Pituitary lactotrophs exhibit a high rate of spontaneous secretion of prolactin in the absence of hypothalamic regulatory factors. It has been postulated that calcium influx due to spontaneous action potentials in these cells is responsible for the high basal secretion of prolactin. The degree of calcium influx during electrical activity is dependent on both the frequency and the duration of action potentials. In order to further understand the role of electrical activity in prolactin secretion, the whole-cell and perforated-patch variation of patch-clamp techniques were used to identify and characterize the background ionic currents which determine interspike potentials and firing frequency, and the repolarizing currents which determine action potential duration.; Removal of extracellular sodium caused a profound hyperpolarization, an increase in membrane input resistance and a cessation of spontaneous action potentials in lactotrophs. Further, this experimental manipulation caused a decrease in cytosolic calcium (simultaneous measurements of electrical activity and calcium) and basal secretion of prolactin (reverse-hemolytic plaque assay). Therefore, a background sodium current is important to drive lactotrophs to the threshold for generation of spontaneous action potentials, and the calcium influx during electrical activity is important for PRL secretion.; A novel M-like potassium current, a voltage-dependent, sustained outward current, active at voltages from {dollar}-45{dollar} to {dollar}-35{dollar} mV was identified in lactotrophs. Thyrotropin-releasing hormone (TRH) caused an inhibition of this current, which was associated with a depolarized shift in voltage of activation from {dollar}-28{dollar} to {dollar}-17{dollar} mV and an acceleration of the deactivation rates. This inhibition of a background hyperpolarizing current would explain the TRH-induced increase in firing frequency and secretion in lactotrophs.; Lactotrophs had long duration, shallow spikes that were initiated from a more positive baseline, compared to {dollar}rm GHsb3{dollar} cells (a model pituitary clonal cell) which had taller and narrower spikes. The use of pharmacological blockers of potassium currents and calcium-buffered electrodes, suggested that repolarization was a calcium-dependent process in lactotrophs. However, in lactotrophs, unlike the {dollar}rm GHsb3{dollar} cells where the repolarization was due to calcium-dependent potassium currents, the repolarization was likely mediated via calcium-induced inactivation of calcium currents and/or the calcium-activated chloride currents.
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