Vol. 302, Issue 1, 212-218, July 2002

Differential Effects of Short and Prolonged Exposure to Carvedilol on Voltage-Dependent Na⁺ Channels in Cultured Bovine Adrenal Medullary Cells

Koji Kajiwara, Toshihiko Yanagita, Yasuhide Nakashima, Akihiko Wada, Futoshi Izumi and Nobuyuki Yanagihara

Second Department of Internal Medicine (K.K., Y.N.) and Department of Pharmacology (N.Y.), University of Occupational and Environmental Health (F. I.), School of Medicine, Kitakyushu, Japan; and Department of Pharmacology (T.Y., A.W.), Miyazaki Medical College, Miyazaki, Japan

We examined the effects of short and prolonged exposure to carvedilol, an antihypertensive and -adrenoceptor blocking drug, on voltage-dependent Na⁺ channels in cultured bovine adrenal medullary cells. Carvedilol (1-100 µM) reduced ²²Na⁺ influx induced by veratridine, an activator of voltage-dependent Na⁺ channels. Carvedilol also suppressed veratridine-induced ⁴⁵Ca²⁺ influx and catecholamine secretion in a concentration-dependent manner similar to that of ²²Na⁺ influx. Prolonged exposure of the cells to 10 µM carvedilol increased [³H]saxitoxin ([³H]STX) binding, which reached a plateau at 12 h and was still observed at 48 to 72 h. Scatchard analysis of [³H]STX binding revealed that carvedilol increased the B_max value (control, 14.9 ± 0.9 fmol/10⁶ cells; carvedilol, 23.8 ± 1.2 fmol/10⁶ 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 [³H]STX binding by carvedilol was prevented by cycloheximide, an inhibitor of protein synthesis, whereas carvedilol changed neither - nor ₁-subunit mRNA levels of Na⁺ channels. The carvedilol-induced increase of [³H]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.