Primary cultures containing a high percentage of lactotrophs were obtained by dissociating the pituitary of rats following 14-18 days of lactation. Lactotrophs with a distinctive appearance were recorded after 1-35 days in vitro and identified by immunocytochemical staining for prolactin. Whole-cell voltage clamp measurements in isotonic KCl solution from a holding potential of -40 mV revealed the presence of inward-rectifying K currents with a time-dependent, Na(+)-independent inactivation at potentials negative to -60 mV. The time for complete inactivation was strikingly different between lactotrophs, varying between 1 sec and more than 5 sec at -120 mV, and was not related to time in culture. The reversal potential shifted 59 mV (25 degree C) for a tenfold change in external K+ concentration, demonstrating the selectivity of the 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 lactotrophs. Thyrotropin-releasing hormone (500 nM) significantly reduced the inward-rectifying K current in about half of the lactotrophs. This current is similar to the inward-rectifying K current previously characterized in clonal somatomammotrophic pituitary cells (GH3B6). The variability of the rate of inactivation of this current in lactotrophs and its responsiveness to TRH is discussed.
