Differential effects of short and prolonged exposure to carvedilol on voltage-dependent Na(+) channels in cultured bovine adrenal medullary cells

We examined the effects of short and prolonged exposure to carvedilol, an antihypertensive and beta-adrenoceptor blocking drug, on voltage-dependent Na(+) channels in cultured bovine adrenal medullary cells. Carvedilol (1-100 microM) reduced (22)Na(+) influx induced by veratridine, an activator of voltage-dependent Na(+) channels. Carvedilol also suppressed veratridine-induced (45)Ca(2+) influx and catecholamine secretion in a concentration-dependent manner similar to that of (22)Na(+) influx. Prolonged exposure of the cells to 10 microM carvedilol increased (3)H]saxitoxin ((3)H]STX) binding, which reached a plateau at 12 h and was still observed at 48 to 72 h. Scatchard analysis of (3)H]STX binding revealed that carvedilol increased the B(max) value (control, 14.9 +/- 0.9 fmol/10(6) cells; carvedilol, 23.8 +/- 1.2 fmol/10(6) cells) (n = 3, P < 0.05) without altering the K(d) value, suggesting a rise in the number of cell surface Na(+) channels. The increase in (3)H]STX binding by carvedilol was prevented by cycloheximide, an inhibitor of protein synthesis, whereas carvedilol changed neither alpha- nor beta(1)-subunit mRNA levels of Na(+) channels. The carvedilol-induced increase of (3)H]STX binding was abolished by brefeldin A and H-89, inhibitors of intracellular vesicular trafficking of proteins from the trans-Golgi network and of cyclic AMP-dependent protein kinase (protein kinase A), respectively. The present findings suggest that short-term treatment with carvedilol reduces the activity of Na(+) channels, whereas prolonged exposure to carvedilol up-regulates cell surface Na(+) channels. This may add new pharmacological effects of carvedilol to our understanding in the treatment of heart failure and hypertension.