Alterations in respiratory muscle activation in the ischemic fatigued canine diaphragm

The purpose of the present study was to examine the respiratory motor response to diaphragm fatigue. Studies were performed using in situ diaphragm muscle strips dissected from the left costal diaphragm in anesthetized dogs. The left inferior phrenic artery was isolated, and diaphragmatic strip fatigue was elicited by occluding this vessel. Strip tension, strip electromyographic activity, parasternal electromyographic activity, and the electromyogram of the right hemidiaphragm were recorded during spontaneous breathing efforts before, during, and after periods of phrenic arterial occlusion. In separate trials, we examined the neuromuscular responses to phrenic arterial occlusion at arterial PCO2 (PaCO2) of 40, 55, and 75 Torr. No fatigue and no alteration in electromyographic activities were observed in trials at PaCO2 of 40 Torr. During trials at PaCO2 of 55 and 75 Torr, however, diaphragm tension fell, the peak height of the diaphragm strip electromyogram decreased, and the peak heights of the parasternal and right hemidiaphragm electromyograms increased. Relief of phrenic arterial occlusion resulted in a return of strip tension and all electromyograms toward base-line values. In additional experiments, the left phrenic nerve was sectioned in the chest after producing fatigue. Phrenic section was followed by an increase in the peak height of the left phrenic neurogram (recorded above the site of section). This latter finding suggests that diaphragm strip motor drive may be reflexly inhibited during the development of fatigue by neural traffic carried along phrenic afferents.     

Mechanics of the canine diaphragm in ascites: a CT study.

Ascites causes an increase in the elastance of the abdomen and impairs the lung-expanding action of the diaphragm, but its overall effects on the pressure-generating ability of the muscle remain unclear. In the present study, radiopaque markers were attached to muscle bundles in the midcostal region of the diaphragm in five dogs, and the three-dimensional locations of the markers during relaxation and during phrenic nerve stimulation in the presence of increasing amounts of ascites were determined using a computed tomographic scanner. From these data, accurate measurements of muscle length and quantitative estimates of diaphragm curvature were obtained, and the changes in transdiaphragmatic pressure (Pdi) were analyzed as functions of muscle length and curvature. With increasing ascites, the resting length of the diaphragm increased progressively. In addition, the amount of muscle shortening during phrenic nerve stimulation decreased gradually. When ascites was 100 ml/kg body wt, therefore, the muscle during contraction was longer, leading to a 20-25% increase in Pdi. As ascites increased further to 200 ml/kg, however, muscle length during contraction continued to increase, but Pdi did not. This absence of additional increase in Pdi was well explained by the increase in the diameter of the ring of insertion of the diaphragm to the rib cage and the concomitant increase in the radius of diaphragm curvature. These observations indicate that the pressure-generating ability of the diaphragm is determined not only by muscle length as conventionally thought but also by muscle shape.     

Effect of age on sleep onset-related changes in respiratory pump and upper airway muscle function.

In normal young men, there is an abrupt fall in ventilation (VE), a rise in upper airway resistance (UAR), and falls in the activities of the diaphragm (Di), intercostals (IC), genioglossus (GG), and tensor palatini (TP) at sleep onset. On waking, there is an abrupt increase in VE and fall in UAR and an increase in the activities of Di, IC, GG, and TP. The aim of this study was to determine whether these changes are age dependent. Nine men aged 20 to 25 yr were compared with nine men aged 42 to 67 yr. Airflow, UAR, Di, and IC surface electromyograms (EMGs) and the intramuscular EMGs of GG and TP were recorded. It was found that the falls in IC, GG, and TP at the transition from alpha to theta electroencephalogram (EEG) activity were significantly greater in the older than in the younger men (P < 0.05) and that the fall in Di was also greater, although this was only marginally significant (P = 0.15). The rise in GG at theta-to-alpha transitions was also greater in the older than in the younger men, and there was a trend for TP to be higher.     

Modulation of laryngeal and respiratory pump muscle activities with upper airway pressure and flow.

The hypothesis that upper airway (UA) pressure and flow modulate respiratory muscle activity in a respiratory phase-specific fashion was assessed in anesthetized, tracheotomized, spontaneously breathing piglets. We generated negative pressure and inspiratory flow in phase with tracheal inspiration or positive pressure and expiratory flow in phase with tracheal expiration in the isolated UA. Stimulation of UA negative pressure receptors with body temperature air resulted in a 10--15% enhancement of phasic moving-time-averaged posterior cricoarytenoid electromyographic (EMG) activity above tonic levels obtained without pressure and flow in the UA (baseline). Stimulation of UA positive pressure receptors increased phasic moving-time-averaged thyroarytenoid EMG activity above tonic levels by 45% from baseline. The same enhancement of posterior cricoarytenoid or thyroarytenoid EMG activity was observed with the addition of flow receptor stimulation with room temperature air. Tidal volume and diaphragmatic and abdominal muscle activity were unaffected by UA flow and/or pressure, whereas respiratory timing was minimally affected. We conclude that laryngeal afferents, mainly from pressure receptors, are important in modulating the respiratory activity of laryngeal muscles.     

Impact of acute ascites on the action of the canine abdominal muscles.

Although ascites causes abdominal expansion, its effects on abdominal muscle function are uncertain. In the present study, progressively increasing ascites was induced in supine anesthetized dogs, and the changes in abdominal (DeltaPab) and airway opening (DeltaPao) pressure obtained during stimulation of the internal oblique and transversus abdominis muscles were measured; the changes in internal oblique muscle length were also measured. As ascites increased from 0 to 100 ml/kg body wt, Pab and muscle length during relaxation increased. DeltaPab also showed a threefold increase (P < 0.001). However, DeltaPao decreased (P < 0.001). When ascites increased further to 200 ml/kg, resting muscle length continued to increase and muscle shortening during stimulation became very small so that active muscle length was 155% of the resting muscle length in the control condition. Concomitantly, DeltaPab returned to the control value, and DeltaPao continued to decrease. Similar results were obtained with the animals in the head-up posture, although the decrease in DeltaPao appeared only when ascites was greater than 125 ml/kg. It is concluded that 1) ascites adversely affects the expiratory action of the abdominal muscles on the lung; 2) this effect results primarily from the increase in diaphragm elastance; and 3) when ascites is severe, the abdomen cross-sectional area is also increased and the abdominal muscles are excessively lengthened so that their active pressure-generating ability itself is reduced.     

A model of the respiratory pump

The interaction of forces that produce chest wall motion and lung volume change is complex and incompletely understood. To aid understanding we have developed a simple model that allows prediction of the effect on chest wall motion of changes in applied forces. The model is a lever system on which the forces generated actively by the respiratory muscles and passively by impedances of rib cage, lungs, abdomen, and diaphragm act at fixed sites. A change in forces results in translational and/or rotational motion of the lever; motion represents volume change. The distribution and magnitude of passive relative to active forces determine the locus and degree of rotation and therefore the effect of an applied force on motion of the chest wall, allowing the interaction of diaphragm, rib cage, and abdomen to be modeled. Analysis of moments allow equations to be derived that express the effect on chest wall motion of the active component in terms of the passive components. These equations may be used to test the model by comparing predicted with empirical behavior. The model is simple, appears valid for a variety of respiratory maneuvers, is useful in interpreting relative motion of rib cage and abdomen and may be useful in quantifying the effective forces acting on the rib cage.     

Hemodynamic effects of nicotine in canine skeletal muscle.

Studies were conducted in 36 artificially ventilated, anesthetized dogs to clarify hemodynamic effects of nicotine in resting gracilis muscle. In Series 1, effects of intravenous nicotine (36 micrograms/kg/min) were evaluated in (i) intact muscles (Condition 1), (ii) denervated muscles (Condition 2), (iii) denervated muscles following local alpha-adrenergic blockade (Condition 3), (iv) denervated muscles following combined local alpha- and beta-adrenergic blockade (Condition 4), and (v) intact muscles with aortic pressure maintained constant (Condition 5). In Series 2, nicotine was infused directly into the gracilis artery at a rate of 3.6 micrograms/kg/min. Muscle blood flow was obtained with an electromagnetic flowmeter and used to calculate vascular resistance and oxygen consumption (Fick equation). Plasma catecholamine levels were determined with a radioenzymatic method. Intravenous nicotine doubled mean aortic pressure under Conditions 1-4. In intact and denervated muscles (Conditions 1 and 2) proportional increases in vascular resistance, reflective of vasoconstriction, held blood flow constant. Muscle oxygen consumption was unchanged. alpha-Adrenergic blockade with phenoxybenzamine following denervation (Condition 3) converted muscle vasoconstriction to vasodilation during nicotine infusion. Additional beta-adrenergic blockade (Condition 4) restored muscle vasoconstriction. Nicotine-induced muscle vasoconstriction was maintained under controlled pressure (Condition 5). Intravenous nicotine significantly increased plasma catecholamine levels. Intra-arterial infusions of nicotine (Series 2) caused no hemodynamic changes in muscle. In conclusion, intravenous nicotine caused vasoconstriction in muscle, which was not due to reduced metabolic demand, pressure-flow autoregulation, or a direct [corrected] effect on vascular smooth muscle, but to stimulation of alpha-adrenergic receptors. Following denervation, this vasoconstriction was maintained by elevated plasma catecholamines. alpha-Adrenergic blockade unmasked nicotine-induced vasodilation mediated by beta-adrenergic receptors, whereas combined alpha- and beta-adrenergic blockade unmasked nicotine-induced vasoconstriction by a nonadrenergic mechanism.     

Cell potential and the sodium-potassium pump in vascular smooth muscle.

An electrogenic sodium-potassium pump appears to contribute materially to the steady-state potential and to certain of the transient potential responses of vascular smooth muscle. Since changes in cell potential in turn can lead to changes in contractile state, the pump is implicated in some of the constriction-dilation responses of blood vessels. The vasodilator action of potassium is explainable, for instance, through an effect on cell potential if (and only if) an electrogenic pump is assumed to be extruding sodium at a faster rate than it takes up potassium. This is supported by the observation that ouabain, an inhibitor of Na,K-ATPase activity, will eliminate or reverse the vascular effect of potassium. Furthermore, when the in vivo and in vitro effects on vascular smooth muscle of altered extracellular potassium concentration are compared to calculated cell potentials based on a model that includes an electrogenic pump, the experimental findings are shown to be logical and predictable.     

Ratio of active to passive muscle shortening in the canine diaphragm.

Active and passive shortening of muscle bundles in the canine diaphragm were measured with the objective of testing a consequence of the minimal-work hypothesis: namely, that the ratio of active to passive shortening is the same for all active muscles. Lengths of six muscle bundles in the costal diaphragm and two muscle bundles in the crural diaphragm of each of four bred-for-research beagle dogs were measured by the radiopaque marker technique during the following maneuvers: a passive deflation maneuver from total lung capacity to functional residual capacity, quiet breathing, and forceful inspiratory efforts against an occluded airway at different lung volumes. Shortening per liter increase in lung volume was, on average, 70% greater during quiet breathing than during passive inflation in the prone posture and 40% greater in the supine posture. For the prone posture, the ratio of active to passive shortening was larger in the ventral and midcostal diaphragm than at the dorsal end of the costal diaphragm. For both postures, active shortening during quiet breathing was poorly correlated with passive shortening. However, shortening during forceful inspiratory efforts was highly correlated with passive shortening. The average ratios of active to passive shortening were 1.23 +/- 0.02 and 1.32 +/- 0.03 for the prone and supine postures, respectively. These data, taken together with the data reported in the companion paper (T. A. Wilson, M. Angelillo, A. Legrand, and A. De Troyer, J. Appl. Physiol. 87: 554-560, 1999), support the hypothesis that, during forceful inspiratory efforts, the inspiratory muscles drive the chest wall along the minimal-work trajectory.     

Bradykinin facilitates noradrenaline spillover during contraction in the canine gracilis muscle.

Noradrenaline spillover from skeletal muscle vascular areas increases during exercise but the underlying mechanisms are not well understood. Muscle contraction itself causes changes in many factors that could affect noradrenaline spillover. For instance, it has been reported that bradykinin is synthesized in skeletal muscle areas during contraction. Because the B2 bradykinin receptor facilitates noradrenaline spillover, it may be involved in the increase associated with contraction. In this experiment, we studied the effect of bradykinin on noradrenaline spillover in the in situ canine gracilis muscle, using the specific B2 antagonist HOE 140. The drug did not modify noradrenaline spillover at rest, but did cause a significant decrease during muscle contraction, from 558 to 181 pg min(-1). As reported previously in the literature, fractional extraction of noradrenaline decreased during muscle contraction. This effect was independent of HOE 140 treatment. In light of our results, it seems that bradykinin formation during muscle contraction may play an important part in the observed increase in noradrenaline spillover but does not affect fractional extraction.     

Analysis of excitation-contraction-coupling components in chronically stimulated canine skeletal muscle.

The chronic stimulation of predominantly fast-twitch mammalian skeletal muscle causes a transformation to physiological characteristics of slow-twitch skeletal muscle. Here, we report the effects of chronic stimulation on the protein components of the sarcoplasmic reticulum and transverse tubular membranes which are directly involved in excitation-contraction coupling. Comparison of protein composition of microsomal fractions from control and chronically stimulated muscle was performed by immunoblot analysis and also by staining with Coomassie blue or the cationic carbocyanine dye Stains-all. Consistent with previous experiments, a greatly reduced density was observed for the fast-twitch isozyme of Ca(2+)-ATPase, while the expression of the slow-twitch Ca(2+)-ATPase was found to be greatly enhanced. Components of the sarcolemma (Na+/K(+)-ATPase, dystrophin-glycoprotein complex) and the free sarcoplasmic reticulum (Ca(2+)-binding protein sarcalumenin and a 53-kDa glycoprotein) were not affected by chronic stimulation. The relative abundance of calsequestrin was slightly reduced in transformed skeletal muscle. However, the expression of the ryanodine receptor/Ca(Ca2+)-release channel from junctional sarcoplasmic reticulum and the transverse tubular dihydropyridine-sensitive Ca2+ channel, as well as two junctional sarcoplasmic reticulum proteins of 90 kDa and 94 kDa, was greatly suppressed in transformed muscle. Thus, the expression of the major protein components of the triad junction involved in excitation-contraction coupling is suppressed, while the expression of other muscle membrane proteins is not affected in chronically stimulated muscle.     

Plasticity in canine airway smooth muscle

The large volume changes of some hollow viscera require a greater length range for the smooth muscle of their walls than can be accommodated by a fixed array of sliding filaments. A possible explanation is that smooth muscles adapt to length changes by forming variable numbers of contractile units in series. To test for such plasticity we examined the muscle length dependence of shortening velocity and compliance, both of which will vary directly with the number of thick filaments in series. Dog tracheal smooth muscle was studied because its cells are arrayed in long, straight, parallel bundles that span the length of the preparation. In experiments where muscle length was changed, both compliance and velocity showed a strong dependence on muscle length, varying by 1.7-fold and 2.2-fold, respectively, over a threefold range of length. The variation in isometric force was substantially less, ranging from a 1.2- to 1.3-fold in two series of experiments where length was varied by twofold to an insignificant 4% variation in a third series where a threefold length range was studied. Tetanic force was below its steady level after both stretches and releases, and increased to a steady level with 5-6 tetani at 5 min intervals. These results suggest strongly that the number of contractile units in series varies directly with the adapted muscle length. Temporary force depression after a length change would occur if the change transiently moved the filaments from their optimum overlap. The relative length independence of the adapted force is explained by the reforming of the filament lattice to produce optimum force development, with commensurate changes of velocity and compliance.     

Proteolytic activation of the canine cardiac sarcoplasmic reticulum calcium pump

Mild trypsin treatment of canine cardiac microsomes consisting largely of sarcoplasmic reticulum vesicles produced a severalfold activation of oxalate-facilitated calcium uptake. The increase in calcium uptake was associated with an increase in ATP hydrolysis. Proteases other than trypsin were also effective although to a lesser degree. Trypsin produced a shift of the Ca2+ concentration dependency curve for calcium uptake toward lower Ca2+ concentrations, which was almost identical with that produced by phosphorylation of microsomes by cyclic AMP dependent protein kinase when the trypsin and the protein kinase were present at maximally activating concentrations. The Hill numbers (+/- SD) of the Ca2+ dependency after treatment of microsomes with trypsin (1.5 +/- 0.1) or protein kinase (1.7 +/- 0.1) were similar and were not significantly different from those for untreated control microsomes (1.6 +/- 0.1 and 1.8 +/- 0.1, respectively). Autoradiograms of sodium dodecyl sulfate-polyacrylamide electrophoretic gels indicate that 32P incorporation into phospholamban (Mr 27.3K) or its presumed monomeric subunit (Mr 5.5K) was markedly reduced when trypsin-treated microsomes were incubated in the presence of cyclic AMP dependent protein kinase and [gamma-32P]ATP compared to control microsomes incubated similarly but pretreated with trypsin inhibitor inactivated trypsin. The activation of calcium uptake by increasing concentrations of trypsin was paralleled by the reduction of phosphorylation of phospholamban. Trypsin treatment of microsomes previously thiophosphorylated in the presence of cyclic AMP dependent protein kinase and [gamma-35S]thio-ATP did not result in a loss of 35S label from phospholamban, which suggests that phosphorylation of phospholamban protects against trypsin attack.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of lung volume on the respiratory action of the canine pectoral muscles.

Lung volume influences the mechanical action of the primary inspiratory and expiratory muscles by affecting their precontraction length, alignment with the rib cage, and mechanical coupling to agonistic and antagonistic muscles. We have previously shown that the canine pectoral muscles exert an expiratory action on the rib cage when the forelimbs are at the torso's side and an inspiratory action when the forelimbs are held elevated. To determine the effect of lung volume on intrathoracic pressure changes produced by the canine pectoral muscles, we performed isolated bilateral supramaximal electrical stimulation of the deep pectoral and superficial pectoralis (descending and transverse heads) muscles in 15 adult supine anesthetized dogs during hyperventilation-induced apnea. Lung volume was altered by application of a negative or positive pressure (+/- 30 cmH2O) to the airway. In all animals, selective electrical stimulation of the descending, transverse, and deep pectoral muscles with the forelimbs held elevated produced negative intrathoracic pressure changes (i.e., an inspiratory action). Moreover, with the forelimbs elevated, increasing lung volume decreased both pectoral muscle fiber precontraction length and the negative intrathoracic pressure changes generated by contraction of each of these muscles. Conversely, with the forelimbs along the torso, increasing lung volume lengthened pectoral muscle precontraction length and augmented the positive intrathoracic pressure changes produced by muscle contraction (i.e., an expiratory action). These results indicate that lung volume significantly affects the length of the canine pectoral muscles and their mechanical actions on the rib cage.     

Quasi-static pressure-volume hysteresis in the canine respiratory system in vivo

We investigated the quasi-static pressure-volume (P-V) hysteresis of the normal canine lung in vivo by performing 15-s flow interruptions at various points throughout the breathing cycle in mechanically ventilated anesthetized paralyzed dogs. By measuring the transpulmonary pressure (Ptp) at 5 s after each interruption, we built up a quasi-static P-V loop of the lungs. We found, however, that the area of the loop was significantly smaller (by a factor of 4-6) than has been reported by others for the isolated canine lung. We also found the hysteresis loop area of the chest wall to be of similar magnitude. If we measured Ptp 10-15 s after interruption, we found it always decreased at a rate expected to result from continuing gas exchange in the lungs. We conclude that 1) the areas of the quasi-static P-V loop in vivo for the total respiratory system, as well as the lungs and chest wall separately, are significantly smaller than has been reported previously for isolated lungs and 2) continuing gas exchange in the lungs places a lower limit on the frequencies (equivalent to flow interruptions of greater than 5- to 7-s duration) at which the P-flow-V behavior of the lungs in vivo can be considered in purely mechanical terms.