M‐currents and other potassium currents in bullfrog sympathetic neurones

1. Bullfrog lumbar sympathetic neurones were voltage‐clamped in vitro through twin micro‐electrodes. Four different outward (K+) currents could be identified: (i) a large sustained voltage‐sensitive delayed rectifier current (IK) activated at membrane potentials more positive than ‐25 mV; (ii) a calcium‐dependent sustained outward current (IC) activated at similar positive potentials and peaking at +20 to +60 mV; (iii) a transient current (IA) activated at membrane potentials more positive than ‐60 mV after a hyperpolarizing pre‐pulse, but which was rapidly and totally inactivated at all potentials within its activation range; and (iv) a new K+ current, the M‐current (IM).

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