AN INACTIVATING INWARD-RECTIFYING K-CURRENT PRESENT IN PROLACTIN CELLS FROM THE PITUITARY OF LACTATING RATS

Primary cultures containing a high percentage of lactotrophs were obta ined by dissociating the pituitary of rats following 14-18 days of lac tation. Lactotrophs with a distinctive appearance were recorded after 1-35 days in vitro and identified by immunocytochemical staining for p rolactin. Whole-cell voltage clamp measurements in isotonic KCl soluti on from a holding potential of -40 mV revealed the presence of inward- rectifying K currents with a time-dependent, Nat independent inactivat ion at potentials negative to -60 mV. The time for complete inactivati on was strikingly different between lactotrophs, varying between 1 sec and more than 5 sec at -120 mV, and was not related to time in cultur e. The reversal potential shifted 59 mV (25 degrees C) for a tenfold c hange in external K+ concentration, demonstrating the selectivity of t he channel for K+ over Na+ The inward-rectifying K current was blocked by 5 mM Ba2+ and partially blocked by 10 mM TEA. Chloramine-T (1 and 2 mM) produced a total block of the inward-rectifying K current in lac totrophs. Thyrotropin-releasing hormone (500 mM) significantly reduced the inward-rectifying K current in about half of the lactotrophs. Thi s current is similar to the inward-rectifying K current previously cha racterized in clonal somatomam-motrophic pituitary cells (GH(3)B(6)). The variability of the rate of inactivation of this current in lactotr ophs and its responsiveness to TRH is discussed.