Thiol regulation of depletion-transformation and release of prolactin by the pituitary of the lactating rat.

We investigated the possibility that thiol-disulfide interchange mechanisms are involved in depletion-transformation (loss of tissue PRL detectability) and release of PRL from adenohypophyses (AP) of lactating rats. The influence of pH, bicarbonate, and Triton X-100 as well as thiol-related compounds on these processes was assessed. Tissue PRL was depleted-transformed by the use of three different conditions: 1) 30 min of suckling after 8 h of nonsuckling; 2) in vitro incubation of APs for 2 h; or 3) in vivo cysteamine (CSH) treatment. Lactating rats nonsuckled for 8 h served as controls (no depletion-transformation). The depletion-transformation phenomenon was unchanged by extraction with Tris-HCl-0.1% Triton X-100 buffers but reversed either by extraction with bicarbonate buffer (pH 8.2 or 9.7) or by incubation of pH 8.2 homogenates for 3 h at 37 C. Reduced glutathione (GSH) added to these homogenates further enhanced PRL detectability. At pH 6.5, however, incubation with or without GSH had the opposite effect and decreased PRL detectability. In AP incubations, depletion was increased in a dose- and time-dependent fashion by the aminothiol CSH, and by GSH, dithiothreitol, or mercaptoethanol but not sodium ascorbate. These agents also inhibited PRL release. Similar results were obtained after injection of CSH (20-120 mg/kg BW) 4 h before death. Depletion and release of PRL in incubated APs were prevented by iodoacetamide and N-ethylmaleimide (0.1-5 mM); GSH or CSH counteracted these effects. In contrast to the alkylating agents, oxidized glutathione and 5,5'-dithio-2-nitrobenzoic acid inhibited PRL depletion but stimulated PRL release. Thus, thiols and aminothiols may preferentially lead to depletion-transformation of PRL, whereas disulfides may inhibit depletion and facilitate PRL release. Although in some experiments increased depletion was dissociated from increased release, nonetheless the data support the concept that shifts in PRL detectability during depletion-transformation, repletion, and release involve thiol-disulfide interchange mechanisms.