Unidirectional flux ratio for potassium ions in depolarized frog skeletal muscle

Abstract

Small bundles of frog skeletal muscle fibers were loaded with 305 mM K+ and 120 mM Cl-, and 42K+ tracer efflux and influx were measured as a function of external K+ concentration ([K+]o) at a resting potential of -2 mV. As [K+]o was lowered from 305 mM, efflux decreased along a markedly sigmoidal curve, reaching a constant nonzero value at low [K+]o. Influx varied linearly with [K+]o at low [K+]o and more steeply at higher [K+]o. The ratio of influx to efflux was described by the equation: influx/efflux = exp[-n(V - VK)F/RT] with n = 2 at high [K+]o, but the ratio approached this equation with n = 1 at low [K+]o. Efflux did not depend on [K+]o when the membrane potential was raised to +36 mV, whereas at low [K+]o decreasing the membrane potential to -19 mV further activated the efflux. The results are discussed in terms of an inwardly rectifying potassium channel with two or more activating sites within the membrane that bind K+ and are accessible from the external solution.

Unidirectional K+ fluxes through recombinant Shaker potassium channels expressed in single Xenopus oocytes.

“…The flux-ratio exponent for these channels had a weak voltage dependence and attained a value as large as 3.3 at -40 mV. Horowicz et al (1968) and Spalding et al (1981) studied K + fluxes through inward rectifier K channels in skeletal muscle. They found that the flux-ratio exponent was near 1.0 at low external K concentrations and increased to 2 for high concentrations.…”

Whole-cell and single channel K+ and Cl- currents in epithelial cells of frog skin.

“…inward rectifier (Adrian, 1962;Horowicz, Gage, and Eisenberg, 1968;Spalding, Senyk, Swift, and Horowicz, 1981), resulting in a crossover of outward currents (Noble and Tsien, 1968;Adrian, 1969). Crossover of the I-V relations are also evident for single channel currents measured at varying pipette K concentrations for inward rectifier channels of MDCK cells (Friedrich, Weiss, Paulmichl, and Lang, 1989) and basolateral membrane of rabbit proximal tubule cells (Parent, Cardinal, and Sauv~, 1988).…”

Influence of external barium and potassium on potassium efflux in depolarized frog sartorius muscles

“…Unidirectional tracer K + efflux decreases as [K+]o is decreased at a constant depolarized V~; when [K+]o = 0, K + efflux is at least one order of magnitude less than its maximum in high [K+]o (Adrian, 1962;Horowicz, Gage & Eisenberg, 1968;Spalding, Senyk, Swift & Horowicz, 1981). In addition, starting in K+-free solutions at constant V~, addition of external Rb + or external K + increases K + efflux (Adrian, 1962;Spalding, Swift, Senyk, and Horowicz, 1982).…”

“…In addition, starting in K+-free solutions at constant V~, addition of external Rb + or external K + increases K + efflux (Adrian, 1962;Spalding, Swift, Senyk, and Horowicz, 1982). For low concentrations of K + or Rb + the increment of K + efflux is proportional to the second power of either [K+]o or [Rb+]o and the proportionality constant depends on V~ (Spalding et al, 1981;. The conclusion from these studies is that the K + inward rectifier channel is activated from a state of low flux to a state of high flux when at least two sites within the membrane associated with the channel are occupied by K + coming from the external solution.…”

“…, 1968;Spalding et al, 1981 (Spalding et al, 1981(Spalding et al, , 1982, this suggests than at least two sites associated with each channel need to be occupied by K + from the external solution for conversion from a low to a high flux state. Since we will examine the possibility that inhibition by external Ba 2+ results from interaction with either of the sites involved in the conversion, we first fit a simple twosite model to data on stimulation of K + efflux by external K +.…”