M‐currents and other potassium currents in bullfrog sympathetic neurones

Abstract

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 (I_K) activated at membrane potentials more positive than ‐25 mV; (ii) a calcium‐dependent sustained outward current (I_C) activated at similar positive potentials and peaking at +20 to +60 mV; (iii) a transient current (I_A) 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 (I_M). 2. I_M was detected as a non‐inactivating current with a threshold at ‐60 mV. The underlying conductance G_M showed a sigmoidal activation curve between ‐60 and ‐10 mV, with half‐activation at ‐35 mV and a maximal value (Ḡ_M) of 84±14 (S.E.M.) nS per neurone. The voltage sensitivity of G_M could be expressed in terms of a simple Boltzmann distribution for a single multivalent gating particle. 3. I_M activated and de‐activated along an exponential time course with a time constant uniquely dependent upon voltage, maximizing at ≃ 150 ms at ‐35 mV at 22 °C. 4. Instantaneous current—voltage (I/V) curves were approximately linear in the presence of I_M, suggesting that the M‐channels do not show appreciable rectification. However, the time‐ and voltage‐dependent opening of the M‐channels induced considerable rectification in the steady‐state I/V curves recorded under both voltage‐clamp and current‐clamp modes between ‐60 and ‐25 mV. Both time‐ and voltage‐dependent rectification in the voltage responses to current injection over this range could be predicted from the kinetic properties of I_M. 5. It is suggested that I_M exerts a strong potential‐clamping effect on the behaviour of these neurones at membrane potentials subthreshold to excitation.