THE CORTICAL ACTIN CYTOSKELETON OF LACTOTROPES AS AN INTRACELLULAR TARGET FOR THE CONTROL OF PROLACTIN SECRETION

We investigated the role of cortical actin filaments (F-actin) in the regulation of PRL secretion in cultured normal anterior pituitary cell s. F-actin dynamics were evaluated by fluorescence microscopy, and PRL secretion from attached cells was measured by the reverse hemolytic p laque assay. F-actin localized to the periphery of lactotropes. PRL-re leasing factors such as TRH, vasoactive intestinal peptide (VIP), and forskolin, or removal of the PRL-inhibiting factor dopamine (DA) from cultures chronically exposed to DA, caused fragmentation, i.e. focal d isassembly of cortical F-actin. Basal, VIP-, and DA withdrawal-induced cortical F-actin disassembly were dependent on extracellular Ca2+ whe reas TRH-and forskolin-induced disassembly were not. Short-term (5 min ) treatment of cells with the F-actin-disrupting agent cytochalasin D (CD) enhanced basal PRL secretion but did not further stimulate TRH-or VIP-induced PRL secretion. The results support the existence of a cau sal link between F-actin disassembly and increased PRL secretion. On t he other hand, exposure of cultures to DA decreased the percentage of cells showing cortical F-actin disassembly within minutes. Longer trea tments (2-4 h) caused stabilization of cortical actin filaments as rev ealed by the protection vis-a-vis the depolymerizing effect of CD. The protective effect was specific for lactotropes and was evident with D A concentrations as low as 50 nM. Chronic exposure of the cells to DA blocked CD-and TRH-evoked actin disassembly and PRL secretion while VI P-induced effects were partially inhibited. Stabilization of F-actin w ith the marine sponge venom, jasplakinolide, also decreased basal and stimulated PRL secretion. In conclusion, our results suggest that, fir st, the cortical actin cytoskeleton of lactotropes is an integrator of the multiple factors regulating PRL secretion directly on the lactotr ope, and second, the tonic inhibition of PRL secretion is mediated, at least in part, by DA-induced stabilization of cortical F-actin.