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Biomedical subjects

M B Reid

Publications and source records attributed to M B Reid.

At least 55 records · Page 3Linked to original sources
PubMed1994

Dimethyl sulfoxide depresses skeletal muscle contractility.

Dimethyl sulfoxide (DMSO) is commonly used in studies of skeletal muscle as a selective antioxidant (DMSO preferentially scavenges hydroxyl radicals) or as a solvent for drugs. The present experiments tested DMSO for direct effects on diaphragm contractile properties. Fiber bundles were removed from anesthetized rats, mounted in vitro at optimal length (37 degrees C), curarized, and stimulated directly. Protocol 1 tested for contractile depression and dose dependence by comparing bundles treated with DMSO (0.6-640 mM) with time- and stimulus-matched controls. Protocol 2 tested reversibility of 220 mM DMSO effects by using each bundle as its own control. DMSO decreased the relative forces developed during twitch and submaximal tetanic (30- and 60-Hz) contractions, shifting the force-frequency relationship down and to the right. These effects were strongly dose dependent and were reversed by DMSO washout. DMSO had no detectable effect on the forces developed during maximal tetany (200 Hz). DMSO depresses contractile function of diaphragm fibers by reversible dose-dependent inhibition of excitation-contraction coupling.

Animals↗
PubMed1994

N-acetylcysteine depresses contractile function and inhibits fatigue of diaphragm in vitro.

We have previously shown that antioxidant enzymes (superoxide dismutase and catalase) depress contractility of unfatigued diaphragm fiber bundles and inhibit development of acute fatigue. In the present study, we tested for similar effects of N-acetyl-cysteine (NAC), a nonspecific antioxidant approved for clinical use. Diaphragms were excised from deeply anesthetized rats. Fiber bundles were removed, mounted isometrically at 37 degrees C, and stimulated directly using supramaximal current intensity. Studies of unfatigued muscle showed that 10 mM NAC reduced peak twitch stress (P < 0.0001), shortened time to peak twitch stress (P < 0.002), and shifted the stress-frequency curve down and to the right (P < 0.05). Fiber bundles incubated in 0.1-10 mM NAC exhibited a dose-dependent decrease in relative stresses developed during 30-Hz contraction (P < 0.0001) with no change in maximal tetanic (200 Hz) stress. NAC (10 mM) also inhibited acute fatigue. Throughout 10 min of intermittent contraction at 30-40 Hz, treated bundles developed higher stresses than time-matched control bundles (P < 0.0001). NAC concentrations > or = 30 mM were toxic, causing a prompt irreversible decrease in maximal tetanic stress (P < 0.0001). Because NAC effects mimic the effects of other antioxidant agents with different mechanisms of action, we conclude that exogenous antioxidants exert stereotypical effects on contractile function that differ between unfatigued and fatiguing muscle. Unlike antioxidant enzymes, however, NAC has been approved for clinical use and may be used in future studies of human muscle fatigue.

Acetylcysteine↗
PubMed1994

N-acetylcysteine inhibits muscle fatigue in humans.

N-acetylcysteine (NAC) is a nonspecific antioxidant that selectively inhibits acute fatigue of rodent skeletal muscle stimulated at low (but not high) tetanic frequencies and that decreases contractile function of unfatigued muscle in a dose-dependent manner. The present experiments test the hypothesis that NAC pretreatment can inhibit acute muscular fatigue in humans. Healthy volunteers were studied on two occasions each. Subjects were pretreated with NAC 150 mg/kg or 5% dextrose in water by intravenous infusion. The subject then sat in a chair with surface electrodes positioned over the motor point of tibialis anterior, an ankle dorsiflexor of mixed-fiber composition. The muscle was stimulated to contract electrically (40-55 mA, 0.2-ms pulses) and force production was measured. Function of the unfatigued muscle was assessed by measuring the forces produced during maximal voluntary contractions (MVC) of ankle dorsiflexor muscle groups and during electrical stimulation of tibialis anterior at 1, 10, 20, 40, 80, and 120 Hz (protocol 1). Fatigue was produced using repetitive tetanic stimulations at 10 Hz (protocol 1) or 40 Hz (protocol 2); intermittent stimulations subsequently were used to monitor recovery from fatigue. The contralateral leg then was studied using the same protocol. Pretreatment with NAC did not alter the function of unfatigued muscle; MVC performance and the force-frequency relationship of tibialis anterior were unchanged. During fatiguing contractions stimulated at 10 Hz, NAC increased force output by approximately 15% (P < 0.0001), an effect that was evident after 3 min of repetitive contraction (P < 0.0125) and persisted throughout the 30-min protocol. NAC had no effect on fatigue induced using 40 Hz stimuli or on recovery from fatigue. N-acetylcysteine pretreatment can improve performance of human limb muscle during fatiguing exercise, suggesting that oxidative stress plays a causal role in the fatigue process and identifying antioxidant therapy as a novel intervention that may be useful clinically.

Acetylcysteine↗
PubMed1993

Development and reversal of fatigue in human tibialis anterior.

The lack of easily measurable, objective physiologic indices of muscle fatigue in humans has hampered the evaluation of interventions that putatively inhibit muscle fatigue. In 6 healthy subjects, isometric fatigue of right and left tibialis anterior (TA) was produced using 180 s of intermittent electrical stimulation to the motor point (40 Hz, 650-ms or 250-ms trains, 1 train/s). The time course, magnitude, and reproducibility of fatigue and recovery from fatigue was established, and the effect of duty cycle measured. During fatigue, force fell rapidly in all subjects over the first 70 s, then fell more slowly. Recovery was also biphasic--force increased rapidly over the initial 30 s then slowly--and was incomplete at 300 s. Comparing duty cycles of 0.65 versus 0.25, the longer duty cycle caused a greater fall in force (-65% versus -29%; P < 0.001) and lower recovery (P < 0.02). In all subjects, left leg muscles fatigued more than right. We conclude that this noninvasive fatigue and recovery protocol is highly reproducible and that it may prove useful for evaluating the effects of pharmacologic or restorative interventions on human muscle fatigue.

Adult↗
PubMed1993

Developmental changes in diaphragm contractile properties.

The contractile properties of pre- and early postnatal respiratory muscles are incompletely understood. We examined the effects of development on isometric contractile properties, with an emphasis on properties at 37 degrees C. One-day-old (n = 10), 3-wk-old (n = 10), and adult (n = 10) rabbits were studied. Isometric contractile properties of costal diaphragm strips were measured in vitro by using direct stimulation. Twitch and maximal, i.e., fused, tetanic force production increased with strip dimension and with age. Maximal tetanic force developed per unit cross-sectional area (stress) was significantly decreased in muscle from 1-day olds, whereas it was greatest in muscle from 3-wk olds. Twitch stress was similar in all three groups. Only when the stimulus duration was prolonged did twitch and fused tetanic force achieve maximal values values for the 1-day-old and 3-wk-old strips, suggesting less effective excitation-contraction coupling in those muscles. We conclude that immature rabbit diaphragm has unique isometric contractile properties and stimulus parameter requirements that cannot be deduced from studies using mature diaphragm.

Animals↗
PubMed1993

Reactive oxygen in skeletal muscle. III. Contractility of unfatigued muscle.

This study tested the hypothesis that reactive oxygen intermediates present in unfatigued skeletal muscle act to enhance contractile function. Fiber bundles from rat diaphragm were incubated with exogenous catalase (an antioxidant enzyme that dehydrates hydrogen peroxide to molecular oxygen and water) to decrease the tissue concentration of reactive oxygen intermediates. Catalase (10(3) U/ml) significantly decreased twitch characteristics (time to peak tension, half-relaxation time, peak force, and twitch-to-tetanus force ratio), thereby shifting the force-frequency relationship to the right. Catalase effects were dose dependent. Concentrations of 1 to 10(5) U/ml progressively depressed submaximal (30-Hz) tetanic stress, whereas concentrations > 10(5) U/ml were toxic, inhibiting maximal (200-Hz) tetanic stress (P < 0.0001). Exogenous hydrogen peroxide (10(-4) to 10(-2)M) increased peak twitch stress (P < 0.03) and lengthened both time to peak tension (P < 0.02) and half-relaxation time (P < 0.02). Selective removal of superoxide anion radicals with the use of superoxide dismutase produced dose-dependent contractile inhibition similar to that produced by catalase. We conclude that the reactive oxygen intermediates present in unfatigued skeletal muscle have a positive effect on excitation-contraction coupling and are obligatory for optimal contractile function.

Animals↗
PubMed1992

On the stability of bovine gamma II crystallin.

Bovine gamma II crystallin, undergoes structural changes when lyophilized. The lyophylized powder does not readily dissolve in buffer, although it can be taken up in distilled water. The lyophylized sample, as opposed to a sample concentrated by vacuum distillation at 30 degrees C, does exhibit different migration on isoelectric focusing gels. The hydration and denaturation properties of the two preparations are different. The lyophylized sample possesses a higher non-freezable water content as a function of concentration than its counterpart. The lyophylized sample also shows less denaturation in differential scanning calorimetry scans, up to 110 degrees C, than its counterpart. This indicates that lyophylization may induce a slight denaturation, due to structural-conformational change. On the other hand, the CD spectra of lyophylized and non-lyophylized gamma II crystallins do not differ significantly. This implies that the conformational changes upon lyophylization do not involve the secondary structure of gamma II crystallin.

Animals↗
PubMed1992

Capillaries measured in canine diaphragm by two methods.

We have measured capillary distribution in costal and crural canine diaphragm using two methods: histochemical processing and perfusion fixation. Each of 18 dogs was deeply anesthetized, the abdomen opened, and the left inferior phrenic artery cannulated. The animal was heparinized and overdosed with pentobarbital. The right hemidiaphragm was frozen, either postexcision (Protocol 1) or intact with no preload (Protocol 2), for histochemical processing. The left hemidiaphragm was fixed by perfusion in situ using 2% glutaraldehyde, either with preload (Protocol 1) or without (Protocol 2). Costal and crural regions of each hemidiaphragm were sampled for analysis. Frozen samples were sectioned and processed for acid-stable (pH 4.0) ATPase activity; perfusion-fixed samples were postfixed, stained, embedded in Epon, and sectioned. Measurements were made using a digital imaging system. We found that muscle fibers had smaller cross-sectional areas in costal than in crural diaphragm; capillary-to-fiber ratio (C:F) did not differ by region and regional differences in capillary density could be attributed to differences in fiber size. Results depended critically on methodology. In perfusion-fixed muscle, fiber area was less, C:F was greater, and capillary density was greater than in histochemically-processed tissue. We conclude that capillary distribution is similar in costal vs. crural diaphragm and that perfusion fixation identifies capillaries more effectively than histochemistry.

Animals↗
PubMed1992

Locomotion in men has no appreciable mechanical effect on breathing.

It has been suggested that the act of taking a stride produces substantial respiratory volume displacement and that this assists the respiratory muscles during locomotion. We measured the flow at the mouth associated with stride in walking and running humans and found it to be 1-2% of respiratory tidal volume, which is too small to make an appreciable contribution to pulmonary ventilation.

Adult↗
PubMed1992

Reactive oxygen in skeletal muscle. I. Intracellular oxidant kinetics and fatigue in vitro.

We hypothesized that muscle fiber bundles produce reactive oxygen intermediates and that reactive oxidant species contribute to muscular fatigue in vitro. Fiber bundles from rat diaphragm were mounted in chambers containing Krebs-Ringer solution. In studies of intracellular oxidant kinetics, bundles were loaded with 2',7'-dichlorofluorescin, a fluorochrome that emits at 520 nm when oxidized; emissions were quantified using a fluorescence microscope. Emissions from unstimulated muscles increased over time (P < 0.001). Accumulation of fluorescence was slowed by addition of catalase (P < 0.001) or superoxide dismutase (P < 0.001) and was accelerated by repetitive muscular contraction (P < 0.05). To determine effects of reactive oxygen intermediates on fatigue, curarized bundles were stimulated to contract isometrically; force was measured. Catalase, superoxide dismutase, and dimethyl sulfoxide were screened for effects on low- and high-frequency fatigue. Antioxidants inhibited low-frequency fatigue [after 5 min of repetitive contractions, force at 30 Hz was 20% greater than control (P < 0.015)] and increased the variability of fatigue at 30 Hz (P < 0.03). Antioxidants did not alter high-frequency (200-Hz) fatigue. We conclude that 1) diaphragm fiber bundles produce reactive oxygen intermediates, including O2-. and H2O2; 2) muscular contraction increases intracellular oxidant levels; and 3) reactive oxygen intermediates promote low-frequency fatigue in this preparation.

Animals↗
PubMed1992

Reactive oxygen in skeletal muscle. II. Extracellular release of free radicals.

We have tested the hypothesis that diaphragm muscle fibers release superoxide anion radicals (O2-.) into the extracellular space. Fiber bundles were isolated from rat diaphragm and incubated in Krebs-Ringer solution containing cytochrome c (10(-5) M), a standard assay for O2-.. Bundles were either passive or active, i.e., directly stimulated to contract rhythmically. After 1 h, absorbance of reduced cytochrome c in the incubation medium was measured at 550 nm. Absorbance was greater in medium exposed to passive muscle than in medium without muscle (P < 0.01), indicating O2-. release by passive muscle. Absorbance was greater in medium exposed to active muscle than in that exposed to passive muscle (P < 0.01), an increase inhibited by superoxide dismutase (10(3) U/ml). Active bundles fatigued; bundles developing the lowest final stresses produced the greatest absorbance increases (P < 0.001), suggesting that the magnitude of fatigue was inversely related to O2-. release. We conclude that O2-. is released by diaphragm myocytes into the interstitium and surrounding medium, a process accelerated by fatiguing muscular contractions.

Animals↗
PubMed1991

Effects of theophylline on canine diaphragmatic contractility and fatigue.

To distinguish the effects of theophylline on respiratory muscle contractility from alterations in respiratory muscle interaction or blood flow, we examined in vitro contractile properties and fatigue of canine diaphragm in two series of experiments. In the first series, a 40-mg/kg aminophylline infusion was given to dogs, and diaphragm strips were removed for in vitro study when stable tissue fixation of the drug was reached. Compared with control bundles examined before aminophylline infusion, no alterations in twitch tension, tetanic tension, or force-frequency characteristics were observed. Moreover, theophylline-treated strips fatigued faster than control strips, whether subjected to repetitive submaximal or maximal contractions (p less than 0.01). In the second series, diaphragm bundles were equilibrated with high theophylline doses (400 mg/L) in vitro, and inotropic effects compared with the results in the first series. Supratherapeutic theophylline concentrations increased force development at low stimulation frequencies (p less than 0.05 at 10 Hz) and significantly elevated twitch-tetanus ratio (p less than 0.01) but did not protect against development of in vitro muscle fatigue. Poor penetration of theophylline in diaphragm bundles in vitro was excluded, since drug concentrations in the muscle bundle and the muscle bath were virtually equal. We conclude that diaphragmatic tissue concentrations correlate well with therapeutic serum and supratherapeutic bath levels and that only high theophylline concentrations increase canine diaphragmatic contractility in vitro. None of the theophylline concentrations studied could protect diaphragm bundles against the development of low- or high-frequency fatigue in vitro.

Animals↗
PubMed1990

'Air hunger' from increased PCO2 persists after complete neuromuscular block in humans.

The tolerance of totally curarized subjects for prolonged breath hold is viewed by many as evidence that respiratory muscle contraction is essential to generate the sensation of breathlessness. Although conflicting evidence exists, none of it was obtained during total neuromuscular block. We completely paralyzed four normal, unsedated subjects with vecuronium (a non-depolarizing neuromuscular blocker). Subjects were mechanically ventilated with hyperoxic gas mixtures at fixed rate and tidal volume. End-expiratory PCO2 (PETCO2) was varied surreptitiously by changing inspired PCO2. Subjects rated their respiratory discomfort or 'air hunger' every 45 sec. At low PETCO2 (median 35 Torr) they felt little or no air hunger. When PETCO2 was raised (median 44 Torr) all subjects reported severe air hunger. They had reported the same degree of air hunger at essentially the same PETCO2 before paralysis. When questioned afterwards all subjects said the sensation could be described by the terms 'air hunger', 'urge to breathe', and 'shortness of breath', and that is was like breath holding. They reported no fundamental difference in the sensation before and after paralysis. We conclude that respiratory muscle contraction is not important in the genesis of air hunger evoked by hypercapnia.

Adult↗
PubMed1990

Alcohol protects the diaphragm during dietary restriction.

We recently reported that alcoholic rat diaphragm develops greater contractile force than diaphragm of pair-fed control animals. The present experiment examines whether alcohol or dietary restriction is the more likely cause of this surprising finding. We conditioned 10 rats using a liquid diet containing ethanol as 36% of calories. Ten pair-fed control animals received an equal amount of isocaloric, ethanol-free liquid diet. Ten ad libitum control animals had unrestricted access to lab chow and water. Rats were killed after 30 weeks. Left costal diaphragm strips were studied in vitro at optimal length using direct stimulation at supramaximal voltage. Isometric force was measured and divided by muscle cross-section to compute stress. Maximal tetanic stresses developed by muscle from pair-fed controls were systematically less than alcoholic and ad libitum control values (p less than 0.0001); this did not depend on temperature (25 degrees vs. 37 degrees; p greater than 0.50). Pair-feeding increased twitch half-relaxation times (p less than 0.03) and shifted the tetanic stress-stimulation frequency relationship leftward by 10 Hz (p less than 0.01). Diaphragm of pair-fed rats continued to generate lower stresses during the fatigue caused by repeated contractions (p less than 0.01). We conclude that dietary restriction associated with pair-feeding compromises diaphragm performance in rats. Chronic alcohol consumption prevents or reverses these changes, since diaphragm function of alcoholic and ad libitum control animals was not different.

Alcoholism↗
PubMed1990

Respiratory changes in diaphragmatic intramuscular pressure.

We attempted to measure diaphragmatic tension by measuring changes in diaphragmatic intramuscular pressure (Pim) in the costal and crural parts of the diaphragm in 10 supine anesthetized dogs with Gaeltec 12 CT minitransducers. During phrenic nerve stimulation or direct stimulation of the costal and crural parts of the diaphragm in an animal with the chest and abdomen open, Pim invariably increased and a linear relationship between Pim and the force exerted on the central tendon was found (r greater than or equal to 0.93). During quiet inspiration Pim in general decreased in the costal part (-3.9 +/- 3.3 cmH2O), whereas it either increased or slightly decreased in the crural part (+3.3 +/- 9.4 cmH2O, P less than 0.05). Similar differences were obtained during loaded and occluded inspiration. After bilateral phrenicotomy Pim invariably decreased during inspiration in both parts (costal -4.3 +/- 6.4 cmH2O, crural -3.1 +/- 0.6 cmH2O). Contrary to the expected changes in tension in the muscle, but in conformity with the pressure applied to the muscle, Pim invariably increased during passive inflation from functional residual capacity to total lung capacity (costal +30 +/- 23 cmH2O, crural +18 +/- 18 cmH2O). Similarly, during passive deflation from functional residual capacity to residual volume, Pim invariably decreased (costal -12 +/- 19 cmH2O, crural -12 +/- 14 cmH2O). In two experiments similar observations were made with saline-filled catheters. We conclude that although Pim increases during contraction as in other muscles, Pim during respiratory maneuvers is primarily determined by the pleural and abdominal pressures applied to the muscle rather than by the tension developed by it.

Animals↗
PubMed1989

Effect of nifedipine on the contractile function of the rat diaphragm in vitro.

The isometric contractile response of the directly-stimulated rat diaphragm was studied before and following addition of the calcium channel blocker, nifedipine. Nifedipine (10 micrograms/ml and 30 micrograms/ml bath concentrations) significantly increased isometric force output during twitch and unfused tetanic stimulation. Force potentiation during unfused tetanic stimulation was equivalent during either high or low voltage stimulation. Nifedipine had no effect on the time to peak force, half relaxation time, or relaxation time during twitch stimulation; thus, both activation and relaxation rates were increased. The force potentiating actions of nifedipine persisted in a calcium-free bathing solution and were enhanced by d-tubocurarine. In contrast to the force enhancing effects found with twitch and unfused tetanic stimulation, nifedipine caused a small but significant reduction in isometric force during maximal fused tetanic stimulation. It is concluded that the force potentiating effects of nifedipine on rat diaphragm are not due to fiber recruitment, enhancement of neuromuscular excitation, or altered inward trans-sarcolemmal calcium flux, but may result from a direct effect of the drug on the rate of activation of the contractile apparatus.

Animals↗
PubMed1989

Inhibition by nifedipine of the indirectly induced contractile response of the rat diaphragm.

The effects of nifedipine (30 micrograms/ml) on isometric force production of in-vitro rat diaphragm were studied during direct and indirect modes of muscle activation. During direct muscle stimulation, nifedipine potentiated isometric force during twitch and unfused tetanic stimulation. Indirectly elicited responses, evoked by stimulation of the phrenic nerve, were uniformly depressed following nifedipine. Inhibition of indirect force production increased with time, while force potentiation with direct activation remained constant. We conclude that inhibitory effects of nifedipine on the phrenic nerve-diaphragm preparation are specific for nerve or neuromuscular junction.

Animals↗