Muscle fiber architecture of the dog diaphragm

Boriek, Aladin M., Charles C. Miller III, and Joseph R. Rodarte. Muscle fiber architecture of the dog diaphragm.J. Appl. Physiol. 84(1): 318-326, 1998.Previous measurements of muscle thickness and length ratio ofcostal diaphragm insertions in the dog (A. M. Boriek and J. R. Rodarte.J. Appl. Physiol. 77: 2065-2070,1994) suggested, but did not prove, discontinuous muscle fiberarchitecture. We examined diaphragmatic muscle fiber architecture usingmorphological and histochemical methods. In 15 mongrel dogs, transversesections along the length of the muscle fibers were analyzedmorphometrically at ×20, by using the BioQuant System IVsoftware. We measured fiber diameters, cross-sectional fiber shapes,and cross-sectional area distributions of fibers. We also determinednumbers of muscle fibers per cross-sectional area and ratio ofconnective tissue to muscle fibers along a course of the muscle fromnear the chest wall (CW) to near the central tendon (CT) for midcostalleft and right hemidiaphragms, as well as ventral, middle, and dorsalregions of the left costal hemidiaphragm. In six other mongrel dogs,the macroscopic distribution of neuromuscular junctions (NMJ) onthoracic and abdominal diaphragm surfaces was determined by stainingthe intact diaphragmatic muscle for acetylcholinesterase activity. Theaverage major diameter of muscle fibers was significantly smaller, andthe number of fibers was significantly larger midspan between CT and CWthan near the insertions. The ratio of connective tissues to musclefibers was largest at CW compared with other regions along the lengthof the muscle. The diaphragm is transversely crossed by multiplescattered NMJ bands with fairly regular intervals offset in adjacentstrips. Muscle fascicles traverse two to five NMJ, consistent withfibers that do not span the entire fascicle from CT to CW. Theseresults suggest that the diaphragm has a discontinuous fiberarchitecture in which contractile forces may be transmitted among themuscle fibers through the connective tissue adjacent to the fibers.

     

Kinematics and mechanics of midcostal diaphragm of dog

Boriek, Aladin M., Joseph R. Rodarte, and Theodore A. Wilson. Kinematics and mechanics of midcostal diaphragm of dog. J. Appl. Physiol. 83(4):1068-1075, 1997.Radiopaque markers were attached to theperitoneal surface of three neighboring muscle bundles in the midcostaldiaphragm of four dogs, and the locations of the markers were trackedby biplanar video fluoroscopy during quiet spontaneous breathing andduring inspiratory efforts against an occluded airway at three lungvolumes from functional residual capacity to total lung capacity inboth the prone and supine postures. Length and curvature of the musclebundles were determined from the data on marker location. Musclelengths for the inspiratory states, as a fraction of length atfunctional residual capacity, ranged from 0.89 ± 0.04 at endinspiration during spontaneous breathing down to 0.68 ± 0.07 duringinspiratory efforts at total lung capacity. The muscle bundles werefound to have the shape of circular arcs, with the three bundlesforming a section of a right circular cylinder. With increasing lungvolume and diaphragm displacement, the circular arcs rotate around theline of insertion on the chest wall, the arcs shorten, but the radiusof curvature remains nearly constant. Maximal transdiaphragmaticpressure was calculated from muscle curvature and maximaltension-length data from the literature. The calculated maximaltransdiaphragmatic pressure-length curve agrees well with the data ofRoad et al. (J. Appl. Physiol. 60:63-67, 1986).

     

Passive mechanics of muscle tendinous junction of canine diaphragm.

The diaphragmatic muscle tendon is a biaxially loaded junction in vivo. Stress-strain relations along and transverse to the fiber directions are important in understanding its mechanical properties. We hypothesized that 1) the central tendon possesses greater passive stiffness than adjacent muscle, 2) the diaphragm muscle is anisotropic, whereas the central tendon near the junction is essentially isotropic, and 3) a gradient in passive stiffness exists as one approaches the muscle-tendinous junction (MTJ). To investigate these hypotheses, we conducted uniaxial and biaxial mechanical loading on samples of the MTJ excised from the midcostal region of dog diaphragm. We measured passive length-tension relationships of the muscle, tendon, and MTJ in the direction along the muscle fibers as well as transverse to the fibers. The MTJ was slack in the unloaded state, resulting in a J-shaped passive tension-strain curve. Generally, muscle strain was greater than that of MTJ, which was greater than tendon strain. In the muscular region, stiffness in the direction transverse to the fibers is much greater than that along the fibers. The central tendon is essentially inextensible in the direction transverse to the fibers as well as along the fibers. Our data demonstrate the existence of more pronounced anisotropy in the muscle than in the tendon near the junction. Furthermore, a gradient in muscle stiffness exists as one approaches the MTJ, consistent with the hypothesis of continuous passive stiffness across the MTJ.     

Three-dimensional reconstruction of human diaphragm with the use of spiral computed tomography

Pettiaux, Nicolas, Marie Cassart, Manuel Paiva, and MarcEstenne. Three-dimensional reconstruction of human diaphragm withthe use of spiral computed tomography. J. Appl.Physiol. 82(3): 998-1002, 1997.We developed atechnique of diaphragm imaging by using spiral computed tomography, andwe studied four normal subjects who had been previously investigatedwith magnetic resonance imaging (A. P. Gauthier, S. Verbanck,M. Estenne, C. Segebarth, P. T. Macklem, and M. Paiva.J. Appl. Physiol. 76: 495-506,1994). One acquisition of 15- to 25-s duration was performed atresidual volume, functional residual capacity, functional residualcapacity plus one-half inspiratory capacity, and total lung capacitywith the subject holding his breath and relaxing. From theseacquisitions, 20 coronal and 30 sagittal images were reconstructed ateach lung volume; on each image, diaphragm contour in the zone ofapposition and in the dome was digitized with the software Osiris, andthe digitized silhouettes were used for three-dimensionalreconstruction with Matlab. Values of length and surface area for thediaphragm, the dome, and the zone of apposition were very similar tothose obtained with magnetic resonance imaging. We conclude thatsatisfactory three-dimensional reconstruction of the in vivo diaphragmmay be obtained with spiral computed tomography, allowing accurate measurements of muscle length, surface area, and shape.

     

Long-term facilitation of upper airway muscle activities in vagotomized and vagally intact cats

Mateika, J. H., and R. F. Fregosi. Long-termfacilitation of upper airway muscle activities in vagotomized andvagally intact cats. J. Appl. Physiol.82(2): 419-425, 1997.The primary purpose of the presentinvestigation was to determine whether long-term facilitation (LTF) ofupper airway muscle activities occurs in vagotomized and vagally intactcats. Tidal volume and diaphragm, genioglossus, and nasal dilatormuscle activities were recorded before, during, and after one carotidsinus nerve was stimulated five times with 2-min trains of constantcurrent. Sixty minutes after stimulation, nasal dilator andgenioglossus muscle activities were significantly greater than controlin the vagotomized cats but not in the vagally intact cats. Tidalvolume recorded from the vagotomized and vagally intact cats wassignificantly greater than control during the poststimulation period.In contrast, diaphragm activities were not significantly elevated inthe poststimulation period in either group of animals. We conclude that1) LTF of genioglossus and nasaldilator muscle activities can be evoked in vagotomized cats;2) vagal mechanisms inhibit LTF inupper airway muscles; and 3) LTF canbe evoked in accessory inspiratory muscles because LTF of inspiredtidal volume was greater than LTF of diaphragm activity.

     

Zone of apposition in the passive diaphragm of the dog

Boriek, Aladin M., Joseph R. Rodarte, and Susan S. Margulies. Zone of apposition in the passive diaphragm of thedog. J. Appl. Physiol. 81(5): 1929-1940, 1996.Wedetermined the regional area of the diaphragmatic zone of apposition(ZAP) as well as the regional craniocaudal extent of the ZAP(ZAP_ht) of the passive diaphragm in six paralyzedanesthetized beagle dogs (8-12 kg) at residual lung volume (RV),functional residual capacity (FRC), FRC + 0.25 and FRC + 0.5 inspiratory capacity, and total lung capacity (TLC) in prone and supinepostures. To identify the caudal boundary of the ZAP, 17 lead markers(1 mm) were sutured to the abdominal side of the costal and cruraldiaphragms around the diaphragm insertion on the chest wall. Two weekslater, the dogs' caudal thoraces were scanned by the use of thedynamic spatial reconstructor (DSR), a prototype fast volumetric X-raycomputer tomographic scanner, developed at the Mayo Clinic. Thethree-dimensional spatial coordinates of the markers were identified(±1.4 mm), and the cranial boundary of the ZAP was determined from30-40 1.4-mm-thick sagittal and coronal slices in each DSR image.We interpolated the DSR data to find the position of the cranial andcaudal boundaries of the ZAP every 5° around the thorax and computedthe distribution of regional variation of area of the ZAP andZAP_ht as well as the total area of ZAP. TheZAP_ht and area of ZAP increased as lung volume decreasedand were largest near the lateral extremes of the rib cage. We measuredthe surface area of the rib cage cephaled to the ZAP(A_L) in both postures in another six beagle dogs(12-16 kg) of similar stature, scanned previously in the DSR. Weestimated the entire rib cage surface area(A_rc = A_ZAP +A_L). The A_ZAP as a percentageof A_rc increased more than threefold as lung volumedecreased from TLC to RV, from ~9 to 29% of A_rc.

     

Patterns of shortening and thickening of the human diaphragm

Wait, J. L., and R. L. Johnson. Patterns of shorteningand thickening of the human diaphragm. J. Appl.Physiol. 83(4): 1123-1132, 1997.To study how the human diaphragm changesconfiguration during inspiration, we simultaneously measured diaphragmthickening using ultrasound and inspired volumes using apneumotachograph. Diaphragm length was assessed by chest radiography.We found that thickening and shortening were greatest during a breathtaken primarily with the abdomen. However, the degree of thickening wasgreater than expected for fiber shortening, assuming parallel musclefibers and no shear. So, to clarify this unexpected finding, weconsidered geometric models of the diaphragm. How a muscle thickens asits fibers shorten is critically dependent on geometry. Thus, if a flatrectangular sheet of muscle shortens along one dimension, surfacearea-to-length ratio along this dimension should remain constant, andthickness would be inversely proportional to length during shortening.The simplest model of the diaphragm, however, is a cylindrical sheet ofmuscle in the zone of apposition capped by a dome; the ratio of surfacearea to radial fiber length in the dome is substantially less than theratio of area to length of the cylindrical zone of apposition; hence,as the zone of apposition shortens while the dome radius remainsconstant, the ratio of total surface area to combined length (i.e.,dome + zone of apposition) must decrease and thickening of the musclecorrespondingly must increase more than expected for a simplerectangular strip. A similar relationship can be derived betweenthickening and length in a muscle sheet with a wedge-shaped insertioninto a thin flat tendon. Comparison of calculations with these types ofmodels to data from human subjects indicates that the unexpectedthickening in the zone of apposition is explained by the peculiargeometry of the diaphragm. The greater thickening of the diaphragm inthe zone of apposition suggests that more of the muscle mass and more sarcomeres are retained in the zone of apposition as the dome descends.Physiologically, this greater thickening may have importance byreducing wall stress in the zone of apposition and reducing the work orenergy requirements per sarcomere.

     

Airway-parenchymal interdependence after airway contraction in rat lung explants

The constrictionof pulmonary airways is limited by the tethering effect exerted byparenchymal attachments. To characterize this tethering effect at thescale of intraparenchymal airways, we studied the pattern ofparenchymal distortion due to bronchoconstriction in a rat lung explantsystem. First, we measured the elastic modulus under tension for 2%(wt/vol) agarose alone (37.6 ± 1.5 kPa) and for agarose-filled lung(5.7 ± 1.3 kPa). The latter is similar to the elastic modulus ofair-filled lung at total lung capacity (4.5-6 kPa) (S. J. Lai-Fook, T. A. Wilson, R. E. Hyatt, and J. R. Rodarte.J. Appl. Physiol. 40: 508-513,1976), suggesting that explants can be used as a model of lung tissuedistortion. Subsequently, confocal microscopic images of fluorescentlylabeled 0.5-mm-thick explants prepared from agarose-filled rat lungsinflated to total lung capacity (48 ml/kg) were acquired. Images weretaken before and after airway constriction was induced by directapplication of 10 mM methacholine, and the pattern of parenchymaldistortion was measured from the displacement of tissue landmarksidentified in each image for 14 explants. The magnitude of the radialcomponent of tissue displacement was calculated as a function ofdistance from the airway wall and characterized by a parameter,b, describing the rate at which tissuemovement decreased with radial distance. The parameterb was 0.994 ± 0.19 (SE), which isclose to the prediction of b = 1 ofmicromechanical modeling (T. A. Wilson. J. Appl.Physiol. 33: 472-478, 1972). There was significantvariability in b, however, which wascorrelated with the fractional reduction in airway diameter (r = 0.496). Additionally, parenchymaldistortion showed significant torsion with respect to the radialdirection. This torsion was similar in concentric zones around theairway, suggesting that it originates from inhomogeneity in theparenchyma rather than inhomogeneous airway constriction. Our resultsdemonstrate the significance of the nonlinear mechanical properties ofalveolar walls and the anisotropy of the parenchyma in determining the nature of airway-parenchymal interdependence.

     

Effect of insulin-like growth factor I and/or growth hormone on diaphragm of malnourished adolescent rats

Lewis, Michael I., Thomas J. LoRusso, and Mario Fournier.Effect of insulin-like growth factor I and/or growthhormone on diaphragm of malnourished adolescent rats.J. Appl. Physiol. 82(4):1064-1070, 1997.Young growing animals appear to havesignificantly reduced "nutritional reserve" to short periods ofunstressed starvation compared with adults, with resultant growtharrest and/or atrophy of diaphragm (Dia) muscle fibers. The aimof this study was to assess in an adolescent rat model of acutenutritional deprivation (ND; 72 h) the impact of insulin-like growthfactor I (IGF-I), with or without added growth hormone (GH), on thecross-sectional areas (CSA) of individual Dia muscle fibers. Fivegroups were studied: 1) control(Ctr); 2) ND;3) ND given IGF-I (ND/IGF-I); 4) ND given GH (ND/GH); and5) ND given a combination of IGF-I and GH (ND/IGF-I/GH). IGF-I was given by a subcutaneously implanted osmotic minipump (200 µg/day), whereas GH was administered twice daily by a subcutaneous injection (250 µg every 12 h). Isometric contractile and fatigue properties of the Dia were determined in vitro.Forces were normalized for muscle CSA (i.e., specific force). Dia fibertype proportions were determined histochemically, and fiber CSA wasquantified by using a computer-based image-processing system. Totalserum IGF-I concentrations were significantly reduced in ND and ND/GHanimals, compared with Ctr, and elevated in the groups receiving IGF-I.The provision of growth factors did not alter the contractile orfatigue properties of ND animals. Dia fiber type proportions weresimilar among the groups. In ND animals, there was a significantreduction in the CSA of types I, IIa, IIx, and IIc Dia fibers comparedwith Ctr. The administration of IGF-I alone or in combination with GHto ND animals significantly diminished the reduction in Dia fiber size.GH alone had no effect on Dia fiber size in ND animals. We concludethat with acute ND the peripheral resistance to the action of GHappears to be bypassed by the administration of IGF-I alone or incombination with GH.

     

Expression of the inducible nitric oxide synthase gene in diaphragm and skeletal muscle

Thompson, Marita, Lisa Becker, Debbie Bryant, Gary Williams,Daniel Levin, Linda Margraf, and Brett P. Giroir. Expression ofthe inducible nitric oxide synthase gene in diaphragm and skeletal muscle. J. Appl. Physiol. 81(6):2415-2420, 1996.Nitric oxide (NO) is a pluripotent molecule thatcan be secreted by skeletal muscle through the activity of the neuronalconstitutive isoform of NO synthase. To determine whether skeletalmuscle and diaphragm might also express the macrophage-inducible formof NO synthase (iNOS) during provocative states, we examined tissuefrom mice at serial times after intravenous administration ofEscherichia coli endotoxin. In thesestudies, iNOS mRNA was strongly expressed in the diaphragm and skeletalmuscle of mice 4 h after intravenous endotoxin and was significantlydiminished by 8 h after challenge. Induction of iNOS mRNA was followedby expression of iNOS immunoreactive protein on Western immunoblots.Increased iNOS activity was demonstrated by conversion of arginine tocitrulline. Immunochemical analysis of diaphragmatic explants exposedto endotoxin in vitro revealed specific iNOS staining in myocytes, inaddition to macrophages and endothelium. These results may be importantin understanding the pathogenesis of respiratory pump failure duringseptic shock, as well as skeletal muscle injury during inflammation ormetabolic stress.

     

Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation

Zhan, Wen-Zhi, Hirofumi Miyata, Y. S. Prakash, and Gary C. Sieck. Metabolic and phenotypic adaptations of diaphragm musclefibers with inactivation. J. Appl.Physiol. 82(4):1145-1153, 1997.We hypothesizedthat metabolic adaptations to muscle inactivity are most pronouncedwhen neurotrophic influence is disrupted. In ratdiaphragm muscle(Dia_m), 2 wk ofunilateral denervation or tetrodotoxin nerve blockade resulted in areduction in succinate dehydrogenase (SDH) activity of type I, IIa, andIIx fibers (~50, 70, and 24%, respectively) and a decrease in SDHvariability among fibers (~63%). In contrast, inactivity induced byspinal cord hemisection at C₂ (ST)resulted in much less change in SDH activity of type I and IIa fibers(~27 and 24%, respectively) and only an ~30% reduction in SDHvariability among fibers. Actomyosin adenosinetriphosphatase (ATPase)activities of type I, IIx, and IIb fibers in denervated andtetrodotoxin-treated Dia_m werereduced by ~20, 45, and 60%, respectively, and actomyosin ATPasevariability among fibers was ~60% lower. In contrast, onlyactomyosin ATPase activity of type IIb fibers was reduced (~20%) inST Dia_m. These results suggestthat disruption of neurotrophic influence has a greater impact onmuscle fiber metabolic properties than inactivity per se.

     

Functional role and structure of the scalene: an accessory inspiratory muscle in hamster

Fournier, Mario, and Michael I. Lewis. Functional roleand structure of the scalene: an accessory inspiratory muscle inhamster. J. Appl. Physiol. 81(6):2436-2444, 1996.Although the scalene muscle (Sca) is a primaryinspiratory muscle in humans, its respiratory function in other speciesis less clear. The electromyographic (EMG) activity of the Sca wasstudied during resting ventilation (eupnea) in both the awake andanesthetized hamster and after a variety of respiratory challenges inthe anesthetized animal. The EMG activities of the medial Sca and thecostal diaphragm were compared. The medial Sca, the major component ofthe Sca, originates from cervical transverse processes 2 to 5 andinserts primarily onto rib 4, with a small segment onto rib 3. In both the anesthetized and awake animal, the Sca was always silent during quiet breathing. WithCO₂-stimulated hyperpnea, the Scawas always recruited during inspiration in phase with the diaphragm.Active recruitment of the Sca was also observed after resistive loading and total airway occlusion. After ipsilateral phrenicotomy, the Sca waspersistently recruited during eupnea. The specificity of the EMGsignals was tested both by excluding cross contamination from other ribcage muscles and by selective denervation studies. Muscle spindles wereidentified in the medial Sca histochemically, suggesting that therespiratory activity of the Sca can also be modulated by changes inmuscle length and/or load. These results indicate that the Scafunctions as an accessory inspiratory muscle in the hamster and mayplay an important role in conditions of chronic load.

     

Instantaneous force-velocity-length relationship in diaphragmatic sarcomere

Coirault, Catherine, Denis Chemla, Jean-Claude Pourny,Francine Lambert, and Yves Lecarpentier. Instantaneousforce-velocity-length relationship in diaphragmatic sarcomere.J. Appl. Physiol. 82(2): 404-412, 1997.The simultaneous analysis of muscle force, length, velocity, andtime has been shown to precisely characterize the mechanicalperformance of isolated striated muscle. We tested the hypothesis thatthe three-dimensional force-velocity-length relationship reflectsmechanical properties of sarcomeres. In hamster diaphragm strips,instantaneous sarcomere length (SL) and muscle length were simultaneously measured during afterloaded twitches. SL was measured by means of laser diffraction. Wealso studied the influence of initialSL, abrupt changes in total load, and2 × 10⁷ M dantrolene.Baseline resting SL at the apex of thelength-active tension curve was 2.2 ± 0.1 µm, whereasSL at peak shortening was 1.6 ± 0.1 µm in the preloaded twitch and 2.1 ± 0.1 µm in the "isometric" twitch. Over the whole load continuum and at anygiven level of isotonic load, there was a unique relationship between instantaneous sarcomere velocity and instantaneousSL. Part of this relationship was timeindependent and initial SL independent and was markedly downshifted after dantrolene. When five different muscle regions were considered, there were no significant variations ofSL and sarcomere kinetics along themuscle. These results indicate that the time- and initiallength-independent part of the instantaneous force-velocity-lengthrelationship previously described in muscle strips reflects intrinsicsarcomere mechanical properties.

     

Simulation of motor unit recruitment and microvascular unit perfusion: spatial considerations

Fuglevand, Andrew J., and Steven S. Segal. Simulationof motor unit recruitment and microvascular unit perfusion: spatial considerations. J. Appl. Physiol.83(4): 1223-1234, 1997.Muscle fiber activity is the principalstimulus for increasing capillary perfusion during exercise. Thecontrol elements of perfusion, i.e., microvascular units (MVUs), supplyclusters of muscle fibers, whereas the control elements of contraction,i.e., motor units, are composed of fibers widely scattered throughoutmuscle. The purpose of this study was to examine how the discordantspatial domains of MVUs and motor units could influence the proportion of open capillaries (designated as perfusion) throughout a muscle crosssection. A computer model simulated the locations of perfused MVUs inresponse to the activation of up to 100 motor units in a muscle with40,000 fibers and a cross-sectional area of 100 mm². The simulation increasedcontraction intensity by progressive recruitment of motor units. Foreach step of motor unit recruitment, the percentage of active fibersand the number of perfused MVUs were determined for several conditions:1) motor unit fibers widely dispersed and motor unit territories randomly located (whichapproximates healthy human muscle),2) regionalized motor unitterritories, 3) reversed recruitmentorder of motor units, 4) denselyclustered motor unit fibers, and 5)increased size but decreased number of motor units. The simulationsindicated that the widespread dispersion of motor unit fibersfacilitates complete capillary (MVU) perfusion of muscle at low levelsof activity. The efficacy by which muscle fiber activity inducedperfusion was reduced 7- to 14-fold under conditions that decreased thedispersion of active fibers, increased the size of motor units, orreversed the sequence of motor unit recruitment. Such conditions aresimilar to those that arise in neuromuscular disorders, with aging, orduring electrical stimulation of muscle, respectively.

     

Gender-specific effects of dexamethasone treatment on rat diaphragm structure and function

Prezant, D. J., M. L. Karwa, B. Richner, D. Maggiore, E. I. Gentry, and J. Cahill. Gender-specific effects of dexamethasone treatment on rat diaphragm structure and function. J. Appl. Physiol. 82(1): 125-133, 1997.The effectsof long-term dexamethasone treatment on diaphragm muscle were studiedin female and male rats. Compared with pair-fed control animals,dexamethasone treatment did not significantly affect estrous cycling orpeak serum estradiol levels; however, testosterone levels weresignificantly increased in females and decreased in males.Dexamethasone significantly reduced body and costal diaphragm weights,but to a lesser extent in females than in males. Reductions indiaphragm weight were proportional to reductions in body weight. Infemales and males, dexamethasone treatment significantly decreaseddiaphragm fiber (types I and II) cross-sectional area and the relativeexpression of myosin heavy chain isoform 2B. With the exception of typeI fiber atrophy, these changes occurred to a lesser extent in females.Dexamethasone did not significantly affect specific forces.Dexamethasone significantly increased twitch one-half relaxation timeand fatigue resistance indexes in males but not in females. Inconclusion, the effects of long-term dexamethasone treatment weregender specific, with significantly fewer effects in females, andchanges in serum testosterone levels were associated with thesefindings.

     

Chest wall muscle cross talk in canine costal diaphragm electromyogram

Sinderby, C., S. Friberg, N. Comtois, and A. Grassino.Chest wall muscle cross talk in the canine costal diaphragm electromyogram. J. Appl. Physiol.81(5): 2312-2327, 1996.The present paper describes the influenceof cross talk from the abdominal and intercostal muscles on the caninediaphragm electromyogram (EMG). The diaphragm EMG was recorded withbipolar surface electrodes placed on the costal portion of thediaphragm (abdominal side), aligned in the fiber direction, andpositioned in a region with a relatively low density of motor endplates. The results indicated that cross talk may occur in thediaphragm EMG, especially during conditions of loaded breathing andlight general anesthesia. The cross-talk signals showed characteristicsthat were entirely different from the diaphragm EMG. Although thediaphragm EMG was typical for signals recorded with electrodes alignedin the fiber direction, the cross-talk signals were characteristic ofthose obtained with electrode pairs not aligned in the direction of themuscle fibers. Alterations in electrode positioning, interelectrodedistance, and/or electrode surface area cannot guarantee theelimination of cross-talk signals, whereas spinal anesthesia at a highthoracic level will paralyze the sources of the cross talk and henceeliminate the cross-talk signals. By taking advantage of thedifferences in EMG signal characteristics for the diaphragm EMG andcross-talk signals, an index that has the capability to detect crosstalk was developed.

     

Diaphragm disuse reduces Ca2+ uptake capacity of sarcoplasmic reticulum

Howell, Sandra, Wen-Zhi Zhan, and Gary C. Sieck.Diaphragm disuse reduces Ca²⁺uptake capacity of sarcoplasmic reticulum. J. Appl.Physiol. 82(1): 164-171, 1997.Chronic phrenictetrodotoxin (TTX) blockade and phrenic denervation (Dnv) of hamsterdiaphragm result in decreased maximum specific tension, prolongedcontraction time, and improved fatigue resistance (W. Z. Zhan and G. C. Sieck. J. Appl. Physiol. 72:1445-1453, 1992). An underlying increased relative contribution oftype I fibers to total muscle mass appears to be consistent with, butdoes not completely account for, changes in contractile and fatigueproperties. The present study was designed to evaluate a potential rolefor altered cellular Ca²⁺metabolism in the adaptive response of the diaphragm to chronic disuse.An analytic method based on simulation and modeling of long-term⁴⁵Ca²⁺efflux data was used to estimateCa²⁺ contents (nmolCa²⁺/g wet wt tissue) and exchangefluxes (nmolCa^{2+ ·} min^{1 ·} g¹)for extracellular and intracellular compartments in the in vitro hamster hemidiaphragm after prolonged disuse. Three groups were compared: control (Con, n = 5),phrenic TTX blockade (TTX, n = 5), andphrenic denervation (Dnv, n = 5).Experimental muscles were loaded with⁴⁵Ca²⁺for 1 h, and efflux data were collected for 8 h by using a flow-through tissue chamber. Compartmental analysis of efflux data estimated thatthe Ca²⁺ contents andCa²⁺ exchange fluxes of thelargest and slowest intracellular compartment (putative longitudinalreticulum) were reduced by ~50% in TTX and Dnv muscle groupscompared with Con. In addition, the kinetic model predicted significantdecreases in total intracellularCa²⁺ and total diaphragmCa²⁺ in TTX and Dnv muscles. Weconclude that the data support the hypothesis that the capac- ity ofthe sarcoplasmic reticulum for Ca²⁺ sequestration is reduced inchronic diaphragm disuse. The impact of this effect on diaphragmcontractile and fatigue properties is discussed.

     

Changes in skeletal muscle biochemistry and histology relative to fiber type in rats with heart failure

Delp, Michael D., Changping Duan, John P. Mattson, andTimothy I. Musch. Changes in skeletal muscle biochemistry and histology relative to fiber type in rats with heart failure.J. Appl. Physiol. 83(4):1291-1299, 1997.One of the primary consequences of leftventricular dysfunction (LVD) after myocardial infarction is adecrement in exercise capacity. Several factors have been hypothesizedto account for this decrement, including alterations in skeletal musclemetabolism and aerobic capacity. The purpose of this study was todetermine whether LVD-induced alterations in skeletal muscle enzymeactivities, fiber composition, and fiber size are1) generalized in muscles orspecific to muscles composed primarily of a given fiber type and2) related to the severity of theLVD. Female Wistar rats were divided into three groups: sham-operatedcontrols (n = 13) and rats withmoderate (n = 10) and severe(n = 7) LVD. LVD was surgicallyinduced by ligating the left main coronary artery and resulted inelevations (P < 0.05) in leftventricular end-diastolic pressure (sham, 5 ± 1 mmHg; moderate LVD,11 ± 1 mmHg; severe LVD, 25 ± 1 mmHg). Moderate LVDdecreased the activities of phosphofructokinase (PFK) and citratesynthase in one muscle composed of type IIB fibers but did not modifyfiber composition or size of any muscle studied. However, severe LVDdiminished the activity of enzymes involved in terminal and-oxidation in muscles composed primarily of type I fibers, type IIAfibers, and type IIB fibers. In addition, severe LVD induced areduction in the activity of PFK in type IIB muscle, a 10% reductionin the percentage of type IID/X fibers, and a corresponding increase inthe portion of type IIB fibers. Atrophy of type I fibers, type IIAfibers, and/or type IIB fibers occurred in soleus and plantarismuscles of rats with severe LVD. These data indicate that rats withsevere LVD after myocardial infarction exhibit1) decrements in mitochondrialenzyme activities independent of muscle fiber composition,2) a reduction in PFK activity in type IIB muscle, 3) transformationof type IID/X to type IIB fibers, and4) atrophy of type I, IIA, and IIBfibers.

     

Task failure with lack of diaphragm fatigue during inspiratory resistive loading in human subjects

McKenzie, D. K., G. M. Allen, J. E. Butler, and S. C. Gandevia. Task failure with lack of diaphragm fatigue during inspiratory resistive loading in human subjects. J. Appl. Physiol. 82(6): 2011-2019, 1997.Taskfailure during inspiratory resistive loading is thought to beaccompanied by substantial peripheral fatigue of the inspiratorymuscles. Six healthy subjects performed eight resistive breathingtrials with loads of 35, 50, 75 and 90% of maximal inspiratorypressure (MIP) with and without supplemental oxygen. MIP measuredbefore, after, and at every minute during the trial increased slightlyduring the trials, even when corrected for lung volume (e.g., for 24 trials breathing air, 12.5% increase, P < 0.05). In some trials, taskfailure occurred before 20 min (end point of trial), and in thesetrials there was an increase in end-tidalPCO₂(P < 0.01), despite the absence of peripheral muscle fatigue. In four subjects (6 trials with task failure), there was no decline in twitch amplitude with bilateral phrenic stimulation or in voluntary activation of the diaphragm, eventhough end-tidal PCO₂ rose by 1.6 ± 0.9%. These results suggest that hypoventilation,CO₂ retention, and ultimate taskfailure during resistive breathing are not simply dependent on impairedforce-generating capacity of the diaphragm or impaired voluntaryactivation of the diaphragm.