An Inactivating Inward-rectifying K Current Present in Prolactin Cells from the Pituitary of Lactating Rats

Abstract.

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°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.