Hypokalemic periodic paralysis: an autosomal dominant muscle disorder caused by mutations in a voltage-gated calcium channel

Hypokalemic periodic paralysis – the importance of patient education

Hypokalemic periodic paralysis (HOKPP) is a rare neuromuscular disorder caused by altered transport of cellular potassium that leads to significant muscle weakness of the extremities. Paralytic attacks are induced by a drop in the serum potassium level and they have been associated with specific triggers. This case describes a 21-year-old male who has had recurrent presentations of acute paralytic attacks following vigorous physical activity. At presentation, this patient exhibited flaccid paralysis of all skeletal muscles below the neck, but was alert and oriented with stable vital signs. The patient was found to have a potassium level of 2.1 mmol/L and an EKG demonstrating U waves (characteristic of hypokalemia). The patient was treated with potassium supplementation with resolution of symptoms. The mainstay of prevention of long term permanent muscle weakness is avoidance of triggers that can lead to hypokalemia. Through education on disease process and lifestyle modifications, we were able to end the cycle of recurrent hospital readmissions and the subsequent financial burden this generated for the patient and his family.

Familial Hypokalemic Periodic Paralysis: Case Report

Hypokalemic periodic paralysis is a congenital disorder which is characterized by intermittent episodes of muscle weakness or paralysis. The attacks can occur everyday or once a year, may last for a few hours or for several days. Serum potassium level is low during the attack. But serum potassium levels are normal between two attacks. There is no potassium deficiency in the whole body. In this report, a 16 years old boy, whose grandfather, father and uncle had the same semptoms, and had his first attack of familial hypokalemic periodic paralysis following a grand exercise were presented according to the clinical and laboratory features.

Thyrotoxic periodic paralysis: case report and review of the literature

Introduction

Thyrotoxic periodic paralysis (TPP) is a rare and potentially lethal complication of hyperthyroidism. It is characterized by sudden onset paralysis associated with hypokalemia. Management includes prompt normalization of potassium, which results in resolution of the paralysis. Definitive treatment of hyperthyroidism resolves TPP completely.

Case presentation

A 23-year-old African American male patient presented to the emergency room at the University of Mississippi Medical Center, USA in November 2016 with sudden onset quadriplegia. He also endorsed a history of weight loss, palpitations, heat intolerance and tremors. The patient reported similar episodes of quadriplegia in the past, which were associated with hypokalemia and resolved with normalization of potassium levels. Physical examination was significant for exophthalmos, smooth goiter with bruit consistent with the diagnosis of Graves’ disease. Laboratory assessment showed severe hypokalemia, hypomagnesemia, suppressed thyroid stimulating hormone (TSH) and high free thyroxine (T4). Urine potassium creatinine ratio was less than one, indicating transcellular shift as the cause of hypokalemia. After normalization of potassium and magnesium, the paralysis resolved in 12 hours. He was started on methimazole. On follow up, the patient was clinically and biochemically euthyroid with no further episodes of paralysis.

Take-away lesson

TPP is a rare and reversible cause of paralysis. Physicians need to be aware of the diagnostic and treatment modalities as delayed recognition in treatment could result in potential harm or unnecessary interventions.

Thyrotoxic periodic paralysis: clinical and molecular aspects

Thyrotoxic periodic paralysis (TPP) is a rare complication of hyperthyroidism that most often affects young East Asian males but increasingly also in other ethnic groups. The typical presentation is acute attacks varying from mild weakness to total paralysis starting at night or in the early morning a few hours after a heavy meal, alcohol abuse or strenuous exercise with complete recovery within 72 h. Signs and symptoms of hyperthyroidism may not be obvious. The hallmark is hypokalemia from increased cellular sodium/potassium-ATPase pump activity with transport of potassium from the extracellular to the intracellular space in combination with reduced potassium output. Recently, KCNJ18 gene mutations which alter the function of an inwardly rectifying potassium channel named Kir2.6 have been detected in 0-33 % of cases. Hence, the pathophysiology in TPP includes a genetic predisposition, thyrotoxicosis and environmental influences and the relative impact from each of these factors may vary. The initial treatment, which is potassium supplementation, should be given with caution due to a high risk of hyperkalemia. Propranolol is an alternative first-line therapeutic option based on the assumption that hyperadrenergic activity is involved in the pathogenesis. If thyroid function tests are unobtainable in the acute situation the diagnosis is supported by the findings of hypokalemia, low spot urine potassium excretion, hypophosphatemia with hypophosphaturia, high spot urine calcium/phosphate ratio, and electrocardiographic abnormalities as tachycardia, atrial fibrillation, high QRS voltage, and atrioventricular block. Definitive treatment is cure of the hyperthyroidism. The underlying mechanisms of TPP remain, however, incompletely understood awaiting further studies.

An atypical phenotype of hypokalemic periodic paralysis caused by a mutation in the sodium channel gene SCN4A

Familial hypokalemic periodic paralysis is an autosomal-dominant channelopathy characterized by episodic muscle weakness with hypokalemia. The respiratory and cardiac muscles typically remain unaffected, but we report an atypical case of a family with hypokalemic periodic paralysis in which the affected members presented with frequent respiratory insufficiency during severe attacks. Molecular analysis revealed a heterozygous c.664 C>T transition in the sodium channel gene SCN4A, leading to an Arg222Trp mutation in the channel protein. The patients described here presented unusual clinical characteristics that included a severe respiratory phenotype, an incomplete penetrance in female carriers, and a different response to medications.

Severe respiratory phenotype caused by a de novo Arg528Gly mutation in the CACNA1S gene in a patient with hypokalemic periodic paralysis

Hypokalemic periodic paralysis (HOKPP) is a rare disorder characterized by episodic muscle weakness with hypokalemia. Mutations in the CACNA1S gene, which encodes the alpha 1-subunit of the skeletal muscle L-type voltage-dependent calcium channel, have been reported to be mainly responsible for HOKPP. The paralytic attacks generally spare the respiratory muscles and the heart. Here, we report the case of a 16-year-old boy who presented with frequent respiratory insufficiency during the severe attacks. Mutational analysis revealed a heterozygous c.1582C>G substitution in the CACNA1S gene, leading to an Arg528Gly mutation in the protein sequence. The parents were clinically unaffected and did not show a mutation in the CACNA1S gene. A de novo Arg528Gly mutation has not previously been reported. The patient described here presents the unique clinical characteristics, including a severe respiratory phenotype and a reduced susceptibility to cold exposure. The patient did not respond to acetazolamide and showed a marked improvement of the paralytic symptoms on treatment with a combination of spironolactone, amiloride, and potassium supplements.

Reduced expression and abnormal localization of the K(ATP) channel subunit SUR2A in patients with familial hypokalemic periodic paralysis

Familial hypokalemic periodic paralysis is an autosomal-dominant channelopathy that features episodic attacks of flaccid paralysis with concomitant hypokalemia. Reduced activity of ATP-sensitive K(+) (K(ATP)) channels is suggested to be responsible for this disorder; however, the molecular mechanisms have not yet been elucidated. In this study, we investigated the molecular mechanism of reduced K(ATP) channel activity in skeletal muscle cells of patients with familial hypokalemic periodic paralysis. We examined the mRNA and protein levels of SUR2A, a K(ATP) channel subunit, in cells from patients (patient cells) and normal individuals (normal cells). Our results demonstrated that normal cells exposed to 50mM potassium buffer, which was used to induce depolarization, did not show significant change in the SUR2A mRNA levels; however, the protein level significantly increased in the cytosolic fraction. When the patient cells were exposed to 50mM potassium buffer, the SUR2A mRNA level significantly decreased. Further, the protein level of SUR2A significantly increased in the membrane fraction but decreased in the cytosolic fraction in patient cells. These findings suggest that abnormal localization of the SUR2A K(+) channel protein leads to reduced K(ATP) channel activity in familial hypokalemic periodic paralysis.

Atypical arrhythmic complications in familial hypokalemic periodic paralysis

Familial hypokalemic periodic paralysis is an autosomal dominant muscle disorder characterized by episodic attacks of muscle weakness, accompanied by a decrease in blood potassium levels. It is based on genetic mutations in the genes CACNA1S (most frequent, encoding the skeletal muscle calcium channel) and SCN4A (10% of cases, encoding the sodium channel). Few cases have been reported with cardiac dysrhythmia. We report a rare case of a patient with a novel SCN4A mutation who presented, on ECG, extreme bradycardia and syncopal sinus arrest that required a temporary pacemaker implant

Presynaptic calcium channels: structure, regulators, and blockers

The central and peripheral nervous systems express multiple types of ligand and voltage-gated calcium channels (VGCCs), each with specific physiological roles and pharmacological and electrophysiological properties. The members of the Ca(v)2 calcium channel family are located predominantly at presynaptic nerve terminals, where they are responsible for controlling evoked neurotransmitter release. The activity of these channels is subject to modulation by a number of different means, including alternate splicing, ancillary subunit associations, peptide and small organic blockers, G-protein-coupled receptors (GPCRs), protein kinases, synaptic proteins, and calcium-binding proteins. These multiple and complex modes of calcium channel regulation allow neurons to maintain the specific, physiological window of cytoplasmic calcium concentrations which is required for optimal neurotransmission and proper synaptic function. Moreover, these varying means of channel regulation provide insight into potential therapeutic targets for the treatment of pathological conditions that arise from disturbances in calcium channel signaling. Indeed, considerable efforts are presently underway to identify and develop specific presynaptic calcium channel blockers that can be used as analgesics.

The genotype and clinical phenotype of Korean patients with familial hypokalemic periodic paralysis

Familial hypokalemic periodic paralysis (HOPP) is a rare autosomal-dominant disease characterized by reversible attacks of muscle weakness occurring with episodic hypokalemia. Mutations in the skeletal muscle calcium (CACNA1S) and sodium channel (SCN4A) genes have been reported to be responsible for familial HOPP. Fifty-one HOPP patients from 20 Korean families were studied to determine the relative frequency of the known mutations and to specify the clinical features associated with the identified mutations. DNA analysis identified known mutations in 12 families: 9 (75%) were linked to the CACNA1S gene and 3 (25%) to the SCN4A gene. The Arg528His mutation in the CACNA1S gene was found to be predominant in these 12 families. Additionally, we have detected one novel silent exonic mutation (1950C>T) in the SCN4A gene. As for a SCN4A Arg669His mutation, incomplete penetrance in a woman was observed. Characteristic clinical features were observed both in patients with and without mutations. This study presents comprehensive data on the genotype and phenotype of Korean families with HOPP.

À propos de deux cas de paralysie périodique hypokaliémique

We report on two cases of hypokaliemic periodic paralysis due to a potassium shift from the extracellular to the intracellular compartment of skeletal muscle cells. The first case occurred in a 15-year-old boy who experienced rapid onset flaccid tetraplegia without neurological abnormalities. Physical exam revealed facial dysmorphy, and EKG a long QT. Biology evidenced shift hypokalemia that was quickly reversible after administration of intravenous potassium. After exclusion of Andersen-Tawil syndrom, hypokalemic familial paralysis (Westphall disease) was diagnosed by molecular genetic testing (disease-causing mutation in CACNA1S) in the proband and in three other family members. The second case occurred in a 24-year-old male who experienced rapid onset flaccid tetraplegia due to intracellular potassium shift that was quickly reversible after administration of intravenous potassium. Biology revealed thyrotoxicosis due to Grave's disease. To the best of our knowledge, this is the first case described in a people from pacific origin. The clinical, biological, and electromyographic findings of the most frequent causes of periodic paralysis are underlined as well as the molecular genetic diagnosis in familial forms.

PATHOMECHANISMS IN CHANNELOPATHIES OF SKELETAL MUSCLE AND BRAIN

Ion channelopathies are a diverse array of human disorders caused by mutations in ion channel genes. This review focuses on the pathogenic mechanisms of channelopathies affecting skeletal muscle and brain arising from mutations of voltage-gated ion channels and fast ligand-gated ion channels expressed at the surface membrane. Derangements in channel function alter the electrical excitability of the cell and thereby increase susceptibility to transient symptomatic attacks including myasthenia, periodic paralysis, myotonic stiffness, seizures, headache, dyskinesia, or episodic ataxia. Although these disorders are rare, they stand out as exemplary cases for which disease pathogenesis can be traced from a point mutation to altered protein function, to altered cellular activity, and to clinical phenotype. The study of these disorders has provided insights on channel structure-function relations, the physiological roles of ion channels, and rational approaches toward therapeutic intervention for many disorders of cellular excitability.

Skeletal muscle dihydropyridine-sensitive calcium channel (CACNA1S) gene mutations in chinese patients with hypokalemic periodic paralysis

BACKGROUND

Thyrotoxic periodic paralysis (TPP), familial periodic paralysis (FPP), and sporadic periodic paralysis (SPP) are common causes of hypokalemic periodic paralysis and have similar clinical presentations, thus possibly sharing the identical mutations.

METHODS

We analyzed the role of the three known CACNA1S gene mutations (R528H, R1239H, and R1239G) in Chinese patients, including two FPP families, 36 TPP patients, 12 SPP patients, and their relatives. Fifty unrelated healthy subjects were also studied. Genomic DNA was prepared from the peripheral blood of all patients, their family members, and healthy subjects. Mutations of the CACNA1S gene were screened using polymerase chain reaction-based restriction analysis.

RESULTS

Two FPP families had the R528H point mutation, but with incomplete penetrance occurring more commonly in men than in women. Only one SPP patient had a de novo mutation (R528H). None of the TPP patients had mutations in the three hot spots.

CONCLUSION

Patients with FPP have R528H mutations in the CACNA1S gene. Only a few patients with SPP may share similar mutations with FPP. TPP patients do not carry any of the three known gene mutations.

A Korean family of hypokalemic periodic paralysis with mutation in a voltage-gated calcium channel (R1239G)

Hypokalemic periodic paralysis (HOPP) is a rare disease characterized by reversible attacks of muscle weakness accompanied by episodic hypokalemia. Recent molecular work has revealed that the majority of familial HOPP is due to mutations in a skeletal muscle voltage-dependent calcium-channel: the dihydropyridine receptor. We report a 13-yr old boy with HOPP from a family in which 6 members are affected in three generations. Genetic examination identified a nucleotide 3705 C to G mutation in exon 30 of the calcium channel gene, CACNA1S. This mutation predicts a codon change from arginine to glycine at the amino acid position #1239 (R1239G). Among the three known mutations of the CACNA1S gene, the R1239G mutation was rarely reported. This boy and the other family members who did not respond to acetazolamide, showed a marked improvement of the paralytic symptoms after spironolactone treatment.

[Hypokalemic paralysis with thyrotoxicosis]

Hypokalemic periodic paralysis as a complication of thyrotoxicosis (thyrotoxic periodic paralysis) most often occurs in east Asian men. It is characterised by recurrent episodes of flaccid paralysis, hypokalemia, and underlying hyperthyroidism. It needs to be distinguished from sporadic and familial forms of periodic hypokalemic paralysis. No disturbances in the acid-base state and no extracorporal potassium loss are present. We report on the typical case of a young Chinese man presenting with hypokalemic periodic paralysis associated with yet unknown Graves' disease. Intravenous substitution of potassium and oral propranolol were administered. Complete remission was achieved after 10 hours. After medical therapy had normalised thyroid hormone levels, no further hypokalemic paralytic attacks occurred.

[Genetic diversity of voltage-gated calcium channels]

Understanding of the properties of normal and diseased voltage-dependent calcium channels has greatly improved these last Years after the extensive development of the patch-clamp and molecular biology studies and the functional expression strategies. The calcium channel diversity is based on the expression of numerous genes that encode pore channel subunits (10 genes) and auxiliary/regulatory subunits (16 genes). In addition, most of these genes are subject to alternative splicing. The study of calcium channels has also benefited from the discovery of genetic diseases linked to calcium channel mutations: the calcium channelopathies. The review describes the recent data and working hypothesis that address the challenging question of how the calcium channel diversity occurs and how alterations in channel function lead to selective cellular dysfunction.

Visualization of the domain structure of an L-type Ca2+ channel using electron cryo-microscopy

The three-dimensional structure of the skeletal muscle voltage-gated L-type calcium channel (Ca(v)1.1; dihydropyridine receptor, DHPR) was determined using electron cryo-microscopy and single-particle averaging. The structure shows a single channel complex with an approximate total molecular mass of 550 kDa, corresponding to the five known subunits of the DHPR, and bound detergent and lipid. Features visible in our structure together with antibody labeling of the beta and alpha(2) subunits allowed us to assign locations for four of the five subunits within the structure. The most striking feature of the structure is the extra-cellular alpha(2) subunit that protrudes from the membrane domain in close proximity to the alpha(1) subunit. The cytosolic beta subunit is located close to the membrane and adjacent to subunits alpha(1), gamma and delta. Our structure correlates well with the functional and biochemical data available for this channel and suggests a three-dimensional model for the excitation-contraction coupling complex consisting of DHPR tetrads and the calcium release channel.

3D structure of the skeletal muscle dihydropyridine receptor

The dihydropyridine receptors (DHPR) are L-type voltage-gated calcium channels that regulate the flux of calcium ions across the cell membrane. Here we present the three-dimensional (3D) structure at approximately 27A resolution of purified skeletal muscle DHPR, as determined by electron microscopy and single particle analysis. Here both biochemical and 3D structural data indicate that DHPR is dimeric. DHPR dimers are composed of two arch-shaped monomers approximately 210A across and approximately 75A thick, that interact very tightly at each end of the arch. The roughly toroidal structure of the two monomers encloses a cylindrical space of approximately 80A diameter, which is then closed on each side by two dome-shaped protein densities reaching over from each monomer arch. The dome-shaped domains have a length of approximately 80-90A and a maximum height of approximately 45A. Small orifices punctuate their exterior surface. The 3D structure disclosed here may have important implications for the understanding of DHPR Ca(2+) channel function. We also propose a model for its in vivo interactions with the calcium release channel at the junctional sarcoplasmic recticulum.

Mutations linked to familial hypokalaemic periodic paralysis in the calcium channel alpha1 subunit gene (Cav1.1) are not associated with thyrotoxic hypokalaemic periodic paralysis

OBJECTIVE

To investigate whether patients with thyrotoxic hypokalaemic periodic paralysis (THPP) have the same molecular defect in the calcium channel gene described in familial hypokalaemic periodic paralysis (FHPP), as the symptoms of both diseases are comparable, we analysed, in patients with THPP, the presence of mutations R528H, R1239H and R1239G on the S4 voltage-sensing transmembrane segment of the alpha1 subunit of the calcium channel gene (Cav1.1).

DESIGN AND PATIENTS

Genomic DNA was extracted from peripheral blood from 14 patients with THPP, 13 sporadic cases and one with a family history. An FHPP family was selected as a positive control. The exons bearing the described mutations were amplified by PCR, screened by single-strand conformation polymorphism (SSCP), and further sequenced.

MEASUREMENTS

THPP was diagnosed both clinically and through laboratory tests, all patients having elevated levels of thyroid hormones (T4, T3 or free T4), suppressed TSH and plasma potassium below 3 small middle dot5 mmol/l.

RESULTS

No evidence of the described mutations was found in patients with THPP. Furthermore, we did not detect any mutations in any of the four full S4 voltage-sensing transmembrane segments of Cav1 small middle dot1 (DIS4, DIIS4, DIIIS4 and DIVS4) by direct sequencing. However, close to the R528H mutation, we identified two single nucleotide polymorphisms at nucleotides 1551 and 1564 in both familial and sporadic cases with THPP. In addition, we were able to detect the R528H mutation in the DIIS4 transmembrane segment in all members of the FHPP family.

CONCLUSION

Mutations linked to familial hypokalaemic periodic paralysis in the calcium channel alpha1 subunit gene (Cav1.1) are not associated with thyrotoxic hypokalaemic periodic paralysis. However, polymorphisms in nucleotides 1551 and 1564 in the exon 11 were found in patients with familial hypokalaemic periodic paralysis and thyrotoxic hypokalaemic periodic paralysis in higher frequency than in controls. The polymorphisms identified within the Cav1.1 gene are associated with thyrotoxic hypokalaemic periodic paralysis and represent a novel finding.

Peripheral channelopathies as targets for potassium channel openers

Potassium channel openers (KCOs) are important tools that are often used to gain a greater understanding of K(+) channels. Agents that can induce or maintain the opening of K(+) channels also offer a therapeutic approach to controlling of cell excitability and offer a means of producing stability in biological systems. The pathogenesis of a broad range of peripheral disorders (e.g., LQT syndrome, hypokalemic periodic paralysis, hyperinsulinism in infancy and erectile dysfunction) are associated with dysfunctional K(+) channels due to mutations in genes encoding channel proteins. The therapeutic potential of KCOs in peripheral K(+) channelopathies is discussed. The identification of K(+) channel subtype-specific openers offers discrete modulation of cellular systems creating a realistic therapeutic advance in the treatment of K(+) channelopathies.

Identification of mutations including de novo mutations in Korean patients with hypokalaemic periodic paralysis

BACKGROUND

Hypokalaemic periodic paralysis (hypoPP) is an autosomal dominant disorder involving the abnormal function of ion channels and it is characterized by paralysis attacks of varying severity, accompanied by a fall in blood potassium levels. Linkage analysis showed that the candidate locus responsible for hypoPP was localized to chromosome 1q31-32, and this locus encoded the muscle dihydropyridine-sensitive calcium channel alpha(1)-subunit (CACNA1S). So far, three different mutations in CACNA1S gene have been identified in patients with hypoPP: Arg528His, Arg1239His and Arg1239Gly in Caucasian patients. However, there are few reports about the mutations of CACNA1S gene in other races.

METHODS

In this study, four Korean families with five hypoPP patients were screened for mutations of CACNA1S gene with polymerase chain reaction-based restriction analysis and single-strand conformation polymorphism analysis. To determine the mode of inheritance, haplotype analysis was done with three microsatellite markers (D1S1726, CACNL1A3, and D1S1723).

RESULTS

Arg528His mutation was detected in three families, and one family had no known mutations. Moreover, for the first time, we detected de novo Arg528His mutations in two out of three families with hypoPP. Haplotype analysis using three microsatellite markers (D1S1726, CACNL1A3, and D1S1723) suggested the occurrence of de novo Arg528His mutations in two of the three families with Arg528His mutation.

CONCLUSIONS

Arg528His mutations of CACNA1S, including de novo Arg528His mutations, were found in Korean patients with hypoPP. These results imply that de novo mutation, in addition to non-penetrance, is one of the genetic mechanisms that can explain the previous clinical observation that hypoPP occurs sporadically without family history.

Impairment of skeletal muscle adenosine triphosphate-sensitive K+ channels in patients with hypokalemic periodic paralysis

The adenosine triphosphate (ATP)-sensitive K+ (KATP) channel is the most abundant K+ channel active in the skeletal muscle fibers of humans and animals. In the present work, we demonstrate the involvement of the muscular KATP channel in a skeletal muscle disorder known as hypokalemic periodic paralysis (HOPP), which is caused by mutations of the dihydropyridine receptor of the Ca2+ channel. Muscle biopsies excised from three patients with HOPP carrying the R528H mutation of the dihydropyridine receptor showed a reduced sarcolemma KATP current that was not stimulated by magnesium adenosine diphosphate (MgADP; 50-100 microM) and was partially restored by cromakalim. In contrast, large KATP currents stimulated by MgADP were recorded in the healthy subjects. At channel level, an abnormal KATP channel showing several subconductance states was detected in the patients with HOPP. None of these were surveyed in the healthy subjects. Transitions of the KATP channel between subconductance states were also observed after in vitro incubation of the rat muscle with low-K+ solution. The lack of the sarcolemma KATP current observed in these patients explains the symptoms of the disease, i.e., hypokalemia, depolarization of the fibers, and possibly the paralysis following insulin administration.

Genetic analysis of voltage-dependent calcium channels

Molecular cloning of calcium channel subunit genes has identified an unexpectedly large number of genes and splicing variants, and a central problem of calcium channel biology is to now understand the functional significance of this genetic complexity. While electrophyisological, pharmacological, and molecular cloning techniques are providing one level of understanding, a complete understanding will require many additional kinds of studies, including genetic studies done in intact animals. In this regard, an intriguing variety of episodic diseases have recently been identified that result from defects in calcium channel genes. A study of these diseases illustrates the kind of insights into calcium channel function that can be expected from this method of inquiry.

The biophysical and pharmacological characteristics of skeletal muscle ATP-sensitive K+ channels are modified in K+-depleted rat, an animal model of hypokalemic periodic paralysis

We evaluated the involvement of the sarcolemmal ATP-sensitive K+ channel in the depolarization of skeletal muscle fibers occurring in an animal model of human hypokalemic periodic paralysis, the K+-depleted rat. After 23-36 days of treatment with a K+-free diet, an hypokalemia was observed in the rats. No difference in the fasting insulinemia and glycemia was found between normokalemic and hypokalemic rats. The fibers of the hypokalemic rats were depolarized. In these fibers, the current of sarcolemmal ATP-sensitive K+ channels measured by the patch-clamp technique was abnormally reduced. Cromakalim, a K+ channel opener, enhanced the current and repolarized the fibers. At channel level, two open conductance states blocked by ATP and stimulated by cromakalim were found in the hypokalemic rats. The two states could be distinguished on the basis of their slope conductance and open probability and were never detected on muscle fibers of normokalemic rats. It is known that insulin in humans affected by hypokalemic periodic paralysis leads to fiber depolarization and provokes paralysis. We therefore examined the effects of insulin at macroscopic and single-channel level on hypokalemic rats. In normokalemic animals, insulin applied in vitro to the muscles induced a glybenclamide-sensitive hyperpolarization of the fibers and also stimulated the sarcolemmal ATP-sensitive K+ channels. In contrast, in hypokalemic rats, insulin caused a pronounced fiber depolarization and reduced the residual currents. Our data indicated that in hypokalemic rats, an abnormally low activity of ATP-sensitive K+ channel is responsible for the fiber depolarization that is aggravated by insulin.