Potassium Contracture Research Articles

Modulation of Ca2+ channels, charge movement and Ca2+ transients by heparin in frog skeletal muscle fibres.

This study is an investigation into the modulatory effects of heparin, a component of the extracellular matrix that binds to dihydropyridine receptors, on contraction and Ca2+ channels in frog skeletal muscle. Using tension and Ca2+ signal measurements in single intact skeletal muscle cells we have found that heparin (100-200 micrograms ml-1) substantially potentiates twitch and tetanic tension (55% and 28%, respectively). In contrast, heparin reduces the amplitude of K+ contractures. Heparin most likely potentiates twitch tension by prolonging action potentials. The ionic basis of this effect was investigated in voltage-clamp experiments. Membrane currents were monitored in voltage-clamped segments of single fibres using the triple Vaseline gap technique. We found that heparin partially blocks delayed rectifier potassium channels. The depressive effects of heparin on K+ contractures prompted us to investigate the effects of heparin on charge movement and Ca2+ currents (ICa) under voltage-clamp. Charge movement was measured using a subtraction procedure that employed a -20 mV control pulse from a holding potential of -100 mV. Heparin depresses the total charge by 25%. We propose that the reduction in the amplitude of potassium contractures is related to a partial blockade of charge movement. Extracellular heparin shifts the ICa-V relation toward more negative voltages and delays the deactivation of tail currents. Double pulse experiments revealed that conditioning depolarizations speed the activation of ICa during test depolarizations. Heparin does not affect this process. The primary action of heparin is to accelerate the activation of ICa during pulses not preceded by conditioning depolarizations. Overall, the kinetic effects of heparin on ICa would increase the Ca2+ influx associated with action potentials. However, mechanical and optical experiments performed in Ca(2+) -free solutions and in the presence of Ca2+ channel blockers revealed that twitch and tetanic potentiation occur even in the absence of Ca(2+) -influx.

High‐frequency fatigue in rat skeletal muscle: Role of extracellular ion concentrations

High-frequency fatigue (HFF), the decline of force during continuous tetanic stimulation (lasting 4-40 s), was studied in isolated bundles of rat skeletal muscle fibers. HFF was slower in slow-twitch soleus fibers than in fast-twitch red or white sternomastoid fibers; denervation accelerated fatigue in soleus. Maximal 200-mmol/L potassium contractures of normal amplitude were induced in fatigued fibers, suggesting that crossbridge cycling and the voltage activation of excitation-contraction coupling could still occur maximally, but that activation by action potentials was impaired. An increase in [Na+]o slowed HFF, while a small increase in [K+]o or reduction in [Cl(-)]o accelerated HFF. Increasing the tetanic stimulation frequency exacerbated fatigue. Recovery from HFF proceeded rapidly since force increased markedly within a few seconds when stimulation ceased. These results support the hypothesis that a redistribution of Na+, K+, and Cl- across the transverse tubular membranes during repeated action potential activity induces fatigue by reducing the amplitude and conduction of action potentials.

Positive inotropic effect of adrenaline on potassium contractures in tonic skeletal muscle fibres of the frog.

K+ contractures were elicited in small bundles of tonic skeletal muscle fibres of the frog. Adrenaline (1 microM) increased the amplitude of K+ contractures in a [K+]o-dependent manner: maximal effects were produced between 20 and 60 mM [K+]o. In contrast, we found no effect of adrenaline on K+ contractures of twitch fibres. The potentiating effect of adrenaline depended on [Ca2+]o. Increasing [Ca2+]o from 1.8 to 10 mM doubled the positive inotropic effect of adrenaline. In a nominally Ca2+ free saline, adrenaline had no potentiating effect. The Ca2+ channel blockers nifedipine (20 microM) and Ni2+ (1.8 mM) reversibly reduced the amplitude of the tonic phase of K+ contractures and blocked the potentiation by adrenaline. The mechanical effects of adrenaline cannot be explained by changes in the membrane potential, as revealed by intracellular recordings at several [K+]o. It was concluded that the potentiating effect of adrenaline in tonic muscle fibres of the frog may be mediated through Ca2+ channels.

Fluo-3 signals associated with potassium contractures in single amphibian muscle fibres.

1. The calcium-sensitive dye fluo-3 AM was used to obtain fluorescence signals and calcium transients associated with K+ contractures, twitches and tetani, in intact single muscle fibres of the tropical toad Leptodactylus insularis. 2. The changes in free calcium concentration in the myoplasm ([Ca2+]i) were calculated using the values of the 'off' (k- = 33.5 s-1) and 'on' (k+ = 13.1 microM-1 s-1) rate constants for the binding of calcium to the dye (dissociation constant, Kd = k-/k+). The mean (+/- S.E.M., n = 7) peak [Ca2+]i value during twitches or tetani was 3.9 +/- 0.3 or 4.1 +/- 0.3 microM, respectively, while during maximal K+ contractures, it was 10.3 +/- 0.8 microM. The threshold [Ca2+]i for tension development was about 1 microM. 3. For responses elicited with high [K+]o (80-190 mM), the calcium transients decayed faster than tension. At lower [K+]o (30-70 mM), the decay was slower, and relaxation was complete when [Ca2+]i was still above contractile threshold values. 4. Following a K+ contracture, recovery of the calcium transients associated with twitches occurred before recovery of tension, indicating an apparent dissociation between [Ca2+]i and tension output. This apparent dissociation between calcium and tension output could be attributed to the desensitization of the contractile proteins to calcium, or, more probably, to the non-uniform behaviour of calcium release and/or uptake sites, leading to an unhomogeneous distribution of active sarcomeres along the fibre length and localized sarcomere relaxation.

β‐Adrenergic potentiation of E–C coupling increases force in rat skeletal muscle

We examined the mechanism(s) which allow terbutaline, a beta 2-adrenergic agonist, to increase isometric force in bundles of normal and denervated rat soleus fibers. Terbutaline (10 mumol/L) potentiated tetanic contractions during exposure to 1 mmol/L ouabain, 10 mumol/L nifedipine, or 0.5 mmol/L iodoacetate. Terbutaline induced equivalent increases in submaximal potassium (K+) contracture and tetanic force: these effects were mimicked by 2 mmol/L dibutyryl-cyclic AMP. Therefore, terbutaline increased force by a cyclic AMP-dependent mechanism other than enhancement of sodium-pump activity, dihydropyridine sensitive Ca2+ currents, glycolysis, or action potentials. Pretreatment with 1 mmol/L caffeine induced submaximal potentiation of peak tetanic force but prevented further potentiation by terbutaline. This suggested that terbutaline did not influence the myofilaments, but acted on the sarcoplasmic reticulum (SR) to increase the myoplasmic Ca2+ concentration and hence force production. We speculate that force is potentiated following beta-adrenoceptor activation by a cyclic AMP-dependent phosphorylation of Ca2+ release channels to facilitate SR calcium release during tetanic stimulation.

Do independent processes control the activation and inactivation of potassium contracture tension in rat skeletal muscle?

Potassium (K+) contracture tension, measured in small bundles of rat soleus muscle fibers during maintained depolarization, increases to a peak value and then decays either to the baseline or to a pedestal level. We have tested the hypothesis that the rise and fall of tension are determined by independent activation and inactivation processes. If the "Independence" hypothesis is correct, tension during the decay of K+ contractures should equal tension predicted from the product of the activation and inactivation parameters determined from the same K+ contractures. Both the measured and predicted tensions decayed to a pedestal level that was increased in amplitude in the presence of perchlorate ions. However, the measured tensions in normal solutions and in the presence of perchlorate were three to five times smaller than the predicted tensions. This result indicates that the activation and inactivation of processes controlling the rise and decay of K+ contracture tension are not independent.

Effects of metabolic inhibition and changes of intracellular pH on potassium permeability and contraction of rat uterus.

1. We have investigated the role of changes of potassium efflux in the inhibition of uterine force produced by cyanide. K+ efflux (86Rb) was measured from pregnant and non-pregnant rat myometrial strips during metabolic inhibition with cyanide and following manoeuvres to displace intracellular pH (pHi). 2. Cyanide greatly reduced or abolished spontaneous contractions. If the membrane was depolarized directly at this stage (by elevating external K+) then contraction redeveloped. This suggests that the initial depression of force is due to a failure of membrane excitation. 3. Cyanide reversibly increased 86Rb efflux (30-35%) in both pregnant and nonpregnant uteri and contraction was reduced. The increase in 86Rb efflux with cyanide was not secondary to changes of membrane potential as it also occurred in both high-K+ and Ca(2+)-free solutions. 4. Glibenclamide (20 microM), an antagonist of K+ATP channels, reduced the cyanide-evoked increase of 86Rb efflux by about 50%. The glibenclamide-insensitive component of efflux persisted in a Ca(2+)-free solution. Despite its action on 86Rb efflux, glibenclamide did not restore contraction. 5. Intracellular pH falls during metabolic inhibition. We therefore investigated whether reducing pHi (in the absence of cyanide) had an effect on 86Rb efflux. Application of the weak acid butyrate (60 mM, at constant external pH, 7.4) had no significant effect on 86Rb efflux. Thus it is unlikely that the acidification in hypoxia contributes to the increased K+ efflux. 6. Intracellular alkalinization produced by the weak base trimethylamine (60 mM) increased the frequency of uterine contraction and the 86Rb efflux. However, there was no effect on the 86Rb efflux in a Ca(2+)-free solution. The increased efflux is therefore presumably a consequence of the increased frequency. 7. It is concluded that metabolic inhibition produced by cyanide, produces an increase in K+ efflux from the myometrium. Part of this efflux is glibenclamide sensitive. This increased K+ efflux will lead to hyperpolarization of the myometrial membrane and thus decrease excitation. Thus reduced surface membrane excitability will contribute to the fall of force in hypoxia; specifically it may cause the initial loss of spontaneous contractions in the uterus.

Pharmacological relaxation of the saphenous vein during harvesting for coronary artery bypass grafting

Spasm of the saphenous vein frequently occurs during harvesting from the leg and high-pressure distention is required to restore an adequate diameter for grafting. Forceful distention has been shown to damage the intima and media and may predispose to subsequent occlusion of the vein graft. Various pharmacologic vasodilators are capable of relaxing veins; in this study, we carried out a systematic investigation to determine the appropriate agents and concentrations for use during vein graft operations. In organ baths, human saphenous vein segments were contracted with potassium or a thromboxane mimetic, and vasodilator agents were then applied. Glyceryl trinitrate, 7.2 μg/mL, or papaverine hydrochloride, 11.9 μg/mL, caused 80% to 100% relaxation of contraction induced by potassium or thromboxane. Verapamil, 15.5 μg/mL, relaxed the potassium contraction by 100% and the thromboxane contraction by 75%. Comparison of the time course of action showed that glyceryl trinitrate had a rapid onset and a short duration of maximal action, whereas verapamil (like papaverine) had delayed onset and a long duration. A mixture of glyceryl trinitrate and verapamil combined rapid onset with long duration of action. We now use a mixture of glyceryl trinitrate and verapamil (pH 7.4) topically and intraluminally during harvesting of the saphenous vein to provide a relaxed conduit for coronary artery bypass grafting.

Characterization of endothelin-A receptors in the cerebral circulation.

Endothelin-1 (ET-1), a 21-amino acid peptide produced by the vascular endothelium, mediates contraction. In the present study we demonstrate that both ET-1 (Emax: 238 +/- 29% of potassium contraction) and ET-2 (Emax: 231 +/- 36%) produce strong concentration-dependent contractions of circular segments of guinea-pig middle cerebral artery, whereas ET-3 has only weak effects (Emax: 32 +/- 13%). FR 139317 (10(-6) M), a selective endothelinA (ETA) receptor antagonist, shifted the ET-1 response curve to the right (pD2:7.86 +/- 0.09 with and 8.76 +/- 0.09 without the antagonist) in a competitive manner (pA2 = 6.83). These findings are the first to show the presence of ETA receptors in cerebral vessels.

Potentiation of sarcoplasmic reticulum Ca2+ release by 2,3-butanedione monoxime in crustacean muscle.

The effect of the chemical phosphatase 2,3-butanedione monoxime (BDM) on various aspects of excitation/contraction coupling in crustacean muscle was investigated. Despite having a depressant effect on vertebrate skeletal and cardiac muscle, BDM was a potentiator of contraction in crustacean muscle. At concentrations of 1-3 mM BDM caused an increase of potassium contractures in bundles of fibers isolated from crayfish muscle. At higher concentrations BDM caused oscillatory contractions by itself. In single voltage-clamped cut muscle fibers loaded with rhod-2, BDM (0.5-2 mM) potentiated the magnitude and duration of intracellular Ca2+ transients elicited by depolarization. At the same time BDM did not affect the rate of Ca2+ removal from the myoplasm under conditions where Ca2+ release was blocked by tetracaine. Nor did BDM increase Ca2+ entry; in fact it caused a decrease in the amplitude of the inward Ca2+ current (ICa). In microsomes isolated from lobster muscle, BDM also potentiated Ca2+ release induced by caffeine and at higher concentrations (above 3 mM) induced release by itself. At the same time it had little effect on Ca2+ uptake. These results indicate that BDM potentiates Ca2+ release in crustacean muscle possibly by dephosphorylation of the Ca(2+)-release channel.

Actions of perchlorate ions on rat soleus muscle fibres.

1. The effects of perchlorate (ClO4-) on contraction have been studied in rat soleus muscle fibres using (i) potassium (K+) contracture and (ii) two-microelectrode-point voltage clamp techniques. 2. Membrane potentials (Vm) at all external [K+] were 3-5 mV more negative in ClO4-. The hyperpolarization could not be attributed to a change in Na+, K+, or Cl- permeability, or to an effect on the Na(+)-K+ pump. 3. ClO4- shifts the voltage dependence of tension activation, and contraction threshold, to more negative membrane potentials without altering maximum tension. Consequently, twitches and submaximal K+ contractures were potentiated, whereas tetanic contractions and 200 mM-K+ contractures were unaltered. 4. The decay of K+ contractures during steady depolarization with ClO4- developed a slow exponential phase with an average time constant of 6.05 +/- 0.76 min at -38 mV, and 1.68 +/- 0.15 min at -19 mV. This slow component was (a) under the rapid control of the surface Vm and (b) did not depend on external Ca2+. 5. Inactivation of E-C coupling was measured with a test 200 mM-K+ depolarization following 3-10 min depolarizations in conditioning solutions containing 20-120 mM-K+. ClO4- induced a negative shift in the curve-relating test K+ contracture amplitude to conditioning Vm but did not alter the rate of repriming of tension upon repolarization. 6. The results suggest that ClO4- increases the amount of activator produced during depolarization and thus allows the slow inactivation step in excitation-contraction (E-C) coupling to be reflected in the decay of K+ contracture tension. 7. A 'perchlorate contracture', which did not depend on the activation of E-C coupling, was observed. The contracture depended on external Ca2+, but not on voltage-dependent Ca2+ channels or Na(+)-Ca2+ exchange.

Excitation-contraction coupling in pigs heterozygous for malignant hyperthermia.

A defect in the skeletal muscle sarcoplasmic reticulum (SR) calcium release channel of malignant hyperthermia-susceptible (MHS) pigs greatly enhances SR calcium release in pigs homozygous for the malignant hyperthermia (MH) gene. In pigs heterozygous at this locus, rates of calcium release from isolated SR stimulated by Ca2+, ATP, or caffeine are intermediate to those of both MHS and normal SR [Mickelson et al. Am. J. Physiol. 257 (Cell Physiol. 26): C787-C794, 1989]. In this study bundles of intact muscle cells dissected from pigs of various genotypes were used to examine the effects of the MH gene on contractile responses to caffeine (direct stimulation of the SR) or to surface membrane (sarcolemma) depolarization (i.e., stimulation by way of the steps in excitation-contraction coupling). The caffeine threshold for contractures in the heterozygous muscles (5 mM) was intermediate to both types of homozygous muscles (2 mM for MHS and 10 mM for normal) as is the case with direct stimulation of calcium release from SR vesicles [Mickelson et al. Am. J. Physiol. 257 (Cell Physiol. 26): C787-C794, 1989]. Sarcolemmal depolarization was elicited by electrical stimuli or elevated extracellular potassium. Control twitch tension for MHS and heterozygous muscles did not differ and was significantly greater in both than in homozygous normal muscles. Potassium-induced contractures were significantly larger in MHS and heterozygous than in normal muscles. Thus, in heterozygous muscles, force production via sarcolemmal depolarization (twitches and potassium contractures) was enhanced as much as in homozygous MHS muscles. This could be the result of feedback from abnormal SR calcium channels producing altered (enhanced) transverse tubule to SR signal transduction.

Ca ions and potassium contracture in the retractor unguis muscle of the cockroach.

Potassium contracture consists of a phasic and a sustained component. Extracellular Ca ions are essential for both components. In the phasic component the Ca-induced Ca release mechanism is activated by the entry of the Ca ions.

Involvement of chloride-bicarbonate exchange in cell volume regulation during potassium contracture of single isolated smooth muscle cells

Involvement of chloride-bicarbonate exchange in cell volume regulation during potassium contracture of single isolated smooth muscle cells

Activation and inactivation of excitation‐contraction coupling in rat soleus muscle.

1. Potassium (K+) contractures have been used to characterize the processes of activation and inactivation of excitation-contraction coupling during prolonged depolarization of fibres in small bundles dissected from rat soleus muscles at 23 degrees C. 2. The smallest measurable K+ contracture tension was recorded with depolarization to -40 mV in 30 mM-K+ and maximum tension was achieved between -26 mV in 80 mM-K+ and -19 mV in 120 mM-K+. 3. The rate of inactivation of K+ contracture tension was voltage dependent. Tension decayed from 80 to 20% of the peak amplitude within 44.0 +/- 2.2 s at -26 mV (in 80 mM-K+), compared with 66.7 +/- 4.8 s at -35 mV (in 40 mM-K+). Results are given as mean +/- 1 S.E.M. 4. The effect of inactivation on maximum tension was determined using a two pulse protocol in which a 'conditioning' depolarization in solutions containing 20-120 mM-K+ was applied for 0.5-10 min before a 'test' depolarization to -8 mV in 200 mM-K+. The amplitude of the test contracture was compared with the mean amplitude of 'control' 200 mM-K+ contractures elicited in normally polarized fibres immediately before and after the two pulse protocol. Conditioning depolarization to -47 mV (in 20 mM-K+) did not reduce test 200 mM-K+ contracture tension. Significant inactivation was seen with further conditioning depolarization to more positive potentials: after 10 min at -40 mV (in 30 mM-K+), or -35 mV (in 40 mM-K+), test 200 mM-K+ contracture tension was reduced by 33 and 70% respectively. 5. In contrast to amphibian muscle, where maximum tension falls to zero within a few minutes of depolarization to potentials positive to -50 mV, test 200 mM-K+ contracture tension in rat soleus fibres fell initially rapidly and then slowly, but was not reduced to zero, even after 10 min at -19 mV in 120 mM-K+. 6. The fast phase of inactivation of test 200 mM-K+ contracture tension occurred during the decay of the conditioning K+ contracture. The slow phase of inactivation reached completion after 10 min of conditioning depolarization and occurred during the period when conditioning tension was reduced to zero or to a plateau level. Both phases of inactivation in rat soleus fibres are slow compared with fast and slow inactivation times of 5-100 s respectively reported for amphibian muscle. 7. When repolarized after prolonged depolarization, the muscle fibres were initially refractory, i.e. unable to produce tension in response to electrical stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

O-200 - Further evidence for the involvement of chloride-bicarbonate exchange reaction in cell volume regulation during potassium contracture of isolated smooth muscle cells.

O-200 - Further evidence for the involvement of chloride-bicarbonate exchange reaction in cell volume regulation during potassium contracture of isolated smooth muscle cells.

Contraction of Rat Soleus Muscle and the Effects of Hypertonic Treatments.

The effects of exposing rat soleus muscle to solutions made hypertonic by sucrose, mannitol, or urea for a period of 60 min and then to normal solutions for a further 30 min (hypertonic treatment) on twitch, tetanus, potassium, and caffeine contractures were examined. Following hypertonic treatments, twitch, tetanic, and potassium contracture tension were altered from control. The exact effects were dependent upon the concentration of solute used and the temperature. Electrically evoked contractions were abolished following hypertonic treatment with 400 mM sucrose or mannitol at 37 degrees C but 800 mM at 22 degrees C. Urea was less effective; only after hypertonic treatment with 800 mM were contractions of rat soleus muscle abolished at 37 degrees C, while at the lower temperature contractions could not be abolished. In general, caffeine contractures were reduced by hypertonic treatments at 37 degrees C but not at 22 degrees C. Hypertonic solutions themselves caused contractures of rat skeletal muscle, the amplitude of which directly depended on the concentration of solute used and the temperature. These contractures appeared to be due to the release of intracellular calcium from its stores. In hypertonic sucrose and mannitol but not urea solutions, the twitch was reduced and then abolished. Changes in contractile responses of rat soleus muscle during and after exposure to hypertonic solutions are suggested to be due to osmotically induced changes.

Effects of amrinone on the electromechanical coupling in frog skeletal muscle fibres

The contractile effects of 1.1 mM amrinone were studied on isolated skeletal muscle fibres of the frog (2-5 degrees C, slack length). Amrinone potentiated the twitch amplitude and the maximum tetanic tension and also enhanced the maximum contracture response elicited by 117.5 mM K+ (mean increase 6.3 +/- 1.8%, n = 6, P less than 0.02). The time to half-relaxation of the potassium contracture was slightly increased (mean change 13.8 +/- 3.5%, n = 7, P less than 0.01). Amrinone furthermore shifted the S-shaped curve relating contracture tension to log K+ to the left, and thus reduced the threshold depolarization (the mechanical threshold) needed to elicit a contracture (mean reduction 11 +/- 1 mV). The duration of the action potential at the -25 mV level was slightly increased by amrinone, whereas the resting membrane potential was unaffected.

X‐ray Diffraction Indices for Relative Quantification of Interlayering in Phyllosilicates

AbstractSimple, numerical methods for routine interpretation of x‐ray diffractograms of soil samples are desirable but largely unavailable due to the prevalence of mineral intergrades. A set of indices is proposed that facilitates a comparative estimation of proportions of K‐collapsible vermiculite and smectite, proportions of hydroxy interlayers, and proportions of swelling interlayers. The indices are based on an estimation of the horizontal displacements of the centroids of x‐ray diffraction areas representing a range of d001 spacings after various saturation and heat treatments. A potassium contraction index (KCI), a contraction index at 300 °C (CI300), and a contraction index at 550 °C (CI550) express the degree of interlayer contraction upon K saturation, heating to 300 °C, and heating to 550 °C, respectively, relative to the Mg‐saturated sample. Subtracting the CI300 from the CI550 yields an (Al + Fe)/Mg interlayer index expressing the relative proportions of Al + Fe‐hydroxy interlayers and Mg‐hydroxy interlayers. The indices can be a useful supplemental tool in the study of mineral transformations when computer analysis of x‐ray diffractograms is unavailable.

Effects of papaverine on human isolated bladder muscle

Papaverine is a non-specific smooth muscle relaxant and is thought to act at a site beyond the receptor sites on the cell membrane. In this study the relaxing properties of papaverine were tested in isolated muscle strips from the human bladder dome. In carbachol-induced contractions papaverine, even in high concentrations of 10(-4) mol/l had virtually no effects on peak tension generation, whereas the fading was accelerated and the steady state tension at 30 min. was reduced by about 54%. In contrast, high potassium-induced contractions were relaxed by papaverine in a concentration-dependent way; a concentration of papaverine of 10(-4) mol/l produced full relaxation. These findings might possibly be explained if it is assumed that papaverine blocks calcium ion channels in the cell membrane. However, the observation that rather high concentrations of papaverine were necessary to fully relax high potassium contractions and the fact, that papaverine affects cellular cAMP levels separate this drug from more selective calcium channel blockers. The calcium movements responsible for the peak tension generation in carbachol-induced contractions are obviously not affected by papaverine. Although papaverine had little effect on carbachol-induced contractions in vitro it cannot be excluded that the drug is effective in diseases were non-cholinergic mechanisms are involved.