Interaction between vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) in stimulating the secretion of prolactin from rat anterior pituitary cells in vitro

Interaction between vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) in regulating the secretion of prolactin (PRL) from the pituitary was investigated in the rat in vitro using two different methods: (1) short incubation of anterior pituitary cells and (2) superfusion of the pituitary cell column. PRL levels in the incubation medium were raised by addition of either VIP or PHI in concentrations of 10(-9) M to 10(-7) M in a dose-related manner. When both peptides were simultaneously added, additive stimulating effect on PRL release was obtained below the VIP concentration of 10(-7) M, in which no additive effect of PHI was revealed. PRL release from superfused rat pituitary cells was stimulated by 5-min pulses of VIP (10(-7) M), PHI (10(-7) M) and TRH (10(-8) M). Infusion of VIP for 200 min in the concentration of 0.3 x 10(-7) M resulted in an increase in basal release of PRL and blunted PRL release induced by not only VIP but also PHI stimulation, whereas PRL release induced by TRH was not affected. Infusion of PHI (0.3 x 10(-7) M, 0.7 x 10(-7) M and 10(-7) M) for 200 min also dose-relatedly suppressed PRL release induced by not only PHI but also VIP without any change in PRL release induced by TRH. These findings suggest that VIP and PHI may act through a common binding site on the pituitary lactotroph in the rat.