The influences of GnRH, oxytocin and vasoactive intestinal peptide on LH and PRL secretion by porcine pituitary cells in vitro.

The aim of the present study was to evaluate the possible direct effects of GnRH, oxytocin (OT) and vasoactive intestinal peptide (VIP) on the release of LH and PRL by dispersed porcine anterior pituitary cells. Pituitary glands were obtained from mature gilts, which were ovariectomized (OVX) one month before slaughter. Gilts randomly assigned to one of the four groups were treated: in Group 1 (n = 8) with 1 ml/100 kg b.w. corn oil (placebo); in Group 2 (n = 8) and Group 3 (n = 8) with estradiol benzoate (EB) at the dose 2.5 mg/100 kg b.w., respectively, 30-36 h and 60-66 h before slaughter; and in Group 4 (n = 9) with progesterone (P4) at the dose 120 mg/ 100 kg b.w. for five consecutive days before slaughter. In gilts of Group 2 and Group 3 treatments with EB have induced the negative and positive feedback in LH secretion, respectively. Isolated anterior pituitary cells (10(6)/well) were cultured in McCoy's 5a medium with horse serum and fetal calf serum for 3 days at 37 degrees C under the atmosphere of 95% air and 5% CO2. Subsequently, the culture plates were rinsed with fresh McCoy's 5A medium and the cells were incubated for 3.5 h at 37 degrees C in the same medium containing one of the following agents: GnRH (100 ng/ml), OT (10-1000 nM) or VIP (1-100 nM). The addition of GnRH to cultured pituitary cells resulted in marked increases in LH release (p < 0.001) in all experimental groups. In the presence of OT and VIP we noted significant increases (p < 0.001) in LH secretion by pituitary cells derived from gilts representing the positive feedback phase (Group 3). In contrast, OT and VIP were without any effect on LH release in Group 1 (placebo) and Group 2 (the negative feedback). Pituitary cells obtained from OVX gilts primed with P4 produced significantly higher amounts (p < 0.001) of LH only after an addition of 100 nM OT. Neuropeptide GnRH did not affect PRL secretion by pituitary cells obtained from gilts of all experimental groups. Oxytocin also failed to alter PRL secretion in Group 1 and Group 2. However, pituitary cells from animals primed with EB 60-66 h before slaughter and P4 produced markedly increased amounts of PRL in the presence of OT. Neuropeptide VIP stimulated PRL release from pituitary cells of OVX gilts primed with EB (Groups 2 and 3) or P4. In contrast, in OVX gilts primed with placebo, VIP was without any effect on PRL secretion. In conclusion, the results of our in vitro studies confirmed the stimulatory effect of GnRH on LH secretion by porcine pituitary cells and also suggest a participation of OT and VIP in modulation of LH and PRL secretion at the pituitary level in a way dependent on hormonal status of animals.