Novel hyperpolarization-activated K+ current mediates anomalous rectification in crayfish muscle

Abstract

The ionic current underlying anomalous rectification in opener muscle fibers of crayfish was studied under two-electrode voltage clamp. Opener muscle fibers showed a mean resting potential (RP) of -64.8 mV and an input resistance of 0.4 MΩ. Hyperpolarizing voltage command pulses from a holding potential (H) of -60 mV evoked an instantaneous voltage-independent linear current (I(L)) followed by a time- and voltage-dependent inward current (I(AB)) that reached a steady state within 500 msec. The reversal potential of I(AB) (E(AB)) was estimated from tail current amplitudes. At an extracellular K+ concentration ([K+](o)) of 5.4 mM the mean E(AB) was - 61.8 mV. E(AB) shifted toward positive potentials by 50.8 mV for a 10-fold increase in [K+](o). The conductance underlying I(AB) (G(AB)) increased sigmoidally with hyperpolarization, starting close to the RP, saturating at a G(AB,max) of about -140 mV, and showing a mean half-activation at -94.4 mV. The activation curve of G(AB) shifted 53.6 mV toward positive potentials with a 10-fold increase in [K+](o). G(AB,max) did not increase in raised [K+](o). The activation and deactivation kinetics of I(AB) were accurately described by single exponentials with similar time constants (< 100 msec). Time constants changed as an exponential function of the membrane potential. I(AB), its time course, G(AB), and E(AB) were not modified in the following conditions: (1) Na+- and Ca2+-free solutions, (2) intracellular EGTA, (3) extracellular (100 mM) or intracellular tetraethylammonium, (4) extracellular Cs+ (up to 50 mM), Rb+ (up to 10 mM), Ba2+ (13.5 mM), or Mn2+ (13.5 mM). However, low extracellular concentrations of Cd2+ or Zn2+ strongly and reversibly reduced both I(L) and I(AB). Therefore, we conclude that anomalous rectification in crayfish muscle is generated by a voltage- and time-dependent K+ current I(AB). This current displayed many electrophysiological and pharmacological characteristics that distinguished it from all others mediating anomalous rectification described previously.