Up‐Regulation of Sodium Channel Subunit mRNAs and Their Cell Surface Expression by Antiepileptic Valproic Acid: Activation of Calcium Channel and Catecholamine Secretion in Adrenal Chromaffin Cells

Abstract

Treatment of cultured bovine adrenal chromaffin cells with a therapeutic concentration (0.6 mM) of valproic acid (VPA) for > 24 h caused a time-dependent (t1/2 = 74 h) increase in [3H]saxitoxin binding up to 1.4-fold without altering the KD value; it was prevented by the simultaneous treatment with cycloheximide (an inhibitor of protein synthesis). VPA also raised Na+ channel alpha- and beta 1-subunit mRNA levels 1.4- and 1.7-fold at 24 h, and 1.6- and 1.8-fold at 72 h, respectively. Chronic (but not acute) exposure to VPA enhanced 22Na+ influx caused by various concentrations of veratridine 1.4-2.1-fold, even when assayed in the presence of Na+,K(+)-ATPase inhibitor, but did not change the EC50 value of veratridine. Ptychodiscus brevis toxin-3 allosterically potentiated veratridine-induced 22Na+ influx by approximately 2-fold in VPA-treated cells as in nontreated cells. Long-term treatment with VPA augmented veratridine-induced 45Ca2+ influx via voltage-dependent Ca2+ channels and catecholamine secretion, but had no effect on 45Ca2+ influx and catecholamine secretion caused by high K+ (a direct activation of voltage-dependent Ca2+ channels). Chronic treatment with VPA also enhanced nicotine-induced 22Na+ influx via the nicotinic receptor-ion channel complex 1.2-1.4-fold with little change in the EC50 value of nicotine, thereby increasing the nicotine-induced 45Ca2+ influx via voltage-dependent Ca2+ channels and catecholamine secretion. These results suggest that chronic treatment with VPA up-regulates cell surface expression of Na+ channels via the transcription/translation-dependent mechanisms, and probably of nicotinic receptors, thereby resulting in the enhancement of Ca2+ channel gating and catecholamine secretion.