Electrical excitability in the rat clonal pituitary cell and its relation to hormone secretion

Membrane electrical properties of the clonal anterior pituitary cell (GH3), were studied using intracellular recording techniques. The resting potential in the GH3 cell was -48.0 +/- 1.1 mV (mean +/- S.E., n=39) in normal saline. The input resistance estimated from the linear portion of the current-voltage relationship was 453 +/- 30 Momega (n=21). The maximum rate of rise of the action potential was 7.0 +/- 1.1 V/sec (n=12) at room temperature (23-25 degrees C). The action potential had both Na and Ca components. The Ca component was abolished by addition of 4 mM Co2+. Sr2+ could substitute for Ca2+ in supporting spike initiation. As the concentration of Sr2+ was increased, the maximum rate of rise of the action potential increased. After replacement of Ca2+ with isomolar Ba2+ the membrane potential shifted to -6.1 +/- 1.1 mV (n=11). In the Ba solution, prolonged action potentials were evoked by a depolarizing current pulse after maintaining the membrane potential more negative than -50 mV. The release of both prolactin and growth hormone was enhanced by increasing the external K+ concentration to 50 mM in the presence of Ca2+. Sr2+ could substitute for Ca2+. Ba2+ enhanced the release of both hormones, even if the K+ concentration was unaltered. The facilitatory effects of high K+ and Ba2+ were markedly suppressed by addition of 2-4 mM Co2+. These results may suggest that the potential-dependent increase in the membrane permeability to Ca2+, responsible for initiation of the Ca spike, plays a significant role in stimulation of hormone secretion in GH3 cells.