 9 days later. Luteal regression was induced by injection of prostaglandin F2 alpha (PGF2 alpha) 4.2 days after the initial plasma injection. Jugular and ovarian venous blood samples were taken every 4 h over the experimental period, and more frequent samples (every 10-15 min for 2-3 h) were taken to examine pulsatile secretory responses from the ovary to GnRH-induced (150 ng i.m. 1 and 4 days after initial treatment) or endogenous LH pulses (24 and 48 h after injection of PGF2 alpha). Plasma FSH concentrations, ovarian steroid secretion and ovarian follicular development were measured. Immunization against inhibin resulted in a two- to threefold increase (P < 0.001) in plasma FSH concentrations, which remained higher than controls until injection of PGF2 alpha. Within 24 h of immunization, there was an increase in the number of small ovarian follicles (P < 0.01) and by 4 days after treatment, immunized ewes had four or five (P < 0.01) large ovarian follicles and, despite little change in the basal steroid secretion, a four- to sixfold increase (P < 0.05) in the amplitude of the steroidogenic response to a GnRH-induced LH pulse.(ABSTRACT TRUNCATED AT 250 WORDS)