Contribution of an electrogenic sodium pump to membrane potential in mammalian skeletal muscle fibres

Effects of TND1128 (a 5-deazaflavin derivative), with self-redox ability, as a mitochondria activator on the mouse brain slice and its comparison with β-NMN

We have no definitive treatment for dementia characterized by prolonged neuronal death due to the enormous accumulation of foreign matter, such as β-amyloid. Since Alzheimer's type dementia develops slowly, we may be able to delay the onset and improve neuronal dysfunction by enhancing the energy metabolism of individual neurons. TND1128, a derivative of 5-deazaflavin, is a chemical known to have an efficient self-redox ability. We expected TND1128 as an activator for mitochondrial energy synthesis. We used brain slices prepared from mice 22 ± 2 h pretreated with TND1128 or β-NMN. We measured Ca²⁺ concentrations in the cytoplasm ([Ca²⁺]_cyt) and mitochondria ([Ca²⁺]_mit) by using fluorescence Ca²⁺ indicators, Fura-4F, and X-Rhod-1, respectively, and examined the protective effects of drugs on [Ca²⁺]_cyt and [Ca²⁺]_mit overloading by repeating 80K exposure. TND1128 (0.01, 0.1, and 1 mg/kg s.c.) mitigates the dynamics of both [Ca²⁺]_cyt and [Ca²⁺]_mit in a dose-dependent manner. β-NMN (10, 30, and 100 mg/kg s.c.) also showed significant dose-dependent mitigating effects on [Ca²⁺]_cyt, but the effect on the [Ca²⁺]_mit dynamics was insignificant. We confirmed the mitochondria-activating potential of TND1128 in the present study. We expect TND1128 as a drug that rescues deteriorating neurons with aging or disease.

Regulation of muscle potassium: exercise performance, fatigue and health implications

This review integrates from the single muscle fibre to exercising human the current understanding of the role of skeletal muscle for whole-body potassium (K⁺) regulation, and specifically the regulation of skeletal muscle [K⁺]. We describe the K⁺ transport proteins in skeletal muscle and how they contribute to, or modulate, K⁺ disturbances during exercise. Muscle and plasma K⁺ balance are markedly altered during and after high-intensity dynamic exercise (including sports), static contractions and ischaemia, which have implications for skeletal and cardiac muscle contractile performance. Moderate elevations of plasma and interstitial [K⁺] during exercise have beneficial effects on multiple physiological systems. Severe reductions of the trans-sarcolemmal K⁺ gradient likely contributes to muscle and whole-body fatigue, i.e. impaired exercise performance. Chronic or acute changes of arterial plasma [K⁺] (hyperkalaemia or hypokalaemia) have dangerous health implications for cardiac function. The current mechanisms to explain how raised extracellular [K⁺] impairs cardiac and skeletal muscle function are discussed, along with the latest cell physiology research explaining how calcium, β-adrenergic agonists, insulin or glucose act as clinical treatments for hyperkalaemia to protect the heart and skeletal muscle in vivo. Finally, whether these agents can also modulate K⁺-induced muscle fatigue are evaluated.

Modifications of excitatory and inhibitory transmission in rat hippocampal pyramidal neurons by acute lithium treatment

The acute effects of high-dose Li(+) treatment on glutamatergic and GABAergic transmissions were studied in the "synaptic bouton" preparation of isolated rat hippocampal pyramidal neurons by using focal electrical stimulation. Both action potential-dependent glutamatergic excitatory and GABAergic inhibitory postsynaptic currents (eEPSC and eIPSC, respectively) were dose-dependently inhibited in the external media containing 30-150 mM Li(+), but the sensitivity for Li(+) was greater tendency for eEPSCs than for eIPSCs. When the effects of Li(+) on glutamate or GABAA receptor-mediated whole-cell responses (IGlu and IGABA) elicited by an exogenous application of glutamate or GABA were examined in the postsynaptic soma membrane of CA3 neurons, Li(+) slightly inhibited both IGlu and IGABA at the 150 mM Li(+) concentration. Present results suggest that acute treatment with high concentrations of Li(+) acts preferentially on presynaptic terminals, and that the Li(+)-induced inhibition may be greater for excitatory than for inhibitory transmission.

Effects of digoxin on muscle reflexes in normal humans

Blockade of the skeletal muscle Na(+)-K(+)-ATPase pump by digoxin could result in a more marked hyperkaliema during a forearm exercise, which in turn could stimulate the mechano- and metaboreceptors. In a randomized, double-blinded, placebo-controlled, and cross-over-design study, we measured mean blood pressure (MBP), heart rate (HR), ventilation (V(E)), oxygen saturation (SpO(2)), muscle sympathetic nerve activity (MSNA), venous plasma potassium and lactic acid during dynamic handgrip exercises, and local circulatory arrest in 11 healthy subjects. Digoxin enhanced MBP during exercise but not during the post-handgrip ischemia and had no effect on HR, V(E), SpO(2), and MSNA. Venous plasma potassium and lactic acid were also not affected by digoxin-induced skeletal muscle Na(+)-K(+)-ATPase blockade. We conclude that digoxin increased MBP during dynamic exercise in healthy humans, independently of changes in potassium and lactic acid. A modest direct sensitization of the muscle mechanoreceptors is unlikely and other mechanisms, independent of muscle reflexes and related to the inotropic effects of digoxin, might be implicated.

The Na(+)-K(+)-ATPase alpha2-subunit isoform modulates contractility in the perinatal mouse diaphragm

This study uses genetically altered mice to examine the contribution of the Na(+)-K(+)-ATPase alpha2 catalytic subunit to resting potential, excitability, and contractility of the perinatal diaphragm. The alpha2 protein is reduced by 38% in alpha2-heterozygous and absent in alpha2-knockout mice, and alpha1-isoform is upregulated 1.9-fold in alpha2-knockout. Resting potentials are depolarized by 0.8-4.0 mV in heterozygous and knockout mice. Action potential threshold, overshoot, and duration are normal. Spontaneous firing, a developmental function, is impaired in knockout diaphragm, but this does not compromise its ability to fire evoked action potential trains, the dominant mode of activation near birth. Maximum tetanic force, rate of activation, force-frequency and force-voltage relationships, and onset and magnitude of fatigue are not changed. The major phenotypic consequence of reduced alpha2 content is that relaxation from contraction is 1.7-fold faster. This finding reveals a distinct cellular role of the alpha2-isoform at a step after membrane excitation, which cannot be restored simply by increasing alpha1 content. Na+/Ca2+ exchanger expression decreases in parallel with alpha2-isoform, suggesting that Ca2+ extrusion is affected by the altered alpha2 genotype. There are no major compensatory changes in expression of sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, or plasma membrane Ca(2+)-ATPase. These results demonstrate that the Na(+)-K(+)-ATPase alpha1-isoform alone is able to maintain equilibrium K+ and Na+ gradients and to substitute for alpha2-isoform in most cellular functions related to excitability and force. They further indicate that the alpha2-isoform contributes significantly less at rest than expected from its proportional content but can modulate contractility during muscle contraction.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Membrane excitability and calcium homeostasis in exercising skeletal muscle

Preservation of the membrane electrochemical gradients for Na, K, and Ca is vital to the maintenance of skeletal muscle structure and function. Muscle excitability may be depressed during contractile activity by changes in the gradients for Na and K, while muscle force may be reduced by an activity-induced increase in free intracellular Ca. Compensatory processes help to maintain ion electrochemical gradients in normal, active muscles, but compensatory mechanisms may be impaired in injured or diseased muscles, contributing to muscle pathology.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Stretch-activated ion channels contribute to membrane depolarization after eccentric contractions

We tested the hypothesis that eccentric contractions activate mechanosensitive or stretch-activated ion channels (SAC) in skeletal muscles, producing increased cation conductance. Resting membrane potentials and contractile function were measured in rat tibialis anterior muscles after single or multiple exposures to a series of eccentric contractions. Each exposure produced a significant and prolonged (>24 h) membrane depolarization in exercised muscle fibers. The magnitude and duration of the depolarization were related to the number of contractions. Membrane depolarization was due primarily to an increase in Na(+) influx, because the estimated Na(+)-to-K(+) permeability ratio was increased in exercised muscles and resting membrane potentials could be partially repolarized by substituting an impermeant cation for extracellular Na(+) concentration. Neither the Na(+)/H(+) antiport inhibitor amiloride nor the fast Na(+) channel blocker TTX had a significant effect on the depolarization. In contrast, addition of either of two nonselective SAC inhibitors, streptomycin or Gd(3+), produced significant membrane repolarization. The results suggest that muscle fibers experience prolonged depolarization after eccentric contractions due, principally, to the activation of Na(+)-selective SAC.

Temporal responses of oxidative vs. glycolytic skeletal muscles to K+ deprivation: Na+ pumps and cell cations

When K+ output exceeds input, skeletal muscle releases intracellular fluid K+ to buffer the fall in extracellular fluid (ECF) K+. To investigate the mechanisms and muscle specificity of the K+ shift, rats were fed K+-deficient chow for 2-10 days, and two muscles at phenotypic extremes were studied: slow-twitch oxidative soleus and fast-twitch glycolytic white gastrocnemius (WG). After 2 days of low-K+ chow, plasma K+ concentration ([K+]) fell from 4.6 to 3.7 mM, and Na+-K+-ATPase alpha2 (not alpha1) protein levels in both muscles, measured by immunoblotting, decreased 36%. Cell [K+] decreased from 116 to 106 mM in soleus and insignificantly in WG, indicating that alpha2 can decrease before cell [K+]. After 5 days, there were further decreases in alpha2 (70%) and beta2 (22%) in WG, not in soleus, whereas cell [K+] decreased and cell [Na+] increased by 10 mM in both muscles. By 10 days, plasma [K+] fell to 2.9 mM, with further decreases in WG alpha2 (94%) and beta2 (70%); cell [K+] fell 19 mM in soleus and 24 mM in WG compared with the control, and cell [Na+] increased 9 mM in soleus and 15 mM in WG; total homogenate Na+-K+-ATPase activity decreased 19% in WG and insignificantly in soleus. Levels of alpha2, beta1, and beta2 mRNA were unchanged over 10 days. The ratios of alpha2 to alpha1 protein levels in both control muscles were found to be nearly 1 by using the relative changes in alpha-isoforms vs. beta1- (soleus) or beta2-isoforms (WG). We conclude that the patterns of regulation of Na+ pump isoforms in oxidative and glycolytic muscles during K+ deprivation mediated by posttranscriptional regulation of alpha2beta1 and alpha2beta2 are distinct and that decreases in alpha2-isoform pools can occur early enough in both muscles to account for the shift of K+ to the ECF.

Characterization of the electrogenic Na+-K+ pump in horizontal cells isolated from the carp retina

The electrogenic Na+-K+ pump current in horizontal cells acutely dissociated from the carp retina was investigated using a nystatin-perforated patch recording configuration under voltage-clamp conditions. In the presence of suitable blockers for known voltage-dependent Na+, K+ and Ca2+ conductances, the pump current was activated in a concentration-dependent manner by adding K+ ions to external solution. The EC50 value and Hill coefficient for the external K+ concentration were 0.66 mM and 1.39, respectively. The pump current did not show any significant voltage dependency at the physiological potential range between -90 and 20 mV either with or without external Na+ ions. In the presence of 120 mM external Na+ concentration, the addition of 3 mM K+ to the external solution induced a steady outward pump current even when the patch-pipette (internal) solution did not contain Na+. A large outward shift of the holding current was observed by removing external Na+. The result thus suggests that continuous Na+ influxes exist across the plasma membrane in the presence of external Na+. When Na+ was removed from both external and internal solutions, a transient outward pump current was observed by adding K+ to the external solution, thus indicating that the transient pump current was activated by the residual intracellular Na+ ions. The pump current was suppressed by ouabain in a concentration-dependent manner, and the ouabain-sensitive inhibition curve was fitted by two components. The IC50 values of high- and low-sensitive pump currents for ouabain were 20 nM and 10.4 microM, respectively, indicating the existence of at least two isoforms of the pump in the horizontal cells.

Characterization of electrogenic Na/K pump in rat neostriatal neurons

The electrogenic Na/K pump current (Ip) was studied in the dissociated neostriatal neurons of the rat by using the nystatin-perforated patch recording mode. The Ip was activated by external K+ in a concentration-dependent manner with an EC50 of 0.7 mM at a holding potential (VH) of -40 mV. Other monovalent cations also caused Ip and the order of potency was Tl+>K+, Rb+>NH4+, Cs+>>>Li+. The Ip decreased with membrane hyperpolarization in an external solution containing 150 mM Na+, while the Ip did not show such voltage dependency without external Na+. Ouabain showed a steady-state inhibition of Ip in a concentration- and temperature-dependent manner at a VH of -40 mV. The IC50 values at 20 and 30 degrees C were 7.1 x 10(-6) and 1.3 x 10(-6) M, respectively. The decay of Ip after adding ouabain well fitted with a single exponential function. At a VH of -40 Mv, the association (k+1) and dissociation (k-1) rate constants estimated from the time constant of the current decay at 20 degrees C were 4.0 x10(2) s-1 M-1 and 6.3 x 10(-3) s-1, respectively. At 30 degrees C, k+1 increased to 2.8 x 10(3) s-1 M-1 while k-1 showed no such change with a value of 1.8 x 10(-3) s-1. A continuous Na+ influx was demonstrated by both the Na+-dependent leakage current and tetrodotoxin-sensitive Na+ current, which resulted in the continuous activation of the Na/K pump. It was thus concluded that the Na/K pump activity was well-maintained in the dissociated rat neostriatal neurons with distinct functional properties and that the activity of the pump was tightly connected with Na+ influxes.

Characterization of the electrogenic Na+ -K+ pump in bipolar cells isolated from carp retina

The electrogenic Na+ -K+ pump current (Ip) in carp bipolar cells was investigated under voltage-clamp conditions. The Ip was activated in a concentration-dependent manner by adding external K+ (Ko+) and was completely suppressed with 10(-4) M ouabain (EC50=1.23 mM; Hill coefficient=1.36). The Ip was suppressed in a concentration-dependent manner by ouabain (IC50=1.90 mM; Hill coefficient=0.93). The Ip did not show a distinct voltage dependency either with or without Na(o)+. A large outward shift of the holding current was observed by completely removing Na(o)+. In the presence of Na(o)+, a steady Ip was observed even in the absence of internal Na+ (Na(i)+). These results suggest that continuous Na+ influxes exist across the membrane. When external and internal Na+ was removed, a transient Ip was observed (half decay time (t1/2) was 5.0+/-0.6 s), thus indicating that the transient Ip was activated by the residual Na(i)+. In the absence of Na(o)+, the transient Ip was also observed with lower than 8 mM Na(i)+. The t1/2 depended on Na(i)+. However, a steady Ip was observed with 10 mM Na(i)+ or more. The functional properties of the Ip are discussed.

Role of ouabain-like compound in the regulation of transmembrane sodium and potassium gradients in rats

A major biologically active Na,K-ATPase inhibitor in the mammalian body may be ouabain-like compound. We investigated the potential roles of circulating ouabain-like compound in the regulation of Na+ and K+ homeostasis in terms of Na+ and K+ distribution between the cells and the extracellular fluid (internal balance). First, we developed a population of rats immunized against ouabain to block the action of ouabain-like compound. We measured plasma and intracellular Na+ and K+ concentrations in skeletal muscle and determined Na+ (extracellular-to-intracellular concentration ratio) and K+ (intracellular-to-extracellular concentration ratio) gradients in immune rats. We examined also the ability to respond to hypertonic NaCl load in immune rats. Consistent lower plasma K+ levels and steeper Na+ and K+ gradients were observed in immune rats. K+ handling in response to hypertonic NaCl load was altered, and lower plasma K+ level was maintained in immune rats. Second, we used PST-2238, a newly developed anti-ouabain agent, to block the action of ouabain-like compound and examined its effect on plasma Na+ and K+ concentrations. Chronic administration of PST-2238 significantly lowered plasma K+ levels in rats with subtotal nephrectomy. These findings collectively suggest that ouabain-like compound may determine at least in part the internal Na+ and K+ distribution and the transmembrane cation gradients in vivo in rats.

Changes of intracellular electrolyte contents in rat skeletal muscle during body suspension

The CNS-mediated inhibition of active Na(+)-K+ transport in both "type S" muscle, soleus (SOL), and "type FF" muscle, extensor digitorum longus (EDL) was investigated in rats suspended horizontally. Plasma Na+ and K+ contents did not change during the suspension period. The relative wet weight of SOL decreased more than that of EDL by suspension. There was significant intracellular Na+ accumulation and K+ loss in both SOL and diaphragm of suspended rats. However, cerebrum, cerebellum, medulla oblongate, ventricle, liver, pancreas, kidney, intestine, aorta and EDL were spared from the intracellular Na+ accumulation and K+ loss. Sciatic nerve sectioning or cervical spinal cord transection recovered the Na+ and K+ contents in the SOL of suspended rats. The results indicate the existence of neural inhibition of the active Na(+)-K+ transport in skeletal muscle of the suspended rats.

Increased Na(+)-K+ pump number and decreased pump activity in soleus muscles in SHR

We have previously demonstrated electrophysiological and contractile abnormalities in soleus muscles of the spontaneously hypertensive rat (SHR). The age-related decrease in force and fatigue resistance observed in SHR muscles may be produced by alterations in sarcolemmal ion conductance and/or Na+ pump function. The experiments reported in the present paper were designed to assess the functional capacity of the Na+ pump in 6- to 8- and 24- to 28-wk-old SHR and Wistar-Kyoto (WKY) soleus muscles and to correlate pump activity with Na+ pump number and binding affinity ([3H]ouabain binding). Functional capacity was determined by measuring the change in resting membrane potential (RMP) of soleus muscle fibers in response to agents that stimulate (epinephrine and insulin) or inhibit (ouabain) the pump and by measuring maximum ouabain-suppressible 86Rb+ uptake in Na(+)-loaded muscles. Na+ pump number and affinity were quantified by determining the specific binding of [3H]ouabain in soleus muscle slices. SHR soleus muscles contain a greater number of Na+ pump sites (ouabain binding sites) than are present in age-matched WKY muscles but also experience a significant decrease in pump activity with age. SHR may upregulate pump number in response to the significantly higher intracellular Na+ concentration found in soleus muscles at all the ages examined. The apparent reduction in pump capacity with age may play a major role in the observed age-related decrease in SHR soleus force and fatigue resistance.

GABAergic synaptic current in dissociated nucleus basalis of Meynert neurons of the rat

gamma-Aminobutyric acid (GABA)-mediated spontaneous inhibitory postsynaptic currents (IPSCs) were recorded from dissociated rat nucleus basalis of Meynert neurons which still had their synaptic boutons attached. The membrane currents were recorded by the whole-cell patch-clamp technique. Elevated extracellular K+ concentration and the addition of the calcium ionophore, A23187, enhanced the amplitude and frequency of spontaneous IPSCs. Ryanodine and Ca(2+)-free external solution containing EGTA or BAPTA markedly decreased the spontaneous IPSC activities. Spontaneous IPSC activities were reversibly reduced by baclofen and increased by phaclofen, indicating that the GABAB receptor regulates the release of GABA from nerve terminals and acts as a negative autoreceptor.

Apparent upregulation of Na+,K+ pump sites in SHR skeletal muscle with reduced transport capacity

Slow-twitch, oxidative skeletal muscles in SHR exhibit several physiological defects, including a reduced ability to maintain force during high frequency repetitive stimulation (1). Muscle fatigue may be produced by one of a variety of factors acting at different levels of the neuromuscular system. Several lines of evidence, however, suggest that SHR soleus fatigues more rapidly than WKY soleus because SHR muscles allow more K+ to accumulate in the extracellular space during repetitive muscle activity. An increase in extracellular K+ can lead to a failure in the generation or conduction of muscle action potentials. Comparison of the compound action potentials recorded from SHR and WKY muscles during repetitive stimulation provided evidence for a decrease in excitability of SHR soleus. Since the K+ released from muscle fibers during exercise is returned to the fiber principally via the activity of the Na+, K+ pump, the increase in extracellular K+ in SHR muscle may reflect a decrease in pump capacity. Measurements including intracellular K+ and Na+ content at rest, the level of hyperpolarization produced by the addition of epinephrine and insulin to SHR soleus and the post-exercise recovery of resting membrane potentials all appear to indicate that Na+, K+ pump capacity is reduced in SHR soleus muscles. Nonetheless, ouabain binding studies show a significantly greater number of pump sites in SHR muscles. The data suggest that Na+ pump activity is decreased in SHR soleus muscles without an apparent reduction in either the number of pump sites or in pump binding affinity.

Membrane potential of rat calvaria bone cells: dependence on temperature

The membrane potentials of bone cells derived from calvaria of new born rats was shown to be strongly dependent on temperature. When we lowered the temperature from 36 degrees C to 26 degrees C, cells with spontaneous resting membrane potentials (MP) of -80 to -50 mV depolarized (mean amplitude 8 mV; n = 33), and the membrane resistance increased by approximately 80% (n = 20). The temperature response depended on the actual MP, the reversal potential being in the range of -80 to -90 mV. With the application of ouabain (0.1-1 mmol/liter; n = 12), cells depolarized. Simultaneously, the reversal potential of the temperature response was shifted towards more positive values and approached the actual MP level of the cells. Consequently, the depolarization amplitudes induced by lowering temperature were reduced at spontaneous MP levels. The rise of the membrane resistance during cooling was unaffected. When the extracellular chloride concentration was reduced from 133 to 9 mmol/liter, temperature-dependent depolarizations persisted at spontaneous MP values (n = 5). The findings indicate that the marked effects of temperature changes on the MP of bone-derived cells are mainly determined by changes of the potassium conductance.

Regulation of the sodium-potassium pump in cultured rat skeletal myotubes by intracellular sodium ions

The properties of the Na-K pump and some of the factors controlling its amount and function were studied in rat myotubes in culture. The number of Na-K pump sites was quantified by measuring the amount of [3H]ouabain bound to whole-cell preparations. Activity of the pump was determined by measurement of ouabain-sensitive 86Rb-uptake and component of membrane potential. Chronic treatment of myotubes with tetrodotoxin (TTX), which lowers [Na]i, decreased the number of Na-K pumps, the ouabain-sensitive 86Rb uptake, and the size of the electrogenic pump component of Em. In contrast, chronic treatment with either ouabain or veratridine, which increases [Na+]i, resulted in an elevated level of Na-K pump sites. This effect was blocked by inhibitors of protein synthesis. Neither rates of degradation nor affinity of pump sites in cells treated with TTX, veratridine, or ouabain differred from those in control cells. The number and activity of Na-K pump sites were unaffected by chronic elevation in [Ca]i or chronic depolarization. We conclude that alterations in the level in intracellular Na ions play the major role in regulation of Na-K pump synthesis in cultured mammalian skeletal muscle.

Change of intracellular K+ activity in rat soleus muscle during hypokalemia

The relationship between intracellular total K+ concentration [( K]i) as determined by a flame spectrophotometer and intracellular K+ activity (aKi) as determined by an ion-selective microelectrode was studied in soleus muscle of rats on a diet deficient in K+ for 40 days. [K]i began to fall immediately from the initial stage of hypokalemia, while aKi was well-maintained for 15 days. Then, aKi decreased gradually. The measured resting potential (Em) hyperpolarized beyond the EK was calculated from aKi in hypokalemic rat muscle from day 20 to 40. A rapid increase in aKi occurred over 3 hours in soleus muscle of hypokalemic rats for 5 to 6 weeks. It was concluded that the bound intracellular K+ acts as a buffer for aKi in hypokalemic rat muscle, that Em exceeds EK because the Na+-K+ pump is stimulated by increased [Na]i and that the increase in aKi after denervation is due to the removal of a Na+-K+ pump inhibitor normally released from nerve ending.

Inhibition of the electrogenic Na,K pump and Na,K-ATPase activity by tetraethylammonium, tetrabutylammonium, and apamin

The K+-induced hyperpolarization of Na-loaded mouse diaphragm muscle, enzymatic activity of Na,K-ATPase and 3H-ouabain binding to rat brain microsomes was measured in the presence of K+ channel blockers tetraethylammonium (TEA), tetrabutylammonium (TBA) and apamin. TBA, and to a lesser extent TEA in millimolar concentrations, inhibited the electrogenic effect of the Na,K pump, Na,K-ATPase activity, and 3H-ouabain binding. The inhibition of 3H-ouabain binding by TEA or TBA was more evident in the presence of ATP and Na+ ions. Apamin in nanomolar concentrations inhibited the electrogenic effect of Na,K pump and Na,K-ATPase but not the 3H-ouabain binding. The hyperpolarizing effects of insulin and NADH, but not that of noradrenaline, were also prevented by apamin. The inhibition of Na,K pump by TEA and TBA is apparently due to both competition with K+ for a binding site on the Na,K-ATPase and a reduction in the number of transporting sites. The site of action of apamin on Na,K-ATPase is different from that of tetra-alkylammonium compounds; it apparently decreases the turnover rate of the enzyme.

Role of ion conductance changes and of the sodium-pump in adrenaline-induced hyperpolarization of rat diaphragm muscle fibres

The ionic mechanism of membrane hyperpolarization induced by adrenaline in rat diaphragm muscle fibres was studied. Removal of the extracellular K+ ([K+]o) from Krebs-Ringer solution initially increased the resting membrane potential and then caused an increase in the intracellular Na+ activity ([Na+]i) and a decrease in the intracellular K+ activity ([K+]i). All the changes were maintained for more than 3 h. Application of ouabain (0.1 mM) or lowering the temperature rapidly reduced the resting potential by about 10 mV in the K+-free solution. It then produced further progressive decreases in resting potential and in [K+]i and a progressive increase in [Na+]i. These observations indicate that an electrogenic Na-pump operates in the K+-free solution. Removal of most of the Cl- in the K+-free solution did not affect the resting potential or the magnitude of the initial decrease produced by ouabain, despite an increased input resistance; this result implies a passive distribution of Cl-. Adrenaline (30-60 microM) either added to the bathing solution or applied to the membrane by ionophoresis produced a hyperpolarization (3-10 mV: adrenaline hyperpolarization), the amplitude of which was decreased with a rise in [K+]o and increased with a reduction in [K+]o, but unaffected by the removal of Cl-. Adrenaline produced an increase in input resistance, the relative magnitude (17-18%) of which was constant whether external K+ or Cl- was removed. In contrast, a conditioning membrane hyperpolarization hardly affected the resistance. Ouabain (0.1 mM) or low temperature (8-10 degrees C) abolished both the hyperpolarization and the increased input resistance induced by adrenaline. The [K+]i, [Na+]i and the peak of the action potential remained unchanged after a 20 min exposure to adrenaline (30 microM). The hyperpolarization induced by the replacement of all Na+ with Tris (Tris-hyperpolarization) in the K+-free solution was depressed by 39% during the early period (4-31 min) of exposure to adrenaline (30 microM), while it was enhanced by 26% during the later period (80-130 min). The initial depression suggested a decrease in the ratio of the membrane permeability for Na+ (PNa) to that for K+ (PK). These results suggest that the adrenaline hyperpolarization is generated largely by a decrease in PNa/PK, which is associated with the activity of the Na-pump.

Active sodium-potassium transports in skeletal muscles of deoxycorticosterone hypertensive rats

CNS-induced suppressions of active Na+, K+ transport was investigated in both 'tonic' muscles, soleus (SOL), and 'twitch' muscle, extensor digitorum longus (EDL) of deoxycorticosterone acetate (DOCA) hypertensive rats. There was a marked K+ loss and Na+ accumulation in the skeletal and smooth muscles of DOCA hypertensive rats. The cellular K+ loss was in the order of SOL greater than EDL greater than diaphragm greater than intestine greater than aorta. However, liver, kidney and CNS organs such as cerebrum, cerebellum and medulla oblongata were spared from this K+ fall. Sciatic nerve sectioning or cervical transection activated the active Na+, K+ transport in SOL during DOCA hypertension but inhibited further the pump activity in EDL. The application of tetrodotoxin on the sciatic nerve also activated the Na+, K+ transport in SOL but inhibited the transport in EDL. The facilitatory effect of denervation on the pump activity in SOL was abolished by pretreatment with ouabain. Injection of curare had no effect on Na+ and K+ contents in both SOL and EDL. These results indicate that the CNS is involved differently on the neural regulations of the active Na+, K+ transport systems in SOL and EDL of DOCA hypertensive rats.

Dependence of the electrogenic pump current of Xenopus oocytes on external potassium

The membrane potential of Xenopus oocytes showed a variable response to an increase of the K+ concentration in the bathing solution, [K+]e, from 2.5 mM to 20 mM. In 54% of the cases (n = 52) the cells hyperpolarized (by max. 70 mV). In the presence of 10(-5) M ouabain, all cells depolarized suggesting that the hyperpolarization was caused by an electrogenic Na+/K+ pump. In cells stored overnight in a Na+-free solution the transition from 2.5 to 20 mM [K+]e always caused depolarization indicating that the stimulation of the pump requires high internal sodium, [Na+]i. Cells stored overnight in a Na+-rich solution had a [Na+]i of 30.7 +/- 7 mM, i.e. the Na+/K+ pump was saturated with sodium (Lafaire and Schwarz 1986). With 9 such cells we determined the K+ activation of the Na+/K+ pump. The activation follows Hill kinetics with Imax = 90.5 nA, Ks = 2.3 mM, and n = 1.68.

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.

Comparison of red and white muscle wet weight changed by age, denervation and morbid states

[Na]i, [K]i and wet weight of the extensor digitrum longus (EDL) and soleus (SOL) muscles of 9- and 52-week-old rats were measured for 7 days after sectioning of the sciatic nerve. The changes in wet weight of the EDL and SOL muscles of rats over 52 weeks and those of morbid state rats were also measured. There was no significant difference in wet weights between the EDL and SOL muscles in infant rats, but the EDL muscle became much heavier than the SOL muscle with aging. The decrease in rate of growth of wet weight of the EDL and SOL muscles caused by denervation, was greater in young rats than in mature rats. In addition, the rate of decrease was greater in the SOL muscles than in the EDL muscles in both young and mature rats. The [Na]i increased while [K]i was decreased by denervation, and the net Na+ increase and the net K+ loss were greater in young rats than in mature rats. The changing rate was more remarkable in the EDL muscles than in the SOL muscles throughout the aging process. During DOCA treatment over 4 weeks, the decrease of muscle wet weight was greater in the EDL muscles. The mechanisms which serve to maintain normal muscle wet weight in the SOL muscle after denervation or treatment with DOCA, were discussed.

Hypothalamic regulation of sodium-potassium pump activity in skeletal muscle

The suppression of active Na+-K+ transport in rat skeletal muscle during hypokalemia was counteracted by bilateral electrolytic lesion of the ventromedial hypothalamic nucleus. This reversal effect was unaffected even after pancreatectomy or adrenalectomy. The anomalous electrolyte content in hypokalemic rat muscles was aggravated by lesion of the dorsomedial hypothalamic nucleus and of the anterior hypothalamus. The results indicate that the hypothalamus is involved in the regulation of the Na+-K+ transport system in skeletal muscle during hypokalemia.

Ionic channels: I. The biophysical basis for ion passage and channel gating

This article reviews the biophysics of ion passage through membrane pores, as well as the physical factors that control the ion selectivity, gating, and conductance of an ionic channel. Different voltage clamp techniques are discussed in detail. The biophysical properties of sodium channels are reviewed.

Hypokalemia modulates alpha- and beta-adrenoceptor bindings in rat skeletal muscle

Changes in the population of adrenergic alpha- and beta-receptors were examined in rat soleus muscles during hypokalemia by their direct determination using radiolabeled ligands. Only beta-adrenoceptors were detected in the normal rat muscles. Hypokalemia led to a pronounced decrease in beta-adrenoceptors, the number of [3H]DHA binding sites, by 50%, as compared with that in the normal rats. There was a genesis of alpha 1-adrenoceptors in hypokalemic rat muscles, since the competitive potency of adrenergic drugs against [3H]prazosin binding was in the order prazosin much greater than phentolamine greater than (+/-)-noradrenaline greater than yohimbine much greater than (+/-)-isoproterenol. The reduction of [3H]DHA binding sites was accompanied by an increase of an approximately equal amount in high-affinity [3H]prazosin binding sites. The Kd determined by kinetic analysis of [3H]prazosin binding was calculated from the ratio K-1/K1 that gave a value of 3.05 nM, which generally agreed with the 1.83 nM determined by saturation experiments (Scatchard plot). This phenomenon of a reduction in the beta-adrenoceptors and the occurrence of alpha 1-adrenoceptors in muscles during hypokalemia is discussed. alpha- and beta-adrenoceptors on soleus muscle membrane may play important but opposite roles in modulating potassium release from the muscle cells.

The electrogenic Na-pump and spontaneous contraction of the hypokalemic rat duodenum

The effects of the electrogenic Na-pump on spontaneous contraction in the isolated, longitudinal muscle of the duodenum of rats which had been on a potassium-deficient diet for 7 weeks, have been investigated. Intracellular levels of Na+ are increased by this diet. The spontaneous contraction of the duodenal muscle was stopped, transiently, by 0.5 to 120 mM-K+ Krebs solution. The period of decrease of tone and amplitude occurring immediately after adding K+ was shortened when the external K+ concentration ([K]o) was increased from 0.5 to 120 mM. The decrease in tone and amplitude induced by K+ was abolished by exposure of the tissue to 0 mM [K]o, by exposure to a temperature below 14 degrees C, and in the presence of ouabain (3 X 10(-5)-10(-4) M). The spontaneous contraction of 'Na-rich' duodenum in bathing medium containing 15 mM K+ and following inhibition of the electrogenic Na-pump with cooling or ouabain was much the same as in the duodenum from rats fed balanced diets: i.e., increase of contractile tone immediately after adding K+. To activate the Na-pump in 'Na-rich' duodenum, the external K+ could be replaced by Rb+, Cs+, NH4+ and Tl3+. The effectiveness was in the order K+ greater than Rb+ greater than Cs+ greater than NH4+ greater than Tl3+. The possible existence of a neuronal or hormonal inhibitory mechanism affecting the active Na-K transport in rat smooth muscle in situ, under conditions of hypokalemia, is discussed.

Effects of hypothalamic lesions on active sodium-potassium transport in the extensor digitorum longus muscles of hypokalemic rat

The CNS-induced suppression on muscle Na+-K+ pump was studied in "twitch" muscle, extensor digitorum longus (EDL), of hypokalemic rats which were fed a K+ deficient diet for several weeks. Peripheral nerve section or bilateral lesion of the ventromedial hypothalamic nucleus had no effect on the Na+ and K+ contents in EDL of hypokalemic rats. However, lesions of the paraventricular nucleus caused the net Na+ loss and the net K+ uptake in the muscles. Lesions in either the dorsomedial nucleus or anterior hypothalamus also caused significant net K+ uptake but the net Na+ loss was not significant. The results were compared with those of "tonic" muscle, soleus, reported previously.

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)

Effects of bicarbonate on resting potentials in mammalian skeletal muscle

In the absence of external HCO3, resting membrane potentials (Vm) in extensor digitorum longus muscle were depolarized as compared to the normal Vm in the presence of HCO3. Removal of Na or Cl form the HCO3-free media induced repolarization. In muscle in HCO3 buffer at 20 degrees C, internal K, Na, and Cl activities were analyzed with liquid ion selective microelectrodes. The averages were respectively, 119.7 +/- 2.1, 6.69 +/- 0.3, and 3.41 +/- 0.06 mM. In a high proportion of cells analyzed, the equilibrium potential for Cl was negative to Vm. Removing external HCO3, decreased internal K while internal Na and Cl increased. An increase in temperature and the application of HCO3 significantly lowered internal activities of both Na and Cl. Removal of HCO3 with temperature held constant caused a rapid depolarization, an increase in internal Na and Cl, and a decrease in internal K. Furosemide (10 microM) induced a repolarization of cells that were previously depolarized in the HCO3-free state, but the drug does not decrease internal Na.

Electrogenic Na-K pump in denervated mouse soleus muscles: prolonged activation by briefly applied acetylcholine

Thin preparations of mouse soleus muscles denervated for 3-11 days were bathed in Cl-free solutions. The membrane potential (microelectrode technique) was an average of -65.6 mV. On application of 10 microM acetylcholine (ACh) the membrane potential fell to -2 to -8 mV. Following the washout of ACh it returned to values 9-24 mV more negative than before ACh. The membrane potential gradually returned toward the initial level during the ensuing 40-60 min. No hyperpolarization occurred when Na ions were absent during the application and washout of ACh or in the presence of 0.1 mM ouabain. The hyperpolarization was enhanced by replacing the Na ions by Li or Tris ions following an application of ACh in the presence of Na+. The hyperpolarization was suppressed by ouabain irrespective of whether the drug was applied in the presence or absence of Na+. The membrane potential was diminished by reducing [K+] from 4 to 1 mM in the absence of Na+ before ACh, but it was increased by the same procedure by up to 20 mV following the application of ACh. This indicates that the hyperpolarization was not entirely due to a K-depleting action of the Na-K pump at the membrane surface.

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.

The facilitating effect of gangliosides on the electrogenic (Na+/K+) pump and on the resistance of the membrane potential to hypoxia in neuromuscular preparation

The effects have been investigated of a mixture of gangliosides from beef brain cortex (GM1, GD1a, GD1b and GT1) either added to the bathing medium or injected intraperitoneally on muscle fibres and nerve terminals in mouse diaphragm. The electrogenic (Na+/K+) pump activity of muscle fibres enriched with sodium was increased by 38% after 2-h pretreatment with gangliosides (5 X 10(-8) mol X 1(-1]. Muscles from animals treated with gangliosides did not show the substantial depolarization of the resting membrane potential (RMP) in K+-free solution (6 h) shown by control muscles. Further, treatment with gangliosides slowed the changes in muscle fibre RMP and frequency of the miniature end-plate potentials in oxygen deprived muscles.

Gangliosides' dual mode of action: a working hypothesis

Using in vitro preparations, we have tested the hypothesis that gangliosides, and more specifically GM1, may prevent progressive neural damage following a trauma by means of complex intracellular mechanisms that might be triggered originally by ganglioside interaction with neuronal membranes. We have recently shown that 2-hr ganglioside incubation in vitro stimulates the membrane Na/K pump in neuromuscular preparations. However, 5-6-hr incubation or in vivo treatment for 3 days with a daily injection of gangliosides at a dose of 1 or 10 mg/kg prevents the depolarization that normally occurs after several hours of exposure to K+-free solutions. In such undepolarized muscles, the electrogenic Na+/K+ pump does not seem to be activated. Hippocampal slices subjected to hypoxia undergo depolarization, which is reversed after oxygen readmission. The recovery phase is characterized by a huge hyperpolarization, probably reflecting electrogenic pump activity. In control preparations the depolarization occurs after 3.15 +/- 0.4 min and has a value of 48.7 +/- 5.7 mV; GM1 treatment for at least 4-5 hr increases the latency to 7.3 +/- 2.3 min, and the depolarization is reduced to 31.8 +/- 4.5 mV. This protective effect is accompanied by a reduced hyperpolarization in treated preparations. The ionic studies performed on neuromuscular preparations indicate that the protective effect may not be solely dependent on K+ leakage; however, the experiments are not conclusive and must be repeated with more direct methods. The results obtained indicate a dual mode of action for gangliosides. The early one seems characterized by membrane-enzyme activation, perhaps in relationship to their incorporation in the membrane, which could be compatible with previously described effects, such as enhancement of neuronal sprouting and neuritogenesis. The late one, occurring 4-5 hr after ganglioside addition in vitro, might reflect intracellular events and be compatible with the protective action exhibited by gangliosides against neural damage.

Effects of denervation on sodium, potassium and [3H]ouabain binding in muscles of normal and potassium-depleted rats

K depletion leads to a selective loss of K from skeletal muscles, which is associated with a decrease in the number of [3H]ouabain binding sites. The significance of the nerve supply for these changes has been assessed in denervation experiments with K-depleted rats. In K-depleted rats (age 4-12 weeks) denervation led to a partial recovery of the K contents in soleus (46-77%), gastrocnemius (23%) and extensor digitorum longus (e.d.l.) muscles (19%) within 24 h. These effects were not prevented by beta-adrenoceptor blockade or mimicked by alpha-adrenoceptor blockade. In K-depleted rats the number of [3H]ouabain binding sites was not increased following denervation. In K-depleted rats 24 h of plaster immobilization of the entire hind limb caused 51% recovery of the total K content in soleus, whereas gastrocnemius and e.d.l. showed 49 and 16% recovery, respectively. Tenotomy for 3 h caused a rise in total K content of 33% in soleus muscles from K-depleted rats. Anaesthesia for 3 h increased the total K content by 23%. The recovery of K induced by denervation, immobilization in plaster, tenotomy or anaesthesia was associated with an equivalent decrease in Na content. Denervation performed before K depletion reduced the loss of K from soleus, but not from gastrocnemius and e.d.l. In both soleus and e.d.l. the number of [3H]ouabain binding sites, however, decreased to the same level as in the contralateral innervated muscles. Denervation reduced, but did not prevent, the increase in the number of [3H]ouabain binding sites seen after re-administration of K to K-depleted rats. It is concluded that the changes in Na-K contents seen after denervation in K-depleted rats are the outcome of cessation of muscle activity. The results give no support to the idea that the effects of K depletion on the K content and the number of [3H]ouabain binding sites in skeletal muscle are mediated by the peripheral nerves.

Neural regulation on the active sodium-potassium transport in hypokalaemic rat skeletal muscles

C.N.S.-induced suppression of muscle Na-pump activity was studied in fast 'twitch' muscle, extensor digitorum longus, of hypokalaemic rats which were fed a K-deficient diet for 0-9 weeks. The results were compared with those of slow 'tonic' muscle, soleus, reported previously. K-deficient diet caused blood hypokalaemia and a considerable K+ loss and Na+ accumulation in the skeletal, heart and smooth muscles. The cellular K+ loss was in the order of soleus greater than extensor digitorum longus greater than diaphragm greater than duodenum greater than auricle greater than ventricle; C.N.S. organs such as cerebrum, cerebellum, medulla oblongata, spinal cord and liver were spared this K+ fall. Skeletal, heart and smooth muscles lost more K+ with prolongation of hypokalaemic periods, whereas plasma K+ concentration did not fall much below 1.6 mM during hypokalaemia. Peripheral nerve section, cervical and brain-stem transection, decerebration and cortical spreading depression with 20% KCl, which activated the active Na+ and K+ transport in soleus muscles during hypokalaemia, could not enhance the pump activity in extensor digitorum longus muscles. Alpha-adrenoreceptor antagonists such as phenoxybenzamine, phentolamine and dibenamine and a specific blocker of post-synaptic alpha 1-adrenoreceptor, prazosin, did not stimulate Na+ and K+ transport in the extensor digitorum longus muscles during hypokalaemia while the beta-adrenoreceptor antagonist, propranolol, also had no effect. The sensitivity of the active Na+ and K+ transport system in rat muscles to ouabain applied intraperitoneally was greater in extensor digitorum longus muscles than in soleus muscles. The binding experiment with a radiolabelled ligand of alpha 1 adrenoreceptor antagonist, [3H]prazosin, demonstrated the presence of alpha 1-adrenergic receptors on the soleus muscle membranes of hypokalaemic rats, but not of normal rats. alpha 1 Adrenergic receptors were not detected on the extensor digitorum longus muscle membranes prepared from either hypokalaemic or normal rats. The correlation between the C.N.S.-induced inhibition on the Na pump in soleus muscle during hypokalaemia and the occurrence of alpha 1 adrenergic receptors on the muscle was discussed.

Recovery from decamethonium in rat muscle and denervated guinea-pig diaphragm

1. In denervated guinea-pig diaphragm the depolarization produced by decamethonium (100 microM) was followed by an initial phase of recovery, and then by a slow restoration of membrane potential in the presence of the drug, with hyperpolarization. Membrane potentials were measured by repeated insertions. The slow phase of spontaneous recovery was not found in the absence of potassium or in the presence of ouabain (100 microM). 2. With 1 microM-decamethonium the net loss of potassium from denervated muscle was 17% by wet weight in 20 min as compared with controls, which represents a loss of over 30 mM in internal concentration. Similar results were obtained with 100 microM-decamethonium. Spontaneous recovery of potassium occurred in the succeeding 2 h in the presence of 1 microM and 100 microM-decamethonium. With 5 nM-decamethonium muscles exposed for 20 min had a potassium content which was not reduced as compared with controls. 3. In rat diaphragm decamethonium (100 microM) also produced depolarization and slow spontaneous recovery which was not seen in the absence of potassium or the presence of ouabain. With 3 mM-decamethonium spontaneous recovery of potential was complete in 5 min. 4. Change from 5 mM-potassium to potassium-free solution produced consistent hyperpolarization in denervated guinea-pig diaphragm. In rat diaphragm at 38 degrees C the results were variable, with some fibres showing hyperpolarization while others showed depolarization.

Membrane potential and contractures in segments cut from rat fast and slow twitch muscles

Contractile responses associated with depolarization caused by an increase in [K]0 or by voltage-clamp steps were compared for fast and slow mammalian twitch muscles. The contractions and contractures of isolated mammalian muscle fibres cut into 0.5-1.0 cm lengths are similar to those obtained from intact cells. The slope of the relationship between the membrane potential and the log [K]0 is similar in slow (46 mV +/- 0.8) and in fast fibres (48 mV +/- 1.1). This relation is not significantly modified in sodium-free or Cl-free solution. The K-contractures of cut sections of slow and fast fibres are transient and a full repriming of the response is only observed when the [K] x [Cl] product is kept constant. The contractile threshold in fast fibres is at 20-30 mM [K]0 (-52 to -43 mV) and in slow muscle at 10-15 mM [K]0 (-62 to -55 mV). Under voltage-clamp conditions, the contractile responses of both types of muscle show two components. In Na-free solution or in presence of TTX (5 x 10(-7) g/ml) the first component is abolished and the second remaining component is similar to that developing during K-contractures. In iliacus fibres, the contracture threshold is between -50.5 mV and -40.5 mV and in soleus fibres between -66 mV and -56 mV; these values are close to those obtained with K-rich depolarizing fluids. The S-shaped curve of the contracture vs membrane potential relation is similar to that found in frog muscles except that the contractile responses are graded over a larger range of membrane potentials (-50 to + 30 mV in fast and -55 to + 10 mV in slow muscle).

Postdenervation changes of intracellular potassium and sodium measured by ion selective microelectrodes in rat soleus and extensor digitorum longus muscle fibres

The intracellular concentration of free [K+]i and [Na+]i in innervated and denervated rat extensor digitorum longus (EDL) and soleus (SOL) muscles was measured by double-barrel glass microelectrodes filled with liquid ion exchanger. In both muscles, postdenervation fall of resting membrane potential was accompanied by a decrease of [K+]i and increase of [Na+]i. The relative permeability, PNa/PK of the muscle fibre membrane increased three times in EDL and 1.5 times in SOL respectively by the third day of denervation and then dropped within 14 days to the values which were only slightly but significantly lower than the control ones.

CNS control of active sodium transport in muscle during progressive hypokalemia in the rat

The degree of plasma hypokalemia was graded with the duration of a potassium deficient diet. The intracellular Na+ and K+ contents ([Na]i and [K]i) of the innervated soleus muscle of hypokalemic rats were determined and plotted as a function of plasma K+ concentration following a potassium deficient diet during 1-8 weeks. The progression of plasma hypokalemia was highly correlated with both [Na]i accumulation and [K]i loss in the muscle. Denervation of muscles in the hypokalemic rat resulted in a rapid activation of the Na-pump in the denervated soleus muscle regardless of the degree of plasma hypokalemia. This pump activation in the denervated muscle was chronically maintained throughout the hypokalemia. Restoration of a normal, K+ containing, diet for 4 days in hypokalemic rats resulted in a complete recovery of normal Na+ and K+ contents in both the innervated muscle and plasma, while the [Na]i and [K]i in the denervated muscle was unaffected. The relationship between the CNS-induced inhibition of the muscle Na-pump and the plasma K+ levels in hypokalemic rats is discussed.