Vasoactive intestinal peptide (VIP) may reduce the removal rate of cytosolic Ca²⁺ after transient elevations in clonal rat lactotrophs

Abstract

The prolactin-producing rat anterior pituitary GH4C1 cells possess Ca2+-activated K channels which are activated by physiological elevations of the cytosolic Ca2+ concentration even at membrane potentials more negative than the normal level of about -50 mV. Whole-cell current recordings showed a marked outward tail current following depolarizing voltage steps to 0 mV from a holding potential close to the normal membrane potential. The half-time of this tail current was about 1.3 s after a 4-s depolarization step. The GH4C1 cells also possess voltage-activated Ca channels, and we conclude that this tail current is a Ca2+-activated K+ current for the following reasons: (1) The reversal potential for the tail current was close to the K+ equilibrium potential for a range of transmembrane K+ gradients. (2) The tail current was blocked by a Ca2+ antagonist, and the voltage dependence of this current closely mirrored the voltage dependence of the isolated Ca2+ current. The time-course of the decline of the tail current thus reflects the removal rate of the Ca2+ entering the cytosol through voltage-dependent Ca channels during the depolarizing voltage step. VIP stimulates prolactin secretion from GH4C1 cells, and this peptide prolonged the half-time of the tail current by about 47% in 63% of the cells. This indicates that VIP may prolong the transient cytosolic Ca2+ elevations following the action potentials in these cells. Such a mechanism might be an important factor for the control of the cytosolic Ca2+ level, and hence hormone secretion.

HERG K+ currents in human prolactin-secreting adenoma cells

“…Off-potentials elicited on the cessation of hyperpolarizing current pulses have often been described in various prolactin-secreting pituitary cells (e.g. [26,35]). In these cells, erg currents are present and the hyperpolarization-induced deactivation of erg currents combined with the slow activation kinetics of the erg currents should support the generation of off-action potentials.…”

Inhibition of BK channels contributes to the second phase of the response to TRH in clonal rat anterior pituitary cells

“…The lower trace was recorded from a spontaneously firing cell, and the action potentials are reflected as outward (upward) patch currents. Action potentials are thus easily detected in cell‐attached patches (Sand et al. 1989, Lang & Ritchie 1990a,b), which makes it possible to study the effect of channel modulators on excitability and channel activity simultaneously.…”

BK channels in intact clonal rat pituitary cells are activated by physiological elevations of the cytosolic Ca2+ concentration at the normal resting potential

“…The GH clonal lines of rat pituitary cells, which generate Ca 2+ -dependent action potentials, possess BK channels (Dubinsky & Oxford 1984, Sand et al 1989. The view that the main functional role of the BK channels is to contribute to the repolarizing phase of the action potential is also adopted for these cell lines (Lang & Ritchie 1990;see Corette et al 1995).…”