In vitro evaluation and in vivo demonstration of a biomimetic, hemocompatible, microfluidic artificial lung

Despite the promising potential of microfluidic artificial lungs, current designs suffer from short functional lifetimes due to surface chemistry and blood flow patterns that act to reduce hemocompatibility. Here, we present the first microfluidic artificial lung featuring a hemocompatible surface coating and a biomimetic blood path. The polyethylene-glycol (PEG) coated microfluidic lung exhibited a significantly improved in vitro lifetime compared to uncoated controls as well as consistent and significantly improved gas exchange over the entire testing period. Enabled by our hemocompatible PEG coating, we additionally describe the first extended (3 h) in vivo demonstration of a microfluidic artificial lung.

Auxotonic contractile responses of rat tracheal and bronchial airway smooth muscle

The purposes of the present study were to directly compare: (1) the degree of trachealis muscle shortening and changes in tracheal dimensions and (2) ACh-mediated auxotonic contraction of trachea and intraparenchymal bronchi. The auxotonic contractile properties of tracheal and bronchial airway smooth muscle were assessed from 1-2 mm thick tracheal sections and;1 mm thick lung slices using videomicrometry in vitro at 37 degrees C. Acetylcholine resulted in reductions in luminal area, perimeter, mean radius, length, and breadth (22.0, 10.0, 11.9, 10.7 and 12.0%, respectively). Trachealis muscle shortening reached a maximum of 39.8+/-4.3%. The K(+)channel blocker 4-aminopyridine significantly augmented the ACh-mediated reductions in tracheal luminal dimensions. In response to ACh (10(-3)m), reductions in bronchial dimensions were significantly greater than those of the trachea for luminal area, perimeter and mean radius (44.6 vs. 18.6, 32.0 vs. 8.0 and 28.9 vs. 9.9%, respectively). These data indicate that auxotonic contractile responses of rat tracheal smooth muscle differ from those previously reported in the dog and guinea pig, that ACh-mediated auxotonic contraction of tracheal smooth muscle is augmented by 4-aminopyridine, and that proportionate reductions in luminal dimensions in response to ACh are considerably greater for bronchial than tracheal airways.

Effect of Temperature on Endplate Potential Rundown and Recovery in Rat Diaphragm

The amplitude of neuromuscular junction end-plate potentials (EPPs) decreases quickly within a train but recovers nearly completely from train to train during intermittent stimulation. Rundown has been shown to be dependent not only on the rate of transmitter release but also on the rate of replenishment of the depleted neurotransmitter at the site of release. Two groups of processes have been proposed for synaptic vesicle recycling, both of which involve multiple energy-requiring steps and enzymatic reactions and which therefore would be expected to be very temperature-sensitive. The present study tested the hypothesis that low temperature therefore increases the rate of EPP amplitude rundown. Studies were performed in vitro on rat diaphragm and used μ-conotoxin to allow normal-sized EPPs to be recorded from intact fibers. EPP amplitude rundown during intermittent stimulation at 20 and 50 Hz (duty cycle 333 ms) was greater at 20°C than it was at 37°C. Initially, temperature affected only intra-train rundown but, over longer periods of stimulation, both intra- and inter-train rundown were significantly accelerated by cold temperature. Cumulative EPP amplitudes were calculated by successively adding the amplitudes of each EPP during the stimulation period to provide an estimate of total neurotransmitter release in the neuromuscular junction. The cumulative EPP amplitude was significantly lower at 20°C than it was at 37°C during both 20 and 50 Hz stimulation. These data indicate that the mechanism involved in EPP amplitude rundown and recovery is temperature-sensitive, with a greater decrement in EPP amplitude at cold than at warm temperatures.

Inotrophic effects of the K(+) channel blocker TEA on dystrophic (mdx and dy/dy) mouse diaphragm

K(+) channels regulate diaphragm resting membrane potential and action potential duration, and hence force. Certain blockers of these channels, e.g. tetraethylammonium (TEA), increase twitch force of normal diaphragm. To further address whether these agents may be useful in the treatment of diaphragm weakness, studies examined the effects of TEA on force of overtly diseased muscle. Diaphragm from two mouse models of muscular dystrophy (mdx and dy/dy) was studied in vitro. Diaphragm from both models was significantly weaker than diaphragm from control animals. TEA (10 mM) increased twitch force of both mdx diaphragm (P<0.005) and dy/dy diaphragm (P<0.0005), as well as force of diaphragm from non-diseased animals. The percent force increase of mdx diaphragm was at least as great as that of non-diseased muscle (15.3 vs 9.2%, P=0.14), and the percent force increase of dy/dy diaphragm was significantly greater than that of non-diseased muscle (22.7 vs 10.2%, P<0.02). Absolute force increases normalized for cross-sectional area were comparable for healthy and diseased diaphragm, however. These findings indicate that TEA increases force of both dystrophin-deficient and merosin-deficient dystrophic mouse diaphragm muscle.

Modulation of diaphragm action potentials by K(+) channel blockers

K(+) channels regulate diaphragm contractility. The present study examined the electrophysiological mechanisms accounting for diversity among K(+) channel blockers in their inotropic actions on the diaphragm. Rat diaphragmatic muscle fibers were recorded intracellularly in vitro at 37 degrees C. Apamin and charybdotoxin (Ca2+)-activated K(+) channel blockers) did not alter resting membrane potential or action potentials. Glibenclamide (ATP-sensitive K(+) channel blocker) slowed action potential repolarization by 12% (P<0.05) and increased action potential area by 25% (P<0.005). Tetraethylammonium (which blocks several types of K(+) channels) increased action potential overshoot by 20% (P<0.01) and prolonged action potential rise time by 17% (P<0.02). 4-Aminopyridine and 3,4-diaminopyridine (which also block several types of K(+) channels) slowed action potential repolarization by 163% (P<0.0001) and 253% (P<0.0001), and increased action potential area by 183% (P<0.0001) and 298% (P<0.0001), respectively. Slowing of repolarization for the aminopyridines was especially marked at voltages approaching resting membrane potential, thereby changing action potential repolarization from a first to a second order decay. Previously reported variability in inotropic effects among K(+) channel blockers correlated significantly with the extent to which they slowed action potential repolarization and increased action potential area, but not with changes in other action potential properties.

Effects on dystrophic (dy/dy) limb muscle of the K+ channel blocker tetraethylammonium

K(+) channel blockers, such as tetraethylammonium (TEA) and 4-aminopyridine, increase force of normal skeletal muscle. To determine whether they also increase force of diseased muscle, effects of TEA were examined on limb muscles of dy/dy dystrophic mice in vitro. TEA significantly increased isometric twitch force of normal and dystrophic soleus muscle by 29.9+/-5.8 and 30.0+/-5.0%, respectively, and significantly increased force of normal and dystrophic extensor digitorum longus muscle by 18.3+/-1.9 and 34. 9+/-10.4%, respectively. Force increases could be attributed only partially to a prolongation of contraction time. These data indicate that TEA augments force of overtly diseased (dystrophic) limb muscle to an extent comparable with that of normal limb muscle.

Catchlike property of rat diaphragm: subsequent train frequency effects in variable-train stimulation

A high-frequency burst of pulses at the onset of a subtetanic train of stimulation allows skeletal muscle to hold force at a higher level than expected from the extra pulses alone because of the catchlike property of muscle. The present study tested the hypothesis that the presence and degree of force increase induced by a high-frequency burst are strongly modulated by the subsequent train frequency. Rat diaphragm muscle strips (studied in vitro at 37 degrees C) underwent two-, three-, or four-pulse bursts [interpulse interval (IPI) of 5 or 10 ms] at the onset of 10- to 50-Hz subtetanic trains. Force was quantified during the train with respect to its peak value (F(peak)), mean value (F(mean)), and force-time integral (F(area)), and it was compared with that produced during subtetanic trains of an equal number of pulses without preceding pulse bursts (Diff-F(peak), Diff-F(mean), Diff-F(area)). F(peak) and F(mean) increased with two-, three-, and four-pulse bursts, and Diff-F(peak) and Diff-F(mean) increased progressively with decreasing frequency of the subtetanic train. F(area), the best reflection of catchlike force augmentation, was increased mainly by the four-pulse bursts with an IPI of 10 ms, and Diff-F(area) was maximal at subsequent train frequencies of 15-25 Hz. The use of incorrect patterns of burst stimulation could also precipitate F(area) decreases, which were observed with the four-pulse, 5-ms IPI paradigm. The time required to reach 80% of maximal force (T(80%)) became shorter for each of the pulse burst stimulation patterns, with maximal reduction of Diff-T(80%) occurring at a subsequent train frequency of 20 Hz in all cases. These data indicate that extra-pulse burst stimulation paradigms need to incorporate the optimal combinations of extra-pulse number, IPI, and the frequency of the subsequent subtetanic train to take greatest advantage of the catchlike property of muscle.

Effect of phasic activation on endplate potential in rat diaphragm

Neuromuscular junction endplate potentials (EPPs) decrease quickly and to a large extent during continuous stimulation. The present study examined the hypothesis that EPP rundown recovers rapidly, thereby substantially preserving neurotransmission during intermittent compared with continuous stimulation. Studies were performed in vitro on rat diaphragm, using mu-conotoxin to allow recording of normal-sized EPPs from intact fibers. During continuous 5- to 100-Hz stimulation, EPP amplitude declined with a biphasic time course. The initial fast rate of decline was modulated substantially by stimulation frequency, whereas the subsequent slow rate of decline was relatively frequency independent. During intermittent 5- to 100-Hz stimulation (duty cycle 0.33), EPP amplitude declined rapidly during each train, but recovered substantially by the onset of the following train. The intra-train declines were substantially greater than the inter-train declines in EPP amplitude. Intra-train reductions in EPP amplitude were stimulation frequency dependent, based on both the total decline and rate constant of EPP decline. In contrast, the degree of recovery from train to train was independent of stimulation frequency, indicating low frequency dependence of inter-train rundown. The substantial recovery of EPP amplitude in between trains resulted in greater cumulative EPP size during intermittent compared with continuous stimulation. During continuous stimulation, EPP drop-out was only seen during 100-Hz stimulation; this was completed mitigated during intermittent stimulation. Miniature EPP size was unaffected by either continuous or intermittent stimulation. The pattern of rapid intra-train rundown and slow inter-train rundown of EPP size during intermittent stimulation is therefore due to rapid changes in the magnitude of neurotransmitter release rather than to axonal block or postsynaptic receptor desensitization. These findings indicate considerable rundown of EPP amplitudes within a stimulus train, with near complete recovery by the onset of the next train. This substantially attenuates the decrement in EPP amplitude during intermittent compared with continuous stimulation, thereby preserving the integrity of neurotransmission during phasic activation.

Attenuation of rat diaphragm low-frequency fatigue by vanadate in vitro

Sodium vanadate inhibits protein tyrosine phosphatases, including in skeletal muscle. Vanadate increases contractile force of airway, vascular and gastrointestinal smooth muscle. The present study tested the hypothesis that vanadate augments skeletal muscle contractility. Rat diaphragm muscle strips (n=26 from 12 animals) were studied in vitro at 37 degrees C. Muscles contracted isometrically while stimulated supramaximally with one of two protocols: 30 min of continuous 0.1 Hz stimulation, or 5 min of intermittent 20 Hz stimulation (duty cycle 0.33). Vanadate (500 microM)-treated muscle strips were compared with untreated muscle. Vanadate did not affect force or isometric twitch kinetics of otherwise quiescent muscle. During prolonged 0.1 Hz stimulation, force of control muscles declined by 17 +/- 4% over 30 min, whereas muscles incubated with vanadate maintained force virtually unchanged. Force over time was significantly greater with than without vanadate (P = 0.03), with values being significantly different during the last 10 min of the 30 min stimulation period. In the absence of vanadate force declined at a rate of approximately 0.6% per min, whereas with vanadate the rate of force decline was less than 0.1% per min (P < 0.02). During intermittent 20 Hz stimulation, the degree of force decline was not affected by vanadate at any time over a course of 5 min. Isometric contractile kinetics were not altered by vanadate during either 0.1 or 20 Hz stimulation. These data suggest that vanadate ameliorates low- but not higher-frequency fatigue in diaphragm, suggesting a role for protein tyrosine phosphorylation in the regulation of muscle fatigue resistance.

Peptide toxin blockers of voltage-sensitive K+ channels: inotropic effects on diaphragm

Agents that block many types of K+ channels (e.g., the aminopyridines) have substantial inotropic effects in skeletal muscle. Specific blockers of ATP-sensitive and Ca2+-activated K+ channels, on the other hand, do not, or minimally, alter the force of nonfatigued muscle, consistent with a predominant role for voltage-gated K+ channels in regulating muscle force. To test this more directly, we examined the effects of peptide toxins, which in other tissues specifically block voltage-gated K+ channels, on rat diaphragm in vitro. Twitch force was increased in response to alpha-, beta-, and gamma-dendrotoxin and tityustoxin Kalpha (17 +/- 6, 22 +/- 5, 42 +/- 14, and 13 +/- 5%; P < 0.05, < 0.01, < 0.05, < 0.05, respectively) but not in response to delta-dendrotoxin or BSA (in which toxins were dissolved). Force during 20-Hz stimulation was also increased significantly by alpha-, beta-, and gamma-dendrotoxin and tityustoxin Kalpha. Among agents, increases in twitch force correlated with the degree to which contraction time was prolonged (r = 0.88, P < 0.02). To determine whether inotropic effects could be maintained during repeated contractions, muscle strips underwent intermittent 20-Hz train stimulation for a duration of 2 min in presence or absence of gamma-dendrotoxin. Force was significantly greater with than without gamma-dendrotoxin during repetitive stimulation for the first 60 s of repetitive contractions. Despite the approximately 55% higher value for initial force in the presence vs. absence of gamma-dendrotoxin, the rate at which fatigue occurred was not accelerated by the toxin, as assessed by the amount of time over which force declined by 25 and 50%. These data suggest that blocking voltage-activated K+ channels may be a useful therapeutic strategy for augmenting diaphragm force, provided less toxic blockers of these channels can be found.

Effects of genetic obesity on rat upper airway muscle and diaphragm contractile properties

The contractile properties of pharyngeal respiratory muscle are altered in sleep apnoea and in conditions associated with sleep apnoea, such as ageing. We hypothesized that the contractile properties of the pharyngeal musculature are also altered by obesity, another factor associated with sleep apnoea. Studies compared a pharyngeal muscle, the sternohyoid, with the diaphragm. These were chosen as representative muscles whose contraction has opposing effects on upper airway patency. Both muscles were removed from nine lean and nine obese male Zucker rats (a genetic model of obesity), and isometric contractile properties were studied in vitro at 37 degrees C. For the sternohyoid muscle, in obese compared to lean animals there were no significant differences in isometric contraction time (15.2 +/- 0.3 vs 14.2 +/- 0.6 ms, respectively), half-relaxation time (13.6 +/- 0.5 vs 12.6 +/- 0.9 ms, respectively), twitch-to-tetanic tension ratio (0.22 +/- 0.02 vs 0.24 +/- 0.02, respectively), force-frequency relationship, fatigue resistance (2 min fatigue index 0.20 +/- 0.03 vs 0.18 +/- 0.02, respectively), or maximal degree of force potentiation during repetitive stimulation (52 +/- 11 vs 74 +/- 20% increase, respectively). For the diaphragm, the only significant effect of obesity was a lowering of the twitch-to-tetanic tension ratio (0.25 +/- 0.01 vs 0.29 +/- 0.02, respectively). In obese, as in lean animals, the sternohyoid had faster isometric twitch kinetics, a larger degree of force potentiation, and lower resistance to fatigue, than the diaphragm. In lean, but not obese, animals the sternohyoid twitch-to-tetanic tension ratio was lower than and the force frequency relationship was located to the right of that of the diaphragm. In this study, genetic obesity in rats was not associated with any significant alterations in the contractile properties of the pharyngeal muscle, and only small changes in the relationship between the contractile properties of the sternohyoid and diaphragm muscle.

Effects of genetic obesity on rat upper airway muscle and diaphragm contractile properties

The contractile properties of pharyngeal respiratory muscle are altered in sleep apnoea and in conditions associated with sleep apnoea, such as ageing. We hypothesized that the contractile properties of the pharyngeal musculature are also altered by obesity, another factor associated with sleep apnoea. Studies compared a pharyngeal muscle, the sternohyoid, with the diaphragm. These were chosen as representative muscles whose contraction has opposing effects on upper airway patency. Both muscles were removed from nine lean and nine obese male Zucker rats (a genetic model of obesity), and isometric contractile properties were studied in vitro at 37 degrees C. For the sternohyoid muscle, in obese compared to lean animals there were no significant differences in isometric contraction time (15.2 +/- 0.3 vs 14.2 +/- 0.6 ms, respectively), half-relaxation time (13.6 +/- 0.5 vs 12.6 +/- 0.9 ms, respectively), twitch-to-tetanic tension ratio (0.22 +/- 0.02 vs 0.24 +/- 0.02, respectively), force-frequency relationship, fatigue resistance (2 min fatigue index 0.20 +/- 0.03 vs 0.18 +/- 0.02, respectively), or maximal degree of force potentiation during repetitive stimulation (52 +/- 11 vs 74 +/- 20% increase, respectively). For the diaphragm, the only significant effect of obesity was a lowering of the twitch-to-tetanic tension ratio (0.25 +/- 0.01 vs 0.29 +/- 0.02, respectively). In obese, as in lean animals, the sternohyoid had faster isometric twitch kinetics, a larger degree of force potentiation, and lower resistance to fatigue, than the diaphragm. In lean, but not obese, animals the sternohyoid twitch-to-tetanic tension ratio was lower than and the force frequency relationship was located to the right of that of the diaphragm. In this study, genetic obesity in rats was not associated with any significant alterations in the contractile properties of the pharyngeal muscle, and only small changes in the relationship between the contractile properties of the sternohyoid and diaphragm muscle.

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.

Effects of theophylline on pharyngeal dilator and diaphragm muscle contractile properties

Theophylline alleviates central and obstructive apneas of prematurity, and may improve adult obstructive sleep apnea. One mechanism of action appears to be a stimulatory effect on the motor output to upper airway dilator muscles. The purpose of the present study was to determine whether theophylline might have a second mechanism of action, namely that of improving the force and/or endurance of the pharyngeal dilator musculature. Rat sternohyoid muscle strips were studied in vitro and compared to diaphragm strips. The isometric twitch force and twitch kinetics of neither muscle were altered by theophylline (100 mg/l). Theophylline significantly slowed the rate at which the diaphragm fatigued during intermittent 40-Hz stimulation (p < 0.001). In contrast, theophylline produced no improvement in the fatigue resistance of the sternohyoid muscle. The degree of force potentiation during the early portion of the fatigue protocol was not altered by theophylline for either muscle. These results suggest that the mechanism by which theophylline improves obstructive apnea is unlikely to be due to a beneficial effect on pharyngeal dilator muscle force or endurance.

Changes in pharyngeal respiratory muscle force produced by K+ channel blockade

The purpose of the present study was to determine whether the contractility of pharyngeal respiratory muscles can be augmented by altering membranous K+ channel conductance. The effects on twitch force of two K+ channel blockers, tetraethylammonium (TEA, 10 mM) and 4-aminopyridine (4-AP, 0.3 mM), were examined in vitro for sternohyoid and diaphragm muscle strips. Both agents augmented isometric twitch force of both muscles. In response to TEA twitch force of the sternohyoid muscle increased significantly more than that of the diaphragm (by 33 +/- 7 vs. 9 +/- 1%, P = 0.004), whereas with 4-AP the increase in twitch force of the sternohyoid muscle was comparable to that of the diaphragm (55 +/- 15 vs. 64 +/- 6%, P = 0.50). 4-AP shifted the force-frequency relationship of both muscles leftward but did not alter peak tetanic force, so that force with 4-AP exceeded that without drug at stimulation frequencies below 60 Hz. In contrast TEA reduced force at stimulation frequencies > 20 Hz. The isometric contraction times of both muscles was variably prolonged, more so with 4-AP (by 30 +/- 15% for the sternohyoid and 32 +/- 3% for the diaphragm) than with TEA (by 9 +/- 2% for the sternohyoid and 5 +/- 2% for the diaphragm). For the group of muscles and K+ channel blockers, the degree of augmentation of twitch force correlated with the degree of prolongation of contraction time (r = 0.82, P < 0.001), consistent with blocking delayed rectifier K+ channels as the mechanism of increasing muscle force.

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.

Effects of temperature on calcium current of bullfrog sympathetic neurons

1. The temperature dependence of whole-cell calcium current was studied in bullfrog sympathetic neurons. 2. The factor by which the peak calcium current increased upon a 10 degrees C increase in temperature (Q10) was 1.6 +/- 0.1 (mean +/- S.E.M., n = 9), for a change from 20 to 30 degrees C. 3. Activation and deactivation were more rapid at a higher temperature. The Q10 was approximately 7 in the middle of the voltage range (near -10 mV), where the kinetics were slowest. Time constants were less temperature dependent at more positive or negative voltages (Q10 approximately 2 at -70 mV). 4. Near -10 mV, activation and deactivation were associated with a large enthalpy and a large positive entropy change. Deactivation at -70 mV reflected a smaller enthalpy change, and almost no change in entropy. 5. Activation is only slightly more temperature dependent than deactivation, when both are measured at the same voltage. 6. The peak current shifts slightly (approximately 5 mV) to more negative voltages upon a change from 20 to 30 degrees C. 7. Inactivation has a Q10 of approximately 2 at -10 mV. 8. Changes in the kinetics of activation or inactivation could be observed during recording at a constant temperature. These changes were generally small, especially for activation kinetics, and could be distinguished from temperature-dependent changes. 9. The changes in entropy and Q10 with voltage suggest that the rate-limiting steps for activation and deactivation are different at extreme voltages vs. the middle of the voltage range.

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.

Central and spinal effects of sodium cyanide on respiratory activity

The pharmacological actions of cyanide on respiratory activity have been known for some time and are attributed mainly to effects on peripheral chemoreceptors. In the studies reported here, we have examined the acute central effects of cyanide when applied topically to the ventral surface of the medulla (VMS) and when administered into the spinal intrathecal space at the C5-T3 level on activities of the phrenic nerve, diaphragm, parasternal intercostal, triangularis sterni, and transversus abdominis muscles. Topical application of 10-100 micrograms (10 microliters of 1-10 mg/ml) cyanide to the intermediate area of the VMS decreased respiratory activity by > 50%, and expiratory muscles were more sensitive to inhibition than inspiratory muscles. The onset of depression of phrenic nerve or respiratory muscle activity occurred within 20 s of administration, and the effects reversed after washout. In contrast, intrathecal administration of cyanide in doses of 10-100 micrograms (100 microliters of 0.1-1 mg/ml) increased electrical activity of the respiratory muscles. Diaphragm activity changed from 17 +/- 2 to 42 +/- 8 (SE) units (P < 0.01), parasternal intercostal activity increased from 18 +/- 3 to 46 +/- 9 units (P < 0.01), and expiratory activity of the chest wall and abdominal muscles increased from 9 +/- 2 to 39 +/- 10 units (P < 0.05). Both topical application on the VMS and intrathecal administration of cyanide caused an increase in arterial blood pressure and a slight insignificant acceleration of heart rate. These data suggest that cyanide acting on the VMS causes respiratory depression and enhancement of sympathetic outflow.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscles of the pharynx: structural and contractile properties

Pharyngeal muscles are the effector organ by which the brainstem regulates pharyngeal airway size and patency during breathing. These muscles have fast contractile rates, and may be susceptible to develop fatigue when driven at the high levels required to overcome structural pharyngeal narrowing, especially under hypoxic conditions. Diseases with an increased prevalence of sleep apnea are associated with changes in pharyngeal muscle properties, and conversely diseases which primarily alter neuromuscular function have a significant prevalence of sleep apnea. However, further studies are needed to define the precise role of pharyngeal muscle fatigue, and of changes in pharyngeal muscle properties with disease, in the pathophysiology of obstructive sleep apnea.

Intrinsic properties of pharyngeal and diaphragmatic respiratory motoneurons and muscles

Breathing is a complex act requiring the coordinated activity of multiple groups of muscles. Thoracic and abdominal respiratory muscles expand and contract the lungs, whereas pharyngeal and laryngeal respiratory muscles maintain upper airway patency and regulate upper airway resistance. An appreciation of the importance of the latter muscle group in maintaining ventilatory homeostasis and in the pathophysiology of sleep apnea has led to extensive studies examining the neural regulation of pharyngeal dilator muscles. The present review examines the role of heterogeneity in motoneuron and muscle properties in determining the diversity in the electrical and mechanical behaviors of thoracic compared with pharyngeal muscle groups. Specifically, phrenic and hypoglossal motoneuron electrophysiological properties influence whether and the extent to which these neurons will fire in response to a given synaptic input arising from chemo- and mechanoreceptors and from respiratory and nonrespiratory pattern generators. Furthermore, thoracic and pharyngeal muscle properties determine the mechanical response to motoneuronal activity, including the speed of contraction, relationships between motoneuron firing frequency and force production, and whether force is maintained during repetitive activation. Heterogeneity in the functional capabilities of these motoneurons and muscles is in turn determined by diversity of their structural and biochemical properties. Thus, intrinsic properties of respiratory motoneurons and muscles act in concert with neuronal drives in defining the complex electrical and mechanical behavior of pharyngeal and thoracic respiratory motor systems.

Breath-to-breath variability in hypoglossal motor unit firing

Instability in the magnitude and timing of motor output to pharyngeal dilator muscles occurs during breathing. This contributes to alterations in upper airway resistance, and is one of several factors that play a role in the pathophysiology of obstructive apneas. To define the motor unit mechanisms accounting for such variability, geniohyoid motor unit activity was recorded simultaneously with diaphragm EMG in anesthetized cats spontaneously breathing 7% CO2 in O2. Variability was quantified with the coefficient of variation [CV = (SD/mean) x 100%]. In this preparation, we confirmed greater breath-to-breath variability of geniohyoid compared to diaphragm peak moving average EMGs. During recordings of geniohyoid motor unit activity, average CV of other respiratory parameters were as follows: peak diaphragm EMG 5.8%, inspiratory time 3.5%, expiratory time 3.8%. The average CV for geniohyoid motor unit activity patterns were substantially higher: spikes per breath 15.6%, mean firing frequency 13.3%, peak firing frequency 19.0%, minimal firing frequency 26.3%, onset time 40.9%, offset time 10.0% and duration of firing 12.8%. Values differed considerably among motor units, even when activity was recorded simultaneously. These findings suggest that variability is present in both intensity and timing of geniohyoid motor unit firing during breathing, and that different geniohyoid motor units appear to have varying degrees of stability during breathing.

Inhaled albuterol powder for pulmonary function testing

To assess the role of albuterol powder in testing for reversibility of airflow obstruction during routine pulmonary function testing, spirometric data from subjects with baseline FEV1/FVC less than 70 percent and FEV1 less than 80 percent predicted who received inhaled albuterol powder (n = 42) were compared with those who received isoetharine aerosol via metered dose inhaler (n = 49). Prebronchodilator lung function was comparable for the albuterol and isoetharine groups. With albuterol powder, 14 (33 percent) of 42 subjects showed reversibility of airflow obstruction (defined as a 15 percent or greater improvement in either FEV1 or FVC) as compared with 30 (61 percent) of 49 subjects with isoetharine aerosol. The significantly (p less than 0.01) lower rate of improvement with albuterol powder was especially prominent in subjects with moderate airflow obstruction (FEV1/FVC of 45 to 59 percent). These data do not support the substitution of inhaled albuterol powder for isoetharine aerosol in assessing for reversibility of airflow obstruction during routine pulmonary function testing.

Contractile properties of feline genioglossus, sternohyoid, and sternothyroid muscles

Despite a wealth of information about the respiratory behavior of pharyngeal dilator muscles such as the genioglossus, sternohyoid, and sternothyroid muscles, little is known about their contractile and endurance properties. Strips of these muscles (as well as of the diaphragm) were surgically removed from anesthetized cats and studied in vitro at 37 degrees C. The isometric contraction times of the muscles were 38 +/- 1, 31 +/- 1, 28 +/- 2, and 35 +/- 1 ms for genioglossus, sternothyroid, sternohyoid, and diaphragm, respectively. Contraction times were significantly longer for the genioglossus than for the sternohyoid and sternothyroid muscles and significantly longer for the diaphragm than for the sternohyoid muscle. Twitch-to-tetanic ratios were largest for the diaphragm and lowest for the sternohyoid muscle, and the force-frequency relationship of the sternohyoid was most rightward positioned and that of the diaphragm was most leftward positioned. During repetitive stimulation, the decrement in force was greatest for the diaphragm and least for the genioglossus muscle, with the force loss of the two hyoid muscles being intermediate in magnitude. The Burke fatigue index was significantly greater for the genioglossus than for the diaphragm, despite similar tension-time indexes during repetitive stimulation. These data indicate heterogeneity among pharyngeal dilator muscles in their contractile and endurance properties, that certain pharyngeal dilator muscle properties differ from diaphragmatic properties, and that pharyngeal muscles have relatively fast contractile kinetics yet reasonable endurance characteristics.

Nonvagal modulation of hypoglossal neural activity

Upper airway dilating muscle activity is characterized by an early-peaking pattern which serves to dilate or stiffen the upper airway at the time when the greatest negative intraluminal pressure is generated by contraction of chest wall muscles. This pattern has been attributed to phasic afferent inputs from pulmonary stretch receptors. The present study examines the hypothesis that nonvagal factors may also influence the discharge pattern and coordination of upper airway and chest wall muscle activities. Therefore, in anesthetized, paralyzed, vagotomized and artificially ventilated cats, we examined the effects of changes in respiratory drive produced by activation of cholinergic and GA-BAergic (gamma-aminobutyric acid) receptors at the ventrolateral aspects of the medulla oblongata on phasic intrabreath discharge patterns of hypoglossal and phrenic nerves. Cholinergic agents (acetylcholine, carbachol, methacholine, physostigmine) applied directly to chemoreceptive areas on the ventral medullary surface increased hypoglossal activity, and in addition converted inspiratory discharge from an augmenting to a decrementing pattern of activity. The reverse effect on the discharge pattern of hypoglossal activity was observed with a decrease in respiratory drive. While the amplitude of the phrenic nerve discharge was also affected by these interventions, the augmenting discharge pattern of phrenic nerve activity did not change. These results suggest that the early peaking pattern of hypoglossal nerve discharge in vagotomized cats also depends on the level of respiratory drive, and is not solely dependent on vagal afferent inputs. In addition, the data suggest that structures near the ventral surface of the medulla are influential in shaping the pattern of hypoglossal nerve activity and maintaining balanced activity of upper airway and chest wall muscles.

Central effects of tachykinin peptide on tracheal secretion

Tachykinin peptides acting on structures located on the ventral surface of the medulla can increase cholinergic outflow to the tracheal smooth muscles and augment respiratory motor output. In the experiments reported here (performed in anesthetized, paralyzed and artificially ventilated dogs), we examined the effects of tachykinin peptides substance P on secretion from submucosal glands. Changes in secretion were measured in an exposed section of tantalum-coated tracheal epithelium. Substances P was administered intracisternally or applied topically on the intermediate area of the ventral surface of medulla (VMS). Intracisternal infusion and the local medullary administration of tachykinin peptide caused a significant increase in tracheal submucosal gland secretion. Atropine given intravenously prevented the secretory changes induced by central action of tachykinins. In addition, prior application of 2% lidocaine to the medullary surface blocked the responses caused by substance P locally applied on the VMS. These findings suggest that substances P acting centrally can tracheal fluid secretion mainly via cholinergic mechanisms, and that the ventral surface of the medulla is one of the site of these action.

Abdominal muscle length during respiratory defensive reflexes

The purpose of the present study was to assess the mechanical behavior of an expiratory muscle during defensive reflexes. Transversus abdominis muscle length changes were measured using sonomicrometry in anesthetized dogs. The abdominal muscle lengthened during the inspiratory phase and shortened at a rapid velocity during the expiratory portion of coughs and sneezes. The mean extent of muscle shortening was not different during coughing compared to breathing (P greater than 0.20) but was approximately double in magnitude during sneezing compared to breathing (P less than 0.005). On the other hand, the peak velocity of muscle shortening was approximately 5-fold greater during coughing (P less than 0.002) and 10-fold greater during sneezing (P less than 0.05) than during breathing. During the largest coughs and sneezes in each animal, peak velocity of muscle shortening averaged 77 +/- 9 and 179 +/- 65% of end-inspiratory length per sec, respectively. Muscle end-inspiratory length during coughs and sneezes differed from values during breathing (range +/- 8%), although for the group of animals the mean changes were small (+/- 1%). Despite these changes in end-inspiratory length, the abdominal muscle continued to operate at lengths both above and below its resting length. These results suggest that during defensive reflexes, greater increases occur in the velocity than in the extent of transversus abdominis muscle shortening relative to during breathing. In addition the transversus abdominis muscle appears to play an active respiratory role during defensive reflexes.

Contractile and endurance properties of feline triangularis sterni muscle

The triangularis sterni muscle has recently been found to play an active role in the modulation of airflow during expiration in several species; furthermore its electrical activity is influenced by many chemical and mechanical stimuli which influence breathing. To determine feline triangularis sterni contractile and endurance properties, strips of triangularis sterni and costal diaphragm muscle were removed from anesthetized ventilated cats, and studied in vitro. The isometric contractile kinetics of the two muscles were similar; contraction times were 37 +/- 2 and 36 +/- 1 ms for the triangularis sterni and diaphragm, respectively. However, the twitch to tetanic tension ratio of the triangularis sterni was lower than that of the diaphragm (0.19 +/- 0.01 versus 0.37 +/- 0.03; P less than 0.001), and the force frequency relationship of the triangularis sterni was located to the right of that of the diaphragm. Repetitive stimulation (40 Hz trains, duty cycle 0.33) produced a greater decline in force for the diaphragm than the triangularis sterni. The fatigue index (ratio of force at 2 min to initial force) was significantly higher for the triangularis sterni (0.31 +/- 0.04) than for the diaphragm (0.18 +/- 0.02; P less than 0.01). These data indicate that the contractile and endurance properties of the feline triangularis sterni are different in some but not all respects from those of the diaphragm, which may reflect adaptations to patterns of use during breathing.

Contractile and endurance properties of geniohyoid and diaphragm muscles

Despite the wealth of information about the neural control of pharyngeal dilator muscles, little is known about their intrinsic physiological properties. In the present study the in situ isometric contractility and endurance of a pharyngeal dilator, the geniohyoid muscle, were compared with properties of the diaphragm in 12 anesthetized artificially ventilated cats. The contraction time (means +/- SE) of the geniohyoid (27 +/- 2 ms) was shorter than that of the diaphragm (36 +/- 3 ms; P less than 0.0005), as was the half-relaxation time (29 +/- 2 vs. 45 +/- 4 ms; P less than 0.002). The faster contraction and relaxation of the geniohyoid compared with the diaphragm were appropriately reflected in the shape of the force-frequency curves for the two muscles, with that of the geniohyoid located to the right of the diaphragm force-frequency curve. The endurance properties of the two muscles were assessed using repetitive stimulation at 40 Hz in trains lasting 0.33 s, with one train repeated every second. The ratio of force at the end of 2 min of repetitive stimulation to initial force was 0.67 +/- 0.06 for the geniohyoid and 0.15 +/- 0.03 for the diaphragm (P less than 0.00001). After the repetitive stimulation, the muscle force generated in response to a range of stimulus frequencies was reduced to a greater extent for the diaphragm than for the geniohyoid muscle. These results indicate that the geniohyoid muscle has a faster physiological profile than does the diaphragm yet is relatively resistant to fatigue when driven at high rates.

Central action of tachykinins on activity of expiratory pumping muscles

The central effects of tachykinins (substance P, neurokinin A, and neurokinin B) on the distribution of the motor activity to rib cage and abdominal expiratory muscles were studied in anesthetized tracheotomized spontaneously breathing dogs and cats. Intracisternal application of substance P (11 dogs) in doses of 10(-5) to 10(-4) M caused diaphragm electrical activity to change insignificantly from 19.3 +/- 1.9 to 24.8 +/- 3.2 units (P greater than 0.05), produced a moderate increase of triangularis sterni activity from 12.6 +/- 2.2 to 19.2 +/- 2.2 units (P less than 0.05), and stimulated a large increase of transversus abdominis activity from 9.4 +/- 2.7 to 28.5 +/- 2.6 units (P less than 0.01). Comparable effects were seen with similar doses of neurokinin A (8 dogs) and neurokinin B (3 dogs) administered intracisternally. Local application of substance P to the ventral medullary surface (5 dogs and 4 cats) also caused expiratory muscle activity to increase more than diaphragm activity, and in addition transversus abdominis activity increased to a larger extent than triangularis sterni activity. Furthermore, administration of the substance P antagonist [D-Pro2,D-Trp7,9]-SP to the ventral medullary surface decreased respiratory motor output, with expiratory muscles activity being attenuated to a greater extent than diaphragm activity. Application of neurotensin and N-methyl-D-asparate to the ventral surface of the medulla produced responses similar to those observed as a result of central administration of tachykinin peptides. The results suggest that 1) mammalian tachykinins are involved in the regulation of thoracic and abdominal expiratory muscle activity, 2) these muscles manifest substantial differences in their electrical responses to excitatory neuropeptides acting centrally, and 3) inputs from modulatory neurons located in this vicinity of the ventral medullary surface seem to be distributed unevenly to different expiratory premotor and/or motoneurons.

Fiber subtype distribution of pharyngeal dilator muscles and diaphragm in the cat

In previous studies differences were frequently found between the pharyngeal dilator muscles and the thoracic respiratory muscles in their patterns of electrical and mechanical activity during the respiratory cycle, with both resting and stimulated breathing. However, little is known about the intrinsic properties of the pharyngeal muscles and how they relate to the intrinsic properties of the diaphragm. In the present study, the fiber subtype distributions of two pharyngeal dilator muscles, the geniohyoid and the sternohyoid, were ascertained histochemically in the cat. The geniohyoid and the sternohyoid muscles had a preponderance of fast glycolytic (FG) fibers (mean 48 and 55%, respectively), a smaller number of fast oxidative-glycolytic (FOG) fibers (mean 36 and 31%, respectively), and few slow oxidative (SO) fibers (mean 16 and 14%, respectively). The percentages of SO fibers of both hyoid muscles were significantly (P less than 0.01) lower than that of the costal diaphragm, and the percentages of FOG and FG fibers were significantly higher than that of the diaphragm. In conclusion, the geniohyoid and sternohyoid muscles have histochemical characteristics usually associated with fast contraction and intermediate endurance properties.

Upper airway muscle and diaphragm responses to hypoxia in the piglet

The neonatal ventilatory response to hypoxia is characterized by initial transient stimulation and subsequent respiratory depression. It is unknown, however, whether this response is also exhibited by the upper airway muscles that regulate nasal, laryngeal, and pharyngeal patency. We therefore compared electromyogram (EMG) amplitudes and minute EMGs for the diaphragm (DIA), alae nasi (AN), posterior cricoarytenoid (PCA), and genioglossus (GG) muscles in 12 anesthetized spontaneously breathing piglets during inhalation of 12% O2 over 10 min. Minute EMG for the DIA responded to hypoxia with an initial transient increase and subsequent return to prehypoxia levels by 10 min. Hypoxia also stimulated all three upper airway muscles. In contrast to the DIA EMG, however, AN, PCA, and GG EMGs all remained significantly above prehypoxia levels after 10 min of hypoxia. We have thus demonstrated that the initial stimulation and subsequent depression of the DIA EMG after 12% O2 inhalation contrast with the sustained increase in AN, PCA, and GG EMGs during hypoxia. We speculate that 1) central inhibition during neonatal hypoxia is primarily distributed to the motoneuron pools regulating DIA activation and 2) peripheral chemoreceptor stimulation and/or central disinhibition induced by hypoxia preferentially influence those motoneuron pools that regulate upper airway muscle activation, causing the different hypoxic responses of these muscle groups in the young piglet.

Diaphragm, genioglossus, and triangularis sterni responses to poikilocapnic hypoxia

Both isocapnic and poikilocapnic hypoxia may elicit a biphasic respiratory response, during which an initial ventilatory stimulation is followed by a reduction in breathing and diaphragm (DIA) electrical activity. To ascertain whether during adulthood other respiratory muscles have biphasic hypoxic responses similar to the DIA, in nine anesthetized cats electromyograms (EMG) were recorded from the DIA, genioglossus (GG), and triangularis sterni (TS) (n = 7) muscles during poikilocapnic hypoxia. DIA and GG EMG started at 60 +/- 4 and 9 +/- 3 units, respectively, during O2 breathing, increased to a maximum of 100 units during the 10-min hypoxic stimulus, and subsequently declined to 81 +/- 6 and 58 +/- 12 units, respectively, by the end of 10 min of hypoxia. The time course of the increase and subsequent decline was similar for the DIA and GG and for GG activity during both inspiration and expiration. Furthermore the degree to which GG EMG declined after reaching its peak activity level correlated with the magnitude of the respective decline in DIA EMG (r = 0.79, P less than 0.02). The TS, in contrast, was maximally active either during O2 breathing or very early during hypoxia, and its activity declined progressively thereafter (to 13 +/- 6% of its peak value at the end of 10 min of hypoxia). The degree to which TS EMG declined did not correlate with the degree to which DIA or GG EMG declined.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tracheal airway occlusion on hyoid muscle length and upper airway volume

Complex relationships exist among electromyograms (EMGs) of the upper airway muscles, respective changes in muscle length, and upper airway volume. To test the effects of preventing lung inflation on these relationships, recordings were made of EMGs and length changes of the geniohyoid (GH) and sternohyoid (SH) muscles as well as of tidal changes in upper airway volume in eight anesthetized cats. During resting breathing, tracheal airway occlusion tended to increase the inspiratory lengthening of GH and SH. In response to progressive hypercapnia, the GH eventually shortened during inspiration in all animals; the extent of muscle shortening was minimally augmented by airway occlusion despite substantial increases in EMGs. SH lengthened during inspiration in six of eight animals under hypercapnic conditions, and in these cats lengthening was greater during airway occlusion even though EMGs increased. Despite the above effects on SH and GH length, upper airway tidal volume was increased significantly by tracheal occlusion under hypercapnic conditions. These data suggest that the thoracic and upper airway muscle reflex effects of preventing lung inflation during inspiration act antagonistically on hyoid muscle length, but, because of the mechanical arrangement of the hyoid muscles relative to the airway and thorax, they act agonistically to augment tidal changes in upper airway volume. The augmentation of upper airway tidal volume may occur in part as a result of the effects of thoracic movements being passively transmitted through the hyoid muscles.

Central modulatory effects of tachykinin peptides on airway tone

Previous studies suggest that structures within 1 mm of the ventral surface of the medulla (VMS) are involved in the regulation of airway resistance. Furthermore, neurons containing tachykinin peptides have been observed near the surface of the VMS. In the present work, we examined the effects of mammalian tachykinins, substance P (SP) and neurokinin A (NKA), applied locally to the intermediate area of the VMS of cats on tracheal tone and phrenic nerve activity. Since neutral endopeptidase (enkephalinase) has been shown to degrade tachykinin peptides in other tissues, we also investigated the effect of the neutral endopeptidase (NEP) inhibitors (thiorphan and phosphoramidon) on airway tone and phrenic nerve responses to tachykinins when the animals were ventilated with 100% O2 and during hyperoxic hypercapnia and isocapnic hypoxia. Experiments were performed in chloralose-anesthetized cats hyperventilated to phrenic neural apnea or so that the end tidal CO2 was just above the apneic threshold. Trachealis smooth muscle tension was assessed by measuring changes in pressure in a balloon placed in a bypassed segment of trachea (Ptseg). Application to the VMS of SP (10(-5)-10(-3) M) significantly increased tracheal muscle tension. Similar effects were found with applications of NKA. In addition, thiorphan and phosphoramidon potentiated the effects of tachykinins and the responses to hypercapnia and hypoxia of tracheal tone and phrenic nerve activity. Pretreatment with atropine (1 mg/kg) blocked tracheal but not phrenic responses to tachykinins. These suggest that (1) tachykinins acting on structures located on the VMS can increase cholinergic outflow to the airways and augment respiratory motor output, and (2) NEP may be one important modulator of tachykinin-induced effects.

Role of the ventral surface of medulla in the generation of Mayer waves

The regions adjacent to the ventrolateral medullary surface (VMS) play critical roles in the regulation of respiratory and cardiovascular function. Furthermore, these areas seem to be important sites for the integration of afferent inputs from certain sensory organs and the source of excitatory inputs to preganglionic sympathetic and parasympathetic neurons. To determine whether the VMS contributes to the generation of nonrespiratory-related periodic oscillations of arterial blood pressure (Mayer waves), excitatory substances, such as N-methyl-D-aspartate (NMDA), cholinergic agonists, and neuropeptides (substance P, neurokinin A, neurotensin), were applied topically to the intermediate area of VMS in anesthetized cats. In addition, the effects of application of lidocaine and inhibitory substances (benzodiazepines) on Mayer waves were studied. After application of excitatory substances to the VMS, we observed oscillations of arterial blood pressure, recurring with a period of 17.8 +/- 10 (SE) s, which had similar characteristics as the Mayer waves recorded during hypercapnia or hypoxia. In addition, cyclic changes in phrenic nerve activity and tracheal tone occurred with the same periodicity as arterial blood pressure oscillation. Application of lidocaine or benzodiazepines on the intermediate area of the VMS abolished Mayer waves observed during hypercapnia, hypoxia, or application of excitatory substances. These findings show for the first time that the VMS can be considered as one of several synaptic relays involved in the generation of arterial blood pressure oscillation, as well as the cyclic changes in phrenic nerve activity and tracheal smooth muscle tone that occur simultaneously.

Benzodiazepines acting on ventral surface of medulla cause airway dilation

The benzodiazepines that have anxiolytic, anticonvulsant, muscle-relaxant, and sedative-hypnotic properties affect respiration possibly by acting on gamma-aminobutyric acid (GABA)ergic receptors. This study investigated the effects of benzodiazepines diazepam and midazolam) applied topically to or microinjected just beneath the ventrolateral medullary surface (VMS) on airway tone in alpha-chloralose-anesthetized, paralyzed, and artificially ventilated cats. Trachealis smooth muscle tension was assessed by measuring the changes in pressure in a balloon placed in a bypassed rostral segment of the trachea. In 21 cats ventilated with 7% CO2 in O2, surface application of benzodiazepines caused a significant decrease in tracheal tone. Similar to topical application, microinjection of midazolam (1 microgram) in the ventral medulla (0.1-0.2 mm from the surface) in six cats decreased tracheal pressure by 13.2 +/- 2.1 cmH2O (P less than 0.01). In addition, application of benzodiazepines on the VMS in animals ventilated with 12% O2 in N2 (n = 5) decreased tracheal pressure from 15.9 +/- 2.2 to 5.2 +/- 2.7 cmH2O (P less than 0.05). Furthermore, in all cats studied (n = 6), the magnitude of lung deflation-induced tracheal contraction was reduced after application of benzodiazepines on the ventral surface of the medulla (from 11.4 +/- 1.6 to 2.2 +/- 0.9 cmH2O; P less than 0.01). The effects of benzodiazepines on tracheal tone were reversed and blocked by application of Ro 15-1788, a specific benzodiazepines antagonist. However, when parasympathetic activity was abolished by atropine and tracheal tone was restored with 5-hydroxytryptamine, benzodiazepines applied on the VMS had no effect on tracheal pressure. These results suggest that benzodiazepines acting centrally, on structures located near the VMS, can cause a decrease in airway smooth muscle tone by diminishing the activity of parasympathetic neurons which project to the airways.

Motor unit regulation of mammalian pharyngeal dilator muscle activity

The present study examined the cellular regulation of one of the pharyngeal dilator muscles, the geniohyoid, by assessing its motor unit (MU) behavior in anesthetized cats. During spontaneous breathing, MU that (a) were active during inspiration only (I-MU) and (b) were active during both inspiration and expiration (I/E-MU) were identified. I-MU had a later inspiratory onset time and a shorter duration of inspiratory firing than did I/E-MU (P less than 0.002 and P less than 0.0001, respectively). I-MU were usually quiescent whereas I/E-MU were usually active during the last 20% of inspiration. I/E-MU fired more rapidly (P less than 0.00001) and for relatively longer periods of time (P less than 0.00001) during inspiration than during expiration. End-expiratory airway occlusion (preventing lung expansion during inspiration) augmented the inspiratory activity of both I-MU and I/E-MU. Conversely, end-expiratory airway occlusion reduced the absolute and relative firing durations (P less than 0.002 and P less than 0.00002, respectively) and the firing frequency (P less than 0.001) of I/E-MU activity during expiration. These results indicate that (a) the complex pattern of pharyngeal dilator muscle activity is due to the integrated activity of a heterogeneous group of MU, (b) changes in the degree to which pharyngeal dilator muscles are active result from combinations of MU recruitment/decruitment and modulations of the frequency and duration of MU firing, and (c) gating of lung-volume afferent information occurs during the respiratory cycle.

Responses of upper-airway dilating muscles and diaphragm activity to end-expiratory pressure loading in anesthetized dogs

The steady-state responses of upper-airway dilating muscles and diaphragm activity to elevation of lung volume induced by positive end-expiratory pressure loading were studied in 9 pentobarbital-anesthetized dogs with vagus nerves intact. The early and late effects of 5 min of expiratory threshold loads upon upper airway dilating muscle activity (the alae nasi, the genioglossus and the posterior cricoarytenoid) were compared to their effects on diaphragm activity. During resting O2 breathing, application of 5 and 10 cm H2O of positive end-expiratory pressure produced no significant change in the peak electrical activity of the upper-airway dilating muscles and diaphragm (p greater than 0.05). No qualitative differences were found in the upper-airway dilating muscles and diaphragm responses to expiratory threshold loads when the animals breathed 3 or 7% CO2 in O2, compared to when they inspired 100% O2. Furthermore, no differences were found in the electrical activity of the upper-airway dilating muscles and diaphragm at any given end-tidal CO2 when unloaded responses were compared with loaded responses during progressive hypercapnia. However, positive end-expiratory pressure loading caused significant prolongation of expiratory duration, which gradually returned toward control levels when the loads were maintained. In animals who developed periodic breathing by increasing levels of anesthesia, positive end-expiratory pressure loading eliminated the periodicity and made the pattern of breathing regular. Based on these results, it can be concluded that under the conditions of these experiments, increases in lung volume produced by expiratory threshold loads do not reduce the activity of upper-airway dilating muscles. The maintenance of the electrical activity of the upper-airway dilating muscles might be caused by excitatory reflex mechanisms or central habituation.

Role of costal and crural diaphragm and parasternal intercostals during coughing in cats

The inspiratory phase of coughs often consists of large inspired volumes and increased motor discharge to the costal diaphragm. Furthermore, diaphragm electrical activity may persist into the early expiratory portion of coughs. To examine the role of other inspiratory muscles during coughing, electromyograms (EMG) recorded from the crural diaphragm (Dcr) and parasternal intercostal (PSIC) muscles were compared to EMG of the costal diaphragm (Dco) in anesthetized cats. Tracheal or laryngeal stimulation typically produced a series of coughs, with variable increases in peak inspiratory EMGs of all three muscles. On average, peak inspiratory EMG of Dco increased to 346 +/- 60% of control (P less than 0.001), Dcr to 514 +/- 82% of control (P less than 0.0002), and PSIC to 574 +/- 61% of control (P less than 0.0005). Augmentations of Dcr and PSIC EMG were both significantly greater than of Dco EMG (P less than 0.05 and P less than 0.002, respectively). In most animals, EMG of Dco correlated significantly with EMG of Dcr and of PSIC during different size coughs. Electrical activity of all three muscles persisted into the expiratory portions of many (but not all) coughs. The duration of expiratory activity lasted on average 0.17 +/- 0.03 s for Dco, 0.25 +/- 0.06 s for Dcr, and 0.31 +/- 0.09 s for PSIC. These results suggest that multiple respiratory muscles are recruited during inspiration of coughs, and that the persistence of electrical activity into expiration of coughs is not unique to the costal diaphragm.

Role of triangularis sterni during coughing and sneezing in dogs

Studies in mammals have found that during breathing the triangularis sterni (TS) muscle regulates expiratory airflow and the end-expiratory position of the rib cage and furthermore that the respiratory activity of this muscle is influenced by a variety of chemical and mechanical stimuli. To assess the role of the TS during coughing and sneezing, electromyograms (EMGs) recorded from the TS were compared with EMGs of the transversus abdominis (TA) in eight pentobarbital-anesthetized dogs. During coughing induced by mechanically stimulating the trachea or larynx (n = 7 dogs), peak EMGs increased from 23 +/- 2 to 74 +/- 5 U (P less than 0.00002) for the TS and from 21 +/- 6 to 66 +/- 4 U (P less than 0.0002) for the TA. During sneezing induced by mechanically stimulating the nasal mucosa (n = 3 dogs), peak EMG of the TS increased from 10 +/- 3 to 66 +/- 7 U (P less than 0.005) and peak EMG of the TA increased from 10 +/- 2 to 73 +/- 7 U (P less than 0.02). For both muscles the shape of the EMG changed to an early peaking form during coughs and sneezes. Peak expiratory airflow during coughs of different intensity correlated more closely with peak TS EMG in three dogs and with peak TA EMG in four dogs; peak expiratory airflow during sneezes of different intensity correlated more closely with peak TS than TA EMG in all three animals. These results suggest that the TS is actively recruited during coughing and sneezing and that different neuromuscular strategies may be utilized to augment expiratory airflow.

Naloxone enhances the response to hypercapnia of spinal and cranial respiratory nerves

To assess the effects of endogenous opiates on respiratory muscle responses to CO2, naloxone was administered intravenously to paralyzed, vagotomized and artificially ventilated cats anesthetized with alpha-chloralose. Neural activity was recorded from the phrenic, hypoglossal (HG), glossopharyngeal (GP) and recurrent laryngeal (RL) nerves. Before naloxone, phasic activity began first in the phrenic at a PETCO2 of 30.0 +/- 1.8 Torr, followed by the RL at a PETCO2 of 33.5 +/- 1.7 Torr, the HG at a PETCO2 of 39.9 +/- 2.1 Torr and the GP at a PETCO2 of 42.5 +/- 2.2 Torr during CO2 rebreathing. Naloxone had no significant effect on the apneic threshold of any of the nerves studied. Naloxone did, however, increase respiratory frequency (P less than 0.01) mainly by causing a significant (P less than 0.01) shortening of TE as it had no significant effect on TI. Naloxone also significantly increased the rate at which peak nerve activity increased with CO2 in the HG (P less than 0.01) and the GP (P less than 0.01) nerves, but not in the phrenic and RL nerves. Instead, the maximum activity produced by hypercapnia and the PETCO2 level at which maximum activity occurred in the phrenic, but not the RL, increased after naloxone. The result of these effects was that naloxone extended the range over which the HG and GP behaved proportionally with the phrenic, but it did not change the curvilinear nature of these relationships.

Tracheal and phrenic responses to neurotensin applied to ventral medulla

Respiratory activity and airway tone can be significantly affected by perturbations confined to superficial areas of the ventrolateral surface of the medulla (VMS). It is not clear which neuromediators are responsible for these changes. Neurotensin (NT), a tridecapeptide, fulfills many of the criteria required for a neurotransmitter or a neuromodulator. In this study, we determined whether NT applied topically to the intermediocaudal area of VMS could alter tracheal tone (Ptseg) and phrenic nerve activity (Ph) in alpha-chloralose-anesthetized cats hyperventilated with O2 to neural apnea. Also, the effects of NT on the responses of tracheal tone and phrenic nerve activity to steady-state hyperoxic hypercapnia (3% CO2 in O2) and isocapnic hypoxia (12% O2) were tested. Application of pledgets containing NT (10(-5)-10(-3) M) caused significant increases in Ptseg and Ph activity without significant changes in blood pressure. Both tracheal and phrenic responses to hypercapnia and hypoxia were also increased by an earlier application of NT. Application of lidocaine (2%) to the VMS rapidly reversed NT-induced responses and prevented them on reapplication of NT. Phosphoramidon, a neutral endopeptidase inhibitor, potentiated responses to NT, suggesting that a mechanism exists at the VMS that could reverse NT effects. Earlier topical administration of hexamethonium bromide to the VMS did not influence the effects of NT, indicating that NT was not acting by causing the release of acetylcholine. Intravenous administration of atropine (1 mg/kg) blocked tracheal but not phrenic responses to NT. These findings suggest that neurotensin may be a neuromodulator involved in central chemosensitivity and that it may participate in the regulation of phrenic activity and parasympathetic tone of airway smooth muscle.

Effect of stimulation of pulmonary C-fiber receptors on canine respiratory muscles

The effects of stimulation of pulmonary C-fiber receptors on the distribution of motor activity to upper airway, rib cage, and abdominal muscles were studied in anesthetized, tracheotomized, spontaneously breathing dogs. Stimulation of pulmonary C-fiber receptors by injection of capsaicin (3-20 micrograms/kg) into the right atrium resulted in complete cessation of electrical activity of the upper airway dilating muscles (UADM) and the inspiratory chest wall pumping muscles. The activity of abdominal muscles was also inhibited. The duration of electrical silence was longer for the diaphragm than for the UADM. Upper airway constricting muscles and expiratory intercostal muscles, including the triangularis sterni, remained tonically active during the apneic period. The responses of these muscles were qualitatively the same when the animals breathed 100% O2, 7% CO2 in O2, or 12% O2 in N2, and without or in the presence of an expiratory threshold load. Bilateral vagotomy abolished the inhibitory effects of capsaicin on UADM, chest wall, and abdominal muscle activity, suggesting that the vagus is the major afferent pathway for the reflex. The qualitative difference in the response of intercostal expiratory muscles and abdominal muscles suggests that these two groups of synergistic muscles may be independently regulated.

Nasal and tracheal responses to chemical and somatic afferent stimulation in anesthetized cats

Respiratory chemical and reflex interventions have been shown to affect nasal resistance or tracheal tone, respectively. In the present study, nasal caliber (assessed from pressure at a constant flow) and tracheal tone (assessed from pressure in a fluid-filled balloon within an isolated tracheal segment) were monitored simultaneously in anesthetized, paralyzed, artificially ventilated (inspired O2 fraction = 100%) cats. We examined the effect of CO2 inhalation and sciatic nerve stimulation as well as the application of nicotine (6 X 10(-4) mol/l) or lidocaine (2% solution) to the intermediate area of the ventral medullary surface (VMS). CO2 and VMS nicotine resulted in a significant increase in tracheal pressure [147 +/- 73 and 91 +/- 86% (SD), respectively]; and a significant reduction in nasal pressure (-35 +/- 10 and -20 +/- 13%, respectively). In contrast, sciatic nerve stimulation resulted in a significant fall in both tracheal (-50 +/- 36%) and nasal pressure (-21 +/- 13%). Application of 2 or 4% lidocaine to the VMS reduced tracheal pressure but did not significantly affect nasal pressure. After VMS lidocaine, nasal and tracheal responses to CO2, sciatic nerve stimulation, or VMS nicotine, when present, were negligible. These results suggest a role for the VMS in the regulation and coordination of nasal and tracheal caliber responses.

Upper airway pressure receptors alter expiratory muscle EMG and motor unit firing

To examine the effects of upper airway negative pressure (UAW NP) afferents on respiratory muscle activity during expiration (TE), diaphragm electromyograms (EMG) and triangularis sterni EMG and single motor unit activity were recorded from supine anesthetized tracheotomized cats while they breathed 100% O2. The period of TE during which the diaphragm was electrically active (TE-1) and the period of TE during which the diaphragm was quiescent (TE-2) were both increased with continuous UAW NP (P less than 0.001 and P less than 0.05, respectively), as was TE-1 as a percent of TE (P less than 0.001). Continuous UAW NP reduced peak triangularis sterni EMG (P less than 0.001) and delayed its expiratory onset (P less than 0.005) but did not alter its duration of firing. Changes in triangularis sterni EMG were due to a combination of complete cessation of motor unit activity (2 of 17 motor units), a reduction in mean motor unit firing frequency (P less than 0.02), and a delay in the expiratory onset of motor unit activity (P less than 0.001). Qualitatively similar results were obtained when UAW NP was applied during inspiration only. We conclude that 1) UAW NP has reciprocal stimulatory and inhibitory influences on diaphragm and triangularis sterni muscle electrical activity, respectively, during expiration, and 2) the reductions in triangularis sterni EMG are due to both motor unit derecruitment and a slowing of motor unit firing frequency.

Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recorded from eight pentobarbital anesthetized cats. Augmented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 +/- 5 to 12 +/- 2 arbitrary units (P less than 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths (P less than 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During resting breathing the motor units had a mean relative expiratory onset time of 46 +/- 4% of expiration, and a mean firing frequency of 19 +/- 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency (P less than 0.05) and number of impulses per breath (P less than 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced during augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.

Transverse abdominis length changes during eupnea, hypercapnia, and airway occlusion

The abdominal muscles accelerate airflow during expiration and may also influence the end-expiratory volume and configuration of the thorax. Although much is known about their electrical activity, the degree to which they change length during the respiratory cycle has not been previously assessed. In the present study we measured respiratory changes in transverse abdominis length using sonomicrometry in 14 pentobarbital sodium-anesthetized supine dogs and compared length changes to simultaneously recorded tidal volume and transverse abdominis electromyograms (EMG). To determine muscle resting length at passive functional residual capacity (LFRC), the animals were hyperventilated to apnea. The transverse abdominis was electrically active in all animals during resting O2 breathing (eupnea). During inspiration the transverse abdominis lengthened above resting length in all 14 dogs by a mean of 3.7 +/- 1.1% LFRC; during expiration the transverse abdominis shortened below resting length in 13 of 14 dogs by a mean of 4.2 +/- 0.9% LFRC. Increasing hyperoxic hypercapnia (produced in 9 animals) progressively heightened transverse abdominis EMG and progressively increased the extent of muscle shortening below resting length (to 12.6 +/- 3.2% LFRC at a PCO2 of 90 Torr). During single-breath airway occlusion substantial inspiratory lengthening of the transverse abdominis occurred, both during O2 breathing and during CO2 rebreathing.(ABSTRACT TRUNCATED AT 250 WORDS)