RECEPTORS FOR VASOACTIVE-INTESTINAL-PEPTIDE IN RAT ANTERIOR-PITUITARY GLANDS - LOCALIZATION OF BINDING TO LACTOTROPES

Vasoactive intestinal peptide (VIP) has been implicated as a physiological PRL-releasing factor; however, characterization of VIP receptors on normal pituitaries using radioligand-binding methods has been problematic. In this study we demonstrated specific receptors for VIP in anterior pituitary glands of female rats using HPLC-purified monoiodinated [Tyr(125I)10]VIP. Binding of VIP was reversible, saturable to receptor and radioligand, regulated by guanine nucleotides, and dependent on time and temperature. Scatchard analysis of competitive binding studies indicated high and low affinity binding sites, with equilibrium dissociation constants (Kd) of 0.19 ± 0.03 and 28 ± 16 nM, respectively. The corresponding maximum numbers of binding sites were 158 ± 34 fmol/mg and 11.7 ± 6.9 pmol/mg. Binding was specific, as peptides with structural homology to VIP were less than 100th as potent as VIP. The rank order of potency of the peptides tested was VIP > rat (r) peptide histidine isoleucine = human (h) PHI > rGRF > bovine GRF = porcine PHI = VIP-(10–28) > hGRF > secretin > apamin > glucagon. Radioligand binding was associated primarily with lactotrope-enriched fractions prepared by unit gravity sedimentation of dispersed anterior pituitary cells. VIP stimulated PRL release from cultured rat anterior pituitary cells, with an ED50of 1 nM. These results, comprising the first identification of specific VIP receptors in normal rat anterior pituitary tissue using radioligand-binding methods, provide additional support for a biological role of VIP in lactotrope function. © 1990 by The Endocrine Society.