Kinetics of exchange and net movement of frog muscle potassium

TRACER AND NON-TRACER POTASSIUM FLUXES IN FROG SARTORIUS MUSCLE AND THE KINETICS OF NET POTASSIUM MOVEMENT

Experiments were performed to test the applicability of permeability kinetics to whole frog sartorius muscle using K⁴² 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 efflux was very nearly equal to influx when the rate constant for K⁴² 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 K⁴² 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 K⁴². 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.

THE INFLUENCE OF NA CONCENTRATION ON NA TRANSPORT ACROSS FROG SKIN

The effects of changes in Na concentration of the bathing solutions on some transport and permeability properties of the isolated frog skin have been examined. Rate coefficients for unidirectional Na movements across the two major barriers in the skin have been estimated as functions of Na concentration. The results indicate that the "apparent Na permeability" of the outer barrier of the skin decreases markedly when Na concentration in the outer solution is increased from 7 to 115 mM. The observed saturation of rate of Na transport with increasing Na concentration can be ascribed, in part, to this permeability change rather than to saturation of the transport system itself. Unidirectional Cl flux across the short-circuited skin was not significantly altered by an increase in Na concentration from 30 to 115 mM suggesting that the changes in membrane properties are relatively specific for the Na ion. The results also suggest that the movement of Na across the outer membrane may not be due entirely to simple passive diffusion of free Na ions.

Transport of electrolytes in muscle

The muscle fiber stands alongside the red blood cell and the giant axon as one of the three classical cell types that have had major application in investigating ion transport processes in cell membranes. Of these three cell types, the muscle fiber was the first to provide definite evidence for a sodium pump. The ability of the sodium pump to produce an electrical potential difference across the cell membrane was also first demonstrated in muscle fibers. This important property of the sodium pump is now known to have physiological significance in many other types of cells. In this review, electrolyte transport investigations in skeletal muscle are traced from their inception to the current state of the field. Applications of major research techniques are discussed and key results are summarized. An overview of electrolyte transport in muscle, this article emphasizes relationships between the muscle fiber membrane potential and ionic transport processes.

Sodium and potassium activities in normal and "sodium-rich" frog skeletal muscle

In frog sartorius muscles immersed for 2 hours at 26 degrees C in normal Ringer solution, the intrafiber potassium concentration, C(K)(in millimoles per liter), was 123 +/- 2 (mean value plus or minus standard error), and the potassium activity, a(K) (in millimoles per liter), was 90 +/- 1.0. The corresponding sodium concentration and activity were 20 +/- 1 and 6.5 +/- 0.4, respectively. After overnight immersion in K+-free Ringer solution the values were: CK, 97 +/- 2; aK, 81.5 +/- 1.6; CNa, 47 +/- 2; and aNa, 11.2 +/- 0.6. The changes in aK and aNa during storage were not consistent with an exchange between predominantly "free" fiber K+ and external Na+. These results suggest that the Na+ taken up during overnight immersion largely replaced adsorbed or sequestered K+ in the fibers.

Factors affecting the permeability of isolated fat-cells from the rat to [42K] potassium and [36Cl] chloride ions

1. The effluxes of (42)K(+) and (36)Cl(-) from isolated fat-cells from the rat were studied under a variety of conditions known to affect the metabolism of the cells. 2. (42)K(+) efflux from isolated fat cells was increased in a Na(+)-free-high-K(+) medium and decreased in a K(+)-free medium. The existence of K(+) exchange diffusion across the fat-cell membrane is suggested. 3. (36)Cl(-) efflux from isolated fat-cells was decreased when the Cl(-) component of the wash medium was replaced by acetate. The basal (36)Cl(-) efflux is suggested to be partly by Cl(-) exchange diffusion and partly in company with a univalent cation. 4. A variety of lipolytic stimuli, adrenaline, adrenocorticotrophic hormone, N-6,O-2'-dibutyryladenosine cyclic 3':5'-monophosphate and theophylline, increased (42)K(+) efflux from isolated fat-cells. The adrenaline stimulation was biphasic; an initial, rapid and transient increase in (42)K(+) loss from the fat-cells was followed by a slower, more prolonged, increase in (42)K(+) efflux. The initial phase was inhibited by phentolamine but not by propranolol. 5. Insulin increased (42)K(+) efflux only after preincubation with the cells.

Potassium accumulation and permeation in the canine carotid artery

Potassium accumulation was studied in slices equilibrated in solutions of varying potassium concentration ([K](ex) = 0-20 mM). Steady-state (42)K uptake was also measured under similar conditions. The accumulated potassium characterized by slow exchange kinetics (half time more than 25 min) exhibited saturation behavior at high external concentrations (maximum, 119 meq/kg dry solid), and exhibited cooperative interaction with sodium. Values calculated from an adsorption isotherm based on solute-protein interaction in a fixed charge system were in agreement with the experimental results. Rubidium competitively inhibited the accumulation of potassium. Studies of the (42)K flux indicated that the rate constants for the slow component decreased with increasing [K](ex). At [K](ex) = 3.33 mM a minimum of about 0.88 x 10(-4) sec(-1) was reached. The potassium flux exhibited saturation behavior at high [K](ex) (maximum 10.5 x 10(-3) meq/kg d.s. per sec). A diffusion coefficient of 1.1 x 10(-5) cm(2) sec(-1) adequately characterized the fast exchanging potassium. A portion of this component exhibited saturation behavior (maximum, 11 meq/kg d.s.) and followed the Langmuir adsorption isotherm. The properties exhibited by potassium accumulation and permeation processes were consistent with those of a fixed charge system as formulated in the "association-induction hypothesis." It is suggested that this model provides an analytical basis for future experimentation.

Effects of sodium extrusion and local anaesthetics on muscle membrane resistance and potential

1. Membrane potentials and resistances of K-depleted muscles were measured in the cold and again after warming in K-containing media so that active ion movements occurred.2. On warming there was a fall of resistance and a gradual rise of potential which passed through a maximum. Later measurements of resistance in a chloride medium showed that values were, if anything, higher than initially in the warm.3. The excess potentials measured approximated to those required to induce passive inward movement of the K ions through the measured K resistance.4. Permeabilities for K(+) and Cl(-) were deduced. When cocaine, procaine, amytal or mepyramine were added or when K(+) was replaced by Rb(+) in the Cl(-)-free solution the K(+) permeability was eventually reduced. The same agents led to an enhanced initial response of potential to warming, but later the potentials in Cl(-)-free media fell to less than the K(+) equilibrium values.5. A method for obtaining the resistivity of the membrane from measurements made in conditions of non-linear voltage-current dependence was applied.

POTASSIUM MOVEMENTS IN RAT UTERUS STUDIED IN VITRO: II. EFFECTS OF METABOLIC INHIBITORS, OUABAIN, AND ALTERED POTASSIUM CONCENTRATIONS

1. Sodium fluoride (10−2 M), 2,4-dinitrophenol (10−4 M), and iodoacetate (10−4 M) caused a slight decrease in potassium uptake by the uterus and fluoride and dinitrophenol caused a larger and immediate increase in efflux, resulting in a net loss of potassium. There was apparently a delayed increase in efflux caused by iodoacetate. The effects of inhibitors on efflux were not prevented by the absence of external potassium. The effects of fluoride suggested that it produced inhomogeneity in uterine potassium and analysis of the longitudinal muscle layer separately from the remainder of the uterus suggested that efflux was speeded more in myometrium than in endometrium. This was attributed to the prolonged contracture induced by fluoride. The depolarization required to explain the increases in efflux produced by fluoride and DNP was sufficient to explain the decreases in influx. It was postulated that these inhibitors act by causing depolarization which might be the result of inhibition of an electrogenic sodium pump. Iodoacetate 10−3 M caused a 50% reduction in potassium influx and probably a large immediate increase in efflux, but no evidence was obtained that this concentration caused contraction.2. Ouabain in concentrations as high as 10−5 M had only minor effects on potassium inward and outward movements and on reaccumulation of potassium and extrusion of sodium during recovery from exposure to the cold. The resistance of rat uteri to cardiac glycosides derives either from insensitivity in rat tissues or from a unique feature of sodium transport in the rat uterus.3. When KCl was added to the Ringer fluid, there was no net gain of cellular potassium relative to dry weight. Osmotic balance was achieved mainly by water loss from cells, but uncertainty as to the extracellular fluid volume prevented a definite conclusion. When KCl was omitted from the Ringer fluid, there was a 50% decrease in efflux, suggesting that a part of the potassium movement was "exchange diffusion". The assumption of exchange diffusion also would aid in explaining the observed flux ratio near unity in view of the values reported for membrane potentials of uterine cells. Owing to the lack of data regarding intracellular activity of potassium and the incompleteness of data on membrane potentials of uterine cells, it was not possible to prove whether active influx of potassium was present or absent in addition to that entering passively either by free diffusion or exchang diffusion.

POTASSIUM MOVEMENTS IN RAT UTERUS STUDIED IN VITRO: II. EFFECTS OF METABOLIC INHIBITORS, OUABAIN, AND ALTERED POTASSIUM CONCENTRATIONS

1. Sodium fluoride (10−2 M), 2,4-dinitrophenol (10−4 M), and iodoacetate (10−4 M) caused a slight decrease in potassium uptake by the uterus and fluoride and dinitrophenol caused a larger and immediate increase in efflux, resulting in a net loss of potassium. There was apparently a delayed increase in efflux caused by iodoacetate. The effects of inhibitors on efflux were not prevented by the absence of external potassium. The effects of fluoride suggested that it produced inhomogeneity in uterine potassium and analysis of the longitudinal muscle layer separately from the remainder of the uterus suggested that efflux was speeded more in myometrium than in endometrium. This was attributed to the prolonged contracture induced by fluoride. The depolarization required to explain the increases in efflux produced by fluoride and DNP was sufficient to explain the decreases in influx. It was postulated that these inhibitors act by causing depolarization which might be the result of inhibition of an electrogenic sodium pump. Iodoacetate 10−3 M caused a 50% reduction in potassium influx and probably a large immediate increase in efflux, but no evidence was obtained that this concentration caused contraction.2. Ouabain in concentrations as high as 10−5 M had only minor effects on potassium inward and outward movements and on reaccumulation of potassium and extrusion of sodium during recovery from exposure to the cold. The resistance of rat uteri to cardiac glycosides derives either from insensitivity in rat tissues or from a unique feature of sodium transport in the rat uterus.3. When KCl was added to the Ringer fluid, there was no net gain of cellular potassium relative to dry weight. Osmotic balance was achieved mainly by water loss from cells, but uncertainty as to the extracellular fluid volume prevented a definite conclusion. When KCl was omitted from the Ringer fluid, there was a 50% decrease in efflux, suggesting that a part of the potassium movement was "exchange diffusion". The assumption of exchange diffusion also would aid in explaining the observed flux ratio near unity in view of the values reported for membrane potentials of uterine cells. Owing to the lack of data regarding intracellular activity of potassium and the incompleteness of data on membrane potentials of uterine cells, it was not possible to prove whether active influx of potassium was present or absent in addition to that entering passively either by free diffusion or exchang diffusion.

Movement of Inorganic Ions across the Membrane of Striated Muscle

The unidirectional fluxes of sodium, chloride, and potassium across the membrane of striated muscle cells are described under a number of experimental conditions. The results are compared with the predicted behavior of simple membranes. With the possible exception of chloride, it is concluded that the movements of these ions cannot be described without postulating variable permeabilities or a variable number of `carriers' or `sites' in the membrane.

Quantitative analysis of movements of potassium in rabbit auricles

When rabbit auricles are isolated in physiological saline solution at 29 or 37 °C, the influx or efflux of 42 K follows a course which can be described by the sum of two exponential terms. The presence of an initial fast component is more evident in beating than in spontaneously quiescent auricles, but the fast component is not due to beating because it occurs also in auricles in which beating is stopped by carbachol (2 x 10 -6 M). A small loss of potassium occurs in untreated auricles and a larger one under the influence of certain drugs, such as ouabain (10 -5 M) or dinitrophenol (10 -4 M). Several model systems are considered, especially those with three compartments all or partially communicating with each other, both with regard to exchange of tracer and to net changes in the total ionic content of the compartments. The simplest model which describes the observations had 79 ± 2 (S. E.) % of the tissue potassium free to exchange with the medium and the rest exchanging more slowly with the main fraction. In a steady state at 29 °C, in quiescent left auricles 1⋅07 % of the main fraction exchanged with the medium and 0⋅37 % with the slow fraction per minute. The corresponding rates for beating right auricles were 2⋅25 and 0⋅20 % per minute. In the presence of ouabain (10 -5 M) uptake from the medium to the main fraction was reduced to about 40% of the normal rate, with corresponding net loss of potassium. Dinitrophenol reduced or stopped uptake to the slow fraction and also accelerated loss from the main fraction to the medium.