Wildfires lead to decreased carbon and increased nitrogen concentrations in upland arctic streams.

The Central Siberian Plateau is undergoing rapid climate change that has resulted in increased frequency of forest fires and subsequent alteration of watershed carbon and nutrient dynamics. Across a watershed chronosequence (3 to >100 years since wildfire) we quantified the effects of fire on quantity and composition of dissolved organic matter (DOM), stream water nutrient concentrations, as well as in-stream nutrient uptake. Wildfires increased concentrations of nitrate for a decade, while decreasing concentrations of dissolved organic carbon and nitrogen (DOC and DON) and aliphatic DOM contribution for five decades. These post-wildfire changes in stream DOM result in lower uptake efficiency of in-stream nitrate in recently burned watersheds. Nitrate uptake (as uptake velocity) is strongly dependent on DOM composition (e.g. polyphenolics), ambient dissolved inorganic nitrogen (DIN), and DOC to DIN ratios. Our observations and experiments suggest that a decade-long pulse of inorganic nitrogen and a reduction of DOC export occur following wildfires in streams draining the Central Siberian Plateau. Increased fire frequency in the region is thus likely to both decrease DOM and increase nitrate delivery to the main stem Yenisei River, and ultimately the Arctic Ocean, in the coming decades.

Publisher Correction: Ice sheets matter for the global carbon cycle.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Ice sheets matter for the global carbon cycle.

The cycling of carbon on Earth exerts a fundamental influence upon the greenhouse gas content of the atmosphere, and hence global climate over millennia. Until recently, ice sheets were viewed as inert components of this cycle and largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon (>104 Pg C) and nutrients. Here we assess the active role of ice sheets in the global carbon cycle and potential ramifications of enhanced melt and ice discharge in a warming world.

Image-Based Tissue Distribution Modeling for Skeletal Muscle Quality Characterization.

The identification and characterization of regional body tissues is essential to understand changes that occur with aging and age-related metabolic diseases such as diabetes and obesity and how these diseases affect trajectories of health and functional status. Imaging technologies are frequently used to derive volumetric, area, and density measurements of different tissues. Despite the significance and direct applicability of automated tissue quantification and characterization techniques, these topics have remained relatively underexplored in the medical image analysis literature. We present a method for identification and characterization of muscle and adipose tissue in the midthigh region using MRI. We propose an image-based muscle quality prediction technique that estimates tissue-specific probability density models and their eigenstructures in the joint domain of water- and fat-suppressed voxel signal intensities along with volumetric and intensity-based tissue characteristics computed during the quantification stage. We evaluated the predictive capability of our approach against reference biomechanical muscle quality (MQ) measurements using statistical tests and classification performance experiments. The reference standard for MQ is defined as the ratio of muscle strength to muscle mass. The results show promise for the development of noninvasive image-based MQ descriptors.

Machine learning classification of OARSI-scored human articular cartilage using magnetic resonance imaging.

OBJECTIVE: The purpose of this study is to evaluate the ability of machine learning to discriminate between magnetic resonance images (MRI) of normal and pathological human articular cartilage obtained under standard clinical conditions. METHOD: An approach to MRI classification of cartilage degradation is proposed using pattern recognition and multivariable regression in which image features from MRIs of histologically scored human articular cartilage plugs were computed using weighted neighbor distance using compound hierarchy of algorithms representing morphology (WND-CHRM). The WND-CHRM method was first applied to several clinically available MRI scan types to perform binary classification of normal and osteoarthritic osteochondral plugs based on the Osteoarthritis Research Society International (OARSI) histological system. In addition, the image features computed from WND-CHRM were used to develop a multiple linear least-squares regression model for classification and prediction of an OARSI score for each cartilage plug. RESULTS: The binary classification of normal and osteoarthritic plugs yielded results of limited quality with accuracies between 36% and 70%. However, multiple linear least-squares regression successfully predicted OARSI scores and classified plugs with accuracies as high as 86%. The present results improve upon the previously-reported accuracy of classification using average MRI signal intensities and parameter values. CONCLUSION: MRI features detected by WND-CHRM reflect cartilage degradation status as assessed by OARSI histologic grading. WND-CHRM is therefore of potential use in the clinical detection and grading of osteoarthritis.

Near infrared spectroscopic evaluation of water in hyaline cartilage.

In diseased conditions of cartilage such as osteoarthritis, there is typically an increase in water content from the average normal of 60-85% to greater than 90%. As cartilage has very little capability for self-repair, methods of early detection of degeneration are required, and assessment of water could prove to be a useful diagnostic method. Current assessment methods are either destructive, time consuming, or have limited sensitivity. Here, we investigated the hypotheses that non-destructive near infrared spectroscopy (NIRS) of articular cartilage can be used to differentiate between free and bound water, and to quantitatively assess water content. The absorbances centered at 5200 and 6890 cm(-1) were attributed to a combination of free and bound water, and to free water only, respectively. The integrated areas of both absorbance bands were found to correlate linearly with the absolute water content (R = 0.87 and 0.86) and with percent water content (R = 0.97 and 0.96) of the tissue. Partial least square models were also successfully developed and were used to predict water content, and percent free water. These data demonstrate that NIRS can be utilized to quantitatively determine water content in articular cartilage, and may aid in early detection of degenerative tissue changes in a laboratory setting, and with additional validations, possibly in a clinical setting.

Characterization of skin abnormalities in a mouse model of osteogenesis imperfecta using high resolution magnetic resonance imaging and Fourier transform infrared imaging spectroscopy.

Evaluation of the skin phenotype in osteogenesis imperfecta (OI) typically involves biochemical measurements, such as histologic or biochemical assessment of the collagen produced from biopsy-derived dermal fibroblasts. As an alternative, the current study utilized non-invasive magnetic resonance imaging (MRI) microscopy and optical spectroscopy to define biophysical characteristics of skin in an animal model of OI. MRI of skin harvested from control, homozygous oim/oim and heterozygous oim/+ mice demonstrated several differences in anatomic and biophysical properties. Fourier transform infrared imaging spectroscopy (FT-IRIS) was used to interpret observed MRI signal characteristics in terms of chemical composition. Differences between wild-type and OI mouse skin included the appearance of a collagen-depleted lower dermal layer containing prominent hair follicles in the oim/oim mice, accounting for 55% of skin thickness in these. The MRI magnetization transfer rate was lower by 50% in this layer as compared to the upper dermis, consistent with lower collagen content. The MRI transverse relaxation time, T2, was greater by 30% in the dermis of the oim/oim mice compared to controls, consistent with a more highly hydrated collagen network. Similarly, an FT-IRIS-defined measure of collagen integrity was 30% lower in the oim/oim mice. We conclude that characterization of phenotypic differences between the skin of OI and wild-type mice by MRI and FT-IRIS is feasible, and that these techniques provide powerful complementary approaches for the analysis of the skin phenotype in animal models of disease.

Modification of osteoarthritis in the guinea pig with pulsed low-intensity ultrasound treatment.

OBJECTIVE: The Hartley guinea pig develops articular cartilage degeneration similar to that seen in idiopathic human osteoarthritis (OA). We investigated whether the application of pulsed low-intensity ultrasound (PLIUS) to the Hartley guinea pig joint would prevent or attenuate the progression of this degenerative process. METHODS: Treatment of male Hartley guinea pigs was initiated at the onset of degeneration (8 weeks of age) to assess the ability of PLIUS to prevent OA, or at a later age (12 months) to assess the degree to which PLIUS acted to attenuate the progression of established disease. PLIUS (30 mW/cm(2)) was applied to stifle joints for 20 min/day over periods ranging from 3 to 10 months, with contralateral limbs serving as controls. Joint cartilage histology was graded according to a modified Mankin scale to evaluate treatment effect. Immunohistochemical staining for interleukin-1 receptor antagonist (IL-1ra), matrix metalloproteinase (MMP)-3, MMP-13, and transforming growth factor (TGF)-beta1 was performed on the cartilage to evaluate patterns of expression of these proteins. RESULTS: PLIUS did not fully prevent cartilage degeneration in the prevention groups, but diminished the severity of the disease, with the treated joints showing markedly decreased surface irregularities and a much smaller degree of loss of matrix staining as compared to controls. PLIUS also attenuated disease progression in the groups with established disease, although to a somewhat lesser extent as compared to the prevention groups. Immunohistochemical staining demonstrated a markedly decreased degree of TGF-beta1 production in the PLIUS-treated joints. This indicates less active endogenous repair, consistent with the marked reduction in cartilage degradation. CONCLUSIONS: PLIUS exhibits the ability to attenuate the progression of cartilage degeneration in an animal model of idiopathic human OA. The effect was greater in the treatment of early, rather than established, degeneration.

Effects of knee injection on skeletal muscle metabolism and contractile force in rats.

OBJECTIVE: We tested the hypothesis that intrusion of the knee joint capsule alters quadriceps muscle metabolism and function independently from the damage induced to knee cartilage. METHODS: Adult rats were separated into four groups: intraarticular injections of saline (SAL; n=9); intraarticular injections of papain, a model for osteoarthritis (PIA; n=7); sham injections (SHAM; n=8); and controls (CTL; n=5). 31P magnetic resonance spectroscopy (31P-MRS) was performed after 2 weeks. Spectra were obtained from the left quadriceps: two at baseline, eight during electrical stimulation with simultaneous measurement of contractile force, and 15 during recovery. 31P-MRS data were presented as the ratio of inorganic phosphate (Pi) to phosphocreatine (PCr), concentrations of PCr [PCr], intramuscular pH, and the rates and time constants of PCr breakdown during stimulation and PCr recovery. Intramuscular cytokine concentrations were measured within the quadriceps. Histologic slides of the knees were scored for severity of cartilage damage. RESULTS: The interventional groups produced values of Pi/PCr ratio, [PCr], contractile force and pH that were significantly different from CTL. These changes in muscle function were accompanied by higher concentrations of interleukin-1 observed with PIA and SAL. We did not observe any effect of cartilage damage on muscle function or metabolism. CONCLUSIONS: Knee joint intrusion alters quadriceps muscle metabolism with accelerated depletion of energy stores and fatigue during stimulation. This study demonstrates that needle intrusion into the knee joint results in muscle dysfunction, independently from the extent of cartilage damage.

Temporal dynamics of inotropic, chronotropic, and metabolic responses during beta1- and beta2-AR stimulation in the isolated, perfused rat heart.

During the beta-adrenergic receptor (beta-AR)-mediated stress response in the heart, the relations between functional responses and metabolism are ill defined, with the distinction between beta1- and beta2-AR subtypes creating further complexity. Specific outstanding questions include the temporal relation between inotropic and chronotropic responses and their metabolic correlates. We sought to elucidate the relative magnitudes and temporal dynamics of the response to beta1- and beta2-AR stimulation and the energy expenditure and bioenergetic state related to these responses in the isolated perfused rat heart. Inotropic [left ventricular developed pressure (LVDP) and dP/dt], chronotropic [heart rate (HR)], and metabolic responses were measured during beta1- (n = 9; agonist: norepinephrine) and beta2- (n = 9; agonist: zinterol) AR stimulation. Myocardial oxygen consumption (MVO2) was measured using fiber-optic oximetry, and high-energy phosphate levels and intracellular pH were measured using 31P NMR spectroscopy. A multiple-dose protocol was used, with near-maximal beta-AR stimulation at the highest doses. In both beta1 and beta2 groups, there were dose-dependent increases in LVDP, dP/dt, HR, and MVO2. The inotropic response showed more rapid onset, washout, and variation during dose than did the chronotropic response and was closely correlated with MVO2. This suggests that the myocardial bioenergetic state is more closely related to the inotropic response than to the chronotropic response. In addition, beta1-AR stimulation resulted in a greater magnitude and rate of onset of inotropic and MVO2 responses than did beta2-AR stimulation during maximal stimulation. However, a similar decrease in intracellular energy charge was seen in the two groups, consistent with a greater rate of oxidative phosphorylation during beta1- than during beta2-AR stimulation.

EPR oxygen mapping (EPROM) of engineered cartilage grown in a hollow-fiber bioreactor.

A novel electron paramagnetic resonance (EPR)-based oxygen mapping procedure (EPROM) is applied to cartilage grown in a single-, hollow-fiber bioreactor (HFBR) system. Chondrocytes harvested from the sterna of 17-day-old chick embryos were inoculated into an HFBR and produced hyaline cartilage over a period of 4 weeks. Tissue oxygen maps were generated according to the EPROM technique (Velan et al., Magn Reson Med 2000;43:804-809) by making use of the line-broadening effects of oxygen on the signal generated from nitroxide spin probes. In addition, the effect on oxygen consumption of the addition of cyanide to the tissue was investigated. Cyanide is a potent inhibitor of oxidative phosphorylation, and accordingly, given the constant provision of oxygen to the tissue, it would be expected to increase oxygen levels within the HFBR. The EPROM measurements showed a significant increase in oxygen concentration in the cartilage after the addition of cyanide. In contrast to other methods for studying oxygen in cartilage, EPROM can provide direct, noninvasive visualization of local concentrations in three dimensions.

The lever-coil: a simple, inexpensive sensor for respiratory and cardiac motion in MRI experiments.

Thoracic and abdominal magnetic resonance imaging studies generally require some type of compensation for respiratory and cardiac motions in order to yield artifact-free images with good signal-to-noise ratio. Most techniques for respiratory compensation require the use of a non-NMR sensing device to monitor the subject's chest motion, while cardiac motion compensation generally requires the use of ECG leads within the magnet. An inductive pickup coil placed on the subject's chest is perhaps the simplest and least expensive means of monitoring respiration in a MR scanner. However, due to inductive coupling between the pickup coil, radio frequency resonator and gradient set, this arrangement often results in both NMR and respiratory signal artifacts and can also present a burn hazard to the subject depending on the placement and orientation of the pickup coil. Moreover, the presence of a pickup coil on the chest can degrade local magnetic field homogeneity and thus degrade image quality. Similar problems arise when ECG leads must be connected to the subject for cardiac monitoring and gating. To preserve the benefits of the simple pickup coil while circumventing these limitations, a "lever-coil" sensor is presented in which a pickup coil is mechanically coupled to the subject but is not located within the resonator or gradient coil. This results in much lower mutual inductance between the pickup coil and the resonator or gradients. The optimization of the geometry of the apparatus is discussed and lever-coil signal traces are shown which demonstrate the sensor's ability to simultaneously detect both respiratory and cardiac motion in mice. Finally, respiratory-gated and cardiac-triggered spin echo images of the rat abdomen and mouse heart are presented to demonstrate the utility of the lever-coil sensor.

Histomorphometry of the embryonic avian growth plate by proton nuclear magnetic resonance microscopy.

Quantitative nuclear magnetic resonance (NMR) microscopy was used to characterize the biochemical and morphological properties of the different zones within the growth plate of an embryonic chick femur. For precalcified tissue, water proton transverse relaxation times (T2) and magnetization transfer values (MT) were directly and inversely dependent, respectively, on tissue cellularity, defined as the intracellular area per unit area on histological sections. T2 values extrapolated for intra- and extracellular water were 96 ms and 46 ms, respectively. The extracellular T2 was comparable with that measured for mature cartilage. The MT values extrapolated for intra- and extracellular compartments were 0.32 and 0.85, respectively. These values were comparable with those values reported in the literature for cell pellets and for mature cartilage tissue. Thus, cellularity dominated the NMR properties of this immature cartilage tissue. Mineral deposits within calcified cartilage and periosteal bone invoked NMR relaxation processes that were dependent on the inorganic mineral phase. Additionally, collagen molecules present in mineralized zones gave rise to a significant MT effect. These results show the utility of water proton NMR microscopy for assessing both the organic and inorganic phases within mineralized tissues.

Imaging of collagen and proteoglycan in cartilage sections using Fourier transform infrared spectral imaging.

OBJECTIVE: To test the hypothesis that Fourier transform infrared (FTIR) spectral imaging, coupled with multivariate data processing techniques, can image the spatial distribution of matrix constituents in native and engineered cartilage samples. METHODS: Tissue sections from native and trypsin-digested bovine nasal cartilage (BNC) and from engineered cartilage, generated by chick sternal chondrocytes grown in a hollow fiber bioreactor, were placed either on calcium fluoride windows for FTIR analysis or gelatinized microscope slides for histologic analysis. Based on the assumption that cartilage is predominantly chondroitin sulfate (CS) and type II collagen, chemical images were extracted from FTIR spectral imaging data sets using 2 multivariate methods: the Euclidean distance algorithm and a least-squares approach. RESULTS: Least-squares analysis of the FTIR data of native BNC yielded a collagen content of 54 +/- 13% and a CS content of 37 +/- 16% (mean +/- SD). Euclidean distance analysis of measurements made on trypsin-digested BNC demonstrated only trace amounts of CS. For engineered cartilage, the CS content was significantly lower (15 +/- 5%), while the collagen content (73 +/- 6%) was significantly higher than biochemically determined values (CS 34%, collagen 5%, protein 61%). These differences are due to the fact that the dimethylmethylene blue assay overestimated the CS content of the tissue because it is not specific for CS, while the FTIR spectral imaging technique overestimated the collagen content because it lacks specificity for different proteins. CONCLUSION: FTIR spectral imaging combines histology-like spatial localization with the quantitative capability of bulk chemical analysis. For molecules with a unique spectral signature, such as CS, the FTIR technique coupled with multivariate analysis can define a unique spatial distribution. However, for some applications, the lack of specificity of this technique for different types of proteins may be a limitation.

Pitfalls in the measurement of metabolite concentrations using the one-pulse experiment in in Vivo NMR: commentary on "On neglecting chemical exchange effects when correcting in vivo (31)P MRS data for partial saturation".

In an article in a previous issue of the Journal of Magnetic Resonance, Ouwerkerk and Bottomley (J. Magn. Reson. 148, pp. 425--435, 2001) show that even in the presence of chemical exchange, the dependence of saturation factors on repetition time in the one-pulse experiment is approximately monoexponential. They conclude from this fact that the effect of chemical exchange on the use of saturation factors when correcting for partial saturation is negligible. We take issue with this conclusion and demonstrate that because saturation factors in the presence of chemical exchange are strongly dependent upon all of the chemical parameters of the system, that is, upon all T(1)'s and M(0)'s of resonances in the exchange network and upon the reaction rates themselves, it is problematic to apply saturation factor corrections in situations in which any of these parameters may change. The error criterion we establish reflects actual errors in quantitation, rather than departures from monoexponentiality.

Bipolar spectral associative memories.

Nonlinear spectral associative memories are proposed as quantized frequency domain formulations of nonlinear, recurrent associative memories in which volatile network attractors are instantiated by attractor waves. In contrast to conventional associative memories, attractors encoded in the frequency domain by convolution may be viewed as volatile online inputs, rather than nonvolatile, off-line parameters. Spectral memories hold several advantages over conventional associative memories, including decoder/attractor separability and linear scalability, which make them especially well suited for digital communications. Bit patterns may be transmitted over a noisy channel in a spectral attractor and recovered at the receiver by recurrent, spectral decoding. Massive nonlocal connectivity is realized virtually, maintaining high symbol-to-bit ratios while scaling linearly with pattern dimension. For n-bit patterns, autoassociative memories achieve the highest noise immunity, whereas heteroassociative memories offer the added flexibility of achieving various code rates, or degrees of extrinsic redundancy. Due to linear scalability, high noise immunity and use of conventional building blocks, spectral associative memories hold much promise for achieving robust communication systems. Simulations are provided showing bit error rates for various degrees of decoding time, computational oversampling, and signal-to-noise ratio.

31P NMR spectroscopy of developing cartilage produced from chick chondrocytes in a hollow-fiber bioreactor.

31P NMR was used to measure the concentrations and spin-lattice relaxation times of phosphorus-containing metabolites in neocartilage developing in an NMR-compatible hollow-fiber bioreactor over four weeks. Separate studies were performed for tissue developing from chondrocytes taken from the proximal and the distal sternum of the chick embryo. The metabolite ratio beta-ATP/Pi did not change significantly with development (proximal: beta-ATP/Pi = 0.38+/- 0.12 at one week, beta-ATP/Pi = 0.44+/-0.07 at four weeks, P< 0.63; distal: beta-ATP/Pi = 0.39+/-0.05 at one week, beta-ATP/Pi = 0.66+/- 0.26 at four weeks, P<0.28). ATP spin-lattice relaxation times were found to be comparable to those in muscle and brain tissue (proximal: T(1)(beta-ATP) = 0.5+/-0.06 sec at one week, T(1)(beta-ATP) = 0.4+/- 0.01 sec at four weeks; distal: T(1)(beta-ATP) = 0.3+/-0.12 sec at one week, T(1)(beta-ATP) = 0.4+/-0.04 sec at four weeks). A large increase in the spin-lattice relaxation time of inorganic phosphate, from 1.2+/-0.13 sec to 3.8+/-0.04 sec (P<0.0001) over four weeks of growth, was observed in tissue developing from chondrocytes harvested from the proximal sternum. No comparable increase in T(1)(Pi) was found in tissue developing from chondrocytes harvested from the distal portion of the sternum, which ossifies later in vivo.

Adenovirus-mediated VEGF(121) gene transfer stimulates angiogenesis in normoperfused skeletal muscle and preserves tissue perfusion after induction of ischemia.

BACKGROUND: Administration of angiogenic factors stimulates neovascularization in ischemic tissues. However, there is no evidence that angiogenesis can be induced in normoperfused skeletal muscles. We tested the hypothesis that adenovirus-mediated intramuscular (IM) gene transfer of the 121-amino-acid form of vascular endothelial growth factor (AdCMV.VEGF(121)) could stimulate neovascularization in nonischemic skeletal muscle and consequently attenuate the hemodynamic deficit secondary to surgically induced ischemia. METHODS AND RESULTS: Rabbits and rats received IM injections of AdCMV.VEGF(121), AdCMV.Null, or saline in the thigh, 4 weeks (rabbits) or 2 weeks (rats) before femoral artery removal in the injected limb. In unoperated rats, at the site of injection of AdCMV.VEGF(121), we found 96% and 29% increases in length density of arterioles and capillaries, respectively. Increased tissue perfusion (TP) to the ischemic limb in the AdCMV.VEGF(121) group was documented, as early as day 1 after surgery, by improved blood flow to the ischemic gastrocnemius muscle measured by radioactive microspheres (AdCMV.VEGF(121)=5.69+/-0.40, AdCMV.Null=2.97+/-0.50, and saline=2.78+/-0.43 mL x min(-1) x 100 g(-1), P<0.001), more angiographically recognizable collateral vessels (angioscore) (AdCMV. VEGF(121)=50.58+/-1.48, AdCMV.Null=29.08+/-4.22, saline=11.83+/-1.90, P<0.0001), and improvement of the bioenergetic reserve of the gastrocnemius muscle as assessed by (31)P NMR spectroscopy. Follow-up studies showed that superior TP to the ischemic limb in the AdCMV.VEGF(121) group persisted until it was equalized by spontaneous collateral vessel development in untreated animals. CONCLUSIONS: IM administration of AdCMV.VEGF(121) stimulates angiogenesis in normoperfused skeletal muscles, and the newly formed vessels preserve TP after induction of ischemia.

Response of engineered cartilage tissue to biochemical agents as studied by proton magnetic resonance microscopy.

OBJECTIVE: To test the hypothesis that magnetic resonance imaging (MRI) results correlate with the biochemical composition of cartilage matrix and can therefore be used to evaluate natural tissue development and the effects of biologic interventions. METHODS: Chondrocytes harvested from day-16 chick embryo sterna were inoculated into an MRI-compatible hollow-fiber bioreactor. The tissue that formed over a period of 2-4 weeks was studied biochemically, histologically, and with MRI. Besides natural development, the response of the tissue to administration of retinoic acid, interleukin-1beta (IL-1beta), and daily dosing with ascorbic acid was studied. RESULTS: Tissue wet and dry weight, glycosaminoglycan (GAG) content, and collagen content all increased with development time, while tissue hydration decreased. The administration of retinoic acid resulted in a significant reduction in tissue wet weight, proteoglycan content, and cell number and an increase in hydration as compared with controls. Daily dosing with ascorbic acid increased tissue collagen content significantly compared with controls, while the administration of IL-1beta resulted in increased proteoglycan content. The water proton longitudinal and transverse relaxation rates correlated well with GAG and collagen concentrations of the matrix as well as with tissue hydration. In contrast, the magnetization transfer value for the tissue correlated only with total collagen. Finally, the self-diffusion coefficient of water correlated with tissue hydration. CONCLUSION: Parameters derived from MR images obtained noninvasively can be used to quantitatively assess the composition of cartilage tissue generated in a bioreactor. We conclude that MRI is a promising modality for the assessment of certain biochemical properties of cartilage in a wide variety of settings.

The relationship between creatine kinase kinetics and exercise intensity in human forearm is unchanged by age.

Using (31)P magnetic resonance spectroscopy, creatine kinase (CK) reaction kinetics was assessed in the forearm flexor digitorum profundus muscle of healthy young (n = 11, age 34.7 +/- 5 yr) and older (n = 20, age 73.5 +/- 8 yr) subjects at rest, intermittent exercise at 20% maximum voluntary contraction (MVC), and 40% MVC. Exercise resulted in a significant increase in the average ratio of inorganic phosphate (P(i)) to phosphocreatine (PCr) from resting values of 0.073 +/- 0.031 (young) and 0.082 +/- 0.037 (older) to 0. 268 +/- 0.140 (young, P < 0.01) and 0.452 +/- 0.387 (older, P < 0. 01) at 40% MVC. At 40% MVC, intracellular pH decreased significantly, from resting values of 7.08 +/- 0.08 (young) and 7.08 +/- 0.11 (older) to 6.84 +/- 0.19 (young, P < 0.05) and to 6.75 +/- 0.25 (older, P < 0.05). Average values of the pseudo-first-order reaction rate k((PCr-->ATP)) at rest were 0.07 +/- 0.04 s(-1) in the young and 0.07 +/- 0.03 s(-1) in the older group. At both exercise levels, the reaction rate constant increased compared with the resting value, but only the difference between the resting value and the 20% MVC value, which showed an 86% higher reaction rate constant in both groups, reached statistical significance (P < 0.05). No difference in the reaction rate constant between the young and older groups was observed at either exercise level. As with k((PCr-->ATP)), the average phosphorus flux through the CK reaction increased during exercise at 20% MVC (P < 0.05 in the older group) but decreased toward resting values at 40% MVC in both groups. The data in our study suggest that normal aging does not significantly affect the metabolic processes associated with the CK reaction.