Vasoactive intestinal polypeptide and thyrotropin-releasing hormone stimulate newly synthesized, not stored,...

Experiments were designed to determine whether vasoactive intestinal polypeptide (VIP), reported to stimulate basal PRL secretion, affects PRL processing by lactotrophs. Initially, rat anterior pituitary quarters were incubated for 2 h with [3H]leucine, with and without 10(-5) M VIP, and immunoreactive and immunoprecipitable rPRL were measured during 56 mM KCl perifusion to determine total and 3H-labeled PRL, respectively. Inclusion of VIP increased immunoreactive PRL (P less than 0.05), decreased immunoprecipitable PRL (P less than 0.01), and, therefore, decreased the specific activity of labeled PRL (P less than 0.001). These results suggested an enhanced release of newly synthesized PRL before KCl depolarization, thus decreasing the release of labeled PRL. To discriminate between the two PRL pools, newly synthesized and storage, pituitary quarters were incubated with and without 10(-5) M VIP for 4 h with [14C]leucine, 2 h in cold medium and 2 h with [3H]leucine. Immunoprecipitable PRL was measured during perifusion with 56 mM KCl. Data were depicted as the 3H/14C disintegrations per min ratio of PRL released/3H/14C 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 less than 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 [3H]- and [14C]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 KCl for 1 h. Inclusion of VIP increased the 3H/14C released/3H/14C total tissue ratio during basal perifusion (P less than 0.04) and TRH exposure (P less than 0.05). Within the control group, the TRH ratio was greater than basal (P less than 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.