Two cases of adynamia episodica hereditaria: in vitro investigation of muscle cell membrane and contraction parameters

A sodium channel defect in hyperkalemic periodic paralysis: potassium-induced failure of inactivation

Hyperkalemic periodic analysis (HPP) is an autosomal dominant disorder characterized by episodic weakness lasting minutes to days in association with a mild elevation in serum K+. In vitro measurements of whole-cell currents in HPP muscle have demonstrated a persistent, tetrodotoxin-sensitive Na+ current, and we have recently shown by linkage analysis that the Na+ channel alpha subunit gene may contain the HPP mutation. In this study, we have made patch-clamp recordings from cultured HPP myotubes and found a defect in the normal voltage-dependent inactivation of Na+ channels. Moderate elevation of extracellular K+ favors an aberrant gating mode in a small fraction of the channels that is characterized by persistent reopenings and prolonged dwell times in the open state. The Na+ current, through noninactivating channels, may cause the skeletal muscle weakness in HPP by depolarizing the cell, thereby inactivating normal Na+ channels, which are then unable to generate an action potential. Thus the dominant expression of HPP is manifest by inactivation of the wild-type Na+ channel through the influence of the mutant gene product on membrane voltage.

Altered gating and conductance of Na+ channels in hyperkalemic periodic paralysis

Electrophysiological studies on muscle fibres from patients with hyperkalemic periodic paralysis with myotonia have shown that the episodes of weakness are caused by a sustained depolarization of the sarcolemma to potentials between -40 and -60 mV. In muscle fibre segments from three such patients this sustained depolarization was caused by noninactivating Na+ channels with reduced single-channel conductance blocked by TTX and procainamide. As the chloride conductance was normal, myotonia may be best explained with the abnormal reopenings of the Na+ channels. The recently described genetic linkage between hyperkalemic periodic paralysis with myotonia and the gene coding for the TTX-sensitive Na+ channel suggests an altered primary structure of this channel causing its abnormal function.

Hyperkalemic periodic paralysis and the adult muscle sodium channel alpha-subunit gene

Hyperkalemic periodic paralysis (HYPP) is an autosomal dominant disorder characterized by episodes of muscle weakness due to depolarization of the muscle cell membrane associated with elevated serum potassium. Electrophysiological studies have implicated the adult muscle sodium channel. Here, portions of the adult muscle sodium channel alpha-subunit gene were cloned and mapped near the human growth hormone locus (GH1) on chromosome 17. In a large pedigree displaying HYPP with myotonia, these two loci showed tight linkage to the genetic defect with no recombinants detected. Thus, it is likely that the sodium channel alpha-subunit gene contains the HYPP mutation.

Characteristics of single Na+ channels of adult human skeletal muscle

The patch-clamp technique was used to study Na+ channels of human skeletal muscle. Preparations were from biopsies of quadriceps muscle from adults who were not suffering from neuromuscular diseases. Activity of Na+ channels was recorded from inside-out patches when the membrane potential was stepped from a holding potential of -110 mV to potential above a threshold of about -65 mV. Single channel activity increased within minutes after hyperpolarizing the patch due to recovery from ultra-slow inactivation. Up to ten Na+ channels were active in individual patches. Macroscopic currents were reconstructed by averaging single channel currents. The time-to-peak current declined from 1.6 ms at -60 mV to 0.5 ms at + 10 mV. The currents decayed mono-exponentially with time constants between 12.1 ms at -60 mV and 0.4 ms at + 10 mV (21 C). The conductance of single Na+ channels was 1.65 pS and the mean open time was voltage-dependent. At -50 mV, the mean open time was 0.4 ms, while positive to -10 mV it increased to values above 1 ms. In the threshold potential range, the number of openings per depolarizing pulse was larger than the number of channels under the patch-clamp pipette, indicating reopening of Na+ channels at this potential. Openings could be observed only rarely 10 ms after onset of depolarization and the macroscopic current produced by late openings was less than 0.1% of the peak current. Human skeletal muscle is thus suitable for investigation with the patch-clamp technique and the determination of properties of Na+ channels with this technique could be the basis for an assessment of possible defects of these channels in diseased muscle.

Paramyotonia congenita or hyperkalemic periodic paralysis? Clinical and electrophysiological features of each entity in one family

The nosological distinction between paramyotonia congenita (PC) and hyperkalemic periodic paralysis (HPP) continues to generate debate. Recently, electrophysiological signs thought to be specific for each entity have been described and have been used to bolster the argument that the two disorders are distinct. We report a particularly instructive family wherein individual members had clinical features of either PC or HPP and electrophysiological features of both. We suggest that PC and HPP represent part of the spectrum of a single genetic disorder. Evoked response testing, with exercise and cold provocation, may be useful in determining the physiologic pattern that predominates in any one individual.

Hypokalemic paralysis in two patients with paramyotonia congenita (PC) and known hyperkalemic/exercise-induced weakness

Two male patients from a single family with known PC and "potassium sensitivity" developed hypokalemic paralysis following generalized anesthesia. These patients confirm previous similar observations. It's significance and management are discussed.

Adynamia episodica hereditaria: what causes the weakness?

The cause of weakness was investigated in a patient with adynamia episodica hereditaria without myotonia. A pattern of exercise and rest produced episodes of hyperkalemic periodic paralysis. In addition, local muscle weakness was induced by forearm cooling. Investigations on isolated intercostal muscle demonstrated that a high potassium concentration in the bathing solution triggered a noninactivating membrane current causing depolarization of the muscle fibers. This current was carried by sodium as it could be inhibited by tetrodotoxin. The abnormal sodium conductance led to an increase of sodium within the fibers. This was demonstrated directly by intracellular recordings. Weakness induced by rest after exercise and cold-induced weakness appeared to have different pathomechanisms. In the cold, the muscle fibers retained a normal resting potential, but their excitability was reduced and their mechanical threshold was increased. These findings also provide evidence that the mechanism of cold-induced weakness in adynamia episodica is distinctly different from the cold-induced weakness that occurs in paramyotonia congenita.

Hyperkalemic periodic paralysis in Gordon's syndrome: a possible defect in atrial natriuretic peptide function

We present the case of a 14-year-old boy who had secondary hyperkalemic periodic paralysis caused by Gordon's syndrome. This syndrome consists of hypertension, tubular acidosis, and hyperkalemia with normal glomerular filtration rate. The pathophysiological mechanism is still unknown. Pathophysiological studies suggest that in this disorder the kidney lacks sensitivity to atrial natriuretic peptide. After treatment with hydrochlorothiazide, serum potassium and plasma aldosterone values, plasma renin activity, and blood pressure became normal and the attacks of periodic paralysis disappeared.

Cromakalim (BRL 34915) restores in vitro the membrane potential of depolarized human skeletal muscle fibres

The purpose of the present study was to analyze the effects of cromakalim (BRL 34915), a potent drug from a new class of drugs characterized as "K+ channel openers", on the electrical activity of human skeletal muscle. Therefore, intracellular recordings were used to measure the effects of cromakalim on the membrane potential and input conductance of fibres from human skeletal muscle biopsies. Cromakalim in a concentration above 1 mumol/l induced an increase in membrane K+ conductance. This effect resulted in a membrane hyperpolarization. The magnitude of this polarization depended on the difference between resting and K+ equilibrium potential. The effect had a rapid onset and was quickly reversible after washing. Fibres from two patients with hyperkalaemic periodic paralysis showed an excessive membrane depolarization during and also after exposure to an slightly elevated extracellular K+ concentration. In the latter situation, cromakalim repolarized the fibres to the normal resting potential. Tolbutamide (1 mmol/l) and Ba2+ (3 mmol/l) strongly antagonized the effect of cromakalim. The data show that cromakalim hyperpolarizes depolarized human skeletal muscle fibres maintained in vitro. The underlying mechanism is probably an activation of otherwise "silent", ATP-regulated K+ channels. Such an effect may be of therapeutic benefit in a situation in which a membrane depolarization causes muscle paralysis.

AAEE minimonograph #27: differential diagnosis of myotonic syndromes

Recent advances in neuromuscular diseases have also widened the diagnostic spectrum of myotonic disorders. Treatment, prognosis, and genetic aspects are different in the various syndromes and mandate a correct diagnosis. The combination of neurologic examination, standard EMG, exercise test, cold exposure, potassium loading, eye examination, and pedigree analysis allows correct classification of nearly all patients with myotonic disorders. In this review emphasis is placed on clinical features and electrophysiologic evaluation.

Membrane defects in paramyotonia congenita (Eulenburg)

Membrane potentials, current-voltage relationships, and component conductances were determined in resting excised external intercostal muscle fibers from five patients with paramyotonia congenita. At 37 degrees C all investigated parameters were normal. At 27 degrees C the resting potentials decreased to about -40 mV, and the fibers were inexcitable. At this stage the membrane currents were much larger than in normal fibers owing to increases in the membrane conductances for Na and Cl ions. The earlier finding that in the cold the Na permeability is abnormally large was confirmed. The Cl permeability was shown to be normal even in the cold. The decrease of the resting potential and the changes in the current-voltage relationship at 27 degrees C could be prevented by the use of the Na channel blocker tetrodotoxin (TTX) or by bathing the fibers in a Na-free solution. Our previous conclusion that the Cl conductance at 27 degrees C was also increased when TTX was present was not confirmed. Exposure of a muscle bundle to 7 mmol/l potassium did not lead to excessive depolarization and paralysis.

Adynamia episodica hereditaria with myotonia: a non-inactivating sodium current and the effect of extracellular pH

To study the mechanism of periodic paralysis, we investigated the properties of intact muscle fibers biopsied from a patient who had adynamia episodica hereditaria with electromyographic signs of myotonia. When the potassium concentration in the extracellular medium, [K]e, was 3.5 mmol/l, force of contraction, membrane resting potential, and intracellular sodium activity were normal, but depolarizing voltage clamp steps revealed the existence of an abnormal inward current. This current was activated at membrane potentials less negative than -80 mV, reached a maximum within 50 msec, and was not inactivated with time. The inward current was completely and reversibly blocked by tetrodotoxin, which indicates that it was carried by sodium ions. In a solution containing 9 mmol/l potassium, normal muscle would depolarize to -63 mV and yet be capable of developing full tetanic force upon stimulation. The muscle from the patient depolarized to -57 mV and became inexcitable, i.e., it was paralyzed. A contracture did not develop. Lowering of the extracellular pH did not influence the resting potential, but it effectively antagonized or prevented the paralytic effect of high [K]e by changing the inactivation characteristics of the sodium channels. Hydrochlorothiazide, which had a therapeutic effect on the patient, did not prevent paralysis in vitro. An abnormal rise of the intracellular sodium activity was recorded when the extracellular potassium concentration was raised to 10 mmol/l.

Spontaneous opening at zero membrane potential of sodium channels from eel electroplax reconstituted into lipid vesicles

The voltage-dependent sodium channel from the eel electroplax was purified and reconstituted into vesicles of varying lipid composition. Isotopic sodium uptake experiments were conducted with vesicles at zero membrane potential, using veratridine to activate channels and tetrodotoxin to block them. Under these conditions, channel-dependent uptake of isotopic sodium by the vesicles was observed, demonstrating that a certain fraction of the reconstituted protein was capable of mediating ion fluxes. In addition, vesicles untreated with veratridine showed significant background uptake of sodium; a considerable proportion of this flux was blocked by tetrodotoxin. Thus these measurements showed that a significant subpopulation of channels was present that could mediate ionic fluxes in the absence of activating toxins. The proportion of channels exhibiting this behavior was dependent on the lipid composition of the vesicles and the temperature at which the uptake was measured; furthermore, the effect of temperature was reversible. However, the phenomenon was not affected by the degree of purification of the protein used for reconstitution, and channels in resealed electroplax membrane fragments or reconstituted solely into native eel lipids did not show this behavior. The kinetics of vesicular uptake through these spontaneously-opening channels was slow, and we attribute this behavior to a modification of sodium channel inactivation.

Paramyotonia congenita: successful treatment with tocainide. Clinical and electrophysiologic findings in seven patients

Seven patients with paramyotonia congenita (PC) from two families were studied. Voluntary exercise of the hand muscles was performed at different hand temperatures, both before and after treatment with tocainide. All patients developed stiffness, prolonged weakness, and small compound muscle action potentials (CMAPs) following exercise; the temperature at which this occurred was individually different. Two patients with PC and associated episodes of generalized weakness underwent potassium loading. A prolonged exercise test was performed both immediately before and 90 minutes after K-loading. Exercise-induced weakness and CMAP-decline occurred only with high serum K levels. Thiazide treatment in these two patients was ineffective. All seven patients responded well to tocainide. Treatment response and side effects were dose-dependent. Good clinical improvement has been maintained in all patients for more than 6 months, with relatively small doses of tocainide (400-1200 mg/day).

Electromyography in infants and children

Electromyography (EMG) is of proven value in the diagnosis of acute and chronic neuromuscular diseases in infants and children. When this technique is combined with nerve conduction studies, including repetitive nerve stimulation studies, it is often possible upon completion of the studies to identify the disorder as one of nerve, neuromuscular junction, or muscle. The purpose of this article is to review the principles and techniques of EMG in infants and children and to describe the EMG findings in several neuromuscular disorders.

Membrane currents in human intercostal muscle at varied extracellular potassium

Hyperpolarizing and depolarizing square steps were imposed on the membrane potential of excised human intercostal muscle fibers by means of a 3-microelectrode voltage clamp. The steady-state amplitudes of the membrane currents inducing such steps were investigated as a function of the membrane potential, while the muscle was bathed in solutions varying in potassium content (Ke = 1, 3.5, 7, 20, and 60 mM). At all potassium concentrations, the membrane acted as a rectifier, both in the inward- and outward-going directions. Inward currents were much reduced when Ke was lowered from 3.5 to 1 mM, and were increased when Ke was raised beyond 3.5 mM. The delayed outward current was reduced when Ke was increased from 3.5 mM to 7 mM and higher potassium concentration. The results were qualitatively similar to those reported for rat skeletal muscle.

Hypokalemic periodic paralysis: in vitro investigation of muscle fiber membrane parameters

To study the mechanism of attacks in familial hypokalemic paralysis, we recorded resting membrane potentials, action potentials, current-voltage relationships, and isometric forces in intercostal muscle fibers from three patients. In normal extracellular medium, the resting potential was reduced, but membrane conductance was not different from control. Excitability was reduced and the action potentials had no overshoot. On exposure to a 1-mM potassium solution, with or without insulin, the cells depolarized to about -50 mV, and became inexcitable. Over the tested membrane potential range from -120 to -40 mV, the slope conductance in the 1-mM potassium solution was not different from that of control fibers in a 1-mM potassium solution. In particular, the potassium component conductance was not reduced. Depolarized fibers could not be completely repolarized by returning to a 3.5-mM potassium solution. An experimentally induced transient shift of the chloride equilibrium potential to a highly negative value caused stable repolarization. Paralysis could also be induced by replacement of extracellular chloride with an impermanent anion, a treatment which causes myotonia in healthy fibers. It was concluded that the basic defects are a reduced excitability and an increased sodium conductance, and that these defects are aggravated on reduction of the extracellular potassium concentration.

Voltage clamp of rat and human skeletal muscle: measurements with an improved loose-patch technique

Intact fibres of human intercostal and rat omohyoid muscles were studied at 23 degree C with a loose-patch voltage-clamp technique that employed two concentric micropipettes to electrically isolate small-diameter (10-15 microns) patches of sarcolemma. This method allows investigation of membrane excitability under highly physiological conditions. Step depolarizations to 0 mV elicited sodium inward currents that reached peak values of up to 20 mA/cm2 within 250 microseconds, and then declined. In human muscle, the reversal potential (ENa) was approximately 40 mV, and maximal conductances (GNa) ranged from 44 to 360 mS/cm2. In rat muscle, ENa was 42 mV and GNa ranged from 100 to 250 mS/cm2. Sodium channels in rat and human muscle were indistinguishable in most aspects of their kinetic behaviour and voltage dependence. Outward potassium currents were small by comparison (usually less than 2 mA/cm2) and saturated at positive potentials. The maximum potassium conductance (GK) ranged from 0 to 19 mS/cm2 (human) and from 4 to 12 mS/cm2 (rat muscle).

The resting membrane parameters of human intercostal muscle at low, normal, and high extracellular potassium

Membrane parameters at the respective resting potentials in low, normal, and high extracellular potassium solutions were determined in intercostal muscle fibers from 15 patients with no known neuromuscular disease. In synthetic interstitial fluid (normal potassium concentration 3.5 mmol/liter), we found the following mean values: resting membrane potential RP = -83.3 mV, space constant lambda = 2364 micron, fiber diameter d = 49.3 micron, fiber input resistance Rin = 795 k omega, specific membrane capacitance Cm = 4.7 muF/cm2, and specific membrane resistance Rm = 5970 omega X cm2. The specific membrane conductance was gm = 168 muS/cm2, 76% of it being chloride conductance, 24% being potassium conductance. The dependence of the membrane parameters on the extracellular potassium concentration followed the predictions by the constant field theory. There was no indication of active chloride transport. The resting membrane conductance decreased with temperature with a Q10 of 1.3. Excitability parameters were nearly independent of temperature between 37 and 27 degrees C.