Abstract
Growth factors and hormones induce differentiation of clonal pheochromocytoma (PC12) cells, which are derived from rat adrenal medulla chromaffin cells. On application of nerve growth factor (NGF), PC12 cells extend neurites and express properties characteristic of autonomic ganglion cells. In contrast, incubation of PC12 cells with a corticosteroid, dexamethasone (DEX), does not induce neurite formation but causes an increase in tyrosine hydroxylase activity, suggesting that the cells become chromaffin cell-like. The ability of NGF and DEX to regulate ionic currents has been less well studied. Therefore, we examined how long-term NGF and DEX treatments affected voltage- dependent Na, Ca, and K currents in PC12 cells. Voltage-dependent Na currents were observed only in a small fraction of the PC12 cells in the absence of NGF or DEX. Virtually all NGF-treated cells expressed Na currents within 7 d. DEX increased the number of cells expressing voltage-dependent Na current slowly over 3 weeks, but, unlike NGF, DEX did not change Na current density. Both NGF and DEX also affected the expression of voltage-dependent Ca currents. Most of the untreated cells had only sustained, high-threshold voltage-dependent Ca currents. Chronic application of NGF or DEX increased the fractions of the cells that showed transient, low-threshold T-type Ca currents in addition to the high-threshold currents. The T-type Ca current density, however, increased significantly only in NGF-treated cells. Neither DEX nor NGF affected the voltage-dependent K currents. These results suggest that the expression of voltage-dependent Na and Ca currents are differentially regulated by NGF and DEX. The distinction between treated and untreated cells decreased after 3 weeks in culture as older untreated cells showed increases in the fraction of cells expressing both Na and low-threshold Ca currents. A PC12 subline selected for adherence to uncoated plastic also showed increased fraction of cells expressing these currents, suggesting that interactions with substrate may also influence ionic current expression.
