Synthesis and release of calcitonin-like immunoreactivity by anterior pituitary cells: evidence for a role in paracrine regulation of prolactin secretion.

Calcitonin (CT) is a potent and specific inhibitor of basal and TRH-induced PRL release and PRL mRNA levels in rat anterior pituitary (AP) cells, an action mediated through specific inhibition of the Ca(2+)-inositol phosphate messenger system. Because CT and CT-like peptides have been reported to be present in the AP, the present studies investigated 1) whether 35S-labeled CT-like substances can be precipitated from rat AP cell lysates, 2) whether immunoreactive CT is secreted from rat AP cells, as assessed from a cell blot assay and from RIA of spent medium from cultured AP cells, and 3) whether the release of PRL from cultured rat AP cells can be influenced by immunoneutralization of endogenous CT. Dispersed rat AP cells were labeled with [35S]cysteine, CT-like substances were immunoprecipitated from the lysate with antihuman CT (anti-hCT) and antisalmon CT (anti-sCT) immunoglobulin G conjugated to protein-A-Sepharose beads, and the immunoprecipitates were fractionated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified by fluorography. The results revealed that both anti-sCT and anti-hCT sera precipitated three major bands of 35S-labeled material from AP cell lysates, with approximate molecular masses of 23, 6.5, and 3.5 kilodaltons. Synthetic sCT completely displaced anti-sCT-precipitable bands and partially displaced anti-hCT precipitates. The cell blot assay revealed the presence of CT-immunopositive AP cells, and secretion of this substance was indicated by zones of secretion surrounding these cells. Immunoreactive CT was also detected in spent medium from cultured rat AP cells, and the average rate of release over 4 days of culture was approximately 1 ng sCT eq/3 million AP cells.24 h. Both anti-sCT and anti-hCT sera significantly stimulated PRL release from rat AP cells, and the stimulatory effect of anti-sCT serum was dose dependent up to a concentration of 1:50. The present findings demonstrate that CT-like peptide(s) is synthesized and released from cultured AP cells and suggest that this peptide may participate in the regulation of PRL secretion via paracrine or autocrine actions.