[Eosinophilic bronchitis: update and review].

Eosinophilic bronchitis (EB) is a common cause of chronic cough, which shares similar airway eosinophilic inflammation with asthma, however, there is no airway hyperresponsiveness and airflow obstruction. The mechanism of the different phenotype between EB and asthma remains unclear. The differences in the location of airway inflammation, the level of inflammatory mediators, the imbalance of important metabolic pathways, and the degree of airway remodeling may result in different pathogenesis between EB and asthma. EB response well to inhaled corticosteroids but recurrence of EB is still high after treatment. The longer duration of treatment with inhaled corticosteroids could decrease the relapse rate. On the prognosis of EB, a long-term follow-up study suggested that EB should be a distinct entity rather than an early stage of asthma or chronic obstructive pulmonary disease.

Synaptic vesicle pools at diaphragm neuromuscular junctions vary with motoneuron soma, not axon terminal, inactivity.

Both spinal hemisection (SH) at C2 and tetrodotoxin (TTX) phrenic nerve blockade result in diaphragm muscle paralysis and inactivity of the phrenic axon terminals. However, phrenic motoneuron somata are inactive with SH but remain active with TTX phrenic nerve blockade. Neuromuscular transmission failure with repeated activation decreases following SH and increases following TTX phrenic nerve blockade, suggesting that matching (or mismatching) of somal and synaptic inactivities of phrenic motoneurons differentially regulates synaptic vesicle pools at diaphragm neuromuscular junctions. At individual type-identified rat diaphragm presynaptic terminals, the size of the releasable pool of synaptic vesicles was analyzed by fluorescence confocal microscopy of N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl) pyridinium dibromide (FM4-64) uptake and synaptic vesicle density at active zones was determined using transmission electron microscopy. After 14 days of SH and TTX-induced diaphragm muscle inactivity, neuromuscular junction size was not different at type I or IIa fibers, but increased at type IIx and/or IIb fibers (by 51% in SH and 35% in TTX) compared with control. With SH, synaptic vesicle pool size and density increased at presynaptic terminals innervating type I or IIa fibers (17 and 63%, respectively; P<0.001) and type IIx and/or IIb fibers (41 and 31%, respectively; P<0.001) when compared with controls. Following TTX, synaptic vesicle pool size and density decreased by 64 and 17%, respectively, at presynaptic terminals innervating type I or IIa fibers, and by 50 and 36%, respectively, at type IIx and/or IIb fibers (P<0.001, for all comparisons). Thus, matching motoneuron soma and axon terminal inactivity (SH) increases the size and density of releasable synaptic vesicle pools at adult rat diaphragm neuromuscular junctions. Mismatching motoneuron soma and axon terminal inactivities (TTX) results in converse presynaptic adaptations. Inactivity-induced neuromuscular plasticity reflects specific adaptations in the size and density of synaptic vesicle pools that depend on motoneuron soma rather than axon terminal (or muscle fiber) inactivity.

Subnuclear organization of the lateral tegmental field of the rat. II: Catecholamine neurons and ventral respiratory group.

Bulbospinal and propriobulbar respiratory neurons of the ventral respiratory group and catecholamine neurons of the A1 and C1 cell groups were simultaneously labelled in the rat medulla by a combination of retrograde tracing and immunohistochemical identification. The ventral respiratory group and catecholamine cell groups form adjacent, parallel cell columns in the lateral tegmental field of the ventrolateral medulla. The ventral respiratory group is located immediately dorsal to the A1 and C1 groups, although some A1 neurons are intermingled with neurons of the rostral ventral respiratory group, and some C1 neurons are intermingled with those of the Bötzinger complex. The proximate populations of respiratory, catecholamine, and (presumptive) cardiovascular neurons identified in this study provide further support to the hypothesis that this region of the lateral tegmental field of the ventrolateral medulla is a site of cardiorespiratory coordination.

Monoaminergic and GABAergic terminations in phrenic nucleus of rat identified by immunohistochemical labeling.

The termination patterns of axons in the phrenic nucleus immunoreactive to synthetic enzymes for catecholamines and for serotonin and GABA were studied in rats. Spinal cord tissue in which phrenic motoneurons were retrogradely labeled with horseradish peroxidase was incubated with antisera against dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, 5-hydroxytryptamine, and GABA to identify presumptive terminations of monoaminergic and GABAergic neurons onto identified phrenic motoneurons. In the C3 to C5 spinal cord, 5-hydroxytryptamine-, dopamine beta-hydroxylase- and GABA-like positive terminals with varicosities formed a dense network, with presumptive synaptic contacts on dendrites and somas of phrenic motoneurons. A similar pattern of terminations was also observed in adjacent (non-respiratory muscle) motoneuron pools. There were fewer phenylethanolamine-N-methyltransferase-positive terminal arborizations in the cervical spinal cord compared to thoracic spinal cord; phenylethanolamine-N-methyltransferase terminals were not seen in the vicinity of phrenic motoneurons. These results suggest that phrenic motoneuronal activity is influenced by multiple supraspinal inputs utilizing different neurotransmitters. These transmitters also mediate inputs to other (nearby) spinal motoneurons and thus are not unique for signal transmission to phrenic motoneurons.

Adaptations of diaphragm and medial gastrocnemius muscles to inactivity.

The effects of 2 wk of inactivity on in vitro contractile properties of diaphragm and medial gastrocnemius (MG) muscles were examined in adult hamsters. In addition, inactivity effects on fiber-type proportions and cross-sectional areas were studied. Inactivity of the right hemidiaphragm or MG muscle was induced by either tetrodotoxin (TTX) blockade of nerve impulses or denervation (DNV). Inactivity effects on diaphragm or MG were compared with corresponding sham (saline-treated or untreated control) muscles. After both TTX- and DNV-induced inactivity, isometric twitch contraction and half-relaxation times were prolonged, maximum tetanic force decreased, and fatigue resistance improved. Proportions of type I and II fibers in both diaphragm and MG were unaffected by TTX- and DNV-induced inactivity. However, in both muscles, type I fibers hypertrophied, whereas type II fibers atrophied. In diaphragm, contractile and morphometric adaptations after DNV were generally more pronounced than those induced by TTX. In addition, compared with corresponding untreated or saline-treated control groups, inactivity effects (both TTX and DNV) on MG were generally greater than those induced in diaphragm, with the exception of hypertrophy of type I fibers. We conclude that inactivity exerts differential effects on type I and II fibers in both diaphragm and MG. Yet, these morphometric adaptations cannot completely account for the adaptations in muscle contractile and fatigue properties after inactivity.

Denervation alters myosin heavy chain expression and contractility of developing rat diaphragm muscle.

We hypothesized that unilateral denervation (DNV) of the rat diaphragm muscle (Dia(m)) in neonates at postnatal day 7 (D-7) alters normal transitions of myosin heavy chain (MHC) isoform expression and thereby affects postnatal changes in maximum specific force (P(o)) and maximum unloaded shortening velocity (V(o)). The relative expression of different MHC isoforms was analyzed electrophoretically. With DNV at D-7, expression of MHC(neo) in the Dia(m) persisted, and emergence of MHC(2X) and MHC(2B) was delayed. By D-21 and D-28, relative expression of MHC(2A) and MHC(2B) was reduced in DNV compared with control (CTL) animals. Expression of MHC(neo) also reappeared in adult Dia(m) by 2-3 wk after DNV, and relative expression of MHC(2B) was reduced. At each age, P(o) was reduced and V(o) was slowed by DNV, compared with CTL. In CTL Dia(m), postnatal changes in P(o) and V(o) were associated with an increase in fast MHC isoform expression. In DNV Dia(m), no such association existed. We conclude that, in the Dia(m), DNV induces alterations in both MHC isoform expression and contractile properties, which are not necessarily causally linked.

Diaphragm disuse reduces Ca2+ uptake capacity of sarcoplasmic reticulum.

Chronic phrenic tetrodotoxin (TTX) blockade and phrenic denervation (Dnv) of hamster diaphragm result in decreased maximum specific tension, prolonged contraction time, and improved fatigue resistance (W. Z. Zhan and G. C. Sieck, J. Appl. Physiol. 72: 1445-1453, 1992). An underlying increased relative contribution of type I fibers to total muscle mass appears to be consistent with, but does not completely account for, changes in contractile and fatigue properties. The present study was designed to evaluate a potential role for altered cellular Ca2+ metabolism in the adaptive response of the diaphragm to chronic disuse. An analytic method based on simulation and modeling of long-term 45Ca2+ efflux data was used to estimate Ca2+ contents (nmol Ca2+/g wet wt tissue) and exchange fluxes (nmol Ca2+.min-1.g-1) 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), and phrenic denervation (Dnv, n = 5). Experimental muscles were loaded with 45Ca2+ for 1 h, and efflux data were collected for 8 h by using a flow-through tissue chamber. Compartmental analysis of efflux data estimated that the Ca2+ contents and Ca2+ exchange fluxes of the largest and slowest intracellular compartment (putative longitudinal reticulum) were reduced by approximately 50% in TTX and Dnv muscle groups compared with Con. In addition, the kinetic model predicted significant decreases in total intracellular Ca2+ and total diaphragm Ca2+ in TTX and Dnv muscles. We conclude that the data support the hypothesis that the capacity of the sarcoplasmic reticulum for Ca2+ sequestration is reduced in chronic diaphragm disuse. The impact of this effect on diaphragm contractile and fatigue properties is discussed.

Adaptations of the diaphragm in emphysema.

In adult male hamsters the influence of emphysema (EMP) on the in vitro contractile and fatigue properties and the histochemical, morphometric, and metabolic properties of muscle fibers in the costal diaphragm was determined 6 mo after the administration of either elastase or saline (controls, CTL). Isometric contractile properties were determined in vitro using supramaximal direct muscle stimulation. Optimal fiber length for force generation was significantly shorter in the EMP than in the CTL diaphragm. Maximum specific force (i.e., force per unit area) was 25% lower than CTL. Fatigue resistance was significantly improved in the EMP diaphragm compared with CTL. Diaphragm muscle fibers were classified as type I or II on the basis of histochemical staining for myofibrillar adenosinetriphosphatase after alkaline preincubation. The proportions of type I and II fibers were similar between the two groups. Cross-sectional areas of type II fibers were 30% larger in EMP than in CTL diaphragms. Succinate dehydrogenase activities of both type I and II fibers were higher in EMP than in CTL diaphragms. The number of capillaries surrounding both type I and II fibers increased with EMP, but in proportion to the hypertrophy of these fibers. Thus, capillary density (number of capillaries per fiber cross-sectional area) remained unchanged. We postulate that these contractile, morphometric, and metabolic adaptations reflect an increased activation of the diaphragm in response to the loads imposed by EMP.

Metabolic and phenotypic adaptations of diaphragm muscle fibers with inactivation.

We hypothesized that metabolic adaptations to muscle inactivity are most pronounced when neurotrophic influence is disrupted. In rat diaphragm muscle (Dia(m)), 2 wk of unilateral denervation or tetrodotoxin nerve blockade resulted in a reduction in succinate dehydrogenase (SDH) activity of type I, IIa, and IIx fibers (approximately 50, 70, and 24%, respectively) and a decrease in SDH variability among fibers (approximately 63%). In contrast, inactivity induced by spinal cord hemisection at C2 (ST) resulted in much less change in SDH activity of type I and IIa fibers (approximately 27 and 24%, respectively) and only an approximately 30% reduction in SDH variability among fibers. Actomyosin adenosinetriphosphatase (ATPase) activities of type I, IIx, and IIb fibers in denervated and tetrodotoxin-treated Dia(m) were reduced by approximately 20, 45, and 60%, respectively, and actomyosin ATPase variability among fibers was approximately 60% lower. In contrast, only actomyosin ATPase activity of type IIb fibers was reduced (approximately 20%) in ST Dia(m). These results suggest that disruption of neurotrophic influence has a greater impact on muscle fiber metabolic properties than inactivity per se.

Myoneural interactions affect diaphragm muscle adaptations to inactivity.

We hypothesized that inactivity effects on diaphragm muscle contractile and morphometric properties are attenuated if phrenic motoneurons are also inactive. Three models of rat diaphragm inactivity were compared: 1) spinal isolation; 2) tetrodotoxin (TTX) nerve blockade; and 3) denervation (Dnv). Motoneuron and muscle fiber inactivities were matched only in spinal isolated animals. After 2 wk, maximum tetanic force decreased in all three groups compared with control group but to a greater extent in TTX and Dnv animals. Fatigue resistance improved, and maximum unloaded shortening velocity slowed only in TTX and Dnv groups. Type IIa fiber proportions decreased in all three groups, and type IIx fiber proportions increased in TTX and Dnv animals. Type I fiber cross-sectional area increased in all three groups but to a greater extent in TTX and Dnv animals. Type IIa fibers hypertrophied, whereas type IIx and IIb fibers atrophied only in TTX and Dnv groups. These results support the hypothesis that muscle adaptations to prolonged inactivity are attenuated when muscle fiber and motoneuron inactivities are matched.

Hypothyroid-mediated changes in adult rat diaphragm muscle contractile properties and MHC isoform expression.

The purpose of the present study was to examine the effect of acute hypothyroidism on myosin heavy chain (MHC) isoform composition and contractile properties in the adult rat diaphragm muscle. Hypothyroidism was induced by the addition of propylthiouracil (0.05%) in the drinking water for a period of 3 wk. MHC isoform composition of control and hypothyroid diaphragm muscles was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In vitro isometric contractile properties of midcostal diaphragm muscle segements were measured at 26 degrees C, whereas the maximal unloaded shortening velocity was measured at 15 degrees C with the "slack test" method. Serum triiodothyronine and thyroxine values were significantly lower in the hypothyroid compared with the control group. A small but significant increase in the percentage of slow MHC isoform in the diaphragm was observed with acute hypothyroidism, whereas the percentage of the fast MHC isoforms (2A, 2X, and 2B) did not significantly differ between groups. Peak twitch force did not differ between groups. However, twitch contraction and half-relaxation times were significantly prolonged in the hypothyroid group compared with control. Maximal specific force was reduced in the hypothyroid compared with the control group, averaging 15.7 and 19.8 N/cm2, respectively (P < 0.05). The maximal unloaded shortening velocity averaged 4.3 and 8.2 muscle lengths/s in the hypothyroid and control groups, respectively (P < 0.05). We conclude that acute hypothyroidism results in alterations in adult diaphragm muscle contractile properties that cannot be attributed solely to changes in MHC isoform composition.

Phrenic motoneuron morphology during rapid diaphragm muscle growth.

In the adult rat, there is a general correspondence between the sizes of motoneurons, motor units, and muscle fibers that has particular functional importance in motor control. During early postnatal development, after the establishment of singular innervation, there is rapid growth of diaphragm muscle (Dia(m)) fibers. In the present study, the association between Dia(m) fiber growth and changes in phrenic motoneuron size (both somal and dendritic) was evaluated from postnatal day 21 (D21) to adulthood. Phrenic motoneurons were retrogradely labeled with fluorescent tetramethylrhodamine dextran (3,000 MW), and motoneuron somal volumes and surface areas were measured using three-dimensional confocal microscopy. In separate animals, phrenic motoneurons retrogradely labeled with choleratoxin B-fragment were visualized using immunocytochemistry, and dendritic arborization was analyzed by camera lucida. Between D21 and adulthood, Dia(m) fiber cross-sectional area increased by approximately 164% overall, with the growth of type II fibers being disproportionate to that of type I fibers. There was also substantial growth of phrenic motoneurons ( approximately 360% increase in total surface area), during this same period, that was primarily attributable to an expansion of dendritic surface area. Comparison of the distribution of phrenic motoneuron surface areas between D21 and adults suggests the establishment of a bimodal distribution that may have functional significance for motor unit recruitment in the adult rat.

Power fatigue of the rat diaphragm muscle.

We hypothesized that decrements in maximum power output (W(max)) of the rat diaphragm (Dia) muscle with repetitive activation are due to a disproportionate reduction in force (force fatigue) compared with a slowing of shortening velocity (velocity fatigue). Segments of midcostal Dia muscle were mounted in vitro (26 degrees C) and stimulated directly at 75 Hz in 400-ms-duration trains repeated each second (duty cycle = 0.4) for 120 s. A novel technique was used to monitor instantaneous reductions in maximum specific force (P(o)) and W(max) during fatigue. During each stimulus train, activation was isometric for the initial 360 ms during which P(o) was measured; the muscle was then allowed to shorten at a constant velocity (30% V(max)) for the final 40 ms, and W(max) was determined. Compared with initial values, after 120 s of repetitive activation, P(o) and W(max) decreased by 75 and 73%, respectively. Maximum shortening velocity was measured in two ways: by extrapolation of the force-velocity relationship (V(max)) and using the slack test [maximum unloaded shortening velocity (V(o))]. After 120 s of repetitive activation, V(max) slowed by 44%, whereas V(o) slowed by 22%. Thus the decrease in W(max) with repetitive activation was dominated by force fatigue, with velocity fatigue playing a secondary role. On the basis of a greater slowing of V(max) vs. V(o), we also conclude that force and power fatigue cannot be attributed simply to the total inactivation of the most fatigable fiber types.

Interactive effects of emphysema and malnutrition on diaphragm structure and function.

Interactive effects of emphysema (EMP) and prolonged nutritional deprivation (ND) on contractile, morphometric, and metabolic properties of hamster diaphragm muscle (DIA) were examined. Six months after induction of EMP (intratracheal elastase), saline-treated controls (CTL) and EMP hamsters of similar body weights were subjected to ND over 6 wk. Isometric contractile and fatigue properties of costal DIA were determined in vitro. DIA fibers were histochemically classified as type I or II, and fiber succinate dehydrogenase activity and cross-sectional area were determined using quantitative microscopic procedures. From histochemical sections, the number of capillaries per fiber (C/F) and per fiber cross-sectional area (C/A) were determined. ND resulted in progressive loss of body weight (ND-CTL, 23.8%; ND-EMP, 28.4%; P = NS). ND did not affect reduction in optimal length (Lo) of DIA fibers in EMP compared with CTL and ND-CTL hamsters. Maximum specific force (i.e., force/unit area) was reduced by approximately 25% in EMP animals compared with CTL. ND did not improve or exacerbate the reduction in specific force with EMP. ND attenuated improved fatigue resistance of DIA in EMP animals. No differences in fiber type proportions were noted among experimental groups. Significant atrophy of type I and II DIA fibers was noted after ND. Atrophy was proportionately greater in type II fibers of ND-EMP when referenced to EMP animals. Thus adaptive hypertrophy of type II DIA fibers in EMP animals was abolished. Fiber succinate dehydrogenase activity was significantly increased in type I and II fibers in EMP DIA. ND did not affect this metabolic adaptation of DIA fibers to persistent loads imposed by EMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Succinate dehydrogenase activity of sexually dimorphic muscles of rats.

Muscles of the male rat perineum attaching to the penis are the ischiocavernosus, the medial bulbocavernosus (BC), and the dorsal BC, also known as the levator ani (LA). The BC and the LA muscles are innervated by a morphologically, neurochemically, and physiologically homogeneous pool of motoneurons, the spinal nucleus of BC (SNB). The purpose of the present study was to determine whether BC and LA muscle fibers constitute histochemically and biochemically homogeneous populations, reflecting the homogeneity of the innervating motoneuron pool. Histochemical fiber type was based on the pH lability of myofibrillar adenosinetriphosphatase. Activity of the mitochondrial enzyme succinate dehydrogenase (SDH) was determined by using a quantitative histochemical procedure. A nonsexually dimorphic, androgen-insensitive muscle, the medial gastrocnemius (MG), was used as control. The superficial regions of the MG, BC, and LA muscles were composed exclusively of type IIb muscle fibers, whereas the fiber type composition of the deep region of the MG was mixed: 28.3% type I, 20.6% type IIa, 40.1% type IIx, and 11.0% type IIb. The SDH activities of type IIb fibers in the deep region of the MG ranged from 1.20 to 9.00 (mean 3.72 +/- 0.40) mmol fumarate.liter tissue-1.min-1. Fiber SDH activities in the superficial region of the MG ranged from 0.04 to 2.70 (mean 1.20 +/- 0.21) mmol fumarate.liter tissue-1.min-1. In the BC muscle, the SDH activities of the type IIb fibers ranged from undetectable to 1.80 with a mean of 0.62 +/- 0.05 mmol fumarate.liter tissue-1.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Congestive heart failure: differential adaptation of the diaphragm and latissimus dorsi.

Diaphragm and latissimus dorsi muscle functions, histochemistries, and morphometries were studied in anesthetized male Yucatan minipigs with congestive heart failure (CHF) induced by supraventricular tachycardia (n = 5). Sham-operated animals served as a control group (n = 5). In CHF animals, transdiaphragmatic pressure measured during supramaximal phrenic stimulation was reduced by 40% at low frequencies (< or = 20 Hz) and by 60% at higher frequencies. Twitch amplitude and half-relaxation time were also decreased. The cross-sectional areas of type I, IIa, and IIb fibers were reduced in the diaphragm. The proportion of type I fibers increased, whereas type IIa fibers decreased. Succinate dehydrogenase activity was elevated in type IIa and IIb fibers, but diaphragmatic fatigability was not altered. CHF reduced latissimus dorsi isometric force by 40% for stimulation frequencies > or = 30 Hz. The cross-sectional area of latissimus dorsi type IIb fibers was decreased, but twitch characteristics, fiber type composition, succinate dehydrogenase activity, and fatigability were unchanged. Experimental CHF appears to cause greater intrinsic adaptive changes in the diaphragm compared with those in the latissimus dorsi in the minipig. For both muscles, reduced contractile function was associated with atrophy. Impaired performance of the diaphragm may also be attributed to an increase in the relative contribution of type I fibers to the total tension-generating capacity of the muscle and to the pathophysiological mechanisms underlying the shortened relaxation time of the twitch response.

Regional adaptations of rabbit diaphragm muscle fibers to unilateral denervation.

We hypothesized that adaptations of the rabbit diaphragm (Dia) after unilateral denervation (DNV) result from removal of a neural influence rather than from passive stress. Length changes of midcostal and sternal Dia regions were measured before and after DNV by using sonomicrometry. Midcostal fibers passively lengthened after DNV, whereas sternal fibers shortened. In both regions, these length changes were associated with minimal stress, as estimated from passive force-length relationships. Morphological and contractile adaptions of midcostal and sternal Dia regions were examined after 1 and 4 wk of DNV. In both Dia regions, type I fibers progressively hypertrophied, whereas type IIb fibers atrophied. After DNV, changes in isometric contraction were similar in both Dia regions. Twitch contraction and half-relaxation times increased, force-frequency relationships shifted leftward, and maximum tetanic force decreased. We conclude that passive length changes and mechanical stress are not the main determinants of the morphological and contractile adaptations of the Dia after unilateral DNV but that these adaptations result from DNV itself.

SDH and actomyosin ATPase activities of different fiber types in rat diaphragm muscle.

In the rat diaphragm muscle, the histochemical classification of type I, IIa, IIb, or IIx fibers was correlated with myosin heavy chain (MHC) immunoreactivity. Expression of MHC isoforms in single dissected fibers was also assessed electrophoretically. Most fibers (approximately 86%) expressed a single MHC isoform, and when present, coexpression of MHC-2X and MHC-2B isoforms was most prevalent. Type I and IIa fibers were the smallest, type IIb fibers were the largest, and type IIx fibers were intermediate. Succinate dehydrogenase (SDH) and calcium-activated myosin adenosinetriphosphatase (actomyosin ATPase) activities were measured with quantitative histochemical procedures. Type I and IIa fibers had the highest SDH activities, followed in rank order by type IIx and IIb fibers. Type I fibers had the lowest actomyosin ATPase activity, followed in rank order by type IIa, IIx, and IIb fibers. Across all fibers, there was an inverse relationship between fiber SDH activity and cross-sectional area and a positive correlation between fiber actomyosin ATPase activity and cross-sectional area. The SDH and actomyosin ATPase activities of muscle fibers were also inversely correlated. These phenotypic differences in SDH and ATPase activities may be important in determining the contractile and fatigue properties of different fiber types in the rat diaphragm muscle.

Hypothyroidism alters diaphragm muscle development.

The impact of hypothyroidism (Hyp) on myosin heavy chain (MHC) isoform expression, maximum specific force (P0), fatigability, and maximum unloaded shortening velocity (V0) was determined in the rat diaphragm muscle (Dia) at 0, 7, 14, 21, and 28 days of age. Hyp was induced by treating pregnant rats with 6-n-propyl-2-thiouracil (0.05% in drinking water) beginning at gestational day 10 and was confirmed by reduced plasma levels of 3,5,3'-triiodothyronine and thyroxine. MHC isoforms were separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and analyzed by densitometry. Isometric P0 and fatigue resistance of the Dia were measured in vitro at 26 degrees C, and V0 was determined at 15 degrees C with the slack test. Compared with control muscles, expression of MHC-slow was higher and expression of adult fast MHC isoforms was lower in Hyp Dia at all ages. The neonatal isoform of MHC continued to be expressed in the Hyp Dia until day 28. At each age, P0 and fatigability were reduced and V0 was slower in the Hyp Dia. We conclude that Hyp-induced alterations in MHC isoform expression do not fully predict the changes in Dia contractile properties.

Interactive effects of denervation and malnutrition on diaphragm structure and function.

The purpose of this study was to examine the interactive effects of unilateral denervation (DN) and prolonged malnutrition (MN) on the structure and function of the diaphragm muscle (Dia). Four groups of rats were studied: control (Con), MN, DN, and DN-MN. MN began 2 wk after DN and lasted 4 wk. In both the DN and DN-MN groups, the relative loss in Dia weight exceeded the relative change in body weight. Compared with the Con group, Dia specific force was reduced by approximately 40% in both the DN and DN-MN groups but was unaffected in the MN group. Dia fatigue resistance improved in all experimental groups but to a greater extent in the DN and DN-MN groups. In both the DN and DN-MN groups, approximately 50% of Dia fibers were classified as type IIc, whereas fiber type proportions did not change in the MN group. In the DN group, only type IIb/x fibers atrophied, whereas all fiber types atrophied in the MN and DN-MN groups. We conclude that in the DN-MN group the reduction in specific force combined with the reduction in total cross-sectional area of the muscle significantly curtails Dia force-generating capacity.