Effects of prolonged sleep deprivation on local rates of cerebral energy metabolism in freely moving rats

Although sleep deprivation interferes with biological processes essential for performance, health, and longevity, previous studies have failed to reveal any structural or functional changes in brain. We have therefore measured local rates of cerebral glucose utilization (ICMRglc) with the quantitative autoradiographic 2-14C-deoxyglucose method in an effort to determine if and, if so, where sleep deprivation might affect function in sleep-deprived rats. Sleep deprivation was maintained for 11-12 d, long enough to increase whole body energy metabolism, thus confirming that pathophysiological processes that might involve brain functions were evolving. Deep brain temperature was also measured in similarly treated rats and found to be mildly elevated relative to core body temperature. Despite the increased deep brain temperature, systemic hypermetabolism, and sympathetic activation, ICMRglc was not elevated in any of the 60 brain structures examined. Average glucose utilization in the brain as a whole was unchanged in the sleep-deprived rats, but regional decreases were found. The most marked decreases in ICMRglc were in regions of the hypothalamus, thalamus, and limbic system. Mesencephalic and pontine regions were relatively unaffected except for the central gray area. The medulla was entirely normal. The effects of sleep deprivation on brain tended, therefore, to be unidirectional toward decreased energy metabolism, primarily in regions associated with mechanisms of thermoregulation, endocrine regulation, and sleep. Correspondence was found between the hypometabolic brain regions and some aspects of peripheral symptoms.

Effects of chronic dietary aluminum on local cerebral glucose utilization in rats

Beginning at 4 weeks of age normal, male, Sprague-Dawley rats were reared on Purina Laboratory Chow and drinking water containing 100 microM AlCl3. After 2 years, local rates of cerebral glucose utilization were determined with the autoradiographic [14C] deoxyglucose method in the brain as a whole and in 25 brain regions in 6 treated rats and 4 age-matched controls. The results indicate that any effects of chronic aluminum in the diet on rates of cerebral glucose utilization are small. In the brain as a whole, the mean rate of glucose utilization in the aluminum-treated rats was 6% lower than that of the controls (p = 0.09). In 21 of the 25 brain regions examined mean rates of glucose utilization were generally lower in the aluminum-treated rats but in none of the region were the effects statistically significant.

The use of spectral analysis to determine regional cerebral glucose utilization with positron emission tomography and [18F]fluorodeoxyglucose: theory, implementation, and optimization procedures

A method for kinetic analysis of dynamic positron emission tomography (PET) data by linear programming that allows identification of the components of a measured PET signal without predefining a compartmental model has recently been proposed by Cunningham and co-workers. The method identifies a small subset of functions from a large input set of feasible functions that best fits the time course of total radioactivity measured by PET. To investigate in detail the properties of this technique, we applied it to PET studies with [18F]fluorodeoxyglucose, a tracer with well-characterized kinetic properties. We examined dynamically acquired data over various time intervals in many brain regions and found that the number of components identified by the method is stable and consistent with the presence of kinetic heterogeneity in every region. We optimized the method for determination of regional rates of glucose utilization; calculated rates were found to be somewhat dependent upon the treatment of noise in the measured tissue data and upon the time interval in which the data were collected. The application of a numerical filter to remove noise in the data resulted in values for regional cerebral glucose utilization that were stable with time and consistent with rates determined by the other established techniques. Based on the results of the current study, we expect that the spectral analysis technique will prove to be a highly flexible tool for kinetic analysis of other tracer compounds; it is capable of producing low-variance, time-stable estimates of physiological parameters when optimized for time interval of application, input spectrum of components, and processing of noise in the data.

Local cerebral blood flow during hibernation, a model of natural tolerance to "cerebral ischemia"

The breakdown of cellular homeostasis and progressive neuronal destruction in cerebral ischemia appears to be mediated by a complex network of causes that are intricately interrelated. We have investigated a physiological state existing normally in nature in which mammals appear to tolerate the ordinarily detrimental effects of ischemia with reduced oxygen availability and to resist activation of self-destructive processes, i.e., mammalian hibernation. Ground squirrels (Spermophilus tridecemlineatus) were chronically implanted with arterial and venous catheters and telemetry devices for electroencephalography, electrocardiography, and monitoring of body temperature. The animals were placed in an environmental chamber at an ambient temperature of 5 degrees C. Entrance into hibernation was characterized by a drop in heart rate followed by a gradual decline in body temperature and an isoelectric electroencephalogram. Cold-adapted active animals that were not hibernating served as controls. Cerebral blood flow (CBF) was measured in both groups with the autoradiographic [14C]iodoantipyrine method. Mean (+/- SD) mass-weighted CBF in the brain was 62 +/- 18 ml 100 g(-1) min (-1) (n = 4) in the control group but was reduced to ischemic levels, 7 +/- 4 ml 100 g(-1) min (-1) (n = 4), in the hibernating animals (p < 0.001) [corrected]. No neuropathological changes were found in similarly hibernating animals aroused from hibernation. Hibernation appears to be actively regulated, and hormonal factors may be involved. The identification and characterization of such factors and of the mechanisms used by hibernating species to increase ischemic tolerance and to blunt the destructive effects of ischemia may enable us to prevent or minimize the loss of homeostatic control during and after cerebral ischemia in other species.

Effects of physostigmine on local cerebral glucose utilization in the central components of the rat visual system

The effects of intravenous administration of physostigmine at doses of 0.03, 0.095, or 0.3 mg/kg on local cerebral glucose utilization (LCGU) were determined in 3 structures of the visual system of the rat brain by means of the quantitative 2-[14C]deoxyglucose method. LCGU was increased in the superior colliculus (superficial gray layer), but unchanged in the visual cortex and the lateral geniculate body. To determine whether the observed effect of physostigmine on the superior colliculus depended on input from the retina, the highest dose of physostigmine was administered to rats which had previously been enucleated bilaterally. Enucleation decreased LCGU in the superior colliculus of the animals not treated with physostigmine and blocked the effect of physostigmine on LCGU. The effect of physostigmine in the superior colliculus appears, therefore, to depend on input from the retina.

Proximal femur of Australopithecus africanus from Member 4, Makapansgat, South Africa

A left proximal femur (MLD 46) from Member 4, Makapansgat, South Africa is described and analyzed. It consists of the head, neck, and a small segment of the shaft that extends to just below the lesser trochanter. The femur exhibits degenerative joint disease in the form of marginal osteophyte formation and thus its taxonomic identity has been somewhat obscured. Consideration of all like-sized mammalian femora from Makapansgat suggests that the femur is that of either a felid or hominid. Comparison of MLD 46 to femora of extent and extinct felids reveals that MLD 46 does not possess two morphological features that are characteristic of felids, namely a deep, prolonged trochanteric fossa and a high neck-shaft angle. Simple shape variables (ratios) and multivariate analyses consistently place MLD 46 with modern and fossil hominids, and most closely align it with the australopithecines. We conclude that the femur is most reasonably attributable to Australopithecus africanus, which is the only hominid yet identified from Makapansgat. Despite its pathological condition, MLD 46 is the most complete proximal femur known for A. africanus, thereby permitting further morphological comparisons with homologues of A. afarensis and Paranthropus. Marginal osteophytes of mammalian femoral heads characteristically occur in individuals of advanced age, suggesting that MLD 46 may have lived some time with the disease. Finally, MLD 46 is considerably larger than the previously described specimen, Sts 14, from Sterkfontein Member 4. There may be as great a contrast in body size in A. africanus as there is between the large and small specimens of A. afarensis.

Function-related changes in energy metabolism in the nervous system: localization and mechanisms

Previous neuroanatomical and electrophysiological methods to localize functional activity in the nervous system focus on perikarya as the sites of the activity. Metabolic mapping of local functional activity in the nervous system provides a new dimension, the activity in the neuropil. Studies of local glucose utilization have shown that energy metabolism is increased by functional activation, but the effects are mainly in the terminal projection zones of the activated pathway. Electrical stimulation of a pathway raises glucose utilization in the projection zones of the pathway in almost direct proportion to the spike frequency. For example, electrical stimulation of the sciatic nerve produces frequency-dependent metabolic activation in the dorsal horn of the lumbar cord, where the axonal terminals of the afferent pathway reside, but no apparent metabolic effects in the cell bodies of the pathway in the dorsal root ganglia. Functional activation of the hypothalamo-hypophysial pathway by salt-loading increases glucose utilization in the pituitary neural lobe, where the terminal axons of the pathway reside, but not in the paraventricular and supraoptic nuclei, the sites of the cell bodies of origin of the pathway. Reflex activation by hypotension of pathways to these nuclei from brain stem structures involved in baroreceptor reflexes, however, increases glucose utilization in these nuclei. Depolarization induced by electrical stimulation, extracellular K+, or opening of Na+ channels with veratridine, stimulates glucose utilization in neural tissues, and this increase is blocked by ouabain, a specific inhibitor of Na+, K(+)-ATPase. Activation of this enzyme to restore ionic gradients across cellular membranes appears to trigger the functional activation of energy metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of pharmacologic doses of 2-deoxy-D-glucose on local cerebral blood flow in the awake, unrestrained rat

Previous studies on the effects of acute insulin-induced hypoglycemia on cerebral blood flow (CBF) have resulted in conflicting results. An alternate approach to the study of glucoprivation is the administration of pharmacologic doses of the glucose analogue, 2-deoxy-D-glucose (2-DG). 2-DG is transported across the blood-brain barrier into brain tissue where it is phosphorylated to 2-deoxy-D-glucose-6-phosphate (2-DG-6-P) but not metabolized further. The 2-DG-6-P accumulates and inhibits the conversion of glucose-6-phosphate to fructose-6-phosphate, thus blocking glycolysis and glucose metabolism. In the present study we have employed the [14C]iodoantipyrine method to examine the effects of a pharmacologic dose (500 mg/kg) of 2-DG on local cerebral blood flow (lCBF) in 29 regions of the brain in conscious, unrestrained, adult male rats. The 2-DG treatment raised arterial plasma glucose levels from 8 to 17 mM without affecting arterial blood pO2, pCO2, or pH but increased lCBF in most brain regions examined. The largest increases were in the cerebral cortex, basal ganglia, and thalamic nuclei (+65 to +157%). Smaller increases were found in most structures of the limbic system, brainstem, and white matter, and no changes in lCBF were seen in the cerebellar cortex and ventral medial hypothalamus. The results indicate that cerebral glucoprivation produced by pharmacological doses of 2-deoxyglucose is accompanied by substantial increase in blood flow in most regions of the brain.

Visually guided reaching with the forelimb contralateral to a "blind" hemisphere: a metabolic mapping study in monkeys

The 2-14C-deoxyglucose method was used to map local cerebral metabolic activity in monkeys performing a unimanual task requiring visually guided arm reaching and key pressing. The study was carried out with monkeys that either had intact brains or had one hemisphere deprived of visual input by unilateral optic tract section combined in some cases with forebrain commissurotomy. The metabolic mapping revealed activation of sensorimotor cortex only in the hemisphere contralateral to the moving forelimb, irrespective of whether this hemisphere was intact or visually deafferented. These results suggest that visually guided reaching with the forelimb contralateral to the "blind" hemisphere is subserved by that hemisphere's sensorimotor cortex and not by the cortex of the ipsilateral, "seeing" hemisphere. Other areas that were more active metabolically in the "blind" than in the "seeing" hemisphere included the supplementary motor, the secondary somatosensory, and certain posterior parietal cortical areas, intraparietal lateral 5 (lateral 5-ip), 7a, and intraparietal 7 (7-ip). It is suggested that the "blind" hemisphere utilizes at least two distinct pieces of information to guide forelimb movements to visual targets: (1) information about the location of the visual target derived from head and eye movements made to this target and mediated via the inferior parietal cortical areas 7a and 7-ip, and (2) information about the instantaneous upper extremity position derived from forelimb proprioceptive mechanisms and mediated via the somatosensory cortex and thereafter via the superior parietal cortical area, lateral 5-ip.

Local glucose utilization and local blood flow in hearts of awake rats

Local cardiac glucose utilization and local cardiac blood flow in rat heart were measured in vivo by quantitative autoradiographic techniques with 2-[14C] deoxyglucose and [14C] iodoantipyrine, respectively. [14C]methylmethacrylate standards were calibrated for quantitative autoradiography of dried sections of heart tissue; the calibration values for heart tissue differed from those for brain by 8%, probably because of differences in self-absorption within the tissues. The lumped constant required by the deoxyglucose method was determined in isolated, perfused, working rat hearts and found to be 1.11 +/- 0.36 (mean +/- SD, n = 21). The heart: blood partition coefficient for iodoantipyrine required by the [14C]iodoantipyrine method was measured and found to be 1.25. The results obtained in awake rats showed: 1) overall cardiac glucose utilization varied considerably among animals with a mean of 53 (left ventricle) and 30 (right ventricle) mumol/100 g/min; 2) cardiac blood flow was less variable among animals with a mean of 592 (left ventricle) and 420 (right ventricle) ml/100 g/min; 3) glucose utilization was found to be particularly high in the papillary muscle; 4) systematic gradients of glucose utilization or blood flow in the ventricular wall were not observed; 5) glucose utilization and blood flow were not closely correlated on a local level. It is concluded that autoradiographic methods are suitable for the quantification of local glucose utilization and local blood flow in the rat heart in vivo. These methods could not demonstrate transmural gradients for glucose utilization and blood flow between epi- and endocardium in awake rats.

Metabolites of 2-deoxy-[14C]glucose in plasma and brain: influence on rate of glucose utilization determined with deoxyglucose method in rat brain

The [14C]deoxyglucose ([14C]DG) method depends upon quantitative trapping of metabolites in brain at the site of phosphorylation, and in the usual procedure it is assumed that all the label in plasma is in free DG. Our previous finding of labeled nonacidic derivatives of DG in plasma raised the possibility that some metabolites of DG might not be fully retained in body tissues and therefore cause overestimation of the integrated specific activity of the precursor pool determined from assay of label in plasma and/or underestimation of the true size of the metabolite fraction in brain. In the present study, metabolism of DG in rat tissues by secondary pathways was examined and found to be more extensive than previously recognized. When 14C-labeled compounds in ethanol extracts of either plasma or brain were separated by anion exchange HPLC, eight fractions were obtained. 14C-labeled metabolites in plasma were detected after a 35-min lag and gradually increased in amount with time after an intravenous pulse. In brain, deoxyglucose-6-phosphate was further metabolized, mainly to deoxyglucose-1-phosphate and deoxyglucose-1,6-phosphate. These are acid-labile compounds and accounted for approximately 20% of the 14C in the metabolite pool in brain. The rate constants for net loss of 14C from the metabolite pool between 45 and 180 min after a pulse were similar (0.4-0.5%/min) in vivo and in intact postmortem brain. The rate constant for loss of deoxyglucose-6-phosphate (DG-6-P) in vivo (approximately 0.7%/min) was, however, about twice that for postmortem brain, suggesting that a significant fraction of the DG-6-P lost in vivo is due to its further metabolism by energy-dependent reactions. 14C-labeled metabolites of [14C]DG in plasma and brain do not interfere with determination of local rates of glucose utilization in brain in normal, conscious rats by the autoradiographic method if the prescribed procedures and a 45-min experimental period are used.

Measurement of regional cerebral glucose utilization with fluorine-18-FDG and PET in heterogeneous tissues: theoretical considerations and practical procedure

Functional tissue heterogeneity, i.e., inclusion of tissues with different rates of blood flow and metabolism within a single region of interest, is an unavoidable problem with PET. Errors in determination of regional cerebral glucose utilization (rCMRglc) with [18F]FDG have resulted from the currently used simplifying assumption that all regions examined are homogeneous. We have established an optimal, yet practical procedure to minimize errors due to tissue heterogeneity in determination of rCMRglc. Effects of applying the three-rate constant kinetic model designed for homogeneous tissues with both dynamic and single-scan procedures and the Patlak plot were evaluated in normal subjects in experimental periods up to 120 min following tracer injection. The procedure with a single scan carried out any time within the interval between 60 and 120 min following tracer injection, combined with population average rate constants determined over a 120-min period, was found to be optimal for quantitative rCMRglc studies.

Microcracks in the calcified layer of articular cartilage

Evidence is presented that basophilic radial hairlines in the calcified layer of articular cartilage are in vivo microcracks. They were present in three of 25 normal patellas from subjects below the age of 50 years, in 20 of 25 recently fractured hips (average patient age, 77.3 years), and in 16 of 25 osteoarthritic tibial plateaus (71.7 years). It is hypothesized that extension of these microfractures beyond the calcified layer mediates remodeling of the osteochondral junction in aging and degenerative joint disease.

Sites and mechanisms of function-related changes in energy metabolism in the nervous system

Traditional neuroanatomical and electrophysiological methods to localize functional activities in the nervous system focus on perikarya as the sites of activity. Metabolic mapping of local functional activity in the nervous system with the deoxyglucose method has directed interest toward the activity in neuropil. Studies of local glucose utilization (lCMRglc) indicate that energy metabolism is increased by functional activation mainly in terminal projection zones of activated pathways. Electrical stimulation of a pathway raises lCMRglc in the projection zones of the pathway in almost direct proportion to the spike frequency. For example, stimulation of the sciatic nerve produces frequency-dependent metabolic activation in the dorsal horn of the lumbar cord, where the axonal terminals of the afferent pathway reside, with no apparent metabolic effects in the cell bodies of the pathway in the dorsal root ganglia. Functional activation of the hypothalamo-hypophysial pathway by salt-loading increases lCMRglc in the neurohypophysis, the site of the terminal axons of the pathway, but not in the paraventricular and supraoptic nuclei, where the cell bodies of origin of the pathway reside. Activation by hypotension of pathways to these nuclei from brain stem structures involved in baroceptor reflexes does, however, increase lCMRglc in these nuclei. Depolarization induced by electrical stimulation, increased extracellular K+, or opening of Na+ channels with veratridine stimulate lCMRglc in neural tissues, and this increase is blocked by ouabain, a specific inhibitor of Na+,K(+)-ATPase. Activation of this enzyme to restore ionic gradients across cellular membranes appears to trigger the function-related increase in energy metabolism. The metabolic activation is the consequence not of the functional activity itself but of processes operating to recover from that activity.

Residual effects of tracer in sequential double label deoxyglucose studies

The validity of sequential double label deoxyglucose (DG) determinations of local metabolic rate for glucose (IMRglc) was examined by quantifying the degree of trapping of residual first DG tracer during the second experimental period. One sciatic nerve was repetitively stimulated for 25 min, beginning either at the time of the DG injection or 25 min later. IMRglc in the ipsilateral dorsal horn of the lumbar spinal cord was found to be 105% and 56%, respectively, greater than that of the contralateral unstimulated side. Attempts to lower the body burden of radioactive DG by exchange blood transfusion failed to reduce this delayed effect. These data indicate that residual effects of the first tracer could obscure possible differences in IMRglc between two sequential experimental states.

Comparison of rates of local cerebral glucose utilization determined with deoxy[1-14C]glucose and deoxy[6-14C]glucose

The activity of the pentose phosphate shunt pathway in brain is thought to be linked to neurotransmitter metabolism, glutathione reduction, and synthetic pathways requiring NADPH. There is currently no method available to assess flux of glucose through the pentose phosphate pathway in localized regions of the brain of conscious animals in vivo. Because metabolites of deoxy[1-14C]glucose are lost from brain when the experimental period of the deoxy[14C]glucose method exceeds 45 min, the possibility was considered that the loss reflected activity of this shunt pathway and that this hexose might be used to assay regional pentose phosphate shunt pathway activity in brain. Decarboxylation of deoxy[1-14C]glucose by brain extracts was detected in vitro, and small quantities of 14C were recovered in the 6-phosphodeoxygluconate fraction when deoxy[14C]glucose metabolites were isolated from freeze-blown brains and separated by HPLC. Local rates of glucose utilization determined with deoxy[1-14C]glucose and deoxy[6-14C]glucose were, however, similar in 20 brain structures at 45, 60, 90, and 120 min after the pulse, indicating that the rate of loss of 14CO2 from deoxy[1-14C]glucose-6-phosphate in normal adult rat brain is too low to permit assay pentose phosphate shunt activity in vivo. Further metabolism of deoxy[1-14]glucose-6-phosphate via this pathway does not interfere during routine use of the deoxyglucose method or explain the progressive decrease in calculated metabolic rate when the experimental period exceeds 45 min.

Determination of regional rates of cerebral protein synthesis adjusted for regional differences in recycling of leucine derived from protein degradation into the precursor pool in conscious adult rats

The quantitative autoradiographic L-[1-14C]leucine method for the determination of regional rates of cerebral protein synthesis in vivo takes into account recycling of unlabeled leucine derived from protein degradation into the precursor pool for protein synthesis. We have evaluated the degree of recycling by measuring the ratio of the apparent steady-state leucine specific activity in the precursor amino acid pool (tRNA-bound leucine) to that in the arterial plasma. In the whole brain of the conscious rat this ratio (lambda WB) equals 0.58. The equivalent ratio for leucine in the acid-soluble pool in whole brain (psi WB) is 0.49. A first-degree polynomial equation for lambda WB as a function of psi WB was fitted from paired determinations. To determine the degree of recycling in local regions of the brain, we have measured in individual brain regions (i) psi i and calculated lambda i assuming that the fitted equation also applies to these localized regions. Our results indicate that the degree of recycling into the precursor pool does vary regionally; lambda i in the individual regions varies from 0.62 in the hypoglossal nucleus to 0.50 in the globus pallidus. Local rates of protein synthesis were then determined by the autoradiographic technique with regional corrections for recycling of unlabeled leucine. Rates of leucine incorporation into protein averaged 6.1 nmol/g of tissue/min in the brain as a whole, with the rates in gray matter about twice those in white matter.

Errors introduced by tissue heterogeneity in estimation of local cerebral glucose utilization with current kinetic models of the [18F]fluorodeoxyglucose method

The effects of tissue heterogeneity on the estimation of regional cerebral glucose utilization (rCMRglc) in normal humans with [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) and positron emission tomography (PET) were compared with respect to the various kinetic models of the [18F]FDG method. The kinetic models were conventional homogeneous tissue models of the [18F]FDG method, with (4K Model) and without (3K Model) a rate constant to account for an apparent loss of [18F]2-fluoro-2-deoxy-D-glucose-6-phosphate ([18F]FDG-6-P), and a tissue heterogeneity model (TH Model). When either of the kinetic models designed for homogeneous tissues was applied to heterogeneous tissues, estimates of the rate constant for efflux of [18F]FDG from the tissue (k2*) and of the rate constant for phosphorylation of [18F]FDG (k3*) decreased as the duration of the experimental period was increased. When the 4K Model was used, estimates of the rate constant for the apparent dephosphorylation of [18F]FDG-6-P (k4*) were significantly greater than zero and fell with increasing duration of the experimental period. Although the TH Model included no term to describe an apparent dephosphorylation of [18F]FDG-6-P, the fit of the TH Model to the time course of total tissue radioactivity was at least as good as and often better than the fit of the 4K Model in the 120-min period following the pulse of [18F]FDG. Hence, the high estimates of k4* found in PET studies of less than or equal to 120 min can be explained as the consequence of measuring radioactivity in a heterogeneous tissue and applying a model designed for a homogeneous tissue; there remains no evidence of significant dephosphorylation of [18F]FDG-6-P in this time period. Furthermore, use of the 4K Model led to an overestimation of rCMRglc; whole-brain glucose utilization calculated with the 4K Model was greater than 20% higher than values usually obtained in normal humans by the model-independent Kety-Schmidt technique. rCMRglc was accurately estimated by the TH Model and, in experimental periods sufficiently long to minimize the effects of tissue heterogeneity, also by the original 3K Model of the deoxyglucose method.

Energy Metabolism and Effects of Energy Depletion or Exposure to Glutamate

The entire program of the first day of the IBRO satellite meeting entitled Ions, Water, and Energy in Brain Cells was devoted to the subject of energy. There were three sessions on the topics of energy metabolism, activation, and development and pathological conditions, followed by a final general discussion on the contents of the day's topics. During this general discussion there were spirited exchanges on the role of glycogen in the energy metabolism of the brain, on the metabolic source of the energy consumed by functional activity, e.g., glycolytic or oxidative energy metabolism, and on the sources of the acid-equivalents that are responsible for the tissue acidosis accompanying cerebral hypoxia. Despite the arguments pro and con presented on all of the issues that were discussed, it is doubtful that a consensus was achieved on most of the issues.Key words: glycogen, glycolysis, oxidative metabolism, acidosis, energy metabolism.

Characterization of a bovine synovial fluid lubricating factor. III. The interaction with hyaluronic acid

Although hyaluronate is not a boundary lubricant in cartilaginous and latex:glass bearings, a distinct interaction with purified synovial lubricating factor (PSLF) was demonstrated by three means: 1) enhancement of lubricating ability in an artificial test system; 2) viscometry; 3) electron microscopy. The interaction was of a physical rather than a specific chemical type; it varied with the degree of purification of PSLF and of hyaluronate. The interaction accounts for retention of the relatively small PSLF molecule (approximately 280 kDa) with the synovial mucin on a 0.22 microns filter. The data provide evidence that hyaluronate and PSLF act synergistically in the boundary lubricating activity of animal joints.

Effect of loading doses of L-valine on relative contributions of valine derived from protein degradation and plasma to the precursor pool for protein synthesis in rat brain

"Flooding" amino acid pools with high doses of labeled amino acids of low specific activity has been proposed to minimize the effects of recycling of amino acids derived from protein degradation on the specific activity of the amino acid precursor pool for protein synthesis. We have examined the influence of recycling on the precursor pool for protein synthesis under conditions in which plasma valine concentrations were normal (0.19 mM) and "flooded" (10-28 mM) by comparing the steady-state specific activity of the tRNA-bound valine with that of the plasma valine. Under normal and "flooding" conditions, the relative contributions of valine from protein degradation to the precursor pool were 63 and 26%, respectively; "flooding" with a plasma level of 28 mM raised the brain acid-soluble pool level to 3.1 mM but was no more effective in decreasing the relative contribution of valine from protein degradation to the precursor pool than "flooding" with a plasma level of 17 mM valine, which raised the brain acid-soluble level only to 2.3 mM. The results of these studies show that "flooding" amino acid pools does indeed reduce the effect of recycling on the precursor amino acid pool for protein synthesis, but it does not totally eliminate it.