Regional rheological differences in locomoting neutrophils

Intracellular rheology is a useful probe of the mechanisms underlying spontaneous or chemotactic locomotion and transcellular migration of leukocytes. We characterized regional rheological differences between the leading, body, and trailing regions of isolated, adherent, and spontaneously locomoting human neutrophils. We optically trapped intracellular granules and measured their displacement for 500 ms after a 100-nm step change in the trap position. Results were analyzed in terms of simple viscoelasticity and with the use of structural damping (stress relaxation follows a power law in time). Structural damping fit the data better than did viscoelasticity. Regional viscoelastic stiffness and viscosity or structural damping storage and loss moduli were all significantly lower in leading regions than in pooled body and/or trailing regions (the latter were not significantly different). Structural damping showed similar levels of elastic and dissipative stresses in body and/or trailing regions; leading regions were significantly more fluidlike (increased power law exponent). Cytoskeletal disruption with cytochalasin D or nocodazole made body and/or trailing regions 50% less elastic and less viscous. Cytochalasin D completely suppressed pseudopodial formation and locomotion; nocodazole had no effect on leading regions. Neither drug changed the dissipation-storage energy ratio. These results differ from those of studies of neutrophils and other cell types probed at the cell membrane via ₂-integrin receptors, which suggests a distinct role for the cell cortex or focal adhesion complexes. We conclude that 1) structural damping well describes intracellular rheology, and 2) while not conclusive, the significantly more fluidlike behavior of the leading edge supports the idea that intracellular pressure may be the origin of motive force in neutrophil locomotion. structural damping; power law; viscoelasticity; optical trap     

Length-dependent modulation of smooth muscle activation: effects of agonist, cytochalasin, and temperature

We tested thehypothesis that mechanical strain modulates agonist sensitivity ofsmooth muscle by measuring myosin phosphorylation and contractile forcein bovine tracheal smooth muscle activated by various concentrations ofthe muscarinic receptor agonist carbachol and at various musclelengths. Increasing carbachol concentration by 10,000-fold did notrestore myosin phosphorylation levels at shorter muscle lengths to thelevel at optimal length(L_o). Maximum levels of myosin phosphorylation induced by carbachol at 0.6, 0.8, and1.0 L_o weresimilar but became lower at <0.6L_o. Cytochalasin D significantly attenuated carbachol-induced contraction by 54%. Inaddition, cytochalasin D treatment induced a parallel downward shift inthe length-myosin phosphorylation relation. Lowering temperature from37 to 23°C did not significantly change the length dependencies ofcarbachol-induced active force and myosin phosphorylation. Theseresults have led us to conclude that1) agonist sensitivity and maximumlevel of activation (as measured by myosin phosphorylation) are targetsof length-dependent modulation, 2)actin filaments involved in contraction and length-dependent modulationare distinct in sensitivity to cytochalasin D, and3) length-dependent modulation isrelatively temperature insensitive.

     

Influence of diet and exercise on skeletal muscle and visceral adipose tissue in men

Ross, Robert, John Rissanen, Heather Pedwell, JenniferClifford, and Peter Shragge. Influence of diet and exercise onskeletal muscle and visceral adipose tissue in men. J. Appl. Physiol. 81(6): 2445-2455, 1996.Theeffects of diet only (DO) and diet combined with either aerobic (DA) orresistance (DR) exercise on subcutaneous adipose tissue (SAT), visceraladipose tissue (VAT), lean tissue (LT), and skeletal muscle (SM) tissue were evaluated in 33 obese men (DO, n = 11; DA, n = 11; DR,n = 11). All tissues were measured byusing a whole body multislice magnetic resonance imaging (MRI) model.Within each group, significant reductions were observed for bodyweight, SAT, and VAT (P < 0.05). Thereductions in body weight (~10%) and SAT (~25%) and VAT volume (~35%) were not different between groups(P > 0.05). For alltreatments, the relative reduction in VAT was greater than in SAT(P < 0.05). For the DA and DR groupsonly, the reduction in abdominal SAT (~27%) was greater(P < 0.05) than thatobserved for the gluteal-femoral region (~20%). Conversely, thereduction in VAT was uniform throughout the abdomen regardless oftreatment (P > 0.05). MRI-LT andMRI-SM decreased both in the upper and lower body regions for the DO group alone (P < 0.05). PeakO₂ uptake (liters) wassignificantly improved (~14%) in the DA group as was muscularstrength (~20%) in the DR group (P < 0.01). These findings indicate that DA and DR result in a greaterpreservation of MRI-SM, mobilization of SAT from the abdominal region,by comparison with the gluteal-femoral region, and improved functionalcapacity when compared with DO in obese men.

     

Comparisons of two-, three-, and four-compartment models of body composition analysis in men and women

This study compared the traditionaltwo-compartment (fat mass or FM; fat free mass or FFM)hydrodensitometric method of body composition measurement, which isbased on body density, with three (FM, total body water or TBW, fatfree dry mass)- and four (FM, TBW, bone mineral mass or BMM,residual)-compartment models in highly trained men(n = 12), sedentary men(n = 12), highly trained women(n = 12), and sedentary women(n = 12). The means andvariances for the relative body fat (%BF) differences between the two-and three-compartment models [2.2 ± 1.6 (SD) % BF;n = 48] were significantlygreater (P  0.02) than those between the three- and four-compartment models (0.2 ± 0.3% BF;n = 48) for all four groups. Thethree-compartment model is more valid than the two-compartmenthydrodensitometric model because it controls for biological variabilityin TBW, but additional control for interindividual variability in BMMvia the four-compartment model achieves little extra accuracy. Thecombined group (n = 48) exhibited greater (P < 0.001) FFM densities(1.1075 ± 0.0049 g/cm³) thanthe hydrodensitometric assumption of 1.1000 g/cm³, which is based on analysesof three male cadavers aged 25, 35, and 46 yr. This was primarilybecause their FFM hydration (72.4 ± 1.1%;n = 48) was lower(P  0.001) than thehydrodensitometric assumption of 73.72%.

     

Motility comparisons of two shape-distinctiveEntamoeba variants

Summary The motility ofEntamoeba sp. (Laredo isolate, LA) is compared to a colchicine-resistant variant (LACR) derived from it and known to exhibit physiological and morphological differences. Both variants possess contractile elements which in glycerinated cell models were activated by Mg²⁺-ATP. Short-term experiments indicated that whereas both variants are insensitive to colchicine, treatment with cytochalasin B induces a change in cell shape and loss of adhesiveness in LA cells leaving LACR relatively unaffected. Quantitative analysis of time-lapse cinematographic recordings revealed that the mean relative surface area change of LA cells was 411% per minute with a mean pseudopod extension rate of 103 m per minute. In contrast, the respective values for LACR cells were 112% per minute und 32 m per minute. Cytochalasin B treatment reduced the relative surface area change of LA cells by approximately 18-fold with a corresponding reduction in LACR cells of only 1.6-fold. Similarly, cytochalasin B inhibited pseudopod extension by 34 and 1.8-fold in LA and LACR cells respectively. Thus, we have demonstrated a clear difference in both pattern and rate of locomotion of the two variants and have shown that the colchicine resistant variant has also become relatively insensitive to treatment with CB.     

Training, muscle volume, and energy expenditure in nonobese American girls

Little is known about the relationship among training,energy expenditure, muscle volume, and fitness in prepubertalgirls. Because physical activity is high in prepubertalchildren, we hypothesized that there would be no effect of training.Forty pre- and early pubertal (mean age 9.1 ± 0.1 yr) nonobesegirls enrolled in a 5 day/wk summer school program for 5 wk and were randomized to control (n = 20) or training groups(n = 20; 1.5 h/day, endurance-type exercise). Totalenergy expenditure (TEE) was measured using doubly labeled water, thighmuscle volume using magnetic resonance imaging, and peak O₂uptake (O_2 peak) using cycle ergometry.TEE was significantly greater (17%, P < 0.02) in thetraining girls. Training increased thigh muscle volume (+4.3 ± 0.9%, P < 0.005) andO_2 peak (+9.5 ± 6%,P < 0.05), effects surprisingly similar to thoseobserved in adolescent girls using the same protocol (Eliakim A,Barstow TJ, Brasel JA, Ajie H, Lee W-NP, Renslo R, Berman N, and CooperDM, J Pediatr 129: 537-543, 1996). We furthercompared these two sample populations: thigh muscle volume per weightwas much lower in adolescent compared with prepubertal girls (17.0 ± 0.3 vs. 27.8 ± 0.6 ml/kg body mass; P < 0.001), and allometric analysis revealed remarkably low scaling factorsrelating muscle volume (0.34 ± 0.05, P < 0.0001), TEE (0.24 ± 0.06, P < 0.0004), andO_2 peak (0.28 ± 0.07, P < 0.0001) to body mass in all subjects. Muscle andcardiorespiratory functions were quite responsive to brief training inprepubertal girls. Moreover, a retrospective, cross-sectional analysissuggests that increases in muscle mass andO_2 peak may be depressed in nonobeseAmerican girls as they mature.

     

Effects of aging on actin sliding speed on myosin from single skeletal muscle cells of mice, rats, and humans

The effects of aging onthe mechanical properties of myosin were measured in 87 fibers frommuscles of humans (n = 40), rats (n = 21), and mice (n = 26) using a single fiber in vitromotility assay. Irrespective of species, an 18-25% aging-relatedslowing in the speed of actin filaments was observed from 62 singlefibers expressing the slow (type I) -myosin heavy chain isoform. The mechanisms underlying the aging-related slowing of motility speed remain unknown, but it is suggested that posttranslationalmodifications of myosin by oxidative stress, glycation, or nitrationplay an important role. The aging-related slowing in the speed of actin filaments propelled by the type I myosin was confirmed in three mammalian species with an ~3,400-fold difference in body size. Motility speed from human myosin was 3-fold slower than from myosin ofthe ~3,400-fold smaller mouse and approximately twofold slower whencompared with the ~130-fold smaller rat, irrespective of age. Astrong correlation was observed between the log values of actin slidingspeed and body mass, suggesting that the effects of scaling is, atleast in part, due to altered functional properties of the motorprotein itself.

     

Effects of resistance training and chromium picolinate on body composition and skeletal muscle in older men

The effects of chromium picolinate (CrPic)supplementation and resistance training (RT) on skeletal muscle size,strength, and power and whole body composition were examined in 18 men(age range 56-69 yr). The men were randomly assigned(double-blind) to groups (n = 9) thatconsumed either 17.8 µmol Cr/day (924 µg Cr/day) as CrPic or alow-Cr placebo for 12 wk while participating twice weekly in ahigh-intensity RT program. CrPic increased urinary Cr excretion~50-fold (P < 0.001). RT-inducedincreases in muscle strength (P < 0.001) were not enhanced by CrPic. Arm-pull muscle power increased withRT at 20% (P = 0.016) but not at 40, 60, or 80% of the one repetition maximum, independent of CrPic.Knee-extension muscle power increased with RT at 20, 40, and 60%(P < 0.001) but not at 80% of onerepetition maximum, and the placebo group gained more muscle power thandid the CrPic group (RT by supplemental interaction,P < 0.05). Fat-free mass(P < 0.001), whole body muscle mass(P < 0.001), and vastus lateralistype II fiber area (P < 0.05)increased with RT in these body-weight-stable men, independent ofCrPic. In conclusion, high-dose CrPic supplementation did not enhancemuscle size, strength, or power development or lean body mass accretionin older men during a RT program, which had significant, independenteffects on these measurements.

     

Differential susceptibility of subsarcolemmal and intermyofibrillar mitochondria to apoptotic stimuli

Apoptosis can be evoked by reactive oxygen species (ROS)-induced mitochondrial release of the proapoptotic factors cytochrome c and apoptosis-inducing factor (AIF). Because skeletal muscle is composed of two mitochondrial subfractions that reside in distinct subcellular regions, we investigated the apoptotic susceptibility of subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria. SS and IMF mitochondria exhibited a dose-dependent release of protein in response to H₂O₂ (0, 25, 50, and 100 µM). However, IMF mitochondria were more sensitive to H₂O₂ and released a 2.5-fold and 10-fold greater amount of cytochrome c and AIF, respectively, compared with SS mitochondria. This finding coincided with a 44% (P < 0.05) greater rate of opening (maximum rate of absorbance decrease, V_max) of the protein release channel, the mitochondrial permeability transition pore (mtPTP), in IMF mitochondria. IMF mitochondria also exhibited a 47% (P < 0.05) and 60% (0.05 < P < 0.1) greater expression of the key mtPTP component voltage-dependent anion channel and cyclophilin D, respectively, along with a threefold greater cytochrome c content, but similar levels of AIF compared with SS mitochondria. Despite a lower susceptibility to H₂O₂-induced release, SS mitochondria possessed a 10-fold greater Bax-to-Bcl-2 ratio (P < 0.05), a 2.7-fold greater rate of ROS production, and an approximately twofold greater membrane potential compared with IMF mitochondria. The expression of the antioxidant enzyme Mn²⁺-superoxide dismutase was similar between subfractions. Thus the divergent protein composition and function of the mtPTP between SS and IMF mitochondria contributes to a differential release of cytochrome c and AIF in response to ROS. Given the relatively high proportion of IMF mitochondria within a muscle fiber, this subfraction is likely most important in inducing apoptosis when presented with apoptotic stimuli, ultimately leading to myonuclear decay and muscle fiber atrophy. reactive oxygen species; skeletal muscle; mitochondrial permeability transition pore; cytochrome c; apoptosis     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.     

Mechanical strain regulates syndecan-4 expression and shedding in smooth muscle cells through differential activation of MAP kinase signaling pathways

Syndecan-4 (S4) belongs to a family of transmembrane proteoglycans, acts as a coreceptor for growth factor binding as well as cell-matrix and cell-cell interactions, and is induced in neointimal smooth muscle cells (SMCs) after balloon catheter injury. We investigated S4 expression in SMCs in response to several force profiles and the role of MAP kinase signaling pathways in regulating these responses. S4 mRNA expression increased in response to 5% and 10% cyclic strain (4 h: 200 ± 34% and 182 ± 17%, respectively; P < 0.05) before returning to basal levels by 24 h. Notably, the SMC mechanosensor mechanism was reset after an initial 24-h "preconditioning" period, as evident by an increase in S4 gene expression following a change in cyclic stress from 10% to 20% (28 h: 181 ± 1%; P < 0.05). Mechanical stress induced a late decrease in cell-associated S4 protein levels (24 h: 70 ± 6%; P < 0.05), with an associated increase in S4 shedding (24 h: 537 ± 109%; P < 0.05). To examine the role of MAP kinases, cells were treated with U-0126 (ERK1/2 inhibitor), SB-203580 (p38 inhibitor), or JNKI I (JNK/SAPK inhibitor). Late reduction in cell-associated S4 levels was attributed to ERK1/2 and p38 signaling. In contrast, accelerated S4 shedding required both ERK1/2 (5-fold reduction in accelerated shedding; P < 0.05) and JNK/SAPK (4-fold reduction; P < 0.05) signaling. Given the varied functions of S4, stress-induced effects on SMC S4 expression and shedding may represent an additional component of the proinflammatory, growth-stimulating pathways that are activated in response to changes in the mechanical microenvironment of the vascular wall. heparan sulfate proteoglycan     

Prey size, prey nutrition, and food handling by shrews of different body sizes

We tested some predictions relating metabolic constraints offoraging behavior and prey selection by comparing food handlingand utilization in four sympatric shrew species: Sorex minutus(mean body mass = 3.0 g), S. araneus (8.0 g), Neomys anomalus(10.0 g), and N. fodiens (14.4 g). Live fly larvae, mealwormlarvae, and aquatic arthropods were offered to shrews as smallprey (body mass <0.1 g). Live earthworms, snails, and smallfish were offered as large prey (>0.3 g). The larvae werethe high-nutrition food (>8 kJ/g), and the other prey werethe low-nutrition food (<4 kJ/g). The smallest shrew, S.minutus, utilized (ate + hoarded) <30% of offered food,and the other species utilized >48% of food. The largerthe shrew, the more prey it ate per capita. However, highlyenergetic insect larvae composed 75% of food utilized by S.minutus and only >40% of the food utilized by the other species. Thus, inverse relationships appeared between shrewbody mass and mass-specific food mass utilization and betweenshrew body mass and mass-specific food energy utilization:the largest shrew, N. fodiens, utilized the least food massand the least energy quantity per 1 g of its body mass. Also,the proportion of food hoarded by shrews decreased with increase in size of shrew. With the exception of S. araneus, the sizeof prey hoarded by the shrews was significantly larger thanthe size of prey eaten. Tiny S. minutus hoarded and ate smallerprey items than the other shrews, and large N. fodiens hoardedlarger prey than the other shrews.     

Training intensity, blood lipids, and apolipoproteins in men with high cholesterol

Crouse, Stephen F., Barbara C. O'Brien, Peter W. Grandjean,Robert C. Lowe, J. James Rohack, John S. Green, and Homer Tolson. Training intensity, blood lipids, and apolipoproteins in men withhigh cholesterol. J. Appl. Physiol.82(1): 270-277, 1997.Twenty-six hypercholesterolemic men (meancholesterol, 258 mg/dl; age, 47 yr; weight, 81.9 kg) completed 24 wk ofcycle ergometer training (3 days/wk, 350 kcal/session) at either high(n = 12) or moderate (n = 14) intensity (80 and 50%maximal O₂ uptake, respectively, randomly assigned) to test the influence of training intensity on bloodlipid and apolipoprotein (apo) concentrations. Allphysiological, lipid, and apo measurements were completed at 0, 8, 16, and 24 wk. Lipid data were analyzed via two × fourrepeated-measures analysis of variance ( = 0.0031). Trainingproduced a significant decrease in body weight and increase in maximalO₂ uptake. No interactions betweenintensity and weeks of training were noted for any lipid or apovariable, and no between-group differences were significant before orthroughout training. Therefore, intensity did not affect the trainingresponse. Regardless of intensity, apo AI and apo B fell 9 and 13%,respectively, by week 16 and remainedlower through week 24 (P < 0.0003). Total cholesterol felltransiently (5.5%) by week 16 (P < 0.0021) but returned to initiallevels by week 24. Triglyceride,low-density-lipoprotein cholesterol, and high-density-lipoprotein (HDL)cholesterol did not change with training. In contrast,HDL₂ cholesterol rose 79% aboveinitial levels by week 8 and 82%above initial levels by week 24 (P < 0.0018);HDL₃ cholesterol fell 8 and 13%over the same training intervals (P < 0.0026). These data show that changes in blood lipid and apoconcentrations that accompany training in hypercholesterolemic men arenot influenced by exercise intensity when caloric expenditure is heldconstant.

     

Reflex responses to regional venous pooling during lower body negative pressure in humans

Halliwill, John R., Lori A. Lawler, Tamara J. Eickhoff,Michael J. Joyner, and Sharon L. Mulvagh. Reflex responses toregional venous pooling during lower body negative pressure in humans.J. Appl. Physiol. 84(2): 454-458, 1998.Lower body negative pressure is frequently used to simulateorthostasis. Prior data suggest that venous pooling in abdominal orpelvic regions may have major hemodynamic consequences. Therefore, we developed a simple paradigm for assessing regional contributions tovenous pooling during lower body negative pressure. Sixteen healthy menand women underwent graded lower body negative pressure protocols to 60 mmHg while wearing medical antishock trousers to prevent venous poolingunder three randomized conditions:1) no trouser inflation (control),2) only the trouser legs inflated, and 3) the trouser legs andabdominopelvic region inflated. Without trouser inflation, heart rateincreased 28 ± 4 beats/min, mean arterial pressure fell 3 ± 2 mmHg, and forearm vascular resistance increased 51 ± 9 units at 60 mmHg lower body negative pressure. With inflation of eitherthe trouser legs or the trouser legs and abdominopelvic region, heartrate and mean arterial pressure did not change during lower bodynegative pressure. By contrast, although the forearm vasoconstrictorresponse to lower body negative pressure was attenuated by inflation ofthe trouser legs (forearm vascular resistance 33 ± 10 units,P < 0.05 vs. control), attenuation was greater with the inflation of the trouser legs and abdominopelvic region (forearm vascular resistance 16 ± 5 units,P < 0.05 vs. control and trouserlegs-only inflation). Thus the hemodynamic consequences of pooling inthe abdominal and pelvic regions during lower body negative pressureappear to be less than in the legs in healthy individuals.

     

Interaction of leg stiffness and surface stiffness during human hopping

Ferris, Daniel P., and Claire T. Farley. Interaction ofleg stiffness and surface stiffness during human hopping.J. Appl.Physiol. 82(1): 15-22, 1997.When mammals run,the overall musculoskeletal system behaves as a single linear "legspring." We used force platform and kinematic measurements todetermine whether leg spring stiffness(k_leg) isadjusted to accommodate changes in surface stiffness(k_surf) whenhumans hop in place, a good experimental model for examiningadjustments tok_leg in bouncinggaits. We found thatk_leg was greatlyincreased to accommodate surfaces of lower stiffnesses. The seriescombination ofk_leg andk_surf[total stiffness(k_tot)]was independent ofk_surf at a givenhopping frequency. For example, when humans hopped at a frequency of 2 Hz, they tripled theirk_leg on the leaststiff surface(k_surf = 26.1 kN/m; k_leg = 53.3 kN/m) compared with the most stiff surface(k_surf = 35,000 kN/m; k_leg = 17.8 kN/m). Values fork_tot were notsignificantly different on the least stiff surface (16.7 kN/m) and themost stiff surface (17.8 kN/m). Because of thek_leg adjustment,many aspects of the hopping mechanics (e.g., ground-contact time andcenter of mass vertical displacement) remained remarkably similardespite a >1,000-fold change ink_surf. This studyprovides insight into howk_leg adjustmentscan allow similar locomotion mechanics on the variety of terrainsencountered by runners in the natural world.

     

Functional characterization of two S-nitroso-L-cysteine transporters, which mediate movement of NO equivalents into vascular cells

In myogenic C₂C₁₂ cells, 5 mM creatine increased the incorporation of labeled [³⁵S]methionine into sarcoplasmic (+20%, P < 0.05) and myofibrillar proteins (+50%, P < 0.01). Creatine also promoted the fusion of myoblasts assessed by an increased number of nuclei incorporated within myotubes (+40%, P < 0.001). Expression of myosin heavy chain type II (+1,300%, P < 0.001), troponin T (+65%, P < 0.01), and titin (+40%, P < 0.05) was enhanced by creatine. Mannitol, taurine, and -alanine did not mimic the effect of creatine, ruling out an osmolarity-dependent mechanism. The addition of rapamycin, the inhibitor of mammalian target of rapamycin/70-kDa ribosomal S6 protein kinase (mTOR/p70^s6k) pathway, and SB 202190, the inhibitor of p38, completely blocked differentiation in control cells, and creatine did not reverse this inhibition, suggesting that the mTOR/p70^s6k and p38 pathways could be potentially involved in the effect induced by creatine on differentiation. Creatine upregulated phosphorylation of protein kinase B (Akt/PKB; +60%, P < 0.001), glycogen synthase kinase-3 (+70%, P < 0.001), and p70^s6k (+50%, P < 0.001). Creatine also affected the phosphorylation state of p38 (–50% at 24 h and +70% at 96 h, P < 0.05) as well as the nuclear content of its downstream targets myocyte enhancer factor-2 (–55% at 48 h and +170% at 96 h, P < 0.05) and MyoD (+60%, P < 0.01). In conclusion, this study points out the involvement of the p38 and the Akt/PKB-p70^s6k pathways in the enhanced differentiation induced by creatine in C₂C₁₂ cells. protein synthesis; insulin-like growth factor; mitogen-activated protein kinase; extracellular signal-regulated kinase 1/2; 70-kDa ribosomal S6 protein kinase     

Copepod egg production, moulting and growth rates, and secondary production, in the Skagerrak in August 1988

Measurements of hydrography, chlorophyll, moulting rates ofjuvenile copepods and egg production rates of adult female copepodswere made at eight stations along a transect across the Skagerrak.The goals of the study were to determine (i) if there were correlationsbetween spatial variations in hydrography, phytoplankton andcopepod production rates, (ii) if copepod egg production rateswere correlated with juvenile growth rates, and (iii) if therewas evidence of food-niche separation among co-occumng femalecopepods The 200 km wide Skagerrak had a stratified water columnin the center and a mixed water column along the margins. Suchspatial variations should lead to a dominance of small phytoplanktoncells in the center and large cells along the margins; however,during our study blooms of Gyrodinium aureolum and Ceratium(three species) masked any locally driven differences in cellsize: 50% of chla was >11 µm, 5% in the 11–50µm fraction and 45% <50 µm. averaged for allstations. Chlorophyll ranged from 0.2 to 2.5 µg l^–1at most depths and stations. Specific growth rates of copepodsaveraged 0.10 day^–1 for adult females and 0.27 day^–1for juveniles The latter is similar to maximum rates known fromlaboratory studies, thus were probably not food-limited. Eggproduction rates were food-limited with the degree of limitationvarying among species: 75% of maximum for Centropages typicus, 50% for Calanus finmarchicus, 30% for Paracalanus parvus and 15% for Acartia longiremis and Temora longicornis. Thedegree of limitation was unrelated to female body size suggestingfood-niche separation among adults. Copepod production, summedover all species, ranged from 3 to 8 mg carbon m^–3day^–1and averaged 4.6 mg carbon m^–1 day^–1. Egg productionaccounted for 25% of the total.     

Mild sustained and intermittent hypoxia induce apoptosis in PC-12 cells via different mechanisms

Episodic hypoxia, a characteristic feature of obstructive sleep apnea, induces cellular changes and apoptosis in brain regions associated with neurocognitive function. To investigate whether mild, intermittent hypoxia would induce more extensive neuronal damage than would a similar degree of sustained hypoxia, rat pheochromocytoma PC-12 neuronal cells were subjected to either sustained (5% O₂) or intermittent (alternating 5% O₂ 35 min, 21% O₂ 25 min) hypoxia for 2 or 4 days. Quantitative assessment of apoptosis showed that while mild sustained hypoxia did not significantly increase cell apoptosis at 2 days (1.31 ± 0.29-fold, n = 8; P = NS), a significant increase in apoptosis occurred after 4 days (2.25 ± 0.4-fold, n = 8; P < 0.002), without increased caspase activation. Furthermore, caspase inhibition with the general caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z-VAD-FMK) did not modify sustained hypoxia-induced apoptosis. In contrast, mild, intermittent hypoxia induced significant increases in apoptosis at 2 days (3.72 ± 1.43-fold, n = 8; P < 0.03) and at 4 days (4.57 ± 0.82-fold, n = 8; P < 0.001) that was associated with enhanced caspase activity and attenuated by Z-VAD-FMK pretreatment. We conclude that intermittent hypoxia induces an earlier and more extensive apoptotic response than sustained hypoxia and that this response is at least partially dependent on caspase-mediated pathways. In contrast, caspases do not seem to play a role in sustained hypoxia-induced apoptosis. These findings suggest that different signaling pathways are involved in sustained and intermittent hypoxia-induced cell injury and may contribute to the understanding of differential brain susceptibility to sustained and intermittent hypoxia. episodic hypoxia; neuronal cell death; caspase; hypoxic adaptation     

Exercise-associated differences in an array of proteins involved in signal transduction and glucose transport

Vastus lateralismuscle biopsies were obtained from endurance-trained (running ~50km/wk) and untrained (no regular physical exercise) men, and theexpression of an array of insulin-signaling intermediates wasdetermined. Expression of insulin receptor and insulin receptorsubstrate-1 and -2 was decreased 44% (P < 0.05), 57%(P < 0.001), and 77% (P < 0.001),respectively, in trained vs. untrained muscle. The downstream signalingtarget, Akt kinase, was not altered in trained subjects. Components ofthe mitogenic signaling cascade were also assessed. Extracellularsignal-regulated kinase 1/2 mitogen-activated protein kinase expressionwas 190% greater (P < 0.05), whereas p38 mitogen-activatedprotein kinase expression was 32% lower (P < 0.05), intrained vs. untrained muscle. GLUT-4 protein expression was twofoldhigher (P < 0.05), and the GLUT-4 vesicle-associatedprotein, the insulin-regulated aminopeptidase, was increased 4.7-fold(P < 0.05) in trained muscle. In conclusion, the expressionof proteins involved in signal transduction is altered in skeletalmuscle from well-trained athletes. Downregulation of early componentsof the insulin-signaling cascade may occur in response to increasedinsulin sensitivity associated with endurance training.

     

Bellows, copulations, and sexual selection in bison (Bison bison)

We provide data on the timing and frequency of bellowing episodesand 261 copulations in North American bison (Bison bison) extractedfrom more than 8500 h of observation to examine Darwin's ideathat by using breeding-season vocalizations the male "endeavoursthus to charm or excite the female." Copulatory status affectedvocalization rates; after mating, the frequency of male bellowingdropped to 16.1% of precopulatory rates (p < .0001). Malesbellowed at a higher rate on days when females were in estrus(p < .025) than when they were in anestrus, but females'probability of offspring production in the next year was notcorrelated with bellowing rate. For estrous females, bellowingwas positively correlated with number of attendant males (p< .0001) and inversely related both to the number of daysindividual males had participated in the rut (p < .05) andto the date during the 6-week breeding season (p < .003).Body size also influenced bellowing rate: with other factorsheld constant, small males bellowed more than large males (p< .03). With respect to Darwin's idea that vocalizationsserve as a male display to females, (1) males bellowed neitherbefore nor after copulating when rivals were absent but (2)they bellowed both before and after copulating when rivals werepresent. Overall, these results suggest that bison bellows donot serve as advertisements to females but function as intrasexualdisplays. [Behav Ecol 1991; 2: l-6]