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

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

K(ATP) channel therapeutics at the bedside

The family of potassium channel openers regroups drugs that share the property of activating adenosine triphosphate-sensitive potassium (K(ATP)) channels, metabolic sensors responsible for adjusting membrane potential-dependent functions to match cellular energetic demands. K(ATP) channels, widely represented in metabolically-active tissue, are heteromultimers composed of an inwardly rectifying potassium channel pore and a regulatory sulfonylurea receptor subunit, the site of action of potassium channel opening drugs that promote channel activity by antagonizing ATP-induced pore inhibition. The activity of K(ATP) channels is critical in the cardiovascular adaptive response to stress, maintenance of neuronal electrical stability, and hormonal homeostasis. Thereby, K(ATP) channel openers have a unique therapeutic spectrum, ranging from applications in myopreservation and vasodilatation in patients with heart or vascular disease to potential clinical use as bronchodilators, bladder relaxants, islet cell protector, antiepileptics and promoters of hair growth. While the current experience in practice with potassium channel openers remains limited, multitude of ongoing investigations aims at defining the benefit of this emerging family of therapeutics in diverse disease conditions associated with metabolic distress.

Drugs to facilitate recovery of neuromuscular blockade and muscle strength

Several drugs that quicken recovery from neuromuscular blockade caused by vecuronium in anesthetized patients are reviewed. Ulinastatin, a protease inhibitor, is thought to promote the release of acetylcholine at the neuromuscular junction and increases hepatic blood flow and urine volume. For this reason, ulinastatin quickens recovery from neuromuscular blockade in anesthetized patients receiving vecuronium. Additionally, pretreatment with ulinastatin avoids prolongation of vecuronium-induced neuromuscular blockade in patients with hepatic cirrhosis. Gabexate mesilate is also a protease inhibitor. During a continuous infusion of gabexate mesilate, recovery from neuromuscular blockade was quickened. Amino acid-enriched solution supplies energy to the skeletal muscles and causes an increase in muscle strength. An infusion of amino acid-enriched solution hastens recovery from neuromuscular blockade in anesthetized patients. When amino acids supply energy to the skeletal muscles, they simultaneously produce heat in the skeletal muscles. This thermal generation may be closely related to fast recovery from neuromuscular blockade. Amino acid-enriched solution makes recovery from neuromuscular blockade quick and avoids hypothermia during general anesthesia. Milrinone, a phosphodiesterase III inhibitor, is supposed to increase the release of acetylcholine at the neuromuscular junction and make the neuromuscular junction sensitive to acetylcholine. Therefore, recovery from neuromuscular blockade is hastened. Nicorandil enhances membrane K+ conductance in skeletal muscle and increases contraction of the skeletal muscle. Thus, nicorandil quickens recovery from neuromuscular blockade.

Nicorandil accelerates recovery of neuromuscular block caused by vecuronium

PURPOSE

To examine the effect of nicorandil, a K ATP channel agonist, on neuromuscular block caused by vecuronium in patients anesthetized with nitrous oxide, oxygen, isoflurane, and fentanyl.

METHODS

Sixty adult patients were allocated to four groups of 15: nicorandil-post-tetanic count (N-PTC), nicorandil-train-of-four (N-TOF), control-post-tetanic count (C-PTC) or control-train-of-four (C-TOF) group. In the N-PTC and N-TOF groups, 0.1 mg kg nicorandil was given as a bolus followed by an infusion at 1 microg x kg(-1) x min(-1). Two minutes after the bolus, 0.1 mg x kg(-1) vecuronium was administered. In the C-PTC or C-TOF group normal saline was given instead of nicorandil. PTC and TOF responses were measured mechanically using a force displacement transducer.

RESULTS

Time from the administration of vecuronium to the onset of neuromuscular block in the N-PTC or N-TOF group did not differ from that in the C-PTC or C-TOF group (241 +/- 33 vs 225 +/- 32 sec, mean +/- SD). Times from vecuronium injection to the return of PTC in the N-PTC and C-PTC groups, and those of T1, T2, T3, and T4 (first, second, third, and fourth stimulation of TOF) in the N-TOF and C-TOF groups did not differ. Recoveries of PTC in the N-PTC and C-PTC groups followed similar time course. T1/control twitch height and TOF ratio (T4/T1) in the N-TOF group were higher than those in the C-TOF group 80-120 min and 100-120 min after administration of vecuronium, respectively.

CONCLUSION

Nicorandil accelerates recovery of neuromuscular block caused by vecuronium.

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.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Trauma-induced changes of skeletal muscle membrane: decreased K+ and increased Na+ permeability

Trauma of skeletal muscle causes membrane depolarization and reduces membrane resistance. The underlying mechanisms were studied in isolated mouse phrenic nerve diaphragms subject to sharp transections of muscle. Depolarization was most marked at the vicinity ( approximately 1 mm) of trauma, where the membrane potential dropped rapidly from about -80 mV to zero and repolarized to about -25 mV. At the end-plate region (located approximately 3 mm away from the cut end), the membrane gradually attained a plateau potential around -45 mV. The magnitude of depolarization was not reduced by inhibition of Na+, Ca2+, or Cl- channel, whereas the progress of depolarization was delayed in low-Na+ medium. Activation of the K+ channel with lemakalim induced some hyperpolarization at damaged site but produced a glybenclamide-sensitive outward current and hyperpolarization of end-plate region to the levels before trauma, as if there was no diminution of transmembrane K+ gradient in this area. Appropriate elevation of extracellular K+ to stimulate K+ conductance also hyperpolarized the end-plate region. The results suggest that depolarization at regions remote from trauma is related to decreased K+ and increased Na+ permeability. The cytoplasma compartmentalization and permeability changes may protect muscle fiber from trauma.

Prevention by cromakalim of spontaneously occurring cardiac necroses in polymyopathic hamsters

Previous studies on the heart necrotizing process at early stages of the hamster polymyopathy have led us to believe that this hereditary disease derives from a defective transmembrane ion flux resulting in myocardial Ca2+ over-load. On the other hand, certain K+ ATP channel openers were shown to prevent cytosolic Ca2+ accumulation in ischemic hearts. Therefore, we investigated the potential beneficial effect of chronic treatment with cromakalim (CR) on the development of necrotic changes in hamster myopathic hearts. Young cardiomyopathic (CM) hamsters were treated parenterally with CR over 4 consecutive weeks. The K+ ATP opener was dissolved in 5% DMSO and injected twice daily (s.c. and i.p. alternatively) at a dose level of 2.5 mg/kg per injection. Microscopic readings were carried out in staged serial paraffin sections of heart ventricles, the diaphragm, and tongue, will all tissues freshly taken at autopsy. In comparison with control untreated hearts, which exhibit numerous necrotic calcific foci, only minute myolytic lesions were found in 5 of 12 hamsters hearts receiving CR (p < 0.0001). Interestingly, the dystrophic process in the tongue was significantly less severe (p < 0.0004) in CR-treated animals. These observations provide evidence for the first time that in vivo sustained treatment with a K+ ATP opener exerts cardioprotection upon development of the hamster hereditary cardiomyopathy.

Channelopathies: the nondystrophic myotonias and periodic paralyses

The term channelopathy does not indicate a new group of neuromuscular conditions, but a re-orientation of well- and long-known muscular conditions, the congenital myotonias, and the periodic paralyses. Although, in the past, they have overlapped clinically here and there, both groups were classified differently, as myotonias and as metabolic myopathies, respectively. The discovery of mutations in several ion channels has rewritten nosography of these disorders and procured a new term, the channelopathy-clinical, electrophysiological, and molecular genetic details of which are discussed in this chapter.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Coexistence of two classes of glibenclamide-inhibitable ATP-regulated K+ channels in avian skeletal muscle

Avian skeletal muscle expresses two types of ATP-sensitive K+ channels which have a unitary conductance of 15pS. These K+ channels can be distinguished pharmacologically by their high or low sensitivity to the antidiabetic sulphonylurea blocker glibenclamide. Both channels are activated by the K+ channel opener cromakalim. Chick skeletal muscle expresses high-affinity binding sites for [3H]glibenclamide (Kd = 0.6nM) which presumably correspond to the ATP-sensitive K+ channels with the greatest sensitivity to glibenclamide. The density of these high-affinity binding sites varies during muscle development. The maximum density (500fmol/mg protein) appears at 16 days in ovo, i.e. at a period when myoblasts have differentiated into myotubes and when innervation of myotubes has started. After this maximum, the level of [3H]glibenclamide-binding sites decreases to a plateau value of 100fmol/mg protein at 2-5 days post-natal. When muscle cells are put in cultures, the high-affinity binding sites disappear rapidly. Neither glibenclamide nor cromakalim have any effect on normal physiological chick muscle contraction. They have no effect on contracture and/or 86Rb+ efflux produced by metabolic poisoning.

Activation of ATP-dependent K+ channels by metabolic poisoning in adult mouse skeletal muscle: role of intracellular Mg(2+) and pH

1. The effects of metabolic poisoning, intracellular Mg(2+) and pH on ATP-dependent K+ (K+ATP) channels were examined in adult mouse isolated skeletal muscle fibres using the patch clamp technique. 2. In cell-attached membrane patches, while openings of one kind of channel could only rarely be detected under control conditions, cell poisoning with fluorodinitrobenzene (FDNB), dinitrophenol (DNP) and cyanide (CN) induced a strong and partially reversible increase in channel activity. 3. Slope conductance and glibenclamide sensitivity of this outward current indicated that the channel activated during poisoning was the K+ATP channel. 4. Single channel current amplitude was reduced during poisoning, but remained unchanged when activation of the K+ATP channel was induced by cromakalim. 5. In inside-out membrane patches, in the absence of intracellular ATP, intracellular application of Mg2+ decreased channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 6. In the absence of intracellular ATP, a decrease in intracellular pH induced a reduction in channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 7. The reduction of single channel current amplitude during poisoning was attributed to an increase in intracellular Mg2+ concentration caused by a fall in intracellular ATP concentration. These results also show that metabolic poisoning causes direct activation of K+ATP channels in skeletal muscle, and that is activation is at least partially mediated through an increase in intracellular Mg(2+) concentration and a decrease in intracellular pH.

Biophysical, pharmacological and developmental properties of ATP-sensitive K+ channels in cultured myotomal muscle cells from Xenopus embryos

Unlike mammalian muscle cells in culture, cultured myotomal muscle cells of Xenopus embryos express ATP-sensitive K+ (KATP) channels. The KATP channels are blocked by internal ATP (half-maximal inhibition K0.5 = 16 microM) and to a lesser extent by internal ADP, are voltage independent, have an inward rectification at positive potentials and are inhibited by glibenclamide (K0.5 = 2 microM). Surprisingly, these KATP channels are not sensitive to K+ channel openers such as cromakalim. Opening of these KATP channels does not occur under normal physiological conditions. It is elicited by metabolic exhaustion of the muscle cell and it precedes the development of an irreversible rigor state. Neither intracellular acidosis nor an increase of intracellular Ca2+ are involved in KATP channel opening. Different types of K+ channels are successively expressed after plating of myotomal muscle cells: (1) sustained delayed-rectifier K+ channels; (2) KATP channels; (3) inward-rectifier K+ channels; (4) transient delayed-rectifier K+ channels. The current density associated with KATP channels far exceeds that of voltage-dependent K+ channels. Innervation controls the expression of these KATP channels. Co-culture of muscle cells with neurons from the neural tube decreases the number of active KATP channels per patch. Similarly, in situ innervated submaxillaris muscle of tadpoles at stage 50-55 has a very low density of KATP channels.

Response of guinea pig smooth and striated urethral sphincter to cromakalim, prazosin, nifedipine, nitroprusside, and electrical stimulation

Prazosin (an alpha-1-adrenergic blocker) and cromakalim (potassium channel opener), given alone, induced significant fatigue of the urethral sphincter at a concentration of 10(-4) M; both drugs combined achieved a significant sphincteric fatigue at a concentration of 10(-5) M each. To 10(-4) M hexamethonium (ganglionic smooth muscle blocker) and 10(-4) M decamethonium (nicotinic blocker of striated muscle) the striated urethral sphincter responded like striated muscle with no detectable function of its smooth muscle component. Therefore, the striated component seems to play a dominant role in sphincteric function. With calcium depletion or in the presence of a calcium channel blocker (10(-4) M nifedipine) the urethral sphincter showed a relative enhancement of response to electrical field stimulation when compared with smooth and skeletal muscle, whose responses were both significantly reduced. This phenomenon could not be explained with calcium-dependent, inhibitory, nitric oxide-releasing nerves, as the NO-synthase blocker N-nitro-L-arginine (10(-5) M to 5 x 10(-5) M) failed to induce the enhancement of sphincter contraction during electrostimulation found with calcium depletion. Still, NO-releasing nerves might play a role in sphincteric relaxation because sodium nitroprusside (10(-5) M) induced a significant relaxation of the urethral sphincter precontracted with 80 mM potassium. The potential to weaken sphincteric closure with drugs, exemplified by the results obtained in response to prazosin and cromakalim, would represent a therapeutic advance in the patient with neurogenic bladder dysfunction.

Actions of cromakalim on outward currents of CA1 neurones in hippocampal slices

1. Membrane effects of cromakalim (Crom; 50-300 microM) were examined in CA1 neurones recorded mainly by intracellular, single-electrode voltage-clamping in slices (from Sprague-Dawley rats) kept in an interface chamber at 33 degrees C. 2. In 14 cells held at -63 +/- 3.5 mV, in the presence of tetrodotoxin, kynurenic acid and (in most cases) bicuculline, bath applied Crom produced no consistent change in holding current (-59 +/- 66 pA) or input conductance (GN) (-3.9 +/- 5.2%). 3. Overall there were no significant changes in instantaneous inward rectification or in Q-current inward relaxations. 4. In 18 out of 22 cells, outward currents, evoked by 0.5 s pulses to voltages > -50 and < -20 mV, were depressed by Crom (by 42 +/- 11%, for n = 22). Because this effect was consistently seen in Ca current-blocking media, containing either Mn and low Ca, or Cd (and also carbachol), the K channels depressed by Crom were probably of the delayed rectifier (IDR) type. 5. The Crom-control difference current (ICrom), obtained with slow depolarizing ramps, had a biphasic character, inward in the voltage (V) range > -50 < -20 mV (where outward currents are depressed by Crom) and tending outward for V > or = -20 mV. 6. In 10 out of 11 cells, Crom potentiated a D-like, slowly-inactivating outward current (by 88 +/- 31%, for n = 11). 7 The effects of Crom and of 2 min periods of anoxia were compared in 12 cells: unlike anoxia, Cromproduced no consistent increases in GN; the currents evoked in the same cells by anoxia differed significantly from those evoked by Crom (by 150 +/- 60 pA); the directions of current changes induced byCrom and anoxia respectively were not significantly correlated. Crom strongly depressed anoxic outward currents (by 80 +/- 12%, n = 4).8 Some Crom-induced effects (increases in D-like current and the outward current elicited at V>- 20 mV) were always reversed by tolbutamide (1 mM), but much less consistently by glibenclamide(10-30 microM).9 In conclusion, the effects of Crom, recorded with intracellular electrodes in CA1 neurones in slices,show little resemblance to the effects of activation of ATP-sensitive K channels.

Mechanism of action of a K+ channel activator BRL 38227 on ATP-sensitive K+ channels in mouse skeletal muscle fibres

1. Investigations were made into the effects of BRL 38227, a potassium channel activator, on ATP-sensitive potassium channels (K+ATP channels) in single fibres dissociated from the flexor digitorum brevis muscle of C57BL/6J mice. 2. In cell-attached patches BRL 38227 (100 microM) caused activation of a glibenclamide-sensitive potassium current. Linear slope conductance of the inward current, partial rectification of the outward current and glibenclamide sensitivity indicate that K+ATP channels are the site of action of BRL 38227. 3. In the absence of ATP at the cytoplasmic side of excised inside-out patches, BRL 38227 caused direct and magnesium-dependent activation of K+ATP channels. The degree of activation diminished with successive applications of BRL 38227. 4. BRL 38227 also caused activation of K+ATP channels in the presence of low (< 100 microM) but not high (1.0 mM) ATP, particularly in patches containing large numbers of channels. 5. BRL 38227 and 5 microM MgATP failed to activate channels following complete run-down. 6. Results show that BRL 38227 caused direct activation of K+ATP in skeletal muscle and that this was mediated through a magnesium-dependent binding site rather than alleviation of inhibition by competitive displacement of ATP from the inhibitory site.

Pharmacological properties of ATP-sensitive K+ channels in mammalian skeletal muscle cells

The patch-clamp technique (single-channel recordings) was used to study the effects of glibenclamide and some channel openers on the KATP channel in mouse skeletal muscle. In outside/out membrane patches, glibenclamide reversibly inhibited KATP channel activity in a dose-dependent manner with an apparent Ki of 190 nM. In inside/out membrane patches, RP 61419 increased KATP channel activity both in the absence and in the presence of internal ATP while other K+ channel openers such as nicorandil and cromakalim required the presence of internal ATP to evoke channel activation. The half-maximal activity effect for cromakalim, with 0.5 mM ATP at the cytoplasmic face, was observed at about 220 microM. Pinacidil was unable to activate the KATP channel in the absence of internal ATP and could even reduce channel opening in situations where activity was high in the control. In the presence of internal Mg2+, activation by pinacidil occurred when ATP or low and weakly activating concentrations of ADP were present at the cytoplasmic side. Pinacidil activation could also be observed in the presence of ATP or ADP when Mg2+ was absent from the internal solution. The mechanism of action of pinacidil is discussed in terms of interactions between the different nucleotide regulatory sites and the K+ channel opener binding site of the KATP channel. Half-maximum activation of the KATP channel in the presence of 0.5 mM ATP at the cytoplasmic face was observed at 125 microM pinacidil.

Effects of ATP-sensitive K(+)-channel activators on transmitter release parameters at the frog neuromuscular junction

The frog neuromuscular junction was used to study KATP channel action and its relation to transmitter release. Diazoxide (10 microM) and cromakalim (300 microM) decreased the amplitude of the end-plate potential (EPP) without significantly affecting the miniature end plate potential (MEPP) amplitude. Thus, there was a significant decrease in the quantal content of EPP after administration of the KATP channel activators. These agents did not alter MEPP frequency or resting membrane potential (RMP). The diazoxide-induced decrease in EPP amplitude was antagonized by the KATP blocker glibenclamide (10 microM) suggesting KATP channel involvement. Glibenclamide (10 microM) by itself caused a significant decrease in the RMP without significantly affecting the parameters of transmitter release. The diazoxide-induced decrease in EPP amplitude was not readily reversible with washing, however, when glibenclamide was added after diazoxide, the effect was easier to reverse. The results indicate that the activators exert their effect predominantly at the presynaptic region.

Nucleotide diphosphates activate the ATP-sensitive potassium channel in mouse skeletal muscle

Patch-clamp techniques were used to study the effects of internal nucleotide diphosphates on the KATP channel in mouse skeletal muscle. In inside-out patches, application of GDP (100 microM) and ADP (100 microM) reversibly increased the channel activity. In the presence of internal Mg2+ (1 mM), low concentrations of ADP (< 300 microM) enhanced channel activity and high concentrations of ADP (> 300 microM) limited channel opening while GDP activated the channel at all concentrations tested. In the absence of internal Mg2+, ADP decreased channel activity at all concentrations tested while GDP had no noticeable effect at submillimolar concentrations and inhibited channel activity at millimolar concentrations. GDP [beta S] (100 microM), which behaved as a weak GDP agonist in the presence of Mg2+, stimulated ADP-evoked activation whereas it inhibited GDP-evoked activation. The K+ channel opener pinacidil was found to activate the KATP channel but only in the presence of internal GDP, ADP and GDP [beta S]. The results are discussed in terms of the existence of multiple nucleotide binding sites, in charge of the regulation of the KATP channel.

Potassium channel openers: pharmacological and clinical aspects

Opening of plasmalemmal K+ channels leads to cellular hyperpolarization which, in excitable tissues possessing voltage-dependent Ca2+ channels, prevents the opening of such channels and thus prevents excitation. In the last few years, an increasing number of compounds have been identified which elicit their effects by opening K+ channels, preferentially in smooth muscle, but also in other excitable tissues. These include the novel benzpyrans, cromakalim and bimakalim, the thioformamide aprikalim, and also well known antihypertensives such as minoxidil sulphate, diazoxide and pinacidil. After a short overview of the various families of K+ channel openers (KCOs), their basic pharmacological properties, including inhibition by the sulfonyl ureas (such as glibenclamide) are presented. The actual discussion concerning the type of K+ channel(s) opened by these compounds and their mechanism(s) of vasorelaxation will be reported. The therapeutic potential of these compounds in the cardiovascular field (as antihypertensives and, in particular, as anti-ischemic agents in heart and skeletal muscle), and in asthma (where they reverse established airway hyperreactivity) will also be discussed. Improved tissue selectivity may be the essential pre-requisite for true clinical success of this class of compounds.

Effects of cromakalim on the membrane potassium permeability of frog skeletal muscle in vitro

1. The effects of the potassium channel opener, cromakalim, and its active enantiomer, lemakalim, have been investigated in frog skeletal muscle. 2. Cromakalim (30-300 microM) increased 86Rb efflux from muscles loaded with the isotope, hyperpolarized the fibres and reduced membrane resistance. 3. These effects were inhibited by the sulphonylureas, glibenclamide and tolbutamide. The IC50 for glibenclamide inhibition of 86Rb efflux was ca. 8 nM. 4. Phentolamine (300 microM) (which blocks responses to cromakalim in smooth muscle and inhibits ATP-sensitive K+ channels in pancreatic beta-cells) had no effect on the reduction in membrane resistance caused by 100 microM lemakalim. 5. Diazoxide (600 microM) had no effect on 86Rb efflux. 6. The similarities of the K+ channel activated by cromakalim in frog skeletal muscle to the channel acted on in smooth muscle and to the ATP-sensitive K+ channel of beta-cells are discussed.

Ca2+ release from isolated sarcoplasmic reticulum of guinea-pig psoas muscle induced by K(+)-channel blockers

A Ca(2+)-sensitive electrode was used to measure the Ca2+ concentration of the medium containing the heavy fraction of the fragmented sarcoplasmic reticulum (SR) prepared from guinea-pig psoas muscle. Among K(+)-channel blockers tested, 4-aminopyridine (4-AP), tetraethylammonium (TEA) and charybdotoxin elicited Ca2+ release from the SR, but apamin and glibenclamide did not. These results suggest that a reduction of SR K+ conductance leads to Ca2+ release from the SR.

ATP-dependent potassium channels of muscle cells: their properties, regulation, and possible functions

ATP-dependent potassium channels are present at high density in the membranes of heart, skeletal, and smooth muscle and have a low Popen at physiological [ATP]i. The unitary conductance is 15-20 pS at physiological [K+]o, and the channels are highly selective for K+. Certain sulfonylureas are specific blockers, and some K channel openers may also act through these channels. KATP channels are probably regulated through the binding of ATP, which may in turn be regulated through changes in the ADP/ATP ratio or in pHi. There is some evidence for control through G-proteins. The channels have complex kinetics, with multiple open and close states. The main effect of ATP is to increase occupancy of long-lived close states. The channels may have a role in the control of excitability and probably act as a route for K+ loss from muscle during activity. In arterial smooth muscle they may act as targets for vasodilators.

Skeletal muscle ATP-sensitive K+ channels recorded from sarcolemmal blebs of split fibers: ATP inhibition is reduced by magnesium and ADP

A new, nonenzymatically treated preparation of amphibian sarcolemmal blebs has been used to study the regulation of skeletal muscle ATP-sensitive K+ [K(ATP)] channels. When a frog skeletal muscle fiber is split in half in a Ca(2+)-free relaxing solution, large hemispherical membrane blebs appear spontaneously within minutes without need for Ca(2+)-induced contraction or enzymatic treatment. These blebs readily formed gigaseals with patch pipettes, and excised inside-out patches were found to contain a variety of K+ channels. Most prominent were K(ATP) channels similar to those found in the surface membrane of other muscle and nonmuscle cells. These channels were highly selective for K+, had a conductance of approximately 53 pS in 140 mM K+, and were blocked by internal ATP. The presence of these channels in most patches implies that split-fiber blebs are made up, at least in large part, of sarcolemmal membrane. In this preparation, K(ATP) channels could be rapidly and reversibly blocked by glibenclamide (0.1-10 microM) in a dose-dependent manner. These channels were sensitive to ATP in the micromolar range in the absence of Mg. This sensitivity was noticeably reduced in the presence of millimolar Mg, most likely because of the ability of Mg2+ ions to bind ATP. Our data therefore suggest that free ATP is a much more potent inhibitor of these channels than MgATP. Channel sensitivity to ATP was significantly reduced by ADP in a manner consistent with a competition between ADP, a weak inhibitor, and ATP, a strong inhibitor, for the same inhibitory binding sites. These observations suggest that the mechanisms of nucleotide regulation of skeletal muscle and pancreatic K(ATP) channels are more analogous than previously thought.

Activation of ATP-sensitive K channels by a K channel opener (SR 44866) and the effect upon electrical and mechanical activity of frog skeletal muscle

To examine the effects of the activation of adenosine 5'-triphosphate (ATP)-sensitive K channels in a skeletal muscle we have applied the ATP-sensitive K channel opener SR44866 whilst recording single ion channels, voltage-clamped membrane currents, evoked action potentials and tension in sartorius muscles of the frog. In excised inside-out membrane patches SR44866 opened channels which could be inhibited by internal ATP and glibenclamide. In voltage-clamped individual muscle fibres SR44866 evoked a glibenclamide-sensitive membrane current which reversed at -70 mV. The effect of SR44866 was dose dependent with an effective concentration for 50% maximal effect (EC50) of 67 microM and a slope factor of 2. SR44866 dose dependently reduced the duration of the spike after-potential, spike overshoot, Vmax, tetrodotoxin-sensitive voltage-gated inward membrane currents and muscle twitch tension. From this evidence it can be concluded that the opening of ATP-sensitive K channels may be associated with the inhibition of contraction of skeletal muscle.

Two different types of potassium channels in human skeletal muscle activated by potassium channel openers

The inside-out patch clamp technique was used to record the effects of K+ channel openers (EMD 52692, RP 49356 and Cromakalim) on single channel currents in membrane blebs of human skeletal muscle. Two types of K+ channels were activated by these drugs: an ATP-sensitive K+ channel which was inhibited by 3 mM ATP and 5 microM Glibenclamide and an ATP insensitive K+ channel. The open probability of both types was strongly increased by K+ channel openers. Glibenclamide antagonized the action of the K+ channel openers.

Tolbutamide excites rat glucoreceptive ventromedial hypothalamic neurones by indirect inhibition of ATP-K+ channels

1. The sulphonylureas, tolbutamide (0.1-10 mM) and glibenclamide (0.1-100 microM) shown not to inhibit ATP-K+ channel currents when applied to inside-out membrane patches excised from rat cultured cerebral cortex or freshly-dispersed ventromedial hypothalamic nucleus (VMHN) neurones. 2. Saturable binding sites for [3H]-glibenclamide, with similar affinity constants are present in rat cerebral cortex and hypothalamic membranes. The density of binding sites was lower in the hypothalamus than cortex. 3. Intracellular recordings from glucoreceptive VMHN neurones in hypothalamic slices were obtained. In the absence of glucose, tolbutamide (0.1 mM) depolarized these cells, increased membrane resistance and elicited action potentials. 4. Tolbutamide (0.1 mM) inhibited ATP-K+ channel currents and induced action current activity in cell-attached recordings from glucoreceptive VMHN neurones. 5. Glibenclamide (10-500 nM) had no effect per se on glucoreceptive VMHN neurones but did antagonize the actions of tolbutamide. 6. It is concluded that the hypothalamic (and perhaps cortical) sulphonylurea receptors are not directly coupled to ATP-K+ channels.

K+ channel openers suppress myotonic activity of human skeletal muscle in vitro

Isolated fibre bundles from myotonic human skeletal muscle showed after-contractions and spontaneous mechanical activity. The K+ channel openers cromakalim (10-100 mumols/l) and EMD 52962 (1-10 mumols/l) completely suppressed these abnormalities in mechanical activity. Voltage-clamp experiments revealed that cromakalim (100 mumols/l) increased the membrane K+ conductance of isolated, non-myotonic human skeletal muscle fibres 4-fold; Cl- conductance was not altered. The data show that myotonia is suppressed by an increase in in membrane K+ conductance.

Effects of potassium channel openers on single potassium channels in mouse skeletal muscle

The patch-clamp technique was used to study the effects of the potassium channel openers cromakalim, pinacidil, RP 49356 and diazoxide on single potassium channels in mouse skeletal muscle. In excised patches in the inside-out configuration, one type of potassium channel, the ATP-sensitive potassium channel, could be activated by internally applied RP 49356 even in the absence of internal ATP. At a concentration of 0.4 and 0.8 mmol/l, RP 49356 increased the open-probability of the channels by a factor of 2.7 and 17.4 respectively. The stimulating effect of cromakalim (0.2-0.8 mmol/l) and pinacidil (0.4 mmol/l) depended on the presence of ATP (0.1 mmol/l) at the cytoplasmic side of the patch membrane. The two drugs were able to restore the open-probability of the channels blocked by internal ATP (0.1 mmol/l) to 50-90% of its value in ATP-free solution. No channel reactivation could be observed at a higher ATP concentration (1 mmol/l). Diazoxide (0.4 mmol/l) had almost no effect. None of these channel openers could stimulate the other prominent type of potassium channel in skeletal muscle, the large-conductance Ca2(+)-activated potassium channel. The results show that cromakalim, pinacidil and RP 49356 are specific openers of ATP-sensitive potassium channels in skeletal muscle. It is suggested that the drugs displace the channel blocker ATP and that RP 49356 in addition recruits inactive channels.

Enhancement of K+ conductance improves in vitro the contraction force of skeletal muscle in hypokalemic periodic paralysis

An abnormal ratio between Na+ and K+ conductances seems to be the cause for the depolarization and paralysis of skeletal muscle in primary hypokalemic periodic paralysis. Recently we have shown that the "K+ channel opener" cromakalim hyperpolarizes mammalian skeletal muscle fibers. Now we have studied the effects of this drug on the twitch force of muscle biopsies from normal and diseased human skeletal muscle. Cromakalim had little effect on the twitch force of normal muscle whereas it strongly improved the contraction force of fibers from patients suffering from hypokalemic periodic paralysis. Recordings of intracellular K+ and Cl- activities in human muscle and isolated rat soleus muscle support the view that cromakalim enhances the membrane K+ conductance (gK+). These data indicate that "K+ channel openers" may have a beneficial effect in primary hypokalemic periodic paralysis.

Resealed fiber segments for the study of the pathophysiology of human skeletal muscle

The usefulness of long fiber segments for the study of the pathophysiology of human skeletal muscle was evaluated. Immediately after biopsy, the fiber segments were depolarized. Within 3 hours the cut ends resealed, and if the segments were greater than or equal to 2.5 cm long they regained normal resting membrane potentials (i.e., negative to -80 mV). Miniature endplate potentials, endplate potentials, action potentials, the current-voltage relationship, and the resting intracellular Ca2+ concentration of the resealed fiber segments were similar to those in fibers that were intact from tendon to tendon. In addition, specific properties of intact fibers obtained from patients with various neuromuscular diseases were preserved in the resealed fiber segments prepared from the same patients or patients with the same diseases. These segments are easily obtained as a routine muscle biopsy performed under local anesthesia; they provide valuable preparations for the study of the pathophysiology of human skeletal muscle as well as for in vitro pharmacological tests.

The effects of pinacidil and tolbutamide in feline pial arteries in situ

The vasomotor effect of the K+ channel opener pinacidil was investigated in feline pial arteries of the parietal cortex. Perivascular microapplication (5 microliters in 40 sex) and an image splitting method for the measurement of vascular diameter were employed. Pinacidil (10(-11) - 10(-7) M) induced concentration-dependent dilatations at 10(-9) M and higher concentrations. A maximal dilatation of about 42% was achieved at 10(-8) M, the dilatation at 10(-7) M was reduced to 22%. The sulphonylurea tolbutamide exerted per se no effect in pial arteries but it blocked concentration-dependently the pinacidil induced dilatation. This is consistent with the presence of ATP-sensitive K+ channels in pial arteries which are closed under resting conditions.

Effects of cromakalim (BRL 34915) on potassium conductances in CA3 neurons of the guinea-pig hippocampus in vitro

The action of the potassium channel activator, cromakalim (BRL 34915), on membrane potential, input resistance and current-voltage-relationship of CA3 neurons in a slice preparation of the guinea-pig hippocampus was investigated by means of intracellular recordings. In the presence of tetrodotoxin, cromakalim (30-100 mumol/l) produced a hyperpolarization up to 4 mV associated with a decrease in input resistance up to 10 MOhms. Determination of the equilibrium potential of the cromakalim action revealed that the hyperpolarization is due to the activation of a potassium conductance. This cromakalim-activated potassium conductance was voltage-dependent, i.e. it increased with hyperpolarization. Among a number of potassium channel blockers tested, only Cs+ (2 mmol/l) and Ba2+ (0.5 mmol/l) were able to inhibit the cromakalim-induced effects. Simultaneously, both cations suppressed the hyperpolarizing inward rectification (anomalous rectification) in these neurons, indicating that cromakalim activated or potentiated an inwardly rectifying potassium conductance. In addition, cromakalim slightly enhanced both amplitude and duration of afterhyperpolarizations following single calcium-dependent action potentials, suggesting that cromakalim might have a weak facilitatory effect on calcium-dependent potassium conductances.