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

Comparison of intermittent injection of nondepolarizing solution with a single flush of UW solution for donor heart preservation

Isolated canine hearts were preserved for 6 h at 5 degrees C followed by normothermic reperfusion for 2 h. The dogs were divided into two groups of nine hearts each; group 1 received a nondepolarizing preservation solution in multidose, and group 2 received a single flush of University of Wisconsin (UW) solution. Serum MB-CK and mitochondrial aspartate aminotransferase (m-AAT) concentrations and calcium overload during reperfusion were lower in group 1 than in group 2. At the end of reperfusion, myocardial ATP and total adenine nucleotide concentrations were higher and mitochondrial morphology appeared more intact in group 1 than in group 2. Left ventricular diastolic function was preserved better in group 1 than in group 2. These results suggest that in 6-h heart preservation, a nondepolarizing solution applied in multidose fashion protects the myocardium from the deleterious effects of hypothermia and cardioplegia better than a single flush of UW solution.

Taurine and hyperexcitable human muscle: effects of taurine on potassium-induced hyperexcitability of dystrophic myotonic and normal muscles

Progressively increasing concentrations of potassium chloride in Evans blue saline were administered to patients affected with myotonic dystrophy and to healthy volunteers before and after parenteral treatment with taurine. Excitability changes of thenar eminence muscles were related to the venous potassium and chloride concentrations. The actual electrolyte concentrations were compared to those to be expected if no infused electrolytes had been transported into cells. The expected concentrations were calculated by means of Evans blue dilution. This method permitted quantification of changes of muscle-excitability in terms of the potassium chloride concentration capable of disturbing the electrical activity of the studied muscles. The method also provided an indirect evaluation of electrolyte movements across muscle membrane in vivo in humans. Dystrophic myotonic muscles appeared highly sensitive to extracellular potassium and, unlike normal muscles, were unable to accumulate potassium-induced muscle hyperexcitability and favored electrolyte accumulation in dystrophic myotonic muscles. The stabilizing effect of taurine is discussed in relation to its ability to increase intracellular potassium concentration, membrane conductance, or both.

Local anesthetic blockade of Ca2+ -mediated action potentials in cardiac muscle

The effects of local anesthetic agents (lidocaine, procaine, cocaine) and diphenylhydantoin (DPH) were studied on the slow electrical responses induced by isoproterenol or caffeine in cardiac muscle preparations rendered inexcitable by tetrodotoxin (TTX) or by partial depolarization with elevated K+ (26 mM). In such inexcitable cells, we previously demonstrated that addition of some positive inotropic agents, such as catecholamines, histamine, and methylxanthines, rapidly increase the number of available slow Ca2+--Na+ channels, thus allowing slowly rising electrical responses resembling the plateau component of the cardiac action potential. In embryonic chick (16-20-day-old) myocardial cells (ventricular) studied as intact perfused hearts or as reaggregated cell cultures of trypsin-dispersed cells, high concentrations (10-(3) M) of all of the above drugs blocked the induced slow responses with their associated contractions; low concentrations (10-(5) M) of these agents reduced the maximal rate of rise (+Vmax) of the slow responses and depressed the contractions. For comparison with their effects on the slow response, the actions of these drugs on the normal action potential were also studied. As with the slow response, all of these drugs depressed the rate of rise of the action potential (10-(4) M) or blocked it at higher concentrations (10-(3) M); in contrast, low concentrations (10-(5) M) of lidocaine and DPH increased +V max. These findings suggest that local anesthetics, which interact with the lipid phase of the cell membrane, lead to blockade of the slow Ca2+--Na+ channels as well as of the fast Na+ channels in the myocardial sarcolemma.

The membrane potential of rat diaphragm muscle fibres and the effect of denervation

1. Resting membrane potentials of rat diaphragm muscles were measured in vitro after previous denervation for 0-10 days. In some experiments denervated muscles were incubated in vitro for 3 hr while in others they were cultured for 15-24 hr to allow adequate exposure to drugs before recording. 2. It was found that resting membrane potentials, within 2-5 mm of the site of nerve section were significantly lower, within 3 hr, than resting membrane potentials measured more than 9 mm away from site of nerve section. This difference could be reduced or abolished by bathing preparations in solutions containing adrenaline (10 muM), noradrenaline (10 muM) or isoprenaline (10 muM) or dibutyryl cyclic AMP (10 muM-0-25 mM in the presence of 2 mM theophylline). Cyclic AMP (0-5 mM) was ineffective. 3. Application of solutions containing dibutyryl cyclic AMP for 3 hr also raised the resting membrane potential of muscles denervated 4-5 days previously. Culture studies showed that this effect was sustained when the time of incubation was 24 hr. 4. Incubating freshly denervated preparations with cycloheximide (22 mug/ml.) or actinomycin D (1 mug/ml.) did not prevent the development of the early (3 hr) fall in resting membrane potential despite a concomitant inhibition of RNA or protein synthesis. Culturing freshly denervated muscles in solutions containing cycloheximide (10 or 25 mug/ml.) which blocked 93% of protein synthesis, did not prevent the expected drop in resting membrane potential after 15 or 24 hr. 5. It was found that exposure to ouabain (1 or 5 mM) produced a rapid (15 min) fall in resting membrane potential in innervated and denervated preparations treated with dibutyryl cyclic AMP but not denervated preparations. After 5 days denervation cyclic AMP levels in muscle were increased by about 40%. 6. It is suggested that upon denervation an electrogenic action of a NA+-pump is blocked and that dibutyryl cyclic AMP and catecholamines are capable of stimulating this pump.

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

1. Relationship between the resting membrane potential and the changes in the intraceullar Na and K concentrations ([Na]i and [K]i) was studied in 'Na-loaded' and K-depleted' soleus (SOL) muscles of rats which had fed a K-free diet for 40 and more days. 2. The extracellular space of the muscles was not significantly different between normal and K-deficient rats. The inulin space in both the 'fresh' and Na-rich' muscles can be determined by the same function relating the space to the muscle weight. 3. Presence of 2-5-15 mM-K in the recovery solution hyperpolarized the 'Na-rich' muscul fibres at the beginning of recovery. The hyperpolarized membrane potential exceeded, beyond the measured potential of 'fresh' muscle fibres, the theoretical potential derived from the ionic theory, or even beyond Ek. Then, the measured membrane potential declined progressively during the immersion in a recovery solution and returned to the steady-state value When a considerable Na extrusion and K uptake took place, the measured membrane potential became equal to Ek. 4.he maximal hyperpolarization occurring immediately after immersion in the recovery solution became smaller and had a shorter duration when increasing the external K concentration ([K]o) from 2-5 to 15mM. 5. The K-sensitive hyperpolarization was completely abolished on exposure to 0mM [K]o, on cooling to ca. 4 degrees C, and in the presence of oubain (10(-4) M). The inhibitory effects were reversed on returning to the control conditions. The membrane potential obtained after inhibition of the electrogenic Na-pump with cooling or ouabain agrees well with that predicted by the 'constant-field' equation. 7. The external Cl ions had a short-circuiting effect on the electrogenic Na-pumping activated on adding K ions. 8. The replacement of Na ions in a recovery solution with Li ions resulted in a faster rate of depolarization from the maximal hyperpolarizationp. It is concluded that the resting membrane potential of 'Na-loaded' and 'K-depleted' SOL muscle fibres is the sum of an ionic diffusion potential predicted by either the Nernst equation or the constant-field equation and of the potential produced by an electrogenic Na-pump.

Activation of the electrogenic sodium pump in guinea-pig auricles by internal sodium ions

1. The effect of various intracellular Na concentrations ([Na](i)) on the membrane potential after hypothermia was studied in guinea-pig auricles.2. For varying [Na](i), the atria were cooled for 4 hr at 4-6 degrees C in a K-poor solution with different concentrations of NaCl. The auricles were rewarmed in normal Tyrode solution at 35 degrees C.3. Extracellular space (ECS), intracellular Na and K concentrations ([Na](i) and [K](i)) and membrane potential of the atria were measured before and after hypothermia.4. The ECS, measured as inulin space, amounted to 350 ml./kg wet wt. at 35 degrees C and to 300 ml./kg wet wt. at 4-6 degrees C.5. [K](i) decreased during cooling and increased during rewarming the auricles. [Na](i) increased during hypothermia in bathing fluids containing NaCl, but decreased in NaCl- and Na-free solutions. At the beginning of rewarming a net Na transport occurred from cells with high [Na](i), while a net Na uptake took place in atria with low [Na](i).6. At the same time, the membrane potential of auricles with increased [Na](i) hyperpolarized beyond the steady-state value recorded at the end of rewarming, or even beyond the calculated K(+) equilibrium potential (E(K)). Afterwards, the hyperpolarization levelled off, while the E(K) values increased further. The membrane potential of atria with decreased [Na](i) showed no transitory hyperpolarization during rewarming.7. The hyperpolarization beyond the steady-state value of membrane potential in rewarmed auricles was significantly correlated to the active Na efflux.8. From these results it is concluded that the membrane potential of guinea-pig atria after hypothermia is affected by an active, electrogenic Na pump activated by intracellular Na ions.

The hyperpolarization of frog skeletal muscle fibres induced by removing potassium from the bathing medium

1. The time course of changes in resting potential after removing K(0) was studied in twenty-four single fibres and in 136 fibres from small bundle (two to four fibres) preparations of frog semitendinosus muscles.2. The initial resting potentials in the control saline ranged between -88 and -98 mV. The potentials returned to nearly the initial values when control conditions were reinstated after 3-8 hr of experimentation. All the fibres twitched at the end of the experiment.3. Only about one third of the fibres hyperpolarized for any length of time on exposure to a K-free saline at room temperature (20-28 degrees C). The hyperpolarization was reversed to depolarization after a variable delay. The resting potential could fall to -50 or -40 mV.4. The remainder of the fibres depolarized with little or no prior hyperpolarization.5. Both patterns of response could be replicated in the different fibres.6. Hyperpolarization induced by K-free solution was reduced or abolished on cooling to ca 10 degrees C; on substitution of Tris or Li for Na; and upon inhibition of the Na pump with DNP (0.025-0.2 mM) or ouabain (0.05 mM). The latter agent was not as effective as the other conditions.7. Only small, slowly developing depolarization occurred when Na was replaced with Tris or Li.8. The various effects in K-free solutions were reversed on returning to the control conditions.9. It is suggested that removal of K(0) itself has little or no direct effect on the resting potential and that the initial hyperpolarization is due to the pumped efflux of Na without a compensatory influx of K. Block of the pump electrogenesis is manifested by depolarization of the fibres as K(1) is depleted and Na(1) increased.10. The Na pump appears to be dependent upon the nutritional status of the frogs and variations of the latter probably cause the different responses of fibres to removal of K(0).

Accumulation of caesium and rubidium in vivo by red and white muscles of the rat

1. Rats were given drinking water containing either 20 mM-CsCl or 20 mM-RbCl for a period of 2 weeks. Samples of blood were then taken from the rats under anaesthetic. They were immediately centrifuged and the plasma taken for analysis. Soleus muscles, diaphragm, extensor digitorum longus, white gastrocnemius and vastus lateralis muscles were then taken from the dead animals and these and the plasma were analysed for potassium, and for caesium or rubidium by means of the flame photometer.2. The concentrations of potassium and rubidium or caesium in the fibre water of these various muscles and in the samples of plasma water were then calculated.3. It was found that the red muscles including soleus and diaphragm generally tended to accumulate caesium and rubidium to a greater extent than did the white muscles such as the white gastrocnemius and vastus lateralis.4. When the concentration ratio [K](i)/[K](o) was divided into the ratio [Rb](i)/[Rb](o) for the different muscles, values of about 1.3 were obtained for the red muscles compared with values about 1.14 for white muscles.5. When in the case of the caesium-treated rats the ratio [K](i)/[K](o) was divided into the ratio [Cs](i)/[Cs](o) values ranged from 1.94 +/- 0.12 for the red soleus to 1.08 +/- 0.09 for the white gastrocnemius.6. When these values in the caesium-treated animals were plotted against the percentage of red fibres in the five muscle types (as obtained from the data of Sreter & Woo, 1963) the graph indicated that the white fibres had similar ionic gradients for Cs(+) and K(+) and that affinity for Cs(+) was confined to the red fibres.7. The membrane potential measured in soleus and extensor muscles immersed in plasma from the same animal was not significantly different from E(K) but was much less than E(Cs).8. These results are interpreted in terms of permeability differences between the slow red fibres and white twitch fibres.

A ouabain-sensitive membrane conductance

1. Changes in membrane conductance and potential of sodium-loaded frog muscle fibres were found when the external recovery solution was changed: from cold to warm, to warm plus ouabain, to cold plus ouabain. Comparisons of these measurements for different external solutions were made by leaving the electrodes implanted in the same fibre during all solution changes. (The recovery solutions contained 10 mM-K and 82 mM-Cl.)2. The membrane potential became more negative on warming, less negative when ouabain was added, and still less negative when the ouabain-containing recovery solution was cooled. The membrane conductance increased on warming, increased further on addition of ouabain, and decreased when the ouabain-containing recovery solution was cooled.3. The increase of conductance which occurred on warming decreased with increasing periods in cold recovery. The increase of conductance which occurred on addition of ouabain decreased if the ouabain was added to the recovery solutions of muscles which were more fully recovered.4. The ouabain-sensitivity of the membrane conductance may be dependent upon the sodium-pump rate, or the extent of recovery of the sodium-loaded muscle fibre in the potassium- and chloride-containing recovery solutions.5. It is suggested that if the potassium conductance of the membrane increases with decreasing sodium-pump rates, then during the initial part of the recovery period a non-electrogenic mechanism must be producing a substantial part of the early net potassium influx.