VASOACTIVE INTESTINAL POLYPEPTIDE AND THYROTROPIN-RELEASING-HORMONE STIMULATE NEWLY SYNTHESIZED, NOT STORED, PROLACTIN

Experiments were designed to determine whether vasoactive intestinal polypeptide (VIP), reported to stimulate basal PRL secretions, affects PRL processing by lactotrophs. Initially, rat anterior pituitary quarters were incubated for 2 h with [H-3] leucine, with and without 10(-5) M VIP, and immunoreactive and immunoprecipitable rPRL were measured during 56 mM KCl perifusion to determine total and H-3-labeled PRL, respectively. Inclusion of VIP increased immunoreactive PRL (P < 0.05), decreased immunoprecipitable PRL (P < 0.01), and, therefore, decreased the specific activity of labeled PRL (P < 0.001). These results suggested an enhanced release of newly synthesized PRL before KCI depolarization, thus decreasing the release of labeled PRL. To discriminate between the PRL pools, newly synthesized and storage, pituitary quarters were incubated with and without 10(-5) M VIP for 4 h with [C-14] leucine, 2 h in cold medium and 2 h with [H-3] leucine. Immunoprecipitable PRL was measured during perifusion with 56 mM KCI. Data were depicted as the H-3/C-14 disintegrations per min ratio of PRL released/H-3/C-14 disintegrations per min of total tissue to account for any differences in tissue labeling. This ratio was greater for tissue labeled in the presence of VIP (P < 0.002). To determine whether VIP, as a secretagogue, differentiates between the newly synthesized and storage pools, VIP was added after pulse chase, as previously described. No preferential release was observed between the two groups. Finally, using the same [H-3]- and [c-14] leucine-labeling protocol with and without 10(-5) M VIP, tissue was perifused with medium 199 for 1 h, with 10(-5) M TRH for 30 min, with medium 199 for 30 min, and with 56 mM KCI for 1 h. Inclusion of VIP increased the H-3/C-14 released/H-3/C-14 total tissue ratio during basal perifusion (P < 0.04) and TRH exposure (P < 0.05). Within the control group, the TRH ratio was greater than basal (P < 0.003). These experiments suggest that newly synthesized PRL is preferentially secreted over stored PRL from tissue incubated with VIP during pulse-chase labeling; however, addition of VIP as a secretagogue did not affect either PRL pool preferentially.