LONG-TERM REGULATION OF CALCIUM CHANNELS IN CLONAL PITUITARY-CELLS BY EPIDERMAL GROWTH-FACTOR, INSULIN, AND GLUCOCORTICOIDS

In rat pituitary GH(3) cells, epidermal growth factor (EGF) and insulin stimulate prolactin production, whereas glucocorticoids exert the opposite effect. In the present study, GH(3) cells were subjected to whole-cell patch clamp to assess the chronic actions of such regulatory factors on voltage-dependent calcium currents. Before the electrical recording, cells were grown 5-6 d either under standard conditions or in the presence of 5 nM EGF, 100 nM insulin, 1 mu M dexamethasone or 5 mu M cortisol. EGF induced a twofold selective increase in high-threshold calcium current density. Insulin and glucocorticoids, on the other hand, specifically regulated lo iu-threshold Ca channels. Current density through these channels increased by 70% in insulin-treated cells, and decreased by 50% in cells exposed to dexamethasone or cortisol. Other Ca channel properties investigated (conductance-voltage curves, deactivation rates, time course and voltage dependence of low-threshold current inactivation) were unaffected by the chemical messengers. The alterations in current density persisted for many hours after removing the regula tory factors from the culture medium. In fact, the stimulatory action of EGF high-threshold current lasted > 3 d. The results suggest that the control of prolactin production by the factors tested invokes regulation of the surface density of functional Ca channels in the plasma membrane.