PROTEIN PHOSPHATASE 2A REVERSES INHIBITION OF INWARD RECTIFYING K-RELEASING-HORMONE IN GH(3) PITUITARY-CELLS( CURRENTS BY THYROTROPIN)

Thyrotropin-releasing hormone (TRH) reduces an inwardly rectifying Kcurrent in whole-cell voltage-clamped GH(3) rat anterior pituitary cel ls. The TRH effect depends on the maintenance of a background level of Ca2+ in the pipette buffer, and is rapidly minimized by the intracell ular dialysis produced under whole-cell conditions. Introduction of AD P-NH-P, a non-hydrolizable ATP analog, in the pipettes, nearly abolish es the TRH-evoked inhibition. The TRH-induced reduction of the inwardl y rectifying current is significantly enhanced by incubation of cells 2-4 h with cholera toxin, but not by inclusion of 1 mM cyclic AMP in t he pipettes. Under control whole-cell conditions, the reduction caused by TRH is not reversed upon washout of the neuropeptide. However, thi s effect is readily reversed by addition of purified catalytic subunit s of protein phosphatase 2A (PP-2A(c)) but not PP-1(c) to the buffer u sed to fill the patch pipettes. Among previous results with PP inhibit ors, these data indicate that PP2A is involved in the phosphorylation/ dephosphorylation mechanism(s) that regulate the delayed TRH effects o n GH(3) cell excitability.