Morphometric analysis of capillary geometry in pigeon pectoralis muscle

The objective of the study was to examine the relationship(s) between the size and the geometry of the capillary network in the flight muscle of pigeon (Columbia livia). To this end, we used morphometry to analyze the degree of anisotropy (i.e., orientation) of capillaries with respect to the axis of the muscle fibers in perfusion-fixed samples of pigeon pectoralis muscles with large difference in capillary density. Capillary number per fiber cross-sectional area (range, 1,491-5,680 mm-2) depended on fiber size (aerobic fibers, 304-782 microns 2; glycolytic, 1,785-2,444 microns 2), as well as sarcomere length (1.69-2.20 microns), and the relative sectional area of aerobic and glycolytic fibers (aerobic, 42-84% of total fiber area). The degree of tortuosity of capillaries, i.e., their bending or sinuosity relative to the muscle fiber axis, was primarily a function of sarcomere length. In spite of large differences in capillary density, capillary orientation at a given sarcomere length was remarkably similar among samples. In addition to capillaries running parallel to the muscle fiber axis, a unique arrangement of branches running perpendicular to the muscle fiber axis was found in all samples. This arrangement yielded a large circumferential distribution of capillary surface around the muscle fibers. Compared to mammalian limb muscles examined over a 10-fold range of capillary density (range, 450-4,670 mm-2), the degree of anisotropy of capillaries was greater in all samples of pigeon M. pectoralis. In the pigeon, there was no increase in the amount of capillary surface area available for exchange per microvessel as a result of a greater degree of capillary tortuosity in samples with larger capillary density (capillary number per fiber cross-sectional area greater than 4,000 mm-2), as compared to samples with a capillary density less than 4,000 mm-2.     

Capillary tortuosity in skeletal muscles of mammals depends on muscle contraction

Capillary orientation (anisotropy) was compared in hindlimb muscles of mammals of different size and/or different aerobic capacity (dog, goat, pony, and calf). All muscles were fixed by vascular perfusion at sarcomere lengths ranging from 1.5 to 2.7 micron. The ratios of capillary counts per fiber cross-sectional area on two sets of sections (0 and 90 degrees) to the muscle fiber axis were used to estimate capillary anisotropy and the coefficient c(K,0) relating 1) capillary counts on transverse sections (a commonly used parameter to assess muscle capillarity) and 2) capillary length per volume of fiber (i.e., capillary length density). Capillary orientation parallel to the muscle fiber axis decreased substantially with muscle fiber shortening. In muscles fixed at sarcomere lengths of 2.69 microns (dog vastus intermedius) and 1.52 microns (dog gastrocnemius), capillary tortuosity and branching added 7 and 64%, respectively, to capillary length density. The data obtained in this study are highly consistent with the previously demonstrated relationship between capillary anisotropy and sarcomere length in extended vs. contracted rat muscles, by use of the same method. Capillary anisotropy in mammalian locomotory muscles is curvilinearly related to sarcomere length. No systematic difference was found in capillary tortuosity with either body size, athletic ability, or aerobic capacity. Capillary tortuosity is a consequence of fiber shortening rather than an indicator of the O2 requirements of the tissue.     

Fiber capillarization relative to mitochondrial volume in diaphragm of shrew.

The objective was to examine fiber capillarization in relation to fiber mitochondrial volume in the highly aerobic diaphragm of the shrew, the smallest mammal. The diaphragms of four common shrews [Sorex araneus; body mass, 8.2 +/- 1.3 (SE) g] and four lesser shrews (Sorex minutus, 2.6 +/- 0.1 g) were perfusion fixed in situ, processed for electron microscopy, and analyzed by morphometry. Capillary length per fiber volume was extremely high, at values of 8,008 +/- 1,054 and 12,332 +/- 625 mm(-2) in S. araneus and S. minutus, respectively (P = 0.012), with no difference in capillary geometry between the two species. Fiber mitochondrial volume density was 28.5 +/- 2.3% (S. araneus) and 36.5 +/- 1.4% (S. minutus; P = 0.025), yielding capillary length per milliliter mitochondria values (S. araneus, 27.8 +/- 1.5 km; S. minutus, 33.9 +/- 2.2 km; P = 0.06) as high as in the flight muscle of the hummingbird and small bats. The size of the capillary-fiber interface (i.e., capillary surface per fiber surface ratio) per fiber mitochondrial volume in shrew diaphragm was also as high as in bird and bat flight muscles, and it was about two times greater than in rat hindlimb muscle. Thus, whereas fiber capillary and mitochondrial volume densities decreased with increased body mass in S. araneus compared with S. minutus Soricinae shrews, fiber capillarization per milliliter mitochondria in both species was much higher than previously reported for shrew diaphragm, and it matched that of the intensely aerobic flight muscles of birds and mammals.     

Sarcomere length dependence of the force-velocity relation in single frog muscle fibers.

The force-velocity relation of single frog fibers was measured at sarcomere lengths of 2.15, 2.65, and 3.15 microns. Sarcomere length was obtained on-line with a system that measures the distance between two markers attached to the surface of the fiber, approximately 800 microns apart. Maximal shortening velocity, determined by extrapolating the Hill equation, was similar at the three sarcomere lengths: 6.5, 6.0, and 5.7 microns/s at sarcomere lengths of 2.15, 2.65, and 3.15 microns, respectively. For loads not close to zero the shortening velocity decreased with increasing sarcomere length. This was the case when force was expressed as a percentage of the maximal force at optimal fiber length or as a percentage of the sarcomere-isometric force at the respective sarcomere lengths. The force-velocity relation was discontinuous around zero velocity: load clamps above the level that kept sarcomeres isometric resulted in stretch that was much slower than when the load was decreased below isometric by a similar amount. We fitted the force-velocity relation for slow shortening (less than 600 nm/s) and for slow stretch (less than 200 nm/s) with linear regression lines. At a sarcomere length of 2.15 microns the slopes of these lines was 8.6 times higher for shortening than for stretch. At 2.65 and 3.15 microns the values were 21.8 and 14.1, respectively. At a sarcomere length of 2.15 microm, the velocity of stretch abruptly increased at loads that were 160-170% of the sarcomere isometric load, i.e., the muscle yielded. However, at a sarcomere length of 2.65 and 3.15 microm yield was absent at such loads. Even the highest loads tested (260%) resulted in only slow stretch.(ABSTRACT TRUNCATED AT 250 WORDS)     

Three-dimensional reconstruction of alveoli in the rat lung for pressure-volume relationships

To determine alveolar pressure-volume relationships, alveolar three-dimensional reconstructions were prepared from lungs fixed by vascular perfusion at various points on the pressure-volume curve. Lungs from male Sprague-Dawley rats were fixed by perfusion through the pulmonary artery following a pressure-volume maneuver to the desired pressure point on either the inflation or deflation curve. Tissue samples from lungs were serially sectioned for determination of the volume fraction of alveoli and alveolar ducts and reconstruction of alveoli. Alveoli from lungs fixed at 5 cmH2O on the deflation curve (approximating functional residual volume) had a volume of 173 X 10(3) microns3, a surface area of 11,529 microns2, a mouth opening diameter of 72.7 microns, and a mean caliper diameter of 91.8 micron (SE). Alveolar shape changes during deflation from total lung capacity to residual volume was first (30 to 10 cmH2O) associated with little change in the diameter of the alveoli (102.7 +/- 2.4 to 100.3 +/- 3.3 microns). In the range overlapping normal breathing (10 to 0 cmH2O) there was a substantial decrease in diameter (100.3 +/- 3.3 to 43.3 +/- 2.3 microns). These measurements and others made on the relative changes in the dimensions of the alveolus suggest that the elastic network, particularly around the alveolar ducts, are predominant in determining lung behavior near the volume expansion limits of the lung while the elastic and surface tension properties of the alveoli are predominant in the volume range around functional residual capacity.     

Respiratory changes in diaphragmatic intramuscular pressure

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

Effect of intravenous papain on tracheal pressure-volume curves in rabbits

To investigate the effects of airway cartilage softening on tracheal mechanics, pressure-volume (PV) curves of excised tracheas were studied in 12 rabbits treated with 100 mg/kg iv papain, whereas 14 control animals received no pretreatment. The animals were killed 24 h after the injection and the excised specimens studied 24 h later. Treated tracheas exhibited decreased ability to withstand negative transmural pressures, reflected in increased collapse compliance: 6.2 +/- 2.1 vs. 2.0 +/- 0.5% peak volume (Vmax)/cmH2O means +/- SD, P less than 0.001, (Vmax = extrapolated maximal tracheal volume), increased kc (exponential constant that reflects the shape of collapse limb of the PV curve): 0.244 +/- 0.077 vs. 0.065 +/- 0.015 (P less than 0.001). The distension limb of the PV curve greater than 2.5 cmH2O transmural pressure (Ptm) was no different. Compliance between 0 and 2.5 cmH2O Ptm was increased in papain-treated rabbits: 4.97 +/- 1.73 vs. 2.30 +/- 0.31% Vmax/cmH2O (P less than 0.001). Tracheal volume, and therefore mean diameter, was decreased at 0 Ptm: 2.7 +/- 0.26 vs. 3.2 +/- 0.27 mm (P less than 0.001). We conclude that airway cartilage softening increases the compliance of the trachea at pressures less than 2.5 cmH2O Ptm.     

Changes in thick filament length in Limulus striated muscle

Here we describe the change in thick filament length in striated muscle of Limulus, the horseshoe crab. Long thick filaments (4.0 microns) are isolated from living, unstimulated Limulus striated muscle while those isolated from either electrically or K+-stimulated fibers are significantly shorter (3.1 microns) (P less than 0.001). Filaments isolated from muscle glycerinated at long sarcomere lengths are long (4.4 microns) while those isolated from muscle glycerinated at short sarcomere lengths are short (2.9 microns) and the difference is significant (P less than 0.001). Thin filaments are 2.4 microns in length. The shortening of thick filaments is related to the wide range of sarcomere lengths exhibited by Limulus telson striated muscle.     

Polarization states of diffracted light. Changes accompanying fiber activation.

Measurement of the state of optical polarization of light diffracted from single, skinned and intact fibers of anterior tibialis muscle from Rana pipiens revealed a dependence upon rigor, activation, and sarcomere length (SL) change. Changes in total birefringence, delta nT, and differential field ratio value, rT, were determined. In a relaxed, skinned fiber the total birefringence value, delta nT, decreases as sarcomere length is increased from 2.1 microns to approximately 2.8-3.0 microns. From there it increases significantly to a value of approximately 1.8 x 10(-3) at a sarcomere length of 3.6 microns. The differential field ratio, rT, also shows a biphasic response to increasing sarcomere length, first exhibiting a rapid decrease over shorter SL and leveling out after the SL is beyond 3.0 microns. In comparison, relaxed intact fibers change substantially less upon sarcomere length change, showing little change in birefringence and a small bi-phasic change in rT. Skinned fibers were activated using a solution that has the same ionic strength as the relaxing solution and allows repeatable, and sustained activation. A decrease in both delta nT and rT was observed upon fiber activation. The decrease in delta nT and rT was slightly larger at shorter sarcomere lengths than at longer lengths. Relaxed fibers placed in rigor showed changes in delta nT and rT similar to those observed in activated fibers. These results are consistent with the hypothesis that, after activation, a significant portion of the thick filament cross-bridges rotate towards the actin filament resulting in redistribution of the interfilament mass content. They are also consistent with an average orientation of crossbridges in the overlap region different from that in the nonoverlap region.     

Functional adaptation of diaphragm to chronic hyperinflation in emphysematous hamsters

Costal strips of diaphragmatic muscle obtained from animals with elastase-induced emphysema generate maximum tension at significantly shorter muscle fiber lengths than muscle strips from control animals. The present study examined the consequences of alterations in the length-tension relationship assessed in vitro on the pressure generated by the diaphragm in vivo. Transdiaphragmatic pressure (Pdi) and functional residual capacity (FRC) were measured in 22 emphysematous and 22 control hamsters 4-5 mo after intratracheal injection of pancreatic elastase or saline, respectively. In 12 emphysematous and 12 control hamsters Pdi was also measured during spontaneous contractions against an occluded airway. To allow greater control over muscle excitation, Pdi was measured during bilateral tetanic (50 Hz) electrical stimulation of the phrenic nerves in 10 emphysematous and 10 control hamsters. Mean FRC in the emphysematous hamsters was 183% of the value in control hamsters (P less than 0.01). During spontaneous inspiratory efforts against a closed airway the highest Pdi generated at FRC tended to be greater in control than emphysematous hamsters. When control hamsters were inflated to a lung volume approximating the FRC of emphysematous animals, however, peak Pdi was significantly greater in emphysematous animals (70 +/- 6 and 41 +/- 8 cmH2O; P less than 0.05). With electrophrenic stimulation, the Pdi-lung volume curve was shifted toward higher lung volumes in emphysematous hamsters. Pdi at all absolute lung volumes at and above the FRC of emphysematous hamsters was significantly greater in emphysematous compared with control animals. Moreover, Pdi continued to be generated by emphysematous hamsters at levels of lung volume where Pdi of control subjects was zero.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of body position and lung volume on in situ operating length of canine diaphragm

The performance of the diaphragm is influenced by its in situ length relative to its optimal force-generating length (Lo). Lead markers were sutured to the abdominal surface of the diaphragm along bundles of the left ventral, middle, and dorsal regions of the costal diaphragm and the left crural diaphragm of six beagle dogs. After 2-3 wk postoperative recovery, the dogs were anesthetized, paralyzed, and scanned prone and supine in the Dynamic Spatial Reconstructor (DSR) at a total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). The location of each marker was digitized from the reconstructed DSR images, and in situ lengths were determined. After an overdose of anesthetic had been administered to the dogs, each marked diaphragm bundle was removed, mounted in a 37 degrees C in vitro chamber, and adjusted to Lo (maximum tetanic force). The operating length of the diaphragm, or in situ length expressed as percent Lo, varied from region to region at the lung volumes studied; variability was least at RV and increased with increasing lung volume. At FRC, all regions of the diaphragm was shorter in the prone posture compared with the supine, but there was no clear gravity-dependent vertical gradient of in situ length in either posture. Because in vitro length-tension characteristics were similar for all diaphragm regions, regional in vivo length differences indicate that the diaphragm's potential to generate maximal force is nonuniform.     

Energetics of activation in frog skeletal muscle at sarcomere lengths beyond myofilament overlap.

Experiments were designed to gain information about the effects of extremely long sarcomere lengths (greater than 3.8 microns) on muscle activation. The amount of energy liberated in an isometric twitch by muscles stretched to sarcomere lengths where myofilament overlap is vanishingly small (greater than 3.6 microns) is thought to be an indirect measure of the Ca2+ cycled during contraction. The effects of altering sarcomere length from 3.8 to 4.3 microns on the amount of Ca2+ cycled was measured using twitch energy liberation as an indicator of the Ca2+ cycled. Twitch energy liberation decreased by approximately 20% over this sarcomere length region, suggesting that the amount of Ca2+ released by a single action potential is not altered dramatically when a muscle is stretched to extreme lengths.     

Influence of passive changes of lung volume on upper airways

The total upper airway resistances are modified during active changes in lung volume. We studied nine normal subjects to assess the influence of passive thoracopulmonary inflation and deflation on nasal and pharyngeal resistances. With the subjects lying in an iron lung, lung volumes were changed by application of an extrathoracic pressure (Pet) from 0 to 20 (+Pet) or -20 cmH2O (-Pet) in 5-cmH2O steps. Upper airway pressures were measured with two low-bias flow catheters, one at the tip of the epiglottis and the other in the posterior nasopharynx. Breath-by-breath resistance measurements were made at an inspiratory flow rate of 300 ml/s at each Pet step. Total upper airway, nasal, and pharyngeal resistances increased with +Pet [i.e., nasal resistance = 139.6 +/- 14.4% (SE) of base-line and pharyngeal resistances = 189.7 +/- 21.1% at 10 cmH2O of +Pet]. During -Pet there were no significant changes in nasal resistance, whereas pharyngeal resistance decreased significantly (pharyngeal resistance = 73.4 +/- 7.4% at -10 cmH2O). We conclude that upper airway resistance, particularly the pharyngeal resistance, is influenced by passive changes in lung volumes, especially pulmonary deflation.     

Effect of resting smooth muscle length on contractile response in resistance airways

We studied the effect of resting smooth muscle length on the contractile response of the major resistance airways (generations 0-5) in 18 mongrel dogs in vivo using tantalum bronchography. Dose-response curves to 10(-10) to 10(-7) mol/kg methacholine (MCh) were generated [at functional residual capacity (FRC)] by repeated intravenous bolus administration using tantalum bronchography after each dose. Airway constriction varied substantially with dose-equivalent stimulation and varied sequentially from trachea (8.8 +/- 2.2% change in airway diam) to fifth-generation bronchus (49.8 +/- 3.0%; P less than 0.001). Length-tension curves were generated for each airway to determine the airway diameter (i.e., resting in situ smooth muscle length) at which maximal constriction was elicited using bolus intravenous injection of 10(-8) mol/kg MCh. A Frank-Starling relationship was obtained for each airway; the transpulmonary pressure at which maximal constriction was elicited increased progressively from 2.50 +/- 1.12 cmH2O for trachea (approximately FRC) to 18.3 +/- 1.05 cmH2O for fifth-generation airways (approximately 50% TLC) (P less than 0.001). A similar relationship was obtained when change in airway diameter was plotted as a function of airway radius. We demonstrate substantial heterogeneity in the lung volumes at which maximal constriction is elicited and in distribution of parasympathomimetic constriction within the first few generations of resistance bronchi. Our data also suggest that lung hyperinflation may lead to augmented airway contractile responses by shifting resting smooth muscle length toward optimum resting smooth muscle length.     

Effects of elastase-induced emphysema on airway responsiveness to methacholine in rats

We examined the effects of elastase-induced emphysema on lung volumes, pulmonary mechanics, and airway responses to inhaled methacholine (MCh) of nine male Brown Norway rats. Measurements were made before and weekly for 4 wk after elastase in five rats. In four rats measurements were made before and at 3 wk after elastase; in these same animals the effects of changes in end-expiratory lung volume on the airway responses to MCh were evaluated before and after elastase. Airway responses were determined from peak pulmonary resistance (RL) calculated after 30-s aerosolizations of saline and doubling concentrations of MCh from 1 to 64 mg/ml. Porcine pancreatic elastase (1 IU/g) was administered intratracheally. Before elastase RL rose from 0.20 +/- 0.02 cmH2O.ml-1.s (mean +/- SE; n = 9) to 0.57 +/- 0.06 after MCh (64 mg/ml). A plateau was observed in the concentration-response curve. Static compliance and the maximum increase in RL (delta RL64) were significantly correlated (r = 0.799, P less than 0.01). Three weeks after elastase the maximal airway response to MCh was enhanced and no plateau was observed; delta RL64 was 0.78 +/- 0.07 cmH2O.ml-1.s, significantly higher than control delta RL64 (0.36 +/- 0.7, P less than 0.05). Before elastase, increase of end-expiratory lung volume to functional residual capacity + 1.56 ml (+/- 0.08 ml) significantly reduced RL at 64 mg MCh/ml from 0.62 +/- 0.05 cmH2O.ml-1.s to 0.50 +/- 0.03, P less than 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pulmonary emphysema on diaphragm capillary geometry

Poole, David C., and Odile Mathieu-Costello. Effect ofpulmonary emphysema on diaphragm capillary geometry.J. Appl. Physiol. 82(2): 599-606, 1997.In emphysema, the diaphragm shortens by losing sarcomeres. Wehypothesized that unless capillaries undergo a similar shortening,capillary geometry must be altered. Without quantifying this geometry,capillary length and surface area per fiber volume, which are criticalmeasurements of the structural potential for blood-tissue exchange,cannot be resolved. Five months after intratracheal elastase (E) orsaline (control; C) instillation, diaphragms from male Syrian goldenhamsters were glutaraldehyde perfusion fixed in situ at reference lungpositions (residual volume, functional residual capacity, total lungcapacity) to provide diaphragms fixed over a range of sarcomerelengths. Subsequently, diaphragms were processed for electronmicroscopy and analyzed morphometrically. Emphysema increased lungvolume changes from 20 to 25 cmH₂O airway pressure (i.e.,passive vital capacity) and excised lung volume (bothP < 0.001). In each region of thecostal diaphragm (i.e., ventral, medial, dorsal), sarcomere number wasreduced (all P < 0.05).Capillary-to-fiber ratio increased (C = 2.2 ± 0.1, E = 2.8 ± 0.1; P < 0.01) and fibershypertrophied (C = 815 ± 35, E = 987 ± 67 µm²;P < 0.05; both values at 2.5 µmsarcomere length). Capillary geometry was markedly altered by the lossof sarcomeres in series. Specifically, the additional capillary lengthderived from capillary tortuosity and branching was increased by 183%at 2.5 µm sarcomere length compared with C values (C, 359 ± 43;E, 1,020 ± 158 mm²,P < 0.01). This significantlyincreased total capillary length (C, 3,115 ± 173; E, 3,851 ± 219 mm² at 2.5 µm,P < 0.05) and surface area (C, 456 ± 13; E, 519 ± 24 cm¹,P < 0.05) per fiber volume. Thusemphysema substantially alters diaphragm capillary geometry andaugments the capillary length and surface area available forblood-tissue exchange.

     

Stiffness of glycerinated rabbit psoas fibers in the rigor state. Filament-overlap relation

The stiffness of glycerinated rabbit psoas fibers in the rigor state was measured at various sarcomere lengths in order to determine the distribution of the sarcomere compliance between the cross-bridge and other structures. The stiffness was determined by measuring the tension increment at one end of a fiber segment while stretching the other end of the fiber. The contribution of the end compliance to the rigor segments was checked both by laser diffractometry of the sarcomere length change and by measuring the length dependence of the Young's modulus; the contribution was found to be small. The stiffness in the rigor state was constant at sarcomere lengths of 2.4 microns or less; at greater sarcomere lengths the stiffness, when corrected for the contribution of resting stiffness, scaled with the amount of overlap between the thick and thin filaments. These results suggest that the source of the sarcomere compliance of the rigor fiber at the full overlapping of filaments is mostly the cross-bridge compliance.     

Effects of lung volume on maximal methacholine-induced bronchoconstriction in normal humans

We examined the effects of lung volume on the bronchoconstriction induced by inhaled aerosolized methacholine (MCh) in seven normal subjects. We constructed dose-response curves to MCh, using measurements of inspiratory pulmonary resistance (RL) during tidal breathing at functional residual capacity (FRC) and after a change in end-expiratory lung volume (EEV) to either FRC -0.5 liter (n = 5) or FRC +0.5 liter (n = 2). Aerosols of MCh were generated using a nebulizer with an output of 0.12 ml/min and administered for 2 min in progressively doubling concentrations from 1 to 256 mg/ml. After MCh, RL rose from a base-line value of 2.1 +/- 0.3 cmH2O. 1-1 X s (mean +/- SE; n = 7) to a maximum of 13.9 +/- 1.8. In five of the seven subjects a plateau response to MCh was obtained at FRC. There was no correlation between the concentration of MCh required to double RL and the maximum value of RL. The dose-response relationship to MCh was markedly altered by changing lung volume. The bronchoconstrictor response was enhanced at FRC - 0.5 liter; RL reached a maximum of 39.0 +/- 4.0 cmH2O X 1-1 X s. Conversely, at FRC + 0.5 liter the maximum value of RL was reduced in both subjects from 8.2 and 16.6 to 6.0 and 7.7 cmH2O X 1-1 X s, respectively. We conclude that lung volume is a major determinant of the bronchoconstrictor response to MCh in normal subjects. We suggest that changes in lung volume act to alter the forces of interdependence between airways and parenchyma that oppose airway smooth muscle contraction.     

Functional characteristics of canine costal and crural diaphragm

We estimated the in situ force-generating capacity of the costal and crural portions of the canine diaphragm by relating in vitro contractile properties and diaphragmatic dimensions to in situ lengths. Piezoelectric crystals were implanted on right costal and left crural diaphragms of anesthetized dogs, via midline laparatomy. With the abdomen reclosed, diaphragm lengths were recorded at five lung volumes. Contractile properties of excised muscle bundles were then measured. In vitro force-frequency and length-tension characteristics of the costal and crural diaphragms were virtually identical; their optimal force values were 2.15 and 2.22 kg/cm2, respectively. In situ, at residual volume, functional residual capacity (FRC), and total lung capacity the costal diaphragm lay at 102, 95, and 60% of optimal length (Lo), whereas the crural diaphragm lay at 88, 84, and 66% of Lo. Muscle cross-sectional area was 40% greater in costal than in crural diaphragms. Considering in situ lengths, cross-sectional areas, and in vitro length-tension characteristics at FRC, the costal diaphragm could exert 60% more force than the crural diaphragm.