Contractile and endurance properties of geniohyoid and diaphragm muscles

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Respiratory control and sternohyoid muscle structure and function in aged male rats: decreased susceptibility to chronic intermittent hypoxia

Obstructive sleep apnoea syndrome (OSAS) is a common respiratory disorder characterized by chronic intermittent hypoxia (CIH). We have shown that CIH causes upper airway muscle dysfunction in the rat due to oxidative stress. Ageing is an independent risk factor for the development of OSAS perhaps due to respiratory muscle remodelling and increased susceptibility to hypoxia. We sought to examine the effects of CIH on breathing and pharyngeal dilator muscle structure and function in aged rats. Aged (18-20 months), male Wistar rats were exposed to alternating cycles of normoxia and hypoxia (90 s each; F(I)O(2)=5% O(2) at nadir) or sham treatment for 8h/day for 9 days. Following CIH exposure, breathing was assessed by whole-body plethysmography. In addition, sternohyoid muscle contractile and endurance properties were examined in vitro. Muscle fibre type and cross-sectional area, and the activity of key oxidative and glycolytic enzymes were determined. CIH had no effect on basal breathing or ventilatory responses to hypoxia or hypercapnia. CIH did not alter succinate dehydrogenase or glycerol phosphate dehydrogenase enzyme activities, myosin heavy chain fibre areal density or cross-sectional area. Sternohyoid muscle force and endurance were unaffected by CIH exposure. Since we have established that this CIH paradigm causes sternohyoid muscle weakness in adult male rats, we conclude that aged rats have decreased susceptibility to CIH-induced stress. We suggest that structural remodelling with improved hypoxic tolerance in upper airway muscles may partly compensate for impaired neural regulation of the upper airway and increased propensity for airway collapse in aged mammals.

Regional differences in length change and electromyographic heterogeneity in sternohyoid muscle during infant mammalian swallowing

A complex sling of muscles moves and stabilizes the hyoid bone during many mammalian behaviors. One muscle in this sling, the sternohyoid, is recruited during food acquisition, processing, and swallowing, and also during nonfeeding behaviors. We used synchronous sonomicrometry and electromyography to investigate regional (intramuscular) changes in length and electromyographic (EMG) activity of the sternohyoid during swallowing in the infant pig. The simple straplike architecture of the sternohyoid led us to hypothesize that limited regional variation in length and muscle activity would be present. We found statistically significant regional differences in EMG activity, and, with respect to length dynamics, the sternohyoid did not behave homogeneously during swallowing. The midbelly region typically shortened while the anterior and posterior regions lengthened, although in a minority of swallows (12.5%) the midbelly lengthened simultaneously with the end-regions. Despite its nonpennate architecture and evolutionarily conservative innervation, the mammalian sternohyoid appears to contain previously unrecognized populations of regionally specialized motor units. It also displays differential contraction patterns, very similar to the sternohyoid of nonmammalian vertebrates.

Effects of nicotine on rat sternohyoid muscle contractile properties

Obstructive sleep apnoea (OSA) is a major clinical disorder characterised by recurring episodes of pharyngeal collapse during sleep. At present, there remains no satisfactory treatment for OSA. Pharmacological therapies as a potential treatment for the disorder are an attractive option and include agents that increase the contractility of the pharyngeal muscles. The aim of the present study was to examine the effects of nicotine on upper airway muscle contractile properties. In vitro isometric contractile properties were determined using strips of rat sternohyoid muscle in physiological salt solution containing nicotine (0-100 microg/ml) at 25 degrees C. Isometric twitch and tetanic tension, contraction time, half-relaxation time and tension-frequency relationship were determined by electrical field stimulation with platinum electrodes. Fatigue was induced by stimulation at 40 Hz with 300 ms trains at a frequency of 0.5 Hz for 5 min. Nicotine at a concentration of 1 microg/ml was associated with a significant increase in sternohyoid muscle specific tension compared to control data. Dose-dependent increases in contractile tension were not observed. Nicotine had effects on tension-frequency relationship and endurance properties of the sternohyoid muscle at some but not all doses. A leftward shift in the tension-frequency relationship was observed at low stimulus frequencies (20-30 Hz) for nicotine at a concentration of 1 and 5 microg/ml and a significant increase in fatigue resistance was observed with nicotine at a concentration of 10 microg/ml. As fatigue of the upper airway muscles has been implicated in obstructive airway conditions, a pharmacological agent that improves muscle endurance may prove useful as a potential treatment for such disorders. Therefore, further studies of the effects of nicotinic agonists on upper airway function are warranted.

Inspiratory muscle maximum relaxation rate measured from submaximal sniff nasal pressure in patients with severe COPD

BACKGROUND

Slowing of the inspiratory muscle maximum relaxation rate (MRR) is a useful index of severe inspiratory muscle loading and potential fatigue and has been measured from the oesophageal pressure during sniffs in patients with chronic obstructive pulmonary disease (COPD). The purpose of this study was to investigate whether it is possible to measure MRR and detect slowing using sniff nasal pressure in patients with COPD and to investigate the relationship between sniff oesophageal and sniff nasal MRR.

METHODS

Eight patients with severe COPD (mean FEV1 0.7 l; 26% predicted) were studied. Each subject performed submaximal sniff manoeuvres before and after walking to a state of severe dyspnoea on a treadmill. Oesophageal and gastric pressures were measured using balloon tipped catheters and nasal pressure was measured using an individually modelled nasal cast. MRR (% pressure fall/10 ms) was determined for each sniff and any change following exercise was reported as percentage of baseline to allow comparison of sniff nasal and oesophageal MRR.

RESULTS

At rest the mean (SE) sniff Poes MRR was 7.1 (0.3) and the mean Pnasal MRR was 8.6 (0.1). At 1 minute following exercise there was a mean decrease in sniff Poes MRR of 33.7% (range 20.7-53.4%) and a mean decrease in sniff Pnasal MRR of 28.2% (range 8.1-52.8%). The degree of slowing and time course of recovery was similar, with both returning to baseline values within 5-10 minutes. A separate analysis of the sniff pressures using only the nasal pressure traces demonstrated a similar pattern of slowing and recovery.

CONCLUSIONS

It is possible to detect slowing of the inspiratory muscles non-invasively using sniff nasal pressures in patients with COPD. This could be a useful technique with which to measure severe and potentially fatiguing inspiratory muscle loading, both in clinical settings and during exercise studies.

Fatiguing contractions of tongue protrudor and retractor muscles: influence of systemic hypoxia

The influence of systemic hypoxia on the endurance performance of tongue protrudor and retractor muscles was examined in anesthetized, ventilated rats. Tongue protrudor (genioglossus) or retractor (hyoglossus and styloglossus) muscles were activated via medial or lateral XII nerve branch stimulation (0.1-ms pulse; 40 Hz; 330-ms trains; 1 train/s). Maximal evoked potentials (M waves) of genioglossus and hyoglossus were monitored with electromyography. Fatigue tests were performed under normoxic and hypoxic (arterial PO(2) = 50 +/- 1 Torr) conditions in separate animals. The fatigue index (FI; %initial force) after 5 min of normoxic stimulation was 85 +/- 6 and 79 +/- 7% for tongue protrudor and retractor muscles, respectively; these values were significantly lower during hypoxia (protrudor FI = 52 +/- 10, retractor FI = 18 +/- 6%; P < 0.05). Protrudor and retractor muscle M-wave amplitude declined over the course of the hypoxic fatigue test but did not change during normoxia (P < 0.05). We conclude that hypoxia attenuates tongue protrudor and retractor muscle endurance performance; potential mechanisms include neuromuscular transmission failure and/or diminished sarcolemmal excitability.

Effect of gender on rat upper airway muscle contractile properties

Obstructive sleep apnoea arises due to upper airway (UA) collapse which is normally counteracted by contraction of UA muscles such as the sternohyoids and geniohyoids. The disorder has a marked male predominance but the effect of gender on UA muscle contractile properties is unknown and these properties have not been compared for the sternohyoid and geniohyoid muscles in the same species. Isometric contractile characteristics were determined using strips of sternohyoid and geniohyoid muscle from male and female rats in Krebs solution at 30 degrees C. For both muscles, there were no differences between male and female contractile kinetics, twitch or tetanic tension, tension-length or tension-frequency relationship or endurance. In both males and females, sternohyoid twitch and tetanic tension was greater than geniohyoid. Sternohyoid endurance was less than geniohyoid but contractile kinetics, tension-length and tension-frequency relationships were similar. Therefore, gender does not affect UA muscle contractile properties and sternohyoid tension is greater and endurance less than that of the geniohyoid.

Metabolic profiles of cat and rat pharyngeal and diaphragm muscles

Pharyngeal muscles play important roles in the maintenance of upper airway patency during sleep. The present study determined the extent of heterogeneity among pharyngeal muscles and the diaphragm in their metabolic profiles, and examined whether differences among muscles may account for previously described differences in their fatigue resistance. Cat and rat sternohyoid, geniohyoid, genioglossus (cat only) and diaphragm muscle were assayed for activities of the mitochondrial enzyme citrate synthase (CS), the glycolytic enzyme phosphofructokinase (PFK) and the cytosolic enzyme lactate dehydrogenase (LDH). CS activity varied among muscles in both species, being highest for genioglossus in cat and highest for diaphragm in rat. PFK activity was highest for genioglossus in cat, but did not differ among muscles in rat. LDH activity was lower for the genioglossus than the sternohyoid and diaphragm in cat. CS and PFK activities correlated positively, and LDH activity correlated negatively, with in vitro fatigue resistance assessed after 5 min of repetitive stimulation in cat. These data indicate close relationships between metabolic profiles, particularly oxidative capacity, and fatigue resistance of pharyngeal muscles in relationship to each other and to the diaphragm.

Comparative effects of aging on pharyngeal and diaphragm muscles

We hypothesized that aging is associated with alterations in pharyngeal muscle structural and contractile properties. Sternohyoid and geniohyoid muscles from young (3-4 months) and old (20-21 months) Fischer 344 rats were compared with diaphragm muscle. The pharyngeal muscles had significantly lower proportions of slow oxidative (SO) fibers compared to the diaphragm, and the percentage of fast glycolytic (FG) fibers was significantly higher in the sternohyoid than in both the geniohyoid and the diaphragm. With senescence, there was a small but significant increase in the proportion of FG fibers and a corresponding reduction in the proportion of fast oxidative glycolytic (FOG) fibers in all three muscles. The sternohyoid muscle had significantly faster isometric contractile kinetics and lower fatigue indexes than the diaphragm. Aging was associated with significant worsening of sternohyoid endurance, but no significant alterations in sternohyoid twitch kinetics or diaphragm properties. These results indicate that in rats the pharyngeal dilator muscles have larger proportions of fast fibers, fast contractile kinetics and worse endurance than the diaphragm. Furthermore, aging was associated with a shift to a higher proportion of FG fibers with a concomitant reduction in proportion of FOG fibers, as well as a decline in pharyngeal muscle endurance.

Inspiratory muscle relaxation rate assessed from sniff nasal pressure

BACKGROUND

Slowing of the maximum relaxation rate (MRR) of inspiratory muscles measured from oesophageal pressure (POES) during sniffs has been used as an index of the onset and recovery of respiratory muscle fatigue. The purpose of this study was to measure MRR at the nose (PNASAL MRR), to investigate its relationship with POES MRR, and to establish whether PNASAL MRR slows with respiratory loading.

METHODS

Five normal subjects were studied. Each performed sniffs before and after two minutes of maximal isocapnic ventilation (MIV). In a separate session the subjects performed submaximal sniffs. POES and PNASAL were recorded during sniffs and the MRR (% pressure fall/10 ms) for each sniff was determined.

RESULTS

Before MIV mean POES MRR was 8.9 and PNASAL MRR was 9.3. The mean (SD) difference between PNASAL MRR and POES MRR during a maximal sniff was 0.48 (0.34) (n = 64) and during submaximal sniffs was 0.28 (0.46) (n = 526). The subjects showed a mean decrease in sniff POES MRR of 27.4% (range 22.5-36%) after MIV and a similar reduction in sniff PNASAL MRR of 28.5% (range 24.1-41.3%). Both returned to control values within 5-10 minutes.

CONCLUSIONS

PNASAL MRR reflects POES MRR over a wide range of sniff pressures, PNASAL MRR of maximal sniffs reflects POES MRR in normal subjects at rest and following MIV, so measurement of PNASAL MRR may be a useful non-invasive method for measuring inspiratory muscle MRR, thereby providing an index of respiratory muscle fatigue.

Pharyngeal dilator muscle contractile and endurance properties in neonatal piglets

Pharyngeal dilator muscles are critical for maintaining upper airway patency in the neonatal period. The present study examined in vitro the contractile properties of a pharyngeal dilator muscle, the sternohyoid, in 1-7-day-old piglets (n = 24). Isometric contraction and half-relaxation times were 36.7 +/- 1.1 and 30.9 +/- 1.2 msec, respectively. Twitch potentiation ('staircase phenomenon') and post-tetanic potentiation were noted following repetitive stimulation. During prolonged repetitive stimulation with a standard (40 Hz) fatigue test, muscle force declined gradually over time, with loss of half of the initial force occurring over 138 +/- 11 sec, and a 2-min fatigue index (ratio of force at 2 min to initial force) of 0.52 +/- 0.03. An additional 10 piglets were studied at ages of 14-20 days. Muscle from older piglets had comparable isometric twitch kinetics as that of younger animals. However, sternohyoid muscle from the older piglets had worse endurance than muscle from the younger animals, as indicated by a shorter time required for force to decrease by half (86 +/- 10 sec, P < 0.01) and a lower 2-min fatigue index (0.36 +/- 0.03, P < 0.01). These data indicate that for the sternohyoid muscle of the newborn piglet (a) physiological properties are consistent with moderate to fast contraction with good endurance, (b) force potentiates during repetitive twitch stimulation and following a brief period of tetanic stimulation, and (c) there is worsening of endurance but no change in isometric twitch kinetics with increasing age during the first weeks of life.