Single soleus muscle fiber function after hindlimb unweighting in adult and aged rats

This investigation compared how hindlimbunweighting (HU) affected the contractile function of single soleusmuscle fibers from 12- and 30-mo-old Fischer 344 Brown Norway F1 Hybridrats. After 1 wk of HU, functional properties of singlepermeabilized fibers were studied, and, subsequently, the fiber typewas established by myosin heavy chain (MHC) analysis. After HU, therelative mass of soleus declined by 12 and 19% and the relative massof the gastrocnemius declined by 15 and 13% in 12- and 30-mo-oldanimals, respectively. In 12-mo-old animals, the peak active force(5.0 ± 0.2 ×10⁴ vs. 3.8 ± 0.2 ×10⁴ N) and thepeak specific tension (92 ± 4 vs. 78 ± 3 kN/m²) were significantlyreduced in the MHC type I fibers by 24 and 15%, respectively. In30-mo-old animals, the peak active force declined by 40% (4.7 ± 0.2 ×10⁴vs. 2.8 ± 0. 3 ×10⁴ N) and the peakspecific tension declined by 30% (79 ± 5 vs. 55 ± 4 kN/m²). The maximal unloadedshortening velocity of the MHC type I fibers increased in 12-mo-oldanimals (from 1.65 ± 0.12 to 2.59 ± 0.26 fiber lengths/s) andin 30-mo-old animals (from 0.90 ± 0.09 to 1.50 ± 0.10 fiberlengths/s) after HU. Collectively, these data suggest thatthe effects of HU on single soleus skeletal muscle fiber function occurin both age groups; however, the single MHC type I fibers from theolder animals show greater changes than do single MHC type I fibersfrom younger animals.

     

Contractile function of single muscle fibers after hindlimb unweighting in aged rats

Thompson, L. V., and J. A. Shoeman. Contractilefunction of single muscle fibers after hindlimb unweighting in aged rats. J. Appl. Physiol. 84(1):229-235, 1998.This investigation determined how muscle atrophyproduced by hindlimb unweighting (HU) alters the contractile functionof single muscle fibers from older animals (30 mo). After 1 wk of HU,small bundles of fibers were isolated from the soleus muscles and thedeep region of the lateral head of the gastrocnemius muscles. Singleglycerinated fibers were suspended between a motor lever and forcetransducer, functional properties were studied, and the myosin heavychain (MHC) composition was determined electrophoretically. After HU, the diameter of type I MHC fibers of the soleus declined (88 ± 2 vs. 80 ± 4 µm) and reductions were observed in peak active force (47 ± 3 vs. 28 ± 3 mg) and peak specific tension(P_o; 80 ± 5 vs. 56 ± 5 kN/m²). The maximal unloadedshortening velocity increased. The type I MHC fibers from thegastrocnemius showed reductions in diameter (14%), peak active force(41%), and P_o (24%), whereas thetype IIa MHC fibers showed reductions in peak active force andP_o. Thus 1 wk ofinactivity has a significant effect on the force-generating capacity ofsingle skeletal muscle fibers from older animals in a fibertype-specific manner (type I MHC > type IIa MHC > type I-IIa MHC).The decline in the functional properties of single skeletal musclefibers in the older animals appears to be more pronounced than what hasbeen reported in younger animal populations.

     

Effect of clenbuterol on sarcoplasmic reticulum function in single skinned mammalian skeletal muscle fibers

We examined the effect of the₂-agonist clenbuterol (50 µM)on depolarization-induced force responses and sarcoplasmic reticulum (SR) function in muscle fibers of the rat (Rattusnorvegicus; killed by halothane overdose) that had beenmechanically skinned, rendering the₂-agonist pathway inoperable.Clenbuterol decreased the peak of depolarization-induced forceresponses in the extensor digitorum longus (EDL) and soleus fibers to77.2 ± 9.0 and 55.6 ± 5.4%, respectively, ofcontrols. The soleus fibers did not recover. Clenbuterol significantlyand reversibly reduced SR Ca²⁺loading in EDL and soleus fibers to 81.5 ± 2.8 and 78.7 ± 4.0%, respectively, of controls. Clenbuterol also producedan ~25% increase in passive leak ofCa²⁺ from the SR of the EDL andsoleus fibers. These results indicate that clenbuterol has directeffects on fast- and slow-twitch skeletal muscle, in the absence of the₂-agonist pathway. Theincreased Ca²⁺ leak in the triadregion may lead to excitation-contraction coupling damage in the soleusfibers and could also contribute to the anabolic effect of clenbuterolin vivo.

     

Velocity, force, power, and Ca2+ sensitivity of fast and slow monkey skeletal muscle fibers

In this study,we determined the contractile properties of single chemically skinnedfibers prepared from the medial gastrocnemius (MG) and soleus (Sol)muscles of adult male rhesus monkeys and assessed the effects of thespaceflight living facility known as the experiment support primatefacility (ESOP). Muscle biopsies were obtained 4 wk before andimmediately after an 18-day ESOP sit, and fiber type was determined byimmunohistochemical techniques. The MG slow type I fiber wassignificantly smaller than the MG type II, Sol type I, and Sol type IIfibers. The ESOP sit caused a significant reduction in the diameter oftype I and type I/II (hybrid) fibers of Sol and MG type II and hybridfibers but no shift in fiber type distribution. Single-fiber peak force(mN and kN/m²) was similarbetween fiber types and was not significantly different from valuespreviously reported for other species. The ESOP sit significantlyreduced the force (mN) of Sol type I and MG type II fibers. Thisdecline was entirely explained by the atrophy of these fiber typesbecause the force per cross-sectional area (kN/m²) was not altered. Peakpower of Sol and MG fast type II fiber was 5 and 8.5 times that of slowtype I fiber, respectively. The ESOP sit reduced peak power by 25 and18% in Sol type I and MG type II fibers, respectively, and, for theformer fiber type, shifted the force-pCa relationship to the right,increasing the Ca²⁺ activationthreshold and the free Ca²⁺concentration, eliciting half-maximal activation. The ESOP sit had noeffect on the maximal shortening velocity(V_o) of anyfiber type. V_o ofthe hybrid fibers was only slightly higher than that of slow type Ifibers. This result supports the hypothesis that in hybrid fibers theslow myosin heavy chain would be expected to have a disproportionatelygreater influence onV_o.

     

Cytoplasm-to-myonucleus ratios and succinate dehydrogenase activities in adult rat slow and fast muscle fibers

The relationship between myonuclear number, cellular size, succinate dehydrogenase activity, and myosin type was examined in single fiber segments (n = 54; 9 ± 3 mm long) mechanically dissected from soleus and plantaris muscles of adult rats. One end of each fiber segment was stained for DNA before quantitative photometric analysis of succinate dehydrogenase activity; the other end was double immunolabelled with fast and slow myosin heavy chain monoclonal antibodies. Mean ± S.D. cytoplasmic volume/myonucleus ratio was higher in fast and slow plantaris fibers (112 ± 69 vs. 34 ± 21 x 10 ³µm ³) than fast and slow soleus fibers (40 ± 20 vs. 30 ± 14 x 10 ³µm ³), respectively. Slow fibers always had small volumes/myonucleus, regardless of fiber diameter, succinate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 µm) fast soleus and plantaris fibers with high succinate dehydrogenase activity appeared to have low volumes/myonucleus while larger diameter (>70 µm) fast fibers with low succinate dehydrogenase activity always had large volume/myonucleus. Slow soleus fibers had significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116 ± 51 vs. 55 ± 22 and 44 ± 23), respectively. These data suggest that the myonuclear domain is more limited in slow than fast fibers and in the fibers with a high, compared to a low, oxidative metabolic capability.     

Maintenance of myonuclear domain size in rat soleus after overload and growth hormone/IGF-I treatment

The purpose ofthis study was to determine the effects of functional overload (FO)combined with growth hormone/insulin-like growth factor I (GH/IGF-I)administration on myonuclear number and domain size in rat soleusmuscle fibers. Adult female rats underwent bilateral ablation of theplantaris and gastrocnemius muscles and, after 7 days of recovery, wereinjected three times daily for 14 days with GH/IGF-I (1 mg/kg each; FO + GH/IGF-I group) or saline vehicle (FO group). Intact rats receivingsaline vehicle served as controls (Con group). Muscle wet weight was32% greater in the FO than in the Con group: 162 ± 8 vs. 123 ± 16 mg. Muscle weight in the FO + GH/IGF-I group (196 ± 14 mg) was59 and 21% larger than in the Con and FO groups, respectively. Meansoleus fiber cross-sectional area of the FO + GH/IGF-I group (2,826 ± 445 µm²) was increasedcompared with the Con (2,044 ± 108 µm²) and FO (2,267 ± 301 µm²) groups. The difference infiber size between the FO and Con groups was not significant. Meanmyonuclear number increased in FO (187 ± 15 myonuclei/mm) and FO + GH/IGF-I (217 ± 23 myonuclei/mm) rats compared with Con (155 ± 12 myonuclei/mm) rats, although the difference between FO and FO + GH/IGF-I animals was not significant. The mean cytoplasmic volume permyonucleus (myonuclear domain) was similar across groups. These resultsdemonstrate that the larger mean muscle weight and fibercross-sectional area occurred when FO was combined with GH/IGF-Iadministration and that myonuclear number increased concomitantly withfiber volume. Thus there appears to be some mechanism(s) that maintainsthe myonuclear domain when a fiber hypertrophies.

     

Fiber-type susceptibility to eccentric contraction-induced damage of hindlimb-unloaded rat AL muscles.

Slow oxidative (SO) fibers of the adductor longus (AL) were predominantly damaged during voluntary reloading of hindlimb unloaded (HU) rats and appeared explainable by preferential SO fiber recruitment. The present study assessed damage after eliminating the variable of voluntary recruitment by tetanically activating all fibers in situ through the motor nerve while applying eccentric (lengthening) or isometric contractions. Muscles were aldehyde fixed and resin embedded, and semithin sections were cut. Sarcomere lesions were quantified in toluidine blue-stained sections. Fibers were typed in serial sections immunostained with antifast myosin and antitotal myosin (which highlights slow fibers). Both isometric and eccentric paradigms caused fatigue. Lesions occurred only in eccentrically contracted control and HU muscles. Fatigue did not cause lesions. HU increased damage because lesioned- fiber percentages within fiber types and lesion sizes were greater than control. Fast oxidative glycolytic (FOG) fibers were predominantly damaged. In no case did damaged SO fibers predominate. Thus, when FOG, SO, and hybrid fibers are actively lengthened in chronically unloaded muscle, FOG fibers are intrinsically more susceptible to damage than SO fibers. Damaged hybrid-fiber proportions ranged between these extremes.     

Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training

McCall, G. E., W. C. Byrnes, A. Dickinson, P. M. Pattany,and S. J. Fleck. Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training.J. Appl. Physiol. 81(5):2004-2012, 1996.Twelve male subjects with recreationalresistance training backgrounds completed 12 wk of intensifiedresistance training (3 sessions/wk; 8 exercises/session; 3 sets/exercise; 10 repetitions maximum/set). All major muscle groupswere trained, with four exercises emphasizing the forearm flexors.After training, strength (1-repetition maximum preacher curl) increasedby 25% (P < 0.05). Magneticresonance imaging scans revealed an increase in the biceps brachiimuscle cross-sectional area (CSA) (from 11.8 ± 2.7 to 13.3 ± 2.6 cm²;n = 8;P < 0.05). Muscle biopsies of thebiceps brachii revealed increases(P < 0.05) in fiber areas for type I(from 4,196 ± 859 to 4,617 ± 1,116 µm²;n = 11) and II fibers (from 6,378 ± 1,552 to 7,474 ± 2,017 µm²;n = 11). Fiber number estimated fromthe above measurements did not change after training (293.2 ± 61.5 × 10³ pretraining; 297.5 ± 69.5 × 10³ posttraining;n = 8). However, the magnitude ofmuscle fiber hypertrophy may influence this response because thosesubjects with less relative muscle fiber hypertrophy, but similarincreases in muscle CSA, showed evidence of an increase in fibernumber. Capillaries per fiber increased significantly(P < 0.05) for both type I(from 4.9 ± 0.6 to 5.5 ± 0.7;n = 10) and II fibers (from 5.1 ± 0.8 to 6.2 ± 0.7; n = 10). Nochanges occurred in capillaries per fiber area or muscle area. Inconclusion, resistance training resulted in hypertrophy of the totalmuscle CSA and fiber areas with no change in estimated fiber number,whereas capillary changes were proportional to muscle fiber growth.

     

Bone hyperemia precedes disuse-induced intracortical bone resorption

An in vivo modelwas used to determine whether bone hyperemia precedes increasedintracortical porosity induced by disuse. Twenty-four adult maleroosters (age 1 yr) were randomly assigned to intact-control,7-days-sham-surgery, 7-days-disuse, and 14-days-disuse groups. Disusewas achieved by isolating the left ulna diaphysis from physical loadingvia parallel metaphyseal osteotomies. The right ulna served as anintact contralateral control. Colored microspheres were used to assessmiddiaphyseal bone blood flow. Bone blood flow was symmetric betweenthe left and right ulnae of the intact-control and sham-surgery groups.After 7 days of disuse, median (±95% confidence interval)standardized blood flow was significantly elevated compared with thecontralateral bone (6.5 ± 5.2 vs. 1.0 ± 0.8 ml · min¹ · 100 g¹;P = 0.03). After 14 days of disuse,blood flow was also elevated but to a lesser extent. Intracorticalporosity in the sham-surgery and 7-days-disuse bones was not elevatedcompared with intact-control bones. At 14 days of disuse, the area ofintracortical porosity was significantly elevated compared with intactcontrol bones (0.015 ± 0.02 vs. 0.002 ± 0.002 mm²;P = 0.03). We conclude that disuseinduces bone hyperemia before an increase in intracortical porosity.The potential interaction between bone vasoregulation and bone celldynamics remains to be studied.

     

Denervation produces different single fiber phenotypes in fast- and slow-twitch hindlimb muscles of the rat

Using a single, mechanically skinned fiber approach, we tested the hypothesis that denervation (0 to 50 days) of skeletal muscles that do not overlap in fiber type composition [extensor digitorum longus (EDL) and soleus (SOL) muscles of Long-Evans hooded rats] leads to development of different fiber phenotypes. Denervation (50 day) was accompanied by 1) a marked increase in the proportion of hybrid IIB/D fibers (EDL) and I/IIA fibers (SOL) from 30% to >75% in both muscles, and a corresponding decrease in the proportion of pure fibers expressing only one myosin heavy chain (MHC) isoform; 2) complex muscle- and fiber-type specific changes in sarcoplasmic reticulum Ca²⁺-loading level at physiological pCa 7.1, with EDL fibers displaying more consistent changes than SOL fibers; 3) decrease by 50% in specific force of all fiber types; 4) decrease in sensitivity to Ca²⁺, particularly for SOL fibers (by 40%); 5) decrease in the maximum steepness of the force-pCa curves, particularly for the hybrid I/IIA SOL fibers (by 35%); and 6) increased occurrence of biphasic behavior with respect to Sr²⁺ activation in SOL fibers, indicating the presence of both slow and fast troponin C isoforms. No fiber types common to the two muscles were detected at any time points (day 7, 21, and 50) after denervation. The results provide strong evidence that not only neural factors, but also the intrinsic properties of a muscle fiber, influence the structural and functional properties of a particular muscle cell and explain important functional changes induced by denervation at both whole muscle and single cell levels. mechanically skinned fibers; myosin heavy chain isoforms; lineage; sarcoplasmic reticulum; Ca²⁺; Sr²⁺ sensitivity; Long-Evans hooded rat     

Fiber type populations and Ca2+-activation properties of single fibers in soleus muscles from SHR and WKY rats

Electrophoretic analyses of muscle proteins in whole musclehomogenates and single muscle fiber segments were used to examine myosin heavy chain (MHC) and myosin light chain 2 (MLC2) isoform composition and fiber type populations in soleus muscles from spontaneously hypertensive rats (SHRs) and their age-matchednormotensive controls [Wistar-Kyoto (WKY) rats], at threestages in the development of high blood pressure (4 wk, 16 wk, and 24 wk of age). Demembranated (chemically skinned with 2% Triton X-100),single fiber preparations were used to determine the maximumCa²⁺-activated force percross-sectional area, calcium sensitivity, and degree of cooperativityof the contractile apparatus andCa²⁺-regulatory system withrespect to Ca²⁺. The results showthat, at all ages examined, 1) SHRsoleus contained a lower proportion of MHCI and MLC2 slow (MLC2s) and ahigher proportion of MHCIIa, MHCIId/x, and MLC2 fast (MLC2f )isoforms than the age-matched controls;2) random dissection of single fibers from SHR and WKY soleus produced four populations of fibers: type I (expressing MHCI), type IIA (expressing MHCIIa), hybrid typeI+IIA (coexpressing MHCI and MHCIIa), and hybrid type IIA+IID (coexpressing MHCIIa and MHCIId/x); and3) single fiber dissection from SHRsoleus yielded a lower proportion of type I fibers, a higher proportionof fast-twitch fibers (types IIA and IIA+IID), and a higher proportionof hybrid fibers (types I+IIA and IIA+IID) than the homologous musclesfrom the age-matched WKY rats. Because the presence of hybrid fibers isviewed as a marker of muscle transformation, these data suggest thatSHR soleus undergoes transformation well into adulthood. Our data showalso that, for a given fiber type, there are no significant differencesbetween SHR and WKY soleus muscles with respect to any of theCa²⁺-activation propertiesexamined. This finding indicates that the lower specific tensionsreported in the literature for SHR soleus muscles are not due tostrain- or hypertension-related differences in the function of thecontractile apparatus or regulatory system.     

Fiber type composition of four hindlimb muscles of adult Fisher 344 rats

 The limb and trunk muscles of adult rats express four myosin heavy chain (MHC) isoforms, one slow (MHCI) and three fast (MHCIIa, MHCIId, and MHCIIb). The distribution of these isoforms correlates with fiber types delineated using myofibrillar actomyosin adenosine triphosphatase (mATPase) histochemistry. For example, type I fibers express MHCI and fiber types IIA, IID, and IIB express MHCIIa, MHCIId, and MHCIIb, respectively. Fibers containing only one MHC isoform have been termed ”pure” fibers. Recent evidence suggests that a population of ”hybrid” fibers exist in rat skeletal muscle which contain two MHC isoforms. The purpose of the present investigation was to document the entire range of histochemically defined ”pure” and ”hybrid” fiber types in untreated muscles of the young adult Fisher 344 rat hindlimb. The selected hindlimb muscles (soleus, tibialis anterior, extensor digitorum longus, and gastrocnemius muscles) were removed from 12 male rats and analyzed for muscle fiber type distribution, cross-sectional area, and MHC content. Care was taken to delineate eight fiber types (I, IC, IIC, IIA, IIAD, IID, IIDB, and IIB) using refined histochemical techniques. Hybrid fibers were found to make up a considerable portion of the muscles examined (a range of 8.8–17.8% of the total). The deep red portion of the gastrocnemius muscle contained the largest number of hybrid fibers, most of which were the fast types IIAD (8.5±2.8%) and IIDB (5.2±2.3%). In conclusion, hybrid fibers make up a considerable portion of normal rat limb musculature and are an important population that should not be ignored. Accepted: 15 October 1998     

Effects of beta 2-agonist administration and exercise on contractile activation of skeletal muscle fibers

Lynch, Gordon S., Alan Hayes, Siun P. Campbell, and David A. Williams. Effects of₂-agonist administration andexercise on contractile activation of skeletal muscle fibers.J. Appl. Physiol. 81(4):1610-1618, 1996.Clenbuterol, a₂-adrenoceptor agonist, hastherapeutic potential for the treatment of muscle-wasting diseases, yetits effects, especially at the single-fiber level, have not been fullycharacterized. Male C57BL/10 mice were allocated to three groups:Control-Treated mice were administered clenbuterol (2 mg ^· kg^{1 ·} day¹)via their drinking water for 15 wk; Trained-Treated mice underwent low-intensity training (unweighted swimming, 5 days/wk, 1 h/day) inaddition to receiving clenbuterol; and Control mice were sedentary anduntreated. Contractile characteristics were determined on membrane-permeabilized fibers from the extensor digitorum longus (EDL)and soleus muscles. Fast fibers from the EDL and soleus muscles ofTreated mice exhibited decreases inCa²⁺ sensitivity. Enduranceexercise offset clenbuterol's effects, demonstrated by similarCa²⁺ sensitivities in theTrained-Treated and Control groups. Long-term clenbuterol treatment didnot affect the normalized maximal tension of fast or slow fibers butincreased the proportion of fast fibers in the soleus muscle. Trainingincreased the proportion of fibers with high and intermediate succinatedehydrogenase activity in the EDL and soleus muscles, respectively. Ifclenbuterol is to be used for treating muscle-wasting disorders, someform of low-intensity exercise might be encouraged such thatpotentially deleterious slow-to-fast fiber type transformations areminimized. Indeed, in the mouse, low-intensity exercise appears toprevent these effects.

     

Effects of the ovarian cycle on sympathetic neural outflow during static exercise

We comparedreflex responses to static handgrip at 30% maximal voluntarycontraction (MVC) in 10 women (mean age 24.1 ± 1.7 yr) during twophases of their ovarian cycle: the menstrual phase (days 1-4) and the follicularphase (days10-12). Changes in muscle sympathetic nerve activity (MSNA; microneurography) in response tostatic exercise were greater during the menstrual compared withfollicular phase (phase effect P = 0.01). Levels of estrogen were less during the menstrual phase(75 ± 5.5 vs. 116 ± 9.6 pg/ml, days 1-4 vs.days 10-12;P = 0.002). Generated tension did not explain differences in MSNA responses (MVC: 29.3 ± 1.3 vs. 28.2 ± 1.5 kg, days 1-4 vs.days 10-12;P = 0.13). In a group of experiments with the use of ³¹P-NMRspectroscopy, no phase effect was observed forH⁺ andH₂PO₄ concentrations(n = 5). During an ischemicrhythmic handgrip paradigm (20% MVC), a phase effect was notobserved for MSNA or H⁺ orH₂PO₄ concentrations,suggesting that blood flow was necessary for the expression of thecycle-related effect. The present studies suggest that, during statichandgrip exercise, MSNA is increased during the menstrual compared withthe follicular phase of the ovarian cycle.

     

Distribution of TmDOTP5- in rat tissues: TmDOTP5- vs. CoEDTA- as markers of extracellular tissue space

The distributionof TmDOTP⁵ in rat tissuewas compared with CoEDTA,an anionic complex previously used as a marker of extracellular space.Heart, liver, muscle, blood, and urine were collected from rats afterinfusion of either complex and were quantitatively analyzed by atomicabsorption spectroscopy. Although totalTmDOTP⁵ in blood and tissuewas consistently lower (0.88 ± 0.04;n = 6) thanCoEDTA after an identicalinfusion protocol (presumably because of some association of thephosphonate complex with bone), a comparison of blood and tissuecontents indicated that the two anionic complexes distributed intoidentical extracellular spaces. Relative extracellular space in the invivo liver, as determined byTmDOTP⁵ andCoEDTA, was 0.18 ± 0.02 and 0.15 ± 0.01, respectively. The corresponding relativeextracellular space values for the in vivo heart reported by the twoagents were identical (0.11 ± 0.02). Experiments were alsoperformed to evaluate the washout kinetics ofTmDOTP⁵ from anesthesizedrats. In rats given a total dose of 0.16 mmol TmDOTP⁵, 81% appeared inurine by 180 min, <2% was found in all remaining soft tissue,leaving ~18% undetected. The rate of Tm appearance in urine was fitto a standard pharmacokinetic model that included four tissuecompartments: plasma, one fast equilbrating space, one slowequilibrating space, and one very slow equilibrating space (presumablybone). The best fit result suggests that the highly chargedTmDOTP⁵ complex is clearedfrom plasma more rapidly than is the typical lower charged Gd-basedcontrast agents and that release from bone is slow compared with renal clearance.

     

H2O2 increases sheep tracheal blood flow, permeability, and vascular response to luminal capsaicin

Wells, U. M., S. Duneclift, and J. G. Widdicombe.H₂O₂increases sheep tracheal blood flow, permeability, and vascular response to luminal capsaicin. J. Appl.Physiol. 82(2): 621-631, 1997.Exogenous hydrogenperoxide(H₂O₂)causes airway epithelial damage in vitro. We have studied the effectsof luminalH₂O₂in the sheep trachea in vivo on tracheal permeability tolow-molecular-weight hydrophilic (technetium-99m-labeleddiethylenetriamine pentaacetic acid;^99mTc-DTPA) and lipophilic([¹⁴C]antipyrine;[¹⁴C]AP) tracers andon the tracheal vascular response to luminal capsaicin, whichstimulates afferent nerve endings. A tracheal artery was perfused, andtracheal venous blood was collected. H₂O₂exposure (10 mM) reduced tracheal potential difference(42.0 ± 6.4 mV) to zero. It increased arterial andvenous flows (56.7 ± 6.1 and 57.3 ± 10.0%,respectively; n = 5, P < 0.01, paired t-test) but not tracheal lymph flow(unstimulated flow 5.0 ± 1.2 µl ^· min^{1 ·} cm¹,n = 4). DuringH₂O₂exposure, permeability to ^99mTc-DTPA increased from2.6 to 89.7 × 10⁷ cm/s(n = 5, P < 0.05), whereas permeability to[¹⁴C]AP (3,312.6 × 10⁷ cm/s,n = 4) was not altered significantly(2,565 × 10⁷cm/s). Luminal capsaicin (10 µM) increased tracheal blood flow (10.1 ± 4.1%, n = 5)and decreased venous ^99mTc-DTPAconcentration (19.7 ± 4.0, P < 0.01), and these effects weresignificantly greater after epithelial damage (28.1 ± 6.0 and45.7 ± 4.3%, respectively,P < 0.05, unpairedt-test). Thus H₂O₂increases the penetration of a hydrophilic tracer into tracheal bloodand lymph but has less effect on a lipophilic tracer. It also enhancesthe effects of luminal capsaicin on blood flow and tracer uptake.

     

Direct measurement of the energy expenditure of physical activity in preterm infants

The energy cost of physical activity (EEA) has been estimated toaccount for 5-17% of total energy expenditure (TEE) in neonates. To directly measure EEA, a force plate was developed and validated tomeasure work outputs ranging from 0.3 to 40 kcal · kg¹ · day¹.By use of this force plate plus indirect calorimetry, TEE and EEA weremeasured and correlated with five activity states in 24 infants withgestational age of 31.6 ± 0.5 (SE) wk and postnatal age of 24.8 ± 3.7 days. TEE and EEA were 69.2 ± 1.5 and 2.4 ± 0.2 kcal · kg¹ · day¹,respectively. EEA per state was 0.5 ± 0.0 (quiet sleep), 2.4 ± 0.2 (active sleep), 2.8 ± 0.4 (quiet awake), 7.5 ± 0.8 (active awake), and 15.1 ± 2.3 (crying)kcal · kg¹ · day¹.This provides the first direct measurement of the contribution ofphysical activity to TEE in preterm infants and will enable measurementof caloric expenditure from muscle activity in various diseaseconditions and development of nursing strategies to minimize unnecessary energy losses.

     

Lung liquid production rates and volumes do not decrease before labor in healthy fetal sheep

Lines, A., S. B. Hooper, and R. Harding. Lung liquidproduction rates and volumes do not decrease before labor in healthy fetal sheep. J. Appl. Physiol. 82(3):927-932, 1997.Previous studies have suggested that the volumeand production rate of fetal lung liquid decrease late in gestation,before the onset of labor, in preparation for the clearance of lungliquid at birth. In contrast, our earlier studies have not shown adecrease in lung liquid volume near term, although these studies werenot continued to the onset of labor. Our aim was to determine the changes in lung liquid volume and production rate in fetal sheep duringthe last 2 wk of gestation up to the onset of labor at term (~147days). In eight chronically catheterized fetal sheep, the volume andproduction rate of fetal lung liquid were measured at 130, 135, and 140 days of gestation and then on every 2nd day until the onset oflabor. Labor was detected by monitoring uterine muscleactivity and intrauterine pressure changes. On the day of labor onset,which occurred at 147 ± 1 days of gestation, fetuses weighed 5.0 ± 0.2 kg. The volume of fetal lung liquid was 40.4 ± 2.7 ml/kgat 19 ± 1 days before labor onset and had not significantly changedby 0.7 ± 0.2 days (44.8 ± 5.1 ml/kg) before labor. Similarly, lung liquid production rates at 19 ± 1 days before labor (5.1 ± 1.8 ml ^· h^{1 ·} kg¹)were not significantly different from those at 0.7 ± 0.2 days before labor (3.4 ± 0.7 ml ^· h^{1 ·} kg¹).We conclude that, in healthy ovine fetuses, lung liquid volumes andproduction rates do not decrease before the onset of labor. Our resultsindicate that the entire volume of fetal lung liquid (~222.5 ± 36.6 ml) must be cleared after the onset of labor.

     

Human skeletal muscle carnitine palmitoyltransferase I activity determined in isolated intact mitochondria

This study was designed to compare theactivity of skeletal muscle carnitine palmitoyltransferase I (CPT I) intrained and inactive men (n = 14) andwomen (n = 12). CPT Iactivity was measured in intact mitochondria, isolated from needlebiopsy vastus lateralis muscle samples (~60 mg). The variability ofCPT I activity determined on two biopsy samples from the same leg onthe same day was 4.4, whereas it was 7.0% on two biopsy samples fromthe same leg on different days. The method was sensitive to the CPT Iinhibitor malonyl-CoA (88% inhibition) and therefore specific for CPTI activity. The mean CPT I activity for all 26 subjects was 141.1 ± 10.6 µmol · min¹ · kgwet muscle (wm)¹ and wasnot different when all men vs. all women (140.5 ± 15.7 and 142.2 ± 14.5 µmol · min¹ · kgwm¹, respectively) were compared. However, CPT Iactivity was significantly higher in trained vs. inactive subjects forboth men (176.2 ± 21.1 vs. 104.1 ± 13.6 µmol · min¹ · kgwm¹) and women (167.6 ± 14.1 vs. 91.2 ± 9.5 µmol · min¹ · kgwm¹). CPT I activity was also significantly correlatedwith citrate synthase activity (all subjects,r = 0.76) and maximal oxygen consumption expressed in milliliters per kilogram per minute (all subjects, r = 0.69). Theresults of this study suggest that CPT I activity can be accurately andreliably measured in intact mitochondria isolated from human musclebiopsy samples. CPT I activity was not affected by gender, and higheractivities in aerobically trained subjects appeared to be the result ofincreased mitochondrial content in both men and women.

     

Effects of short-term inactivity on glucose tolerance, energy expenditure, and blood flow in trained subjects

The purpose of this investigation was to examinethe effects of 7-10 days of inactivity (IA) on glucose tolerance(GT), resting metabolic rate (RMR), thermic effect of a meal (TEM), andlimb blood flow in endurance-trained men. Eight highly trained (peak O₂ consumption 64 ± 2 ml · kg¹ · min¹)endurance athletes participated in this study involving two identicaltest days, one ~24 h after a normal training bout (Tr) and the secondafter 7-10 days of IA. The following tests were conducted at eachvisit: 75-g oral glucose tolerance test (OGTT), RMR, and TEM andmeasurements of calf and forearm blood flow (BF) by using venousocclusive plethysmography. Body weight remained unchangedduring this short period of IA (Tr, 78.5 ± 1 kg; IA, 78.7 ± 1 kg). The area under the glucose and insulin curves increased 65% (Tr,3,375 ± 877 vs. IA, 5,559.4 ± 621 mg · dl¹ · 180 min¹) and 73% (Tr,2,182.5 ± 270 vs. IA, 3,793.1 ± 739 µU · ml¹ · 180 min¹) after IA,respectively (P < 0.01). RMRdecreased significantly (4%; 1.5 ± 0.02 vs. 1.44 ± 0.02 kcal/min; P < 0.05) and respiratory exchange ratio during the OGTT increased (4%, 0.812 ± 0.011 vs. 0.842 ± 0.009; P < 0.05) afterIA, whereas TEM increased similarly in the Tr and IA states. In the Trstate, mean calf BF increased by 22% (3.17 ± 0.22 vs. 3.87 ± 0.38 ml · 100 ml¹ · min¹;P < 0.05) during the OGTT butremained unchanged after IA, whereas no differences at rest or duringOGTTs existed between the two conditions for forearm BF. Incrementalinsulin area above fasting during the OGTT was correlated with meancalf BF in the Tr (r = 0.76, P < 0.05) and IA(r = 0.72, P < 0.05) states. In conclusion, 7-10 days of IA results in a deterioration in GT and a reduction in RMR. After glucose ingestion, calf BF was elevated compared withresting levels in the Tr state but was unchanged in the IA state;however, limb BF was not related to GT or RMR. Thus our findings raisequestions regarding the relative contribution of BF in modulatingglucose tolerance and energy expenditure in endurance athletes in theirhabitual Tr or IA state.