Inhibition of protein phosphatase 2A (PP2A) mimics suckling-induced sensitization of mammotropes: Involvement of a pertussis toxin (PTX) sensitive G-protein and the adenylate cyclase (AC)

In lactating rats, suckling renders mammotropes more responsive to prolacti n (PRL)-releasing stimuli and less responsive to PRL-inhibiting secretagogu es. We have previously shown that a decrease in the activity of protein pho sphatase 2A (PP2A) may be responsible for the decrease in responsiveness to the inhibitory secretagogue dopamine (DA), In our present experiments, we have studied the involvement of the adenylate cyclase (AC), stimulatory and inhibitory GTP-binding proteins and also the role of PP2A in the sensitiza tion phenomenon. Pituitary cells obtained from mother rats separated from t heir pups for 4 h prior to dispersion (non-suckled), suckled for 10 or 30 m in after the separation period (suckled) and without separation (continual suckling) were incubated in the presence of different doses of forskolin to activate AC and DA. In a further study, pituitary cells of non-suckled rat s were pretreated with cholera toxin (CTX) or pertussis toxin (PTX) and tes ted for the stimulatory action of forskolin or TRH on PRL release. Ocadaic acid (OA) pretreatment has been used to investigate the involvement of PP2A . Hormone secretion was measured by the reverse hemolytic plaque assay (RHP A). Our results have shown that cells from non-suckled rats were unresponsi ve to forskolin. A 10-min suckling stimulus sensitizes pituitary mammotrope s to respond with a PRL release to a dose-dependent activation of AC by for skolin. This sensitization of AC becomes a permanent feature of the cells w hen suckling continues for an additional 20 min. We have also found that pi tuitary mammotropes from non-suckled dams respond to forskolin or TRH with PRL release when they were preincubated with either PTX or the PP2A inhibit or OA. It clearly indicates that the non-responsive pituitary can be shifte d to the responsive stage by uncoupling of inhibitory G-protein from its re ceptor as well as by inhibition of PP2A. This latter finding, consonant wit h our previous results, suggests that suckling may cause selective changes in the function of Gi of mammotropes due to a rapid phosphorylation which c an remove tonic, GTP-dependent inhibitory function. (C) 1999 Elsevier Scien ce Ireland Ltd. All rights reserved.