Experiments were performed to test the applicability of permeability kinetics to whole frog sartorius muscle using K ¢ ions as tracers of potassium flux. The whole muscle was found to obey closely the kinetic laws expected to hold for single cellular units in which the potassium fluxes are membrane-limited and intracellular mixing is rapid enough not to introduce serious error. In a 5 mM K Ringer's solution, potassium effiux was very nearly equal to influx when the rate constant for K a loss was applied to the whole of the muscle potassium. Over a fairly wide range of external potassium concentration, the assumed unidirectional fluxes measured with tracer K a showed good agreement with net potassium changes determined analytically. The specific activity of potassium lost from labeled muscles to an initially K-free Ringer's solution was measured as a test of the adequacy of intracellular mixing. The results were those expected for a population of cells with uniformly distributed intracellular K ¢. A small deviation was encountered which can be attributed either to a dispersion of fiber sizes in the sartorius or to a possible small additional cellular compartment in each individual fiber. The additional cellular compartment, should it exist, contains from 0.5 to 1 per cent of the muscle potassium. This is evidently not large enough to interfere seriously with the applicability of permeability kinetics to the whole muscle.
I N T R O D U C T I O NI o n fluxes have been previously measured in whole muscle b y m a n y investigators. T h e work has led to two main kinetic models for whole muscles. O n e model regards whole muscle as a population of single cellular units each obeying simple first-order permeability kinetics (Keynes, 1954; Creese, 1954). According to this model, the whole muscle will, with possible minor p e r t u r b ations, o b e y the same kinetics. T h e other model proposed regards whole muscle as an assemblage of cellular units in which simple permeability kinetics are not obeyed due to the presence of large perturbations caused b y slow diffusion intracellularly (Harris, 1957; Harris and Steinbach, 1956). For con-605
The potassium exchange properties of glycerol-treated sartorius muscles of the frog were determined. Potassium (K) uptake, efflux, and net flux were measured in the presence of glycerol and at various times after exposure to glycerol and return to isotonic Ringer solution. Potassium uptake was not altered by the presence of glycerol but was reduced on the average 53 % after glycerol treatment. Efflux transiently increased in the presence of glycerol and was reduced 37 % after glycerol removal. Consequently, there was a net loss of intracellular potassium as well as a gain of sodium. In contrast to the irreversible alterations of potassium exchange induced by glycerol treatment, action potentials with normal negative afterpotentials (N.A.P.) were elicited 4-5 hr after glycerol removal. The reappearance of the N.A.P. was associated with a return of the membrane potential to normal values (90 4 2 my). However, the response of these muscles to reduced extracellular potassium was anomalous. In K+-free Ringer solution the average resting membrane potential was 74 4-3 my and a positive afterpotential of 11 + 3 mv was associated with the action potential.
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