A 14-KILODALTON PROLACTIN-LIKE FRAGMENT IS SECRETED BY THE HYPOTHALAMONEUROHYPOPHYSEAL SYSTEM OF THE RAT

The recently described expression of the PRL gene, and the occurrence of a 14-kilodalton (kDa) PRL-like immunoreactive protein in the hypoth alamo-neurohypophyseal system of the rat have raised the possibility t hat PRL variants are released from neurohypophyseal terminals into the blood. In this study, we investigated the local production of a hypot halamo-neurohypophyseal 14-kDa PRL-like protein by showing an independ ent origin from adenohypophyseal PRL. No 14-kDa PRL-like protein was d etected in adenohypophyseal extracts by Western blots, whereas chronic hypophysectomy produced no change in the immunocytochemical detection of PRLs in supraoptic and paraventricular magnocellular neurons. In a ddition, a 14-kDa immunoreactive PRL-like protein was released into th e medium by incubated neurohypophyseal lobes. Western blot analysis sh owed that significantly more of this 14-kDa protein was released into calcium-containing medium (1.8 mM) than into calcium-free medium. Furt hermore, depolarizing concentrations of potassium (56 mM) increased by 3-fold the release of immunoreactive PRL by incubated hypothalamo-neu rohypophyseal explants. In addition, a 14-kDa PRL-like antigen was det ected in the circulation of the rat by Western blot analysis. These re sults are consistent with the local synthesis and calcium-dependent re lease of neurohypophyseal PRL-like proteins that include a predominant 14-kDa form.