Up-regulation of cell surface sodium channels by cyclosporin A, FK506, andrapamycin in adrenal chromaffin cells

Treatment of cultured bovine adrenal chromaffin cells with cyclosporin A (C sA) increased cell surface [H-3] saxitoxin ([H-3]STX) binding by 56% in a t ime (t(1/2) = 15.2 h)- and concentration (EC50 = 2.9 muM)- dependent manner but did not change the K-d value. In CsA-treated cells, veratridine-induce d Na-22(+) influx was augmented with no change in the EC50 of veratridine; also, alpha- and beta -scorpion venom and Ptychodiscus brevis toxin-3 enhan ced veratridine-induced Na-22(+) influx in a more than additive manner, as in nontreated cells. CsA treatment for 1 to 24 h inhibited calcineurin acti vity, measured by the in vitro assay, with the IC50 of 0.6 muM but did not alter cellular level of calcineurin. FK506 or rapamycin elevated [H-3] STX binding by 36 or 25%, whereas GPI-1046, an immunophilin ligand incapable to inhibit calcineurin, or okadaic acid, an inhibitor of protein phosphatases 1 and 2A, had no increasing effect. The rise of [H-3] STX binding by CsA w as attenuated by the coincident treatment with brefeldin A (BFA), an inhibi tor of vesicular exit from the trans-Golgi network. The internalization rat e of cell surface Na+ channels, as determined in the presence of BFA, was d ecreased in CsA (but not rapamycin)-treated cells (t(1/2) = 20.3 h), compar ed with nontreated cells (t(1/2) = 13.7 h). CsA treatment, however, did not elevate cellular levels of Na+ channel alpha -subunit and Na+ channel alph a- and beta (1)-subunit mRNAs. In CsA-treated cells, veratridine-induced Ca -45(2+) influx via voltage-dependent Ca2+ channels and catecholamine secret ion were enhanced, whereas high K+-induced Ca-45(+) influx was not. Thus, t he inhibition of calcineurin or rapamycin-binding protein causes up-regulat ion of cell surface functional Na+ channels via modulating externalization and internalization of Na+ channels, thus enhancing Ca2+ channel gating and catecholamine secretion.