Potassium depletion and sodium block of potassium currents under hyperpolarization in frog sartorius muscle.

Abstract

A 3-electrode voltage-clamp method was used to investigate the mechanism of the fall in resting K permeability which occurs under extreme hyperpolarization in frog sartorius muscle fibers. This permeability change is due to a potential dependent block by Na+ ions present in the external solution. Inward K-currents recorded on hyperpolarization turned off exponentially with time. With Na the steady-state current-voltage relation had a region of negative slope beyond -140 mV. This negative-slope region was removed when Na was replaced by TMA [tetramethyl ammonium], Tris or Li. Increasing [Na] to 140 mM shifted the negative-slope region to less negative membrane potentials; reducing [Na] to 14 mM shifted the region to more negative potentials. The time constant for the turn-off of the currents (.tau.) was the same in Na and TMA-containing solutions at membrane potentials positive to -140 mV. At more negative membrane potentials the .tau. in Na became progressively shorter than those in TMA. Increasing [Na] to 140 mM (from 70 mM) gave smaller .tau. at all potentials. If fibers were hyperpolarized to -240 mV and then repolarized to -160 mV in 70 mM-Na the current recorded during the 2nd pulse turned on with time often reaching a value greater than that at the end of the 1st pulse. This behavior was removed when Na was replaced by TMA or Tris. An estimate of the steady-state relationship between the degree of block and membrane potential was obtained, and could be fitted by an expression for a potential-dependent ionic block with a very low affinity binding site for Na+ in the membrane. The recovery after hyperpolarization of K-currents at the holding potential was examined in 2-pulse experiments. In 70 mM-TMA recovery occurred at the same rate whether the initial hyperpolarization was to -120 or to -210 mV. In 70 mM-Na recovery after an initial pulse to -120 mV occurred at the same rate as in TMA, but recovery after a pulse to -210 mV occurred about 9 times faster. K may be from the lumen of the T[tubule]-system dominating the turnoff of K currents in TMA and in Na for the hyperpolarization to -120 mV, but a different mechanism is involved for the -210 mV pulse in Na. A 3-compartment model is presented which attempts to describe the depletion of K from the T-system. The model accurately predicts the time-course for the decline of inward K-currents, both in 10 and 80 mM-K solutions.