Factors affecting the contribution of the catecholamine activated electrogenic sodium pump to the membrane potential of rat soleus muscle fibers

Experimental approaches to therapy and prophylaxis for heat stress and heatstroke

New developments in the fields of biochemistry, physiology, sepsis, cancer therapy, and molecular genetics have led to opportunities for the development of new therapies and prophylaxes for heat illnesses and for improving human performance during conditions of environmental stress. These include antilipopolysaccharide agents, anticytokines, potassium channel agents, a diet rich in omega-3 fatty acids, and psychological conditioning. This review summarizes the backgrounds and recent findings in the above fields and provides specific suggestions for potential therapy and prophylaxis for classic and exertional heatstroke and for improving athletic performance.

Role of Na-K ATPase in regulation of resting membrane potential of cultured rat skeletal myotubes

The role of Na-K ATPase in the determination of resting membrane potential (Em) as a function of extracellular K ion concentration was investigated in cultured rat myotubes. The Em of control myotubes at 37 degrees C varied as a function of (K+)0 with a slope of about 58-60 mV per ten-fold change in (K+)0. Inhibition of the Na-K pump with ouabain or by reduced temperature revealed that this relation consists of two components. One, between (K+)0 of 10 and 100 mM, remains unchanged by alterations in enzyme activity; The second, between (K+)0 of 1 and 10 mM, is related to the amount of Na-K pump activity, the slope decreasing as pump activity decreases. Indeed, with complete inhibition of the Na-K pump, Em does not change over the range of (K+)0 1 to 10 mM. Measurements of 86Rb efflux and input resistance of individual myotubes showed that membrane permeability does not change as (K+)0 increases from 1 to 10 mM but increases as (K+)0 increases further. Monensin, which increases Na ion permeability, increases Em at values of external K+ below 10 mM, and is without effect at higher values of K+ concentration. The effect of monensin is blocked by ouabain. Tetrodotoxin, which blocks voltage-dependent Na+ channels, decreases Em at low (2-10 mM) K+. We conclude that changes in Em as a function of extracellular K+ concentration in the physiological range are not adequately explained by the diffusion potential hypothesis of Em, and that other theories (electrogenic pump, surface-absorption) must be considered.

Norepinephrine depolarizes sartorius but hyperpolarizes soleus muscles in vitro

Having previously found that in vivo administration of norepinephrine (NE) depolarizes the membrane of hamster sartorius muscle cells, the present study evaluated NE effects in vitro where alterations in blood flow were eliminated. Muscles were submerged in a temperature-controlled chamber, held at their resting length, and impaled with glass microelectrodes. Norepinephrine addition (maximum concentration of 10-20 microM at the muscle surface) induced significant depolarization in sartorius muscle cells and significant hyperpolarization in soleus muscle cells (P less than 0.05). Ascorbic acid (final concentration 15-25 microM), the vehicle for NE, evoked no significant alteration in resting membrane potentials of either muscle. Isoproterenol, a beta-adrenoceptor agonist, elicited responses similar to those of NE; phenylephrine, an alpha-adrenoceptor agonist, had no significant effect. The data suggest that the NE response is mediated through the beta-adrenergic pathway and that in the hamster, both hyperpolarization and depolarization can be observed under the same experimental conditions, depending on the muscle in question.

Contribution of electrogenic sodium-potassium ATPase to resting membrane potential of cultured rat skeletal myotubes

The contribution of electrogenic Na+ -K+ ATPase to resting membrane potential (Em) of mature and developing rat skeletal myotubes in culture was determined by examining effects of inhibition of this enzyme on Em. Ouabain, a specific Na+-K+ ATPase inhibitor, caused resting Em to decrease within 30 s by 5-8 mV and reach a minimum value of about -60 mV after 5 min. The decrease in Em was not accompanied by a decrease in input resistance for up to 15 min after application. Resting Em was found to be dependent on the temperature of the recording medium with maximum values of Em ranging from -85 to -90 mV at a temperature of 35-37 degrees C and minimum values about -60 mV at 10-15 degrees C. Ouabain (1 mM), added to cultures at low temperature (10-15 degrees C) did not further decrease Em but did prevent the increase in Em that occurs with increasing temperature up to 37 degrees C. Resting Em of cultured myotubes was reduced to about -60 mV by reducing the supply of ATP either with 2,4 dinitrophenol (DNP), which inhibits oxidative phosphorylation or with fluorodinitrobenzene (FDNB), which inhibits creatine phosphokinase. Neither of these compounds, when added to cultures in the presence of ouabain, reduced resting Em to a value lower than that obtained with ouabain alone.(ABSTRACT TRUNCATED AT 250 WORDS)

On the mechanism of catecholamine-induced hyperpolarization of skeletal muscle cells

Catecholamines (noradrenaline, adrenaline and isoprenaline) were tested for their effect on the resting membrane potential of mouse skeletal muscle cells. In freshly isolated muscles incubated in the normal solution containing 5 mol . l-1, catecholamines increased the resting membrane potential (RMP) by 3-5 mV. In Na+-loaded muscles incubated in a K+-free solution, however, catecholamines increased the RMP by 13-16 mV; consequent application of K+ to these muscles did not hyperpolarize the membrane further. A significant decrease of input membrane resistance was observed during the noradrenaline-induced hyperpolarization. This indicates that the passive membrane permeability for K+ ions was apparently increased. Noradrenaline-induced hyperpolarization requires the presence of calcium ions in the incubation solution. We therefore assume that catecholamines hyperpolarize the membrane by Ca2+-dependent K+-channels activation. The action of catecholamines on the resting membrane potential of skeletal muscle exhibits a 50% nonspecific effect as far as the adrenergic receptor is concerned, and the rest may be blocked by adrenergic blocking agents.

Serotonin-activated adenylate cyclase and the possible role of cyclic AMP in modulation of buccal muscle contraction in Aplysia

The possible role of cyclic AMP in mediating opposite modulatory effects of serotonin (5-HT) on Aplysia buccal mass muscles E1 and E2 was examined. Serotonin enhances E1 contractions and inhibits E2 contractions. Adenylate cyclase in membranes of both E1 and E2 is stimulated approximately 180% by 10(-6) M 5-HT and 300% by 10(-3) M 5-HT. Dibutyryl cyclic AMP and 8-benzylthio cyclic AMP mimicked the effect of 5-HT on E1 but had no effect on E2. Theophylline (Th) and isobutylmethylxanthine (IBMX) mimicked the effect of 5-HT on E1 at high concentrations. Concentrations of Th and IBMX low enough not to have any direct effect on contraction increased both the magnitude and duration of the effect of 5-HT on E1 contraction. Neither Th nor IBMX had a direct effect on E2 contraction, although Th produced a small increase in the effect of 5-HT on E2. These data are consistent with the hypothesis that cyclic AMP mediates the enhancement effect of 5-HT on E1 contraction. Other mechanisms probably mediate the effect of 5-HT on E2 contraction.

The effect of alpha-, beta adrenergic receptor agonists and antagonists on the efflux of 22Na and uptake of 42K by rat brain cortical slices

The effects of norepinephrine on ion fluxes in rat brain cortical slices have now been ascertained. 22Na efflux and 42K influx are enhanced by norepinephrine. The increase in ion fluxes can be blocked by ouabain, phentolamine and propranolol, suggesting that the catecholamine activates a membrane sodium pump by a receptor-mediated step. The facilitation of 22Na efflux is stereospecific as demonstrated by the very weak action of D-norepinephrine at 10(-5) M concentration. Various alpha-adrenergic and beta-adrenergic receptor agonists, including oxymetazoline, naphazoline, clonidine, tramazoline, methoxamine, phenylephrine, L-isoproterenol and methoxyphenamine are potent stimulants of the sodium pump as demonstrated by their enhancement of ion fluxes in rat brain cortical slices. Our results are consistent with the hypothesis that norepinephrine hyperpolarizes central neurons by activating an ouabain-sensitive, receptor-mediated sodium pump.