Requirement of oestrogens for the sensitivity of prolactin cells to vasoactive intestinal peptide in rats and man

Vasoactive intestinal peptide (VIP) is a prolactin-releasing hormone which is involved in the multifactorial modulation of prolactin secretion in mammals. Intravenous injection of VIP (1 microgram/kg) to fertile women increased plasma prolactin levels and heart rate and reduced diastolic pressure. The same treatment to menopausal women caused similar cardiovascular effects but did not modify plasma prolactin levels. In contrast, TRH (200 micrograms, i.v.) induced a significant increase in plasma prolactin levels in both fertile and menopausal women. The relevance of oestrogens in affecting VIP-stimulated prolactin secretion was evaluated in vitro by measuring prolactin release from pituitary cells of control and ovariectomized rats. The sensitivity of rat mammotrophs to VIP, but not to TRH, was completely suppressed 3 or 4 weeks after ovariectomy. Furthermore, implantation of rats with a silastic capsule containing oestradiol-17 beta during ovariectomy, preserved the cell responsiveness to VIP. The prolactin-releasing property of VIP was also restored when pituitary cells from ovariectomized rats were cultured for 3 days in the presence of 10 nmol oestradiol-17 beta/l before being used for prolactin release experiments. The present study shows that the ability of prolactin-secreting cells to respond to the stimulatory action of VIP requires high levels of circulating oestrogens, both in man and rats.