A model of whole-body protein turnover based on leucine kinetics in rodents

The measurement of fractional synthesis rate is based on the following assumptions: amino acids for protein synthesis are supplied by an intracellular pool; amino acids from protein degradation are not recycled preferentially to protein synthesis; and proteins turn over at a homogeneous rate. To test these assumptions, a mechanistic, theoretical model of protein turnover for a nongrowing 26-g mouse was developed on the basis of data from the literature. The model consisted of three protein pools turning over at fast (102 micromol Leu, t1/2= 11.5 h), medium (212 micromol Leu, t1/2 = 16.6 h) or slow (536 micromol Leu, t1/2 = 71.5 h) rates and extracellular (1.69 micromol Leu), leucyl-tRNA (0.0226 micromol Leu) and intracellular (5.72 micromol Leu) amino acid pools that exchanged amino acids. The flow of amino acids from the protein pools to the leucyl-tRNA pool determined the amount of recycling. The flow of amino acids from the extracellular pool to aminoacyl tRNA determined the amount of channeling. Two flooding dose data sets were used to evaluate specific radioactivity changes predicted by the model. Predictions of specific radioactivities using flooding dose, pulse dose or continuous infusion methods indicated that the model can be a useful tool in estimating the rates of channeling and recycling. However, it was found that use of data from flooding dose experiments might cause inaccurate predictions of certain fluxes.     

Differential response of fast hindlimb extensor and flexor muscles to exercise in adult spinalized cats

Adult cats were spinal transected (T12-13) and maintained for approximately 6 months. Spinal cats were either not trained (N-T) or trained for 30 min/day to either step on a treadmill (Stp-T) or stand (Std-T). Spinalization resulted in a decrease in the mass and maximum tension potential of the medial gastrocnemius (MG), a fast ankle extensor. These adaptations were ameliorated in Std-T but not Stp-T cats. The maximum rate of shortening was elevated by 18 (ns), 34, and 19 (ns)% in the N-T, Std-T, and Stp-T cats, respectively, a finding consistent with a shift in the percentage of fast fibers, a decrease in the percentage of fibers expressing only type I myosin heavy chain, and an increase in myofibrillar adenosine triphosphatase activity. The shift toward a faster fiber type profile in the tibialis anterior (TA), a fast ankle flexor, was of a lesser magnitude than in the MG. There were no significant effects on the contractile properties of the TA in any group of spinal cats. The greater preservation of muscle mass, shift toward faster physiological and biochemical properties, and fatigability in the MG of Std-T than Stp-T cats suggest that factors other than the level of activation and force generation must play a role in muscle homeostasis. From a clinical perspective, the results indicate that muscles innervated by motor neurons below the level of a complete spinal cord lesion are affected differentially by specific neuromuscular activity patterns.     

Reverse transport of glutamate during depolarization in immature hippocampal slices

We studied the source of extracellular glutamate released by hippocampal slices obtained from P14 or adult rats, during 50 mM K+ depolarization by using two potent inhibitors of Na+-dependent glutamate transport: l-trans-pyrrolidine-2,4-dicarboxylate (PDC), which is a relatively non-selective inhibitor of various glutamate transporter subtypes and dihydrokainic acid (DHK), a specific inhibitor of the glial transporter, GLT-1. Most depolarization-induced glutamate release was Ca2+-dependent in adults, while in P14 slices most glutamate release was Ca2+-independent. PDC decreased depolarization-induced glutamate release in P14 slices but not in adults. DHK increased glutamate release in adults but not in P14 slices. These data suggest that most depolarization-induced glutamate release in immature hippocampal slices is due to reversal of transport through a PDC-sensitive Na+-dependent glutamate transporter, presumably acting on presynaptic or cytoplasmic neuronal pools, and is not due to exocytosis from vesicular pools.     

Screening for depression in African-Caribbean elders

BACKGROUND: There are increasing numbers of older African-Caribbeans in the UK. Primary care staff often feel less confident about diagnosing depression in this group. Screening instruments may assist in making diagnoses in cross-cultural consultations. OBJECTIVE: We aimed to determine the sensitivity and specificity of screening instruments for depression in older African-Caribbean people in Manchester, UK. METHODS: We carried out a two-stage study to compare three screening instruments for depression (Geriatric Depression Scale, Brief Assessment Schedule Depression Cards, Caribbean Culture Specific Screen), with a computerized diagnostic interview for mental health disorders in older adults (Geriatric Mental State). The study was set in inner-city Manchester. The subjects were community-resident African-Caribbeans aged 60 years and over; 227 subjects were approached. Of the 160 people screened, 130 agreed to diagnostic interview. The main outcome measures were Spearman correlation coefficients; these were calculated between each screening instrument and the diagnostic interview. Receiver-operating characteristic (ROC) curve analysis was used to determine appropriate sensitivity and specificity for each instrument. RESULTS: The results for the largest subgroup, the Jamaicans (n = 96/130), demonstrated highly significant correlations between screening instruments and diagnostic interview (P < 0.001). Each instrument had a high sensitivity: Brief Assessment Schedule depression cards (cut-off > or =6; sensitivity 90.9% (95% CI 58.8-99.8), specificity 82.1% (95% CI 74.0-90.3)), Caribbean Culture Specific Screen (cut-off > or =6; sensitivity 90.9% (95% CI 58.8-99.8), specificity 74.1% (95% CI 64.8-83.4)), and Geriatric Depression Scale (cut-off > or =4; sensitivity 100% (95% CI 97.1-100), specificity 69.1% (95% CI 59.6-79.2)). CONCLUSIONS: These screening instruments demonstrate high sensitivity levels, if an appropriate cut-off point is used. The culture-specific instrument did not perform better than the traditional instruments. Health professionals should approach the consultation in a culturally sensitive manner and use the validated instrument they are most familiar with.     

Rural Health Research: Rural and Urban Physicians: Does the Content of Their Medicare Practices Differ?

Rural and urban areas have significant differences in the availability of medical technology, medical practice structures and patient populations. This study uses 1994 Medicare claims data to examine whether these differences are associated with variation in the content of practice between physicians practicing in rural and urban areas. This study compared the number of patients, outpatient visits, and inpatient visits per physician in the different specialties, diagnosis clusters, patient age and sex, and procedure frequency and type for board-certified rural and urban physicians in 12 ambulatory medical specialties. Overall, 14.4 percent of physicians in the 12 specialties practiced exclusively in rural Washington, with great variation by specialty. Rural physicians were older and less likely to be female than urban physicians. Rural physicians saw larger numbers of elderly patients and had higher volumes of outpatient visits than their urban counterparts. For all specialty groups except general surgeons and obstetrician-gynecologists, the diagnostic scope of practice was specialty-specific and similar for rural and urban physicians. Rural general surgeons had more visits for gastrointestinal disorders, while rural obstetrician-gynecologists had more visits out of their specialty domain (e.g., hypertension, diabetes) than their urban counterparts. The scope of procedures for rural and urban physicians in most specialties showed more similarities than differences. While the fund of knowledge and outpatient procedural training needed by most rural and urban practitioners to care for the elderly is similar, rural general surgeons and obstetrician-gynecologists need training outside their traditional specialty areas to optimally care for their patients.     

Circumvention of prey defense by a predator: ant lion vs. ant

The pit-dwelling ant lion Myrmeleon carolinus, although topically sensitive to formic acid, is able to prey on formic acid-spraying ants (Camponotus floridanus). It kills the ants without inducing them to spray, and it sucks out the ant's body contents without puncturing the acid sac. Ordinarily, when Camponotus is attacked it retaliates by simultaneously biting and spraying, but it usually refrains from spraying until it has secured a grip with the mandibles. When Myrmeleon pulls Camponotus into the sand at the bottom of the pit, the ant is seemingly unable to grasp the ant lion and it is killed without being induced to spray. When feeding on the ant, the ant lion sucks up the contents of the nutrient-laden crop. How the ant lion differentiates between crop and acid sac, managing to spare the latter while rupturing the former, remains unknown.     

Immune response of the draining and distal lymph nodes during the progressive grwoth of a chemically-induced transplantable rat hepatoma.

The immunological and histological changes occurring in the lymph node draining the site of a progressively growing intramuscular tumour (D192A) implant were monitored during a 4-week time course. Cell-mediated cytotoxicity against hepatoma-D192A and 15-day rat embryo cell targets, was detected with cells derived from the draining "lumbar" lymph node 4 days after tumour implantation and persisted up to the 2nd week of tumour growth, decreasing rapidly during the 3rd week. The observed lymph-node anergy demonstrated in cytotoxicity tests correlated with the histological findings, in that an initial marked paracortical (T-dependent) response also declined towards the end of the 3rd week of tumour growth. The B-dependent cortex showed active lymphocyte follicles in the 2nd week of the time course, and plasma-cell production continued until the experiment was terminated. These changes were shown to occur with the progressive increase in lymph-node mass. Serum antibody specific for the developing tumour was detected during the latter stages of tumour growth. The immunological and histological changes displayed were out of phase with those shown by the draining lymph node.     

Influence of the cardioprotective agent dexrazoxane on doxorubicin pharmacokinetics in the dog

Summary The influence of dexrazoxane on doxorubicin pharmacokinetics was investigated in four dogs using the two treatment sequences of saline/doxorubicin or dexrazoxane/doxorubicin. Intravenous doses of 1.5 mg/kg doxorubicin and 30 mg/kg (the 20-fold multiple) dexrazoxane were given separately, with doxorubicin being injected within 1 min of the dexrazoxane dose. Both doxorubicin and its 13-dihydro metabolite doxorubicinol were quantified in plasma and urine using a validated high-performance liquid chromatographic (HPLC) fluorescence assay. The doxorubicin plasma concentration versus time data were adequately fit by a three-compartment model. The mean half-lives calculated for the fast and slow distributive and terminal elimination phases in the saline/doxorubicin group were 3.0±0.5 and 32.2±12.8 min and 30.0±4.0 h, respectively. The model-predicted plasma concentrations were virtually identical for the saline and dexrazoxane treatment groups. Analysis of variance of the area under the plasma concentration-time curve (AUC_0–), terminal elimination rate (_Z), systemic clearance (CL _s), and renal clearance (CL _r) for the parent drug showed no statistically significant difference (P<0.05) between the two treatments. Furthermore, the doxorubicinol plasma AUC_0– value and the doxorubicinol-to-doxorubicin AUC_0– ratio showed no significant difference, demonstrating that dexrazoxane had no effect on the metabolic capacity for formation of the 13-dihydro metabolite. The total urinary excretion measured as parent drug plus doxorubicinol and the metabolite-to-parent ratio in urine were also unaffected by the presence of dexrazoxane. The myelosuppressive effects of doxorubicin as determined by WBC monitoring revealed no apparent difference between the two treatments. In conclusion, these results show that drug exposure was similar for the two treatment arms. No kinetic interaction with dexrazoxane suggests that its coadministration is unlikely to modify the safety and/or efficacy of doxorubicin.     

Dynamic SPECT imaging of dopamine D2 receptors in human subjects with iodine-123-IBZM.

We studied the uptake, distribution, metabolism and washout of the dopamine D2 receptor ligand [123I]IBZM in healthy subjects (n = 12) with dynamic brain SPECT. The highest radioactivity level was detected in the striatum. Operationally-defined striatal "specific" uptake peaked at 69 min postinjection of radioligand and showed a gradual decline of 15% per hour thereafter. "Specific" uptake at maximal counts represented 53% of the total striatal radioactivity. Two subjects received haloperidol (20 micrograms/kg i.v.) 80 min postinjection of radioligand. Haloperidol caused a 2.6-fold increase in the rate of washout of specific striatal activity in comparison to that in the 10 control subjects and was consistent with drug-induced displacement of radioligand from the dopamine D2 receptor. Two classes of metabolites were detected in plasma and urine: a polar fraction, not extracted by ethyl acetate, and a nonpolar, extractable fraction consisting of parent compound and two compounds having shorter retention times on reversed-phase HPLC. Greater than half the plasma parent was metabolized within 10-15 min after administration. The volume of distribution, estimated from the peak arterial plasma concentration at 50-75 sec, was 7.7-10.2 l; the free (nonprotein bound) fraction of [123I]IBZM after in vitro incubation with blood or plasma was 4.4% +/- 0.4%. These results suggest that [123I]IBZM exhibits uptake in brain regions with high D2 receptor density and shows a relatively stable washout during which drugs affecting dopaminergic transmission may be administered.     

The effect of hypothermic cardiopulmonary bypass and total circulatory arrest on cerebral metabolism in neonates, infants, and children

Cardiopulmonary bypass management in neonates, infants, and children often requires the use of deep hypothermia at 18 degrees C with occasional periods of circulatory arrest and represents marked physiologic extremes of temperature and perfusion. The safety of these techniques is largely dependent on the reduction of metabolism, particularly cerebral metabolism. We studied the effect of hypothermia on cerebral metabolism during cardiac surgery and quantified the changes. Cerebral metabolism was measured before, during, and after hypothermic cardiopulmonary bypass in 46 pediatric patients, aged 1 day to 14 years. Patients were grouped on the basis of the different bypass techniques commonly used in children: group A--moderate hypothermic bypass at 28 degrees C; group B--deep hypothermic bypass at 18 degrees to 20 degrees C with maintenance of continuous flow; and group C--deep hypothermic circulatory arrest at 18 degrees C. Cerebral metabolism significantly decreased under hypothermic conditions in all groups compared with control levels at normothermia, the data demonstrating an exponential relationship between temperature and cerebral metabolism and an average temperature coefficient of 3.65. There was no significant difference in the rate of metabolism reduction (temperature coefficient) in patients cooled to 28 degrees and 18 degrees C. From these data we were able to derive an equation that numerically expresses a hypothermic metabolic index, which quantitates duration of brain protection provided by reduction of cerebral metabolism owing to hypothermic bypass over any temperature range. Based on this index, patients cooled to 28 degrees C have a predicted ischemic tolerance of 11 to 19 minutes. The predicted duration that the brain can tolerate ischemia ("safe" period of deep hypothermic circulatory arrest) in patients cooled to 18 degrees C, based on our metabolic index, is 39 to 65 minutes, similar to the safe period of deep hypothermic circulatory arrest known to be tolerated clinically. In groups A and B (no circulatory arrest), cerebral metabolism returned to control in the rewarming phase of bypass and after bypass. In group C (circulatory arrest), cerebral metabolism and oxygen extraction remained significantly reduced during rewarming and after bypass, suggesting disordered cerebral metabolism and oxygen utilization after deep hypothermic circulatory arrest. The results of this study suggest that cerebral metabolism is exponentially related to temperature during hypothermic bypass with a temperature coefficient of 3.65 in neonates infants and children. Deep hypothermic circulatory arrest changes cerebral metabolism and blood flow after the arrest period despite adequate hypothermic suppression of metabolism.