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J Appl Physiol 85: 601-608, 1998;
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Vol. 85, Issue 2, 601-608, August 1998

ATP-sensitive K+ channel blocker glibenclamide and diaphragm fatigue during normoxia and hypoxia

Erik Van Lunteren, Michelle Moyer, and Augusto Torres

Departments of Medicine and Neurosciences, Case Western Reserve University, and Cleveland Veterans Affairs Medical Center, Cleveland, Ohio 44106

    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

The role of ATP-sensitive K+ channels in skeletal muscle contractile performance is controversial: blockers of these channels have been found to not alter, accelerate, or attenuate fatigue. The present study reexamined whether glibenclamide affects contractile performance during repetitive contraction. Experiments systematically assessed the effects of stimulation paradigm, temperature, and presence of hypoxia and in addition compared intertrain with intratrain fatigue. Adult rat diaphragm muscle strips were studied in vitro. At 37°C and normoxia, glibenclamide did not significantly affect any measure of fatigue during continuous 5- or 100-Hz or intermittent 20-Hz stimulation but progressively prolonged relaxation time during 20-Hz stimulation. At 20°C and normoxia, neither force nor relaxation rate was affected significantly by glibenclamide during 20-Hz stimulation. At 37°C and hypoxia, glibenclamide did not significantly affect fatigue at 5-Hz or intertrain fatigue during 20-Hz stimulation but reduced intratrain fatigue and prolonged relaxation time during 20-Hz stimulation. These findings indicate that, although ATP-sensitive K+ channels may be activated during repetitive contraction, their activation has only a modest effect on the rate of fatigue development.

diaphragm; skeletal muscle; potassium; ATP-sensitive K+ channels; contraction; temperature

    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

ATP-SENSITIVE K+ channels (KATP) are found in high density in many tissues, including skeletal muscle (7). In intact skeletal muscle fibers these channels are generally closed under resting normoxic conditions (2, 3, 15). However, KATP open under conditions of low ATP concentration ([ATP]), low intracellular pH, and metabolic poisoning (2, 7, 9, 22). It has been postulated that KATP become activated during repetitive muscle contraction especially during high-intensity contractions and/or under hypoxic stress, which thereby contributes to K+ efflux and the development of skeletal muscle fatigue (2, 7).

In support of the above postulate, studies utilizing openers of KATP generally concur that these agents accelerate fatigue, especially under hypoxic conditions (12, 28, 30). In contrast, studies utilizing blockers of KATP have found variable effects on fatigue: many studies have found no significant effect on fatigue (5, 12, 16, 28, 30), although an improvement (13, 14, 30) and a worsening (6) of fatigue have also been reported. The methodology of these studies varies considerably with respect to temperature, stimulation paradigm, presence of hypoxia, and data-analysis strategies, which may potentially account for some of the discrepant findings.

The purpose of the present study was to reexamine the issue of whether blocking KATP affects muscle performance during repetitive contractions leading to fatigue by 1) systematically addressing effects of stimulation paradigm, temperature, and presence of hypoxia; 2) comparing intertrain with intratrain fatigue; and 3) assessing the rate of muscle relaxation, which is known to slow during fatigue (10, 11, 16, 19, 26). We found that the KATP blocker glibenclamide significantly improves intratrain but not intertrain fatigue but only under hypoxic and not normoxic conditions and that it slows rate of muscle relaxation during fatigue under both normoxic and hypoxic conditions but not at low temperature. These findings indicate that KATP may be activated during repetitive contraction, especially during higher intensity contractions and/or under hypoxic conditions.

    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Male Sprague-Dawley rats (250-350 g) were anesthetized with intraperitoneal urethan (1-1.5 g/kg), the diaphragms were removed surgically, and two to four small strips (diameter ~1-1.5 mm) were cut per animal, with care taken to preserve the attachment of the muscle to the central tendon and ribs. The muscle strips were mounted in physiological solution at optimal length and were stimulated via platinum electrodes by using a pulse width of 1 ms and supramaximal voltages (Grass Instruments, West Warwick, RI). The aerated (95% O2-5% CO2) physiological solution contained (in mM) 135 NaCl, 5 KCl, 2.5 CaCl2, 1 MgSO4, 1 NaH2PO4, 15 NaHCO3, and 11 glucose, with the pH adjusted to 7.35-7.45. Bath temperature was controlled at 20 or 37°C by circulating water of the appropriate temperature through the outer jacket of the tissue baths (Radnoti Glass, Monrovia, CA). Isometric tension was measured with a high-sensitivity transducer (Kent Scientific/Radnoti Glass, Monrovia, CA). Twitch forces of ~0.5 kg/cm2 are obtained with this methodology in rat diaphragm (25). Force records were digitized, collected online with a computer (Axotape software, Axon Instruments, Foster City, CA), and stored for later data analysis. Drugs and reagents were obtained from Sigma Chemical (St. Louis, MO). Glibenclamide was dissolved as a 2 mM stock solution in 0.05 M NaOH, the proper volume of which was added to the bath to produce a final concentration of 100 µM (16).

Diaphragm muscle strips were allowed to equilibrate and subsequently underwent twitch stimulation at 0.1 Hz for 3 min. Muscle strips were accepted for study only if twitch force varied by no more than 5% during the 3-min baseline period. Six separate experiments were performed, the conditions of which are summarized in Table 1. Muscle strips were randomized across arms of a given experiment but not across experiments. That is, muscle strips were randomized to receive drug or no drug under a given set of experimental conditions; assignment of muscle strips was not randomized across all six experimental conditions. Care was taken to ensure that muscle strips from a given animal were assigned to both drug and no drug. Each muscle strip was used only once. Experiments A-C and E-F were conducted at 37°C, whereas experiment D was conducted at 20°C. After the baseline period, glibenclamide (100 µM) or vehicle (containing an equal volume of 0.05 M NaOH) was added to the bath for all experiments, which was followed by an equilibration period of 4 min. For experiments E-F only, the gas with which the solution was aerated was subsequently switched to 95% N2-5% CO2 followed by an equilibration period of 4 min. Bath oxygen tension was measured in some of the hypoxia studies with a dissolved-oxygen meter (ISO-2, World Precision Instruments, Sarasota, FL) and averaged 3.8 ± 0.8% at the end of the 4-min equilibration period. The muscles were stimulated at 0.1 Hz to monitor twitch tension during all of the above. Finally, the muscle strips underwent one of three stimulation paradigms: continuous 5-Hz stimulation (experiments A and E), intermittent 20-Hz stimulation (train duration 0.33 s, with 1 train delivered every second) (experiments B, D, and F), or continuous 100-Hz stimulation (experiment C). Only limited studies were done at 20°C because it would be highly unusual for mammalian muscle to be contracting in vivo at this cold a temperature, whereas tissue hypoxia and alterations in motoneuronal firing frequency can be seen under a variety of circumstances.

                              
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  		Table 1.   Stimulation frequencies, bath conditions, and sample sizes of the six experiments

Force records were analyzed offline with use of manually positioned cursors displayed on the computer screen. Isometric tension was measured in grams and subsequently normalized for each muscle strip to the average of the last three twitches during the (predrug) baseline period. Normalization was performed to minimize the confounding effects of interstrip variability in size and hence baseline force and to reduce the influences of slight variations in dissection technique affecting baseline force. This method of normalization is consistent with approaches used by us (24, 25, 27) and others (30) in studies of K+ channel blockers. Other studies of KATP blockers have normalized force to postdrug, prefatigue values (e.g., 13, 16, 28), which is similar to the present approach in that KATP blockers in the concentrations used generally have minimal effects on baseline force. Intratrain fatigue was assessed during 20-Hz stimulation by measuring the force at the end of the 330-ms-long train and expressing this as a percentage of the maximum force within the same tetanus (force-330) (26; as modified from Ref. 16). During 0.1- and 5-Hz stimulation, contraction time was assessed as the amount of time for twitch force to reach its peak, and half relaxation time was assessed as the amount of time for twitch force to decay to one-half of the peak value. During 20-Hz trains, contraction time was assessed from the first twitch of the train, and relaxation time was assessed from the decay in force at the end of the train.

All values presented are means ± SE. Statistical analysis of the effects of glibenclamide on prefatigue isometric twitch kinetics was performed with the unpaired t-test. Statistical analysis of the effects of glibenclamide on peak force, force-330, and half relaxation times during fatigue runs was performed with two-way ANOVA for repeated measures, followed in the event of a significant ANOVA by the Newman-Keuls test. The criterion for statistical significance was set at P < 0.05 (2 tailed).

    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Normoxic conditions (37°C). Under normoxic conditions in nonfatigued muscle, glibenclamide (100 µM) did not significantly affect isometric twitch contraction or half relaxation times, although there was a nonsignificant trend for the latter to be prolonged (Table 2). In response to repetitive stimulation, glibenclamide did not significantly affect peak force over time during 5-, 20-, or 100-Hz stimulation under normoxic conditions at 37°C (Fig. 1). Force-330, an evaluation of the ability of the muscle to maintain force during the plateau phase within the same tetanic stimulation (evaluated during 20-Hz trains), was slightly but not significantly improved by glibenclamide under normoxic conditions (Fig. 2). However, the extent to which the half relaxation time progressively prolonged during repetitive stimulation was augmented significantly by glibenclamide during 20-Hz and to a lesser extent 5-Hz stimulation (Fig. 3).

                              
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  		Table 2.   Effects of glibenclamide on diaphragm isometric twitch contraction and half relaxation times and on twitch force


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  		Fig. 1.   Changes in peak diaphragm force over time during repetitive 5- (A), 20- (B), and 100-Hz (C) stimulation in presence and absence of glibenclamide (100 µM) under normoxic conditions and a temperature of 37°C. Force values are means ± SE and are normalized to the value for twitch force immediately before addition of drug or no drug as described in METHODS. There were no significant effects of glibenclamide at 5 Hz (P = 0.41), 20 Hz (P = 0.24) or 100 Hz (P = 0.75).


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  		Fig. 2.   Effects of glibenclamide (100 µM) on ability of diaphragm to maintain force during plateau phase within the same tetanic stimulation (force-330) during 20-Hz stimulation under normoxic conditions and a temperature of 37°C. Values are means ± SE. Force-330, a measure of intratrain fatigue, was determined by evaluating force of the last contraction in the train as a percentage of the maximum tetanic force of the same tetanic contraction at each time point. There was a nonsignificant trend for glibenclamide to improve force-330 (P = 0.12).


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  		Fig. 3.   Diaphragm half relaxation time over the course of repetitive 5- (A) and 20-Hz (B) stimulation in the presence and absence of glibenclamide (100 µM) under normoxic conditions and a temperature of 37°C. Values are means ± SE. * Significant differences between glibenclamide and no drug, P < 0.05.

Effects of lowering temperature (normoxia). Effects of lowering temperature to 20°C were assessed during fatigue produced by 20-Hz stimulation. At this temperature, glibenclamide had no significant effect on baseline twitch kinetics (Table 2), peak force over time (Fig. 4, left), force-330 (Fig. 4, middle), or rate of relaxation (Fig. 4, right).


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  		Fig. 4.   Effect of glibenclamide (100 µM) on diaphragm during 20-Hz stimulation under normoxic conditions and a temperature of 20°C. Values are means ± SE. Changes in peak force (left), force-330 (middle), and half relaxation time (right) are indicated. Peak force was normalized to the value for force immediately before addition of drug or no drug, as described in METHODS. Glibenclamide had no significant effects on peak force (P = 0.81), force-330 (P = 0.94), or half relaxation time (P = 0.36).

Effects of hypoxia (37°C). Glibenclamide had no significant effect on isometric twitch contraction and half relaxation times under hypoxic conditions (Table 2). The change in peak force over time during 5- and 20-Hz stimulation was not affected significantly by glibenclamide under hypoxic conditions, although there was a trend for peak force to be improved by glibenclamide during 20-Hz stimulation (Fig. 5). In contrast to during normoxia, force-330 (assessed during 20-Hz trains) was improved significantly by glibenclamide during hypoxia (Fig. 6). The extent to which relaxation rate slowed during repetitive 20-Hz stimulation was generally more prominent during hypoxia than during normoxia, and this was augmented significantly by glibenclamide during 20- but not 5-Hz stimulation (Fig. 7).


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  		Fig. 5.   Alterations in peak diaphragm force over time during repetitive 5- (A) and 20-Hz (B) stimulation in presence and absence of glibenclamide (100 µM) under hypoxic conditions and a temperature of 37°C. Force values are means ± SE and are normalized to the value for twitch force immediately before addition of drug or no drug as described in METHODS. There were no significant effects of glibenclamide during 5-Hz stimulation, but there was a nonsignificant trend for force to improve during 20-Hz stimulation (P = 0.07).


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  		Fig. 6.   Effects of glibenclamide (100 µM) on ability of diaphragm to maintain force during plateau phase within same tetanic stimulation (force-330) during 20-Hz stimulation under hypoxic conditions and a temperature of 37°C. Values are means ± SE. Force-330, a measure of intratrain fatigue, was determined by evaluating force of the last contraction in the train as percentage of maximum tetanic force of the same tetanic contraction at each time point. * Significant differences between glibenclamide and no drug, P < 0.05.


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  		Fig. 7.   Diaphragm half relaxation time over the course of repetitive 5- (A) and 20-Hz (B) stimulation in presence and absence of glibenclamide (100 µM) under hypoxic conditions and a temperature of 37°C. Values are means ± SE. Half relaxation times were difficult to quantify accurately when force values became very small toward the end of fatiguing stimulation (see Fig. 5) and hence are reported only for the first 2 min of the 3-min stimulation period. * Significant differences between glibenclamide and no drug, P < 0.05. Contraction time was not affected significantly by glibenclamide (P = 0.39).

    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

Methodological issues. There are a number of agents that block KATP, several of which have been used in previous studies of muscle contractility. Gibenclamide was chosen for the present study on the basis of two major considerations. First, glibenclamide at the concentration used in this study (100 µM) blocks rat skeletal muscle KATP but not voltage-gated K+ channels or Ca2+-activated K+ channels (17); comparable data on sensitivity and specificity in rat skeletal muscle are not available for the other KATP blockers. However, tolbutamide affects muscle excitability, suggesting that it may have effects in addition to blocking KATP (5). Thus it was felt best to pick the agent for which specificity for KATP was best established in rat skeletal muscle. Second, glibenclamide has been used in the majority of previous studies examining muscle fatigue and KATP. Among eight studies, six used glibenclamide (6, 12-14, 16, 30), two used glyburide (5, 28), and one study each used phentolamine (30), ciclazindol (30), and tolbutamide (5). It is easier to compare the present data with other data by choosing the agent used most commonly in previous studies (glibenclamide) rather than another agent (e.g., tolbutamide, glyburide, ciclazinol).

Light and French (17) examined the sensitivity to glibenclamide of KATP reconstituted from rat skeletal muscle. They noted a concentration for one-half inhibition of open probability (Ki) of 3-5 µM and found that a dose of 10-100 µM was sufficient to fully eliminate visible channel openings. The value for Ki in rat muscle is higher than that of mouse muscle (Ki of 190 nM) (1). Light and French (17) also found that 100 µM glibenclamide had no effects on voltage-gated or Ca2+-activated K+ channels, suggesting good specificity for KATP (consistent with studies in other tissues). A glibenclamide concentration of 100 µM was chosen over 10 µM in the present study for several reasons. First, a concentration of glibenclamide was desired that would definitely block KATP so that any absent effects of glibenclamide on fatigue could not be attributed to a concentration that was possibly too low. Second, the present study used muscle strips, whereas French and Light (17) studied biplanar layers; a concentration higher than the minimal amount needed would ensure that an adequate concentration of drug would reach the center of the muscle strip. Third, a concentration of 100 µM was used in the most detailed previous study of glibenclamide and muscle fatigue (which also included data on action potentials) (16) so that direct comparisons could most easily be made by using the same drug concentration.

In the present study, glibenclamide was dissolved in NaOH as a stock solution before it was added to the bath. Glibenclamide (and glyburide) do not dissolve readily in water or saline. Previous studies of KATP blockers and muscle contraction have dissolved glibenclamide and glyburide in either DMSO (28, 30) or NaOH (6, 16). The latter was chosen for the present study because DMSO affects free radicals and thereby muscle contractile performance and, hence, could have greater confounding effects than a slight increase in pH induced by NaOH. Control and drug-treated muscle strips had an equal amount of NaOH added to the bath so that any potential effects of acid-base changes would be similar.

An incubation period for glibenclamide of 4 min was used in the present study. Light et al. (16) examined effects of glibenclamide (100 µM) on muscle action potential repolarization at a temperature of 20°C. They found that action potential repolarization was slowed by fatigue and that glibenclamide further slowed action potential repolarization. Furthermore, the mean values of the half repolarization time after fatigue in the presence of glibenclamide were the same whether the drug was applied 60 min before fatigue or 60 s before the end of fatigue. The latter data suggest a fast rate of diffusion and a fast onset of action of glibenclamide (<= 60 s) in skeletal muscle tissue. Light et al. studied muscle fiber bundles with diameters of 1-1.5 mm, which is the same size used in the present study. They used a temperature of 20°C, whereas the present study used a temperature of 37°C; diffusion and onset of drug action should be faster at the higher temperature. Based on these data, 4 min should be sufficiently long for equilibration after drug addition.

Effects of glibenclamide on muscle contraction. Effects of KATP blockers on muscle fatigue have been assessed by utilizing a variety of stimulation frequencies, ranging from 0.2 to 140 Hz (5, 6, 12, 13, 16, 28, 30). In addition, one study used a spontaneously breathing model to test diaphragm fatigue (14), in which motoneuronal firing frequency was not assessed but would be expected to vary among motor units and over time. During muscle contraction, K+ efflux and Na+ influx and the resultant alteration in transmembranous K+ and Na+ concentration gradients may lead to sarcolemmal depolarization, especially in the T tubules in which diffusion of ions is slower than at the outer surface of the muscle (21, 29). Much of the K+ efflux during contraction occurs via delayed rectifier K+ channels (29), although KATP has been postulated to contribute to the K+ efflux under conditions of depleted intracellular [ATP] and concommitant acidosis (8, 21). The Na+-K+-ATPase will restore the membranous ion gradients back to normal, but at high rates of muscle contraction the active transport is overwhelmed (4). Thus the role of K+ channels in regulating muscle fatigue is believed to be most prominent during intense muscle activation. Hence K+ channel blockers should improve high-frequency more than low-frequency fatigue and intratrain more than intertrain fatigue. That one of the studies with the greatest beneficial effects of KATP blockade on fatigue used a low stimulation rate of 0.25 Hz (13) is therefore surprising and may very well reflect other methodological differences (e.g., use of an in vivo preparation in which vascular or other systemic effects of glibenclamide may have contributed to the findings) compared with the other studies of KATP blockers (5, 6, 12, 16, 28, 30).

In the present study we found no significant effects of glibenclamide on fatigue during continuous 5- or 100-Hz stimulation or on intertrain fatigue during intermittent 20-Hz stimulation under normoxic or hypoxic conditions, consistent with all of the other in vitro studies of glibenclamide and fatigue (5, 12, 16, 28, 30). The only in vitro study reporting an improvement of intertrain fatigue with KATP blockers noted a modest improvement in fatigue with ciclazinol but not with glibenclamide (30), suggesting that ciclazinol may be a more effective blocker of KATP or may have additional effects in addition to blocking KATP (e.g., blocking other K+ channels). In the present study we found no significant effects of glibenclamide on intratrain fatigue during normoxia (although there was a trend toward improvement), but we found an attenuation of intratrain fatigue during hypoxia. The former finding (normoxic conditions) is consistent with two previous studies (5, 16), neither of which, however, examined intratrain fatigue under hypoxic conditions. The present finding of glibenclamide significantly attenuating only intratrain fatigue and only during hypoxia suggests that the contribution of KATP to fatigue is small and is limited to conditions expected to lead to profound ATP depletion and/or intracellular acidosis.

The rate of muscle relaxation slows with fatigue and especially does so under hypoxic conditions (10, 11, 16, 19, 26). Two previous studies have found that glyburide and glibenclamide slow the rate of action potential repolarization in resting and fatigued muscle (5, 16). Surprisingly, the rate of muscle relaxation was not found to be affected by glibenclamide in either resting or fatigued muscle in a previous study despite changes in action potential repolarization rate (16). Data on muscle relaxation rate were not provided for glyburide (5), nor have other studies of glibenclamide and fatigue reported values for rate of muscle relaxation. The present data concur with those of Light et al. (16), who found that that KATP blockade does not significantly alter rate of relaxation of resting muscle. In contrast to Light et al., we found a slowing of relaxation rate by glibenclamide during fatigue produced during 20-Hz stimulation under both normoxic and hypoxic conditions. These data suggest that KATP may be activated during fatiguing stimuli but to an insufficient extent to affect peak force production. This is consistent with a previously proposed explanation for glibenclamide not affecting fatigue but delaying the recovery from fatigue (16).

The mechanism by which altering K+ channel conductance affects muscle relaxation rate is unlikely to be a direct effect on either the rate of Ca2+ reuptake by the sarcoplasmic reticulum or the rate of Ca2+ binding by parvalbumin. More likely, the effects of K+ channels on the rate of relaxation are mediated by altering the rate of action potential repolarization. Normally, membrane potential repolarizes very quickly during an action potential. As a result, there is only a brief period of time during repolarization when there is continued Ca2+ influx while Ca2+ is simultaneously being taken back up by the sarcoplasmic reticulum and/or being bound to intracellular Ca2+ buffers. If action potential repolarization is slowed (e.g., with K+ channel blockade), this period can be prolonged, thereby slowing the rate at which intracellular Ca2+ concentration ([Ca2+]) falls and hence slowing the rate of relaxation. If the degree of action potential repolarization slowing is small, it may not be sufficient to affect mechanical relaxation. This could explain why Light et al. (16) found action potential prolongation but no slowing of relaxation with glibenclamide. On the other hand, if the degree of action potential slowing is large, either contraction or relaxation time could be slowed depending on the kinetics of the changes in intracellular [Ca2+] relative to the kinetics of actin-myosin interactions. As muscle fatigues, action potential repolarization slows and relaxation rate slows. Under these circumstances, effects of K+ channel blockers on rate of relaxation may become more manifest, as was found for glibenclamide in the present study. This is consistent with previous studies of the K+ channel-blocking aminopyridines, which do not slow relaxation rate in nonfatigued muscle but markedly augment slowing of relaxation rate as muscle undergoes fatiguing contractions (25, 26).

Effects of KATP blockers have been assessed at 20°C (5, 16), 30°C (28), or 37-38°C (6, 12-14, 30). Studies at 20°C utilized frog muscle, whereas studies performed at 30-38°C utilized mammalian muscle so that the influence of temperature on muscle contractile responses to KATP blockers cannot be inferred directly from previous work. Of note, however, is that both of the studies at a cool temperature found no effect of KATP blockers on fatigue, whereas the studies at warmer temperatures have noted variable effects of KATP blockers on fatigue. Ion channels are very sensitive to temperature, with rates of activation and deactivation having especially high values for Q10 compared with peak current; furthermore, values for Q10 may vary as a function of membrane potential (see, e.g., Refs. 20, 23). In the present study of 20-Hz stimulation during normoxia, we found that at neither warm nor cold temperature was there a significant effect of glibenclamide on either intertrain or intratrain fatigue. This suggests that differences among previous studies regarding whether fatigue is attenuated with glibenclamide are unlikely to be due to differences in muscle temperature. On the other hand, we found that, during fatiguing 20-Hz stimulation, glibenclamide led to an augmentation of the relaxation rate prolongation at 37°C but not at 20°C. Therefore, a low temperature may explain why Light et al. (16), having conducted their studies at 20°C, did not find that glibenclamide prolonged relaxation rate during fatigue despite a high stimulation frequency.

Pharmacological agents that block many types of K+ channels (e.g., tetraethylammonium, 4-aminopyridine, 3,4-diaminopyridine) have greater effects on muscle contractile performance than have been noted for the specific blockers of KATP. Reported increases in twitch force of resting muscle range from ~10 to 150% with single agents and several hundred percent with combinations of agents, and these increases are accompanied by prolongations of contraction time and for the aminopyridines a leftward shift in the force-frequency relationship (18, 24, 25, 27). Furthermore, the aminopyridines improve both peak force and maintenance of intratrain force during repetitive 20-Hz stimulation, reduce the neurotransmission failure contribution to fatigue, and augment the slowing of relaxation rate during fatigue (all tested under normoxic conditions) (24, 25). Therefore, KATP appears to play a relatively small role compared with other K+ channels (e.g., delayed rectifier K+ channels) in regulating muscle contractile performance. However, there may be other conditions under which KATP play a more prominent role in modulating muscle force. Specifically, ATP consumption may be higher during repetitive dynamic contractions (contractions associated with muscle shortening) than during isometric contractions, which could lead to a greater fall in intracellular [ATP] and hence a greater activation of KATP. To test this, future studies are needed to determine the extent to which glibenclamide modulates muscle shortening velocity during fatiguing isotonic contractions.

In conclusion, KATP contributes to fatigue production during isometric contractions only under extreme conditions, such as intense stimulation during hypoxia. These channels are activated under less extreme conditions as evidenced by a prolonged relaxation time as fatigue develops, but the degree of activation is not sufficient to substantially decrease force production. Intense stimulation during hypoxia may result in an acidic intracellular environment that is partially depleted of ATP. Under these circumstances, ATP-dependent Na+-K+ pumps are no longer sufficient to restore normal transmembrane ion gradients so that K+ efflux through KATP may contribute to fatigue.

    ACKNOWLEDGEMENTS

This study was supported in part by the Veterans Affairs Medical Research Service and by National Heart, Lung, and Blood Institute Specialized Center of Research Grant HL-42215.

    FOOTNOTES

Address for reprint requests: E. van Lunteren, Pulmonary Sect. 111J(W), Cleveland VA Medical Center, 10701 East Boulevard, Cleveland, OH 44106 (E-mail: exv4{at}po.cwru.edu).

Received 12 June 1997; accepted in final form 1 April 1998.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Allard, B., and M. Lazdunski. Pharmacological properties of ATP-sensitive K+ channels in mammalian skeletal muscle. Eur. J. Pharmacol. 236: 419-426, 1993[Medline].

2.   Allard, B., M. Lazdunski, and O. Rougier. Activation of ATP-dependent K+ channels by metabolic poisoning in adult mouse skeletal muscle: role of intracellular Mg2+ and pH. J. Physiol. (Lond.) 485: 283-296, 1995.

3.   Brinkmeier, H., E. Zachar, and R. Rudel. Voltage-dependent K+ channels in the sarcolemma of mouse skeletal muscle. Pflügers Arch. 419: 486-491, 1991[Medline].

4.   Clausen, T. Regulation of active Na+-K+ transport in skeletal muscle. Physiol. Rev. 66: 542-580, 1986[Free Full Text].

5.   Comtois, A., P. Light, J. M. Renaud, and M. K. Kong. Tolbutamide, but not glyburide, affects the excitability and contractility of unfatigued frog sartorius muscle. Eur. J. Pharmacol. 242: 65-73, 1993[Medline].

6.   Comtois, A., C. Sinderby, N. Comtois, A. Grassino, and J. M. Renaud. An ATP-sensitive potassium channel blocker decreased diaphragmatic circulation in anesthetized dogs. J. Appl. Physiol. 77: 127-134, 1994[Abstract/Free Full Text].

7.   Davies, N. W. Modulation of ATP-sensitive K+ channels in skeletal muscle by intracellular protons. Nature 343: 375-377, 1990[Medline].

8.   Davies, N. W., N. B. Standen, and P. R. Stanfield. ATP-dependent potassium channels of muscle cells: their properties, regulation, and possible functions. J. Bioenerg. Biomembr. 23: 509-535, 1991[Medline].

9.   Davies, N. W., N. B. Standen, and P. R. Stanfield. The effect of intracellular pH on ATP-dependent potassium channels of frog skeletal muscle. J. Physiol. (Lond.) 445: 549-568, 1992[Abstract/Free Full Text].

10.   Dawson, M. J., D. G. Gadian, and D. R. Wilkie. Mechanical relaxation rate and metabolism studied in fatiguing muscle by phosphorus nuclear magnetic resonance. J. Physiol. (Lond.) 299: 465-484, 1980[Abstract/Free Full Text].

11.   Esau, S. A., F. Bellemare, A. Grassino, S. Permutt, C. Roussos, and R. L. Pardy. Changes in relaxation rate with diaphragm fatigue in humans. J. Appl. Physiol. 54: 1353-1360, 1983[Abstract/Free Full Text].

12.   Gurden, J. M., and V. J. Hart. Effects of cromakalim and glibenclamide on rabbit isolated tenuissimus muscle during hypoxia or normoxia (Abstract). Br. J. Pharmacol. 114: 237P, 1995.

13.   Gutierrez, G., G. Arbelaez, R. A. Cabezas, and D. Lee. Glibenclamide ameliorates skeletal muscle fatigue (Abstract). Am. J. Respir. Crit. Care Med. 151: A811, 1995.

14.   Gutierrez, G., D. Lee, and R. Cabezas. Glibenclamide prevents rabbit diaphragmatic fatigue (Abstract). Am. J. Respir. Crit. Care Med. 153: A687, 1996.

15.   Hussain, M., A. C. Wareham, and S. I. Head. Mechanism of action of a K+ channel activator BRL 38227 on ATP-sensitive K+ channels in mouse skeletal muscle fibers. J. Physiol. (Lond.) 478: 523-532, 1994.

16.   Light, P. E., A. S. Comtois, and J. M. Renaud. The effect of glibenclamide on frog skeletal muscle: evidence for K+ATP channel activation during fatigue. J. Physiol. (Lond.) 475: 495-507, 1994[Abstract/Free Full Text].

17.   Light, P. E., and R. J. French. Glibenclamide selectively blocks ATP-sensitive K+ channels reconstituted from skeletal muscle. Eur. J. Pharmacol. 259: 219-222, 1994[Medline].

18.   Lin-Shiau, S. Y., S. Y. Day, and W. M. Fu. Use of ion channel blockers in studying the regulation of skeletal muscle contractions. Naunyn Schmiedebergs Arch. Pharmacol. 344: 691-697, 1991[Medline].

19.   Metzger, J. M., and R. H. Fitts. Fatigue from high- and low-frequency muscle stimulation: role of sarcolemma action potentials. Exp. Neurol. 93: 320-333, 1986[Medline].

20.   Nobile, M., E. Carbone, H. D. Lux, and H. Zucker. Temperature sensitivity of Ca currents in chick sensory neurons. Pflügers Arch. 415: 658-663, 1990[Medline].

21.   Sjogaard, G. Role of exercise-induced potassium fluxes underlying muscle fatigue: a brief review. Can. J. Physiol. Pharmacol. 69: 238-245, 1991[Medline].

22.   Spruce, A. E., N. B. Standen, and P. R. Stanfield. Voltage-dependent ATP-sensitive potassium channels of skeletal muscle membrane. Nature 316: 736-738, 1985[Medline].

23.   Van Lunteren, E., K. S. Elmslie, and S. W. Jones. Effects of temperature on calcium current of bullfrog sympathetic neurons. J. Physiol. (Lond.) 466: 81-93, 1993[Abstract/Free Full Text].

24.   Van Lunteren, E., and M. Moyer. Effects of 3,4-diaminopyridine on diaphragm force and fatigue, including fatigue due to neurotransmission failure. J. Appl. Physiol. 81: 2214-2220, 1996[Abstract/Free Full Text].

25.   Van Lunteren, E., M. Moyer, and A. Torres. Effect of K+ channel blockade on fatigue in rat diaphragm muscle. J. Appl. Physiol. 79: 738-747, 1995[Abstract/Free Full Text].

26.   Van Lunteren, E., A. Torres, and M. Moyer. Effects of hypoxia on diaphragm relaxation rate during fatigue. J. Appl. Physiol. 82: 1472-1478, 1997[Abstract/Free Full Text].

27.   Van Lunteren, E., H. Vafaie, and M. Moyer. Changes in pharyngeal respiratory muscle force produced by K+ channel blockade. Respir. Physiol. 99: 331-340, 1995[Medline].

28.   Wesselcouch, E. O., C. Sargent, M. W. Wilde, and M. A. Smith. ATP-sensitive potassium channels and skeletal muscle function in vitro. J. Exp. Pharmacol. Ther. 267: 410-416, 1993[Abstract/Free Full Text].

29.   Westerblad, H., J. A. Lee, J. Lannergren, and D. G. Allen. Cellular mechanisms of fatigue in skeletal muscle. Am. J. Physiol. 261 (Cell Physiol. 30): C195-C209, 1991[Abstract/Free Full Text].

30.   Wickendon, A. D., H. Prior, E. Kelly, K. Russell, S. M. Poucher, and P. Kumar. The effects of pharmacological modulation of KATP on the guinea-pig isolated diaphragm. Eur. J. Pharmacol. 302: 79-88, 1996[Medline].

J APPL PHYSIOL 85(2):601-608
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