Caffeine-induced inactivation of the M-current in bullfrog sympathetic neurons.

Neurons from bullfrog sympathetic ganglia were voltage-clamped using a single microelectrode in Ringer solution with tetrodotoxin. The M-current (Adams et al. 1982 a) was markedly reduced during the rhythmic outward potassium current induced by caffeine (1-5 mM). This indicates that an elevation of the intracellular concentration of calcium ions by caffeine not only activates the calcium-activated potassium current but also inactivates another potassium current, the M-current. In Ringer solution, simultaneous activation and inactivation of potassium currents could be observed as a small but significant reduction of the M-current during the calcium-activated of terhyperpolarization which follows the action potential. Bullfrog sympathetic neurons possess at least two subtypes of potassium currents which are activated by an influx of calcium ions during the action potential. These currents, referred to as Ic and IAHP, contribute to the repolarization and the of terhyperpolarization of the action potential, respectively (Adams et al. 1982 b; Kuba et al. 1983; MacDermott and Weight, 1982; Minota, 1974; Pennef ather et al. 1985; Tanaka et al. 1986; Tokimasa, 1984, 1985 a). A potassium current, identical to IAHP, could be activated by calcium release from intracellular organelles which is triggered by caffeine or intracellular anions such as citrate (Endo et al. 1970; Ford and Podolsky, 1970; Koketsu et al. 1982; Kuba, 1980; Kuba and Nishi, 1976; Morita et al. 1980; Smith et al. 1983; Tokimasa, 1985 a). It has recently been demonstrated that an action potential generated by calcium ions is followed by a depolarizing after-