Enhancement of rabbit cardiac sodium channels by β-adrenergic stimulation

Voltage-dependent sodium channels from a variety of tissues are known to be phosphorylated by the cAMP-dependent protein kinase, protein kinase A. However, the functional significance of sodium channel phosphorylation is not clearly understood. Using whole-cell voltage-clamp techniques, we show that sodium currents (I_Nas) in rabbit cardiac myocytes are enhanced by isoproterenol (ISO). This enhancement of I_Na by ISO 1) is holding potential dependent, 2) can be mimicked by forskolin and dibutyryl cAMP, and 3) is accompanied by an increase in the rate of Na⁺ channel inactivation. In single-channel, inside-out patch experiments, the catalytic subunit of protein kinase A also enhances I_Na and increases the rate of inactivation, suggesting that cardiac Na⁺ channel phosphorylation may be physiologically important. Addition of the protein kinase A inhibitor to the pipette solution in whole-cell experiments blocks the stimulatory effect of forskolin without blocking the effect of ISO, suggesting that ISO also enhances I_Na through a cAMP-independent pathway. To determine if ISO may stimulate I_Na through a direct G protein pathway, single channels were recorded in the presence of the G_s-activating GTP analogue, GTPβS, and the stimulatory G protein subunit, G_sα. Both of these agents enhanced I_Na without affecting the rate of Na⁺ channel inactivation. These results suggest that ISO enhances rabbit cardiac I_Na through a dual (direct and indirect) G protein regulatory pathway.