The Obstetrical Care and Delivery Experience of Women with Epilepsy in the MONEAD Study

OBJECTIVE

 We examined mode of delivery among pregnant women with epilepsy (PWWE) versus pregnant controls (PC). We hypothesize that PWWE are more likely to deliver by cesarean.

STUDY DESIGN

 The Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs (MONEAD) study is an observational, prospective, multicenter investigation of pregnancy outcomes funded by the National Institute of Health (NIH). MONEAD enrolled patients from December 2012 through January 2016. PWWE were matched to PC in a case:control ratio of 3:1. This analysis had 80% power to detect a 36% increase in cesarean frequency assuming a baseline rate of 30% among PC at an α = 0.05.

RESULTS

 This report analyzed 331 PWWE (76%) and 102 PC (24%) who gave birth while enrolled in the study. PWWE and PC had similar rates of cesarean delivery (34.7 vs. 28.6%; p = 0.27). Of women with cesarean, rates of cesarean without labor were similar between groups for those delivering in recruitment hospitals (48.2 vs. 50.0%) but in nonrecruitment hospitals, cesarean rates without labor were over two-fold higher among PWWE than those of PC (68.8 vs. 30.8%; p = 0.023). Receipt of a cesarean after labor did not differ for PWWE compared to PC or by type of antiepileptic drug among the PWWE.

CONCLUSION

 These findings suggest that the obstetrical experiences of PWWE and PC are similar. An interesting deviation from this observation was the mode of delivery with higher unlabored cesarean rates occurring among PWWE in nonrecruitment hospitals. As the study recruitment hospitals were tertiary academic centers and nonrecruitment hospitals tended to be community-based institutions, differences in perinatal expertise might contribute to this difference.

KEY POINTS

· Unlabored cesarean rates higher among women with epilepsy.. · Provider preference may influence delivery mode among women with epilepsy.. · Type and amount of antiepileptic drug was not associated with mode of delivery..

Two-Year-Old Cognitive Outcomes in Children of Pregnant Women With Epilepsy in the Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs Study

Importance

The neurodevelopmental risks of fetal exposure are uncertain for many antiseizure medications (ASMs).

Objective

To compare children at 2 years of age who were born to women with epilepsy (WWE) vs healthy women and assess the association of maximum ASM exposure in the third trimester and subsequent cognitive abilities among children of WWE.

Design, Setting, and Participants

The Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs (MONEAD) study is a prospective, observational, multicenter investigation of pregnancy outcomes that enrolled women from December 19, 2012, to January 13, 2016, at 20 US epilepsy centers. Children are followed up from birth to 6 years of age, with assessment at 2 years of age for this study. Of 1123 pregnant women assessed, 456 were enrolled; 426 did not meet criteria, and 241 chose not to participate. Data were analyzed from February 20 to December 4, 2020.

Main Outcomes and Measures

Language domain score according to the Bayley Scales of Infant and Toddler Development, Third Edition (BSID-III), which incorporates 5 domain scores (language, motor, cognitive, social-emotional, and general adaptive), and association between BSID-III language domain and ASM blood levels in the third trimester in children of WWE. Analyses were adjusted for multiple potential confounding factors, and measures of ASM exposure were assessed.

Results

The BSID-III assessments were analyzed in 292 children of WWE (median age, 2.1 [range, 1.9-2.5] years; 155 female [53.1%] and 137 male [46.9%]) and 90 children of healthy women (median age, 2.1 [range, 2.0-2.4] years; 43 female [47.8%] and 47 male [52.2%]). No differences were found between groups on the primary outcome of language domain (-0.5; 95% CI, -4.1 to 3.2). None of the other 4 BSID-III domains differed between children of WWE vs healthy women. Most WWE were taking lamotrigine and/or levetiracetam. Exposure to ASMs in children of WWE showed no association with the language domain. However, secondary analyses revealed that higher maximum observed ASM levels in the third trimester were associated with lower BSID-III scores for the motor domain (-5.6; 95% CI, -10.7 to -0.5), and higher maximum ASM doses in the third trimester were associated with lower scores in the general adaptive domain (-1.4; 95% CI, -2.8 to -0.05).

Conclusions and Relevance

Outcomes of children at 2 years of age did not differ between children of WWE taking ASMs and children of healthy women.

Trial Registration

ClinicalTrials.gov Identifier: NCT01730170.

Folate fortification of food: Insufficient for women with epilepsy

OBJECTIVE

Folic acid supplementation during the periconceptual period has been shown to improve cognitive outcomes in children of women with epilepsy taking anti-seizure medications (ASMs). The dose of folic acid necessary to provide positive cognitive outcomes is unclear. In many countries including the United States, food is fortified with folic acid, but no data exist on how food fortification may affect cognition in children with fetal-ASM exposure. This study evaluated the effect of dietary folate from natural folates plus folic acid fortification, separate from folic acid vitamin supplements, on age-6 year IQ in children with fetal-ASM exposure.

METHODS

Data from the Neurodevelopmental Effects of Antiepileptic Drugs (NEAD) study were retrospectively analyzed for this investigation. Assessment of nutrient intake was conducted using the Block Food Frequency Questionnaire-98. The primary outcome of the present study was to assess association of maternal prepregnancy nutrient levels to child age-6 IQ.

RESULTS

Folate from food alone without supplement was not associated with improvement of age-6 IQ in children with fetal ASM exposure (95% CI: -11.7-2.3, p = 0.187). Periconceptual folate supplement use was associated with a 10.1-point higher age-6 IQ (95% CI: 5.2-15.0, p < .001). Total combined folate from food plus supplement also showed that higher intake of folate was associated with higher age-6 IQ (Coefficient: 4.5, 95% CI: 2.0-6.9, p < .001). Six other nutrients from food and supplements were analyzed (Vitamin C, Vitamin D, Vitamin E, Omega 3, Gamma Tocopherol, and Vitamin B12) and had no significant association with age 6-IQ.

SIGNIFICANCE

Dietary content of folate, even in a country where food is fortified with folic acid, is not sufficient to provide improved cognitive outcomes for children of women taking ASMs during pregnancy. Folate supplementation is needed for significant improvement in cognitive outcomes, specifically age-6 IQ.

Changes in Seizure Frequency and Antiepileptic Therapy during Pregnancy

BACKGROUND

Among women with epilepsy, studies regarding changes in seizure frequency during pregnancy have been limited by the lack of an appropriate nonpregnant comparator group to provide data on the natural course of seizure frequency in both groups.

METHODS

In this prospective, observational, multicenter cohort study, we compared the frequency of seizures during pregnancy through the peripartum period (the first 6 weeks after birth) (epoch 1) with the frequency during the postpartum period (the following 7.5 months after pregnancy) (epoch 2). Nonpregnant women with epilepsy were enrolled as controls and had similar follow-up during an 18-month period. The primary outcome was the percentage of women who had a higher frequency of seizures that impaired awareness during epoch 1 than during epoch 2. We also compared changes in the doses of antiepileptic drugs that were administered in the two groups during the first 9 months of epoch 1.

RESULTS

We enrolled 351 pregnant women and 109 controls with epilepsy. Among the 299 pregnant women and 93 controls who had a history of seizures that impaired awareness and who had available data for the two epochs, seizure frequency was higher during epoch 1 than during epoch 2 in 70 pregnant women (23%) and in 23 controls (25%) (odds ratio, 0.93; 95% confidence interval [CI], 0.54 to 1.60). During pregnancy, the dose of an antiepileptic drug was changed at least once in 74% of pregnant women and in 31% of controls (odds ratio, 6.36; 95% CI, 3.82 to 10.59).

CONCLUSIONS

Among women with epilepsy, the percentage who had a higher incidence of seizures during pregnancy than during the postpartum period was similar to that in women who were not pregnant during the corresponding epochs. Changes in doses of antiepileptic drugs occurred more frequently in pregnant women than in nonpregnant women during similar time periods. (Funded by the National Institutes of Health; MONEAD ClinicalTrials.gov number, NCT01730170.).

Antiepileptic Drug Exposure in Infants of Breastfeeding Mothers With Epilepsy

Importance

There is limited information on infant drug exposure via breastfeeding by mothers who are receiving antiepileptic drug therapy.

Objective

To provide direct, objective information on antiepileptic drug exposure through breast milk.

Design, Setting, and Participants

This prospective cohort study was conducted between December 2012 to October 2016, with follow-up in children until 6 years of age at 20 sites across the United States. Data were collected via an observational multicenter investigation (Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs [MONEAD]) of outcomes in pregnant mothers with epilepsy and their children. Pregnant women with epilepsy who were aged 14 to 45 years, had pregnancies that had progressed to less than 20 weeks' gestational age, and had measured IQ scores of more than 70 points were enrolled and followed up through pregnancy and 9 postpartum months. Their infants were enrolled at birth. Data were analyzed from May 2014 to August 2019.

Exposures

Antiepileptic drug exposure in infants who were breastfed.

Main Outcomes and Measures

The percentage of infant-to-mother concentration of antiepileptic drugs. Antiepileptic drug concentrations were quantified from blood samples collected from infants and mothers at the same visit, 5 to 20 weeks after birth. Concentrations of antiepileptic drugs in infants at less than the lower limit of quantification were assessed as half of the lower limit. Additional measures collected were the total duration of all daily breastfeeding sessions and/or the volume of pumped breast milk ingested from a bottle.

Results

A total of 351 women (of 865 screened and 503 eligible individuals) were enrolled, along with their 345 infants (179 female children [51.9%]; median [range] age, 13 [5-20] weeks). Of the 345 infants, 222 (64.3%) were breastfed; the data collection yielded 164 matching infant-mother concentration pairs from 138 infants. Approximately 49% of all antiepileptic drug concentrations in nursing infants were less than the lower limit of quantification. The median percentage of infant-to-mother concentration for all 7 antiepileptic drugs and 1 metabolite (carbamazepine, carbamazepine-10,11-epoxide, levetiracetam, lamotrigine, oxcarbazepine, topiramate, valproate, and zonisamide) ranged from 0.3% (range, 0.2%-0.9%) to 44.2% (range, 35.2%-125.3%). In multiple linear regression models, maternal concentration was a significant factor associated with lamotrigine concentration in infants (Pearson correlation coefficient, 0.58; P < .001) but not levetiracetam concentration in infants.

Conclusions and Relevance

Overall, antiepileptic drug concentrations in blood samples of infants who were breastfed were substantially lower than maternal blood concentrations. Given the well-known benefits of breastfeeding and the prior studies demonstrating no ill effects when the mother was receiving antiepileptic drugs, these findings support the breastfeeding of infants by mothers with epilepsy who are taking antiepileptic drug therapy.

Fetal loss and malformations in the MONEAD study of pregnant women with epilepsy

OBJECTIVE

To examine occurrence of severe adverse fetal outcomes (SAO), including fetal loss and major congenital malformations (MCMs), in pregnant women with epilepsy (PWWE) vs healthy pregnant women (HPW).

METHODS

The Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs (MONEAD) study is an NIH-funded, prospective, observational, multicenter investigation of pregnancy outcomes for both mother and child, which enrolled women December 2012 through January 2016.

RESULTS

The 351 PWWE had 365 conceptions, and 105 HPW had 109 conceptions. SAOs occurred more often in PWWE (7.9%) vs HPW (1.9%) (p = 0.025) with odds ratio (OR) 4.45 (95% confidence intervals [CI] 1.04-19.01). There were no significant differences for fetal loss (2.8% vs 0%, p = 0.126) or MCMs (5.2% vs 1.9%, p = 0.185; OR 2.86, 95% CI 0.65-12.53) individually. No fetal losses in PWWE appeared to be related to acute seizures. Outcomes were not affected by periconceptional folate, unplanned/unwanted pregnancies, prior maternal pregnancy history, or antiepileptic drug (AED) blood levels, except for an AED level effect for fetal loss that appeared to be due to polytherapy. Combined maternal or paternal family history of MCM was marginally associated with increased SAOs (p = 0.046).

CONCLUSIONS

The findings provide additional information on risks of SAOs in PWWE, assessing effects of both AED levels and periconceptional folate. Group differences in average enrollment gestational age could have affected fetal loss results. Analyses are limited by small sample sizes as the MONEAD study was not powered for these secondary outcomes. The large majority of pregnancies in women with epilepsy do not have SOAs.

Effects of periconceptional folate on cognition in children of women with epilepsy: NEAD study

OBJECTIVE

Emerging evidence suggests potential positive neuropsychological effects of periconceptional folate in both healthy children and children exposed in utero to antiseizure medications (ASMs). In this report, we test the hypothesis that periconceptional folate improves neurodevelopment in children of women with epilepsy by re-examining data from the Neurodevelopmental Effects of Antiepileptic Drugs (NEAD) study.

METHODS

The NEAD study was an NIH-funded, prospective, observational, multicenter investigation of pregnancy outcomes in 311 children of 305 women with epilepsy treated with ASM monotherapy. Missing data points were imputed with Markov chain Monte Carlo methods. Multivariate analyses adjusted for multiple factors (e.g., maternal IQ, ASM type, standardized ASM dose, and gestational birth age) were performed to assess the effects of periconceptional folate on cognitive outcomes (i.e., Full Scale Intelligence Quotient [FSIQ], Verbal and Nonverbal indexes, and Expressive and Receptive Language indexes at 3 and 6 years of age, and executive function and memory function at 6 years of age).

RESULTS

Periconceptional folate was associated with higher FSIQ at both 3 and 6 years of age. Significant effects for other measures included Nonverbal Index, Expressive Language Index, and Developmental Neuropsychological Assessment Executive Function at 6 years of age, and Verbal Index and Receptive Language Index at 3 years of age. Nonsignificant effects included Verbal Index, Receptive Index, Behavior Rating Inventory of Executive Function-Parent Questionnaire Executive Function, and General Memory Index at 6 years of age, and Nonverbal Index and Expressive Index at 3 years of age.

CONCLUSIONS

Use of periconceptional folate in pregnant women with epilepsy taking ASMs is associated with better cognitive development.

CLINICALTRIALSGOV IDENTIFIER

NCT00021866.

Metabolome-wide association study of anti-epileptic drug treatment during pregnancy

Pregnant women with epilepsy (PWWE) require continuous anti-epileptic drug (AED) treatment to avoid risk to themselves and fetal risks secondary to maternal seizures, resulting in prolonged AED exposure to the developing embryo and fetus. The objectives of this study were to determine whether high-resolution metabolomics is able to link the metabolite profile of PWWE receiving lamotrigine or levetiracetam for seizure control to associated pharmacodynamic (PD) biological responses. Untargeted metabolomic analysis of plasma obtained from 82 PWWE was completed using high-resolution mass spectrometry. Biological alterations due to lamotrigine or levetiracetam monotherapy were determined by a metabolome-wide association study that compared patients taking either drug to those who did not require AED treatment. Metabolic changes associated with AED use were then evaluated by testing for drug-dose associated metabolic variations and pathway enrichment. AED therapy resulted in drug-associated metabolic profiles recognizable within maternal plasma. Both the parent compounds and major metabolites were detected, and each AED was correlated with other metabolic features and pathways. Changes in metabolites and metabolic pathways important to maternal health and linked to fetal neurodevelopment were detected for both drugs, including changes in one‑carbon metabolism, neurotransmitter biosynthesis and steroid metabolism. In addition, decreased levels of 5-methyltetrahydrofolate and tetrahydrofolate were detected in women taking lamotrigine, which is consistent with recent findings showing increased risk of autism spectrum disorder traits in PWWE using AED. These results represent a first step in development of pharmacometabolomic framework with potential to detect adverse AED-related metabolic changes during pregnancy.

Changes in antiepileptic drug-prescribing patterns in pregnant women with epilepsy

OBJECTIVE

We analyzed current prescribing patterns for antiepileptic drugs (AEDs) in pregnant women with epilepsy (PWWE) at 20 USA tertiary epilepsy centers.

METHODS

The Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs (MONEAD) study is an NIH-funded, prospective, observational, multicenter investigation of pregnancy outcomes for both mother and child, which enrolled women from December 2012 to January 2016. Inclusion criteria for PWWE included ages 14-45 years and up to 20 weeks gestational age. Exclusion criteria included history of psychogenic nonepileptic spells, expected intelligence quotient (IQ) <70, other major medical illness, progressive cerebral disease, and switching AEDs in pregnancy prior to enrollment.

RESULTS

Three hundred fifty-one PWWE were enrolled in the MONEAD study, which included 259 (73.8%) on monotherapy, 77 (21.9%) on polytherapy, and 15 (4.3%) on no AEDs. The most common AED monotherapy regimens were lamotrigine (42.1% of monotherapies), levetiracetam (37.5%), carbamazepine (5.4%), zonisamide (5.0%), oxcarbazepine (4.6%), and topiramate (3.1%). All other individual monotherapies were each <1%. The most common AED polytherapy combination was lamotrigine + levetiracetam (42.9% of polytherapies), followed by lacosamide + levetiracetam (6.5%), lamotrigine + zonisamide (5.2%), and all other remaining combinations (each <4%); only 5.2% of polytherapy subjects were on ≥3 AEDs (1.1% of total PWWE). Only four subjects (1.1%) were on valproate (1 monotherapy, 3 polytherapy).

CONCLUSIONS

The distribution of AED use likely reflects current prescribing patterns for PWWE cared for in USA tertiary epilepsy centers. This distribution has changed markedly since the turn of the century, but changes in the general population remain uncertain.

A Single-Cell Roadmap of Lineage Bifurcation in Human ESC Models of Embryonic Brain Development

During human brain development, multiple signaling pathways generate diverse cell types with varied regional identities. Here, we integrate single-cell RNA sequencing and clonal analyses to reveal lineage trees and molecular signals underlying early forebrain and mid/hindbrain cell differentiation from human embryonic stem cells (hESCs). Clustering single-cell transcriptomic data identified 41 distinct populations of progenitor, neuronal, and non-neural cells across our differentiation time course. Comparisons with primary mouse and human gene expression data demonstrated rostral and caudal progenitor and neuronal identities from early brain development. Bayesian analyses inferred a unified cell-type lineage tree that bifurcates between cortical and mid/hindbrain cell types. Two methods of clonal analyses confirmed these findings and further revealed the importance of Wnt/β-catenin signaling in controlling this lineage decision. Together, these findings provide a rich transcriptome-based lineage map for studying human brain development and modeling developmental disorders.

Phase 1 testing of detoxified LPS/group B meningococcal outer membrane protein vaccine with and without synthetic CPG 7909 adjuvant for the prevention and treatment of sepsis

BACKGROUND

Gram-negative bacteria (GNB) are a leading cause of nosocomial infection and sepsis. Increasing multi-antibiotic resistance has left clinicians with fewer therapeutic options. Antibodies to GNB lipopolysaccharide (LPS, or endotoxin) have reduced morbidity and mortality as a result of infection and are not subject to the resistance mechanisms deployed by bacteria against antibiotics. In this phase 1 study, we administered a vaccine that elicits antibodies against a highly conserved portion of LPS with and without a CpG oligodeoxynucleotide (ODN) TLR9 agonist as adjuvant.

METHODS

A vaccine composed of the detoxified LPS (dLPS) from E. coli O111:B4 (J5 mutant) non-covalently complexed to group B meningococcal outer membrane protein (OMP). Twenty healthy adult subjects received three doses at 0, 29 and 59 days of antigen (10 μg dLPS) with or without CPG 7909 (250 or 500 μg). Subjects were evaluated for local and systemic adverse effects and laboratory findings. Anti-J5 LPS IgG and IgM antibody levels were measured by electrochemiluminesence. Due to premature study termination, not all subjects received all three doses.

RESULTS

All vaccine formulations were well-tolerated with no local or systemic events of greater than moderate severity. The vaccine alone group achieved a ≥ 4-fold "responder" response in IgG and IgM antibody in only one of 6 subjects. In contrast, the vaccine plus CPG 7909 groups appeared to have earlier and more sustained (to 180 days) responses, greater mean-fold increases, and a higher proportion of "responders" achieving ≥ 4-fold increases over baseline.

CONCLUSIONS

Although the study was halted before all enrolled subjects received all three doses, the J5dLPS/OMP vaccine, with or without CpG adjuvant, was safe and well-tolerated. The inclusion of CpG increased the number of subjects with a ≥ 4-fold antibody response, evident even after the second of three planned doses. A vaccine comprising J5dLPS/OMP antigen with CpG adjuvant merits further investigation.

CLINICAL TRIALS REGISTRATION

ClinicalTrials.gov Identifier: NCT01164514.

Genome engineering of isogenic human ES cells to model autism disorders

Isogenic pluripotent stem cells are critical tools for studying human neurological diseases by allowing one to study the effects of a mutation in a fixed genetic background. Of particular interest are the spectrum of autism disorders, some of which are monogenic such as Timothy syndrome (TS); others are multigenic such as the microdeletion and microduplication syndromes of the 16p11.2 chromosomal locus. Here, we report engineered human embryonic stem cell (hESC) lines for modeling these two disorders using locus-specific endonucleases to increase the efficiency of homology-directed repair (HDR). We developed a system to: (1) computationally identify unique transcription activator-like effector nuclease (TALEN) binding sites in the genome using a new software program, TALENSeek, (2) assemble the TALEN genes by combining golden gate cloning with modified constructs from the FLASH protocol, and (3) test the TALEN pairs in an amplification-based HDR assay that is more sensitive than the typical non-homologous end joining assay. We applied these methods to identify, construct, and test TALENs that were used with HDR donors in hESCs to generate an isogenic TS cell line in a scarless manner and to model the 16p11.2 copy number disorder without modifying genomic loci with high sequence similarity.

Joint modeling of longitudinal and survival data with missing and left-censored time-varying covariates

We propose a joint model for longitudinal and survival data with time-varying covariates subject to detection limits and intermittent missingness at random. The model is motivated by data from the Multicenter AIDS Cohort Study (MACS), in which HIV+ subjects have viral load and CD4 cell count measured at repeated visits along with survival data. We model the longitudinal component using a normal linear mixed model, modeling the trajectory of CD4 cell count by regressing on viral load, and other covariates. The viral load data are subject to both left censoring because of detection limits (17%) and intermittent missingness (27%). The survival component of the joint model is a Cox model with time-dependent covariates for death because of AIDS. The longitudinal and survival models are linked using the trajectory function of the linear mixed model. A Bayesian analysis is conducted on the MACS data using the proposed joint model. The proposed method is shown to improve the precision of estimates when compared with alternative methods.

Longitudinal study of echocardiography-derived tricuspid regurgitant jet velocity in sickle cell disease

Although echocardiography-derived tricuspid regurgitant jet velocity (TRV) is associated with increased mortality in sickle cell disease (SCD), its rate of increase and predictive markers of its progression are unknown. We evaluated 55 subjects (median age: 38 years, range: 20-65 years) with at least two measurable TRVs, followed for a median of 4·5 years (range: 1·0-10·5 years) in a single-centre, prospective study. Thirty-one subjects (56%) showed an increase in TRV, while 24 subjects (44%) showed no change or a decrease in TRV. A linear mixed effects model indicated an overall rate of increase in the TRV of 0·02 m/s per year (P = 0·023). The model showed that treatment with hydroxycarbamide was associated with an initial TRV that was 0·20 m/s lower than no such treatment (P = 0·033), while treatment with angiotensin converting enzyme inhibitors and angiotensin receptor blockers was associated with an increase in the TRV (P = 0·006). In summary, although some patients have clinically meaningful increases, the overall rate of TRV increase is slow. Treatment with hydroxycarbamide may decrease the progression of TRV. Additional studies are required to determine the optimal frequency of screening echocardiography and the effect of therapeutic interventions on the progression of TRV in SCD.

Change-point models to estimate the limit of detection

In many biological and environmental studies, measured data is subject to a limit of detection. The limit of detection is generally defined as the lowest concentration of analyte that can be differentiated from a blank sample with some certainty. Data falling below the limit of detection is left censored, falling below a level that is easily quantified by a measuring device. A great deal of interest lies in estimating the limit of detection for a particular measurement device. In this paper, we propose a change-point model to estimate the limit of detection by using data from an experiment with known analyte concentrations. Estimation of the limit of detection proceeds by a two-stage maximum likelihood method. Extensions are considered that allow for censored measurements and data from multiple experiments. A simulation study is conducted demonstrating that in some settings the change-point model provides less biased estimates of the limit of detection than conventional methods. The proposed method is then applied to data from an HIV pilot study.

Bone-density testing interval and transition to osteoporosis in older women

BACKGROUND

Although bone mineral density (BMD) testing to screen for osteoporosis (BMD T score, -2.50 or lower) is recommended for women 65 years of age or older, there are few data to guide decisions about the interval between BMD tests.

METHODS

We studied 4957 women, 67 years of age or older, with normal BMD (T score at the femoral neck and total hip, -1.00 or higher) or osteopenia (T score, -1.01 to -2.49) and with no history of hip or clinical vertebral fracture or of treatment for osteoporosis, followed prospectively for up to 15 years. The BMD testing interval was defined as the estimated time for 10% of women to make the transition to osteoporosis before having a hip or clinical vertebral fracture, with adjustment for estrogen use and clinical risk factors. Transitions from normal BMD and from three subgroups of osteopenia (mild, moderate, and advanced) were analyzed with the use of parametric cumulative incidence models. Incident hip and clinical vertebral fractures and initiation of treatment with bisphosphonates, calcitonin, or raloxifene were treated as competing risks.

RESULTS

The estimated BMD testing interval was 16.8 years (95% confidence interval [CI], 11.5 to 24.6) for women with normal BMD, 17.3 years (95% CI, 13.9 to 21.5) for women with mild osteopenia, 4.7 years (95% CI, 4.2 to 5.2) for women with moderate osteopenia, and 1.1 years (95% CI, 1.0 to 1.3) for women with advanced osteopenia.

CONCLUSIONS

Our data indicate that osteoporosis would develop in less than 10% of older, postmenopausal women during rescreening intervals of approximately 15 years for women with normal bone density or mild osteopenia, 5 years for women with moderate osteopenia, and 1 year for women with advanced osteopenia. (Funded by the National Institutes of Health.).

Effect of observing change from comparison mammograms on performance of screening mammography in a large community-based population

PURPOSE

To evaluate the effect of comparison mammograms on accuracy, sensitivity, specificity, positive predictive value (PPV(1)), and cancer detection rate (CDR) of screening mammography to determine the role played by identification of change on comparison mammograms.

MATERIALS AND METHODS

This HIPAA-compliant and institutional review board-approved prospective study was performed with waiver of patient informed consent. A total of 1,157,980 screening mammograms obtained between 1994 and 2008 in 435,183 women aged at least 40 years were included. Radiologists recorded presence of comparison mammograms and change, if seen. Women were followed for 1 year to monitor cancer occurrence. Performance measurements were calculated for screening with comparison mammograms versus screening without comparison mammograms and for screening with comparison mammograms that showed a change versus screening with comparison mammograms that did not show a change while controlling for age, breast density, and data clustering.

RESULTS

Comparison mammograms were available in 93% of examinations. For screening with comparison mammograms versus screening without comparison mammograms, CDR per 1000 women was 3.7 versus 7.1; recall rate, 6.9% versus 14.9%; sensitivity, 78.9% versus 87.4%; specificity, 93.5% versus 85.7%; and PPV(1), 5.4% versus 4.8%. For screening with comparison mammograms that showed a change versus screening with comparison mammograms that did not show a change, CDR per 1000 women was 25.4 versus 0.8; recall rate, 41.4% versus 2.0%; sensitivity, 96.6% versus 43.5%; specificity, 60.4% versus 98.1%; and PPV(1), 6.0% versus 3.9%. Detected cancers with change were 21.1% ductal carcinoma in situ and 78.9% invasive carcinoma. Detected cancers with no change were 19.3% ductal carcinoma in situ and 80.7% invasive carcinoma.

CONCLUSION

Performance is affected when change from comparison mammograms is noted. Without change, sensitivity is low and specificity is high. With change, sensitivity is high, with a high false-positive rate (low specificity). Further work is needed to appreciate changes that might indicate cancer and to identify changes that are likely not indicative of cancer.

Maximum likelihood estimation in generalized linear models with multiple covariates subject to detection limits

The analysis of data subject to detection limits is becoming increasingly necessary in many environmental and laboratory studies. Covariates subject to detection limits are often left censored because of a measurement device having a minimal lower limit of detection. In this paper, we propose a Monte Carlo version of the expectation-maximization algorithm to handle large number of covariates subject to detection limits in generalized linear models. We model the covariate distribution via a sequence of one-dimensional conditional distributions, and sample the covariate values using an adaptive rejection metropolis algorithm. Parameter estimation is obtained by maximization via the Monte Carlo M-step. This procedure is applied to a real dataset from the National Health and Nutrition Examination Survey, in which values of urinary heavy metals are subject to a limit of detection. Through simulation studies, we show that the proposed approach can lead to a significant reduction in variance for parameter estimates in these models, improving the power of such studies.

The Arp2/3 complex: a central regulator of the actin cytoskeleton

In recent years the Arp2/3 complex has emerged as a central regulator of actin dynamics, assembling and cross-linking actin filaments to produce a diverse array of cellular structures. Here I discuss our current state of knowledge about this actin-remodelling machine. The predicted structure of the Arp2/3 complex can be directly correlated with its ability to nucleate, cap and cross-link actin filaments. A growing family of Arp2/3 complex activators such as the WASP family, type I myosins, and the newly identified activators cortactin and Abplp tightly regulate this activity within the cell. Localised activation of the Arp2/3 complex produces structures such as lamellipodia or actin patches via a process termed dendritic nucleation. Furthermore, several pathogenic microorganisms have evolved strategies to 'hijack' the Arp2/3 complex to their own advantage. Finally, I discuss some of the questions which remain unanswered about this fascinating complex.

Cryptosporidium parvum infection requires host cell actin polymerization

The intracellular protozoan parasite Cryptosporidium parvum accumulates host cell actin at the interface between the parasite and the host cell cytoplasm. Here we show that the actin polymerizing proteins Arp2/3, vasodilator-stimulated phosphoprotein (VASP), and neural Wiskott Aldrich syndrome protein (N-WASP) are present at this interface and that host cell actin polymerization is necessary for parasite infection.

Plagiarism and pathogenesis: common themes in actin remodeling

The involvement of Nck in pedestal formation by EPEC highlights the similar strategies adopted by this bacterium and the Vaccinia virus to hijack the host cell's cytoskeleton.

Phagocytosis and the actin cytoskeleton

The process of engulfing a foreign particle - phagocytosis - is of fundamental importance for a wide diversity of organisms. From simple unicellular organisms that use phagocytosis to obtain their next meal, to complex metazoans in which phagocytic cells represent an essential branch of the immune system, evolution has armed cells with a fantastic repertoire of molecules that serve to bring about this complex event. Regardless of the organism or specific molecules concerned, however, all phagocytic processes are driven by a finely controlled rearrangement of the actin cytoskeleton. A variety of signals can converge to locally reorganise the actin cytoskeleton at a phagosome, and there are significant similarities and differences between different organisms and between different engulfment processes within the same organism. Recent advances have demonstrated the complexity of phagocytic signalling, such as the involvement of phosphoinostide lipids and multicomponent signalling complexes in transducing signals from phagocytic receptors to the cytoskeleton. Similarly, a wide diversity of ‘effector molecules’ are now implicated in actin-remodelling downstream of these receptors.

The Arp2/3 complex is essential for the actin-based motility of Listeria monocytogenes

Actin polymerisation is thought to drive the movement of eukaryotic cells and some intracellular pathogens such as Listeria monocytogenes. The Listeria surface protein ActA synergises with recruited host proteins to induce actin polymerisation, propelling the bacterium through the host cytoplasm [1]. The Arp2/3 complex is one recruited host factor [2] [3]; it is also believed to regulate actin dynamics in lamellipodia [4] [5]. The Arp2/3 complex promotes actin filament nucleation in vitro, which is further enhanced by ActA [6] [7]. The Arp2/3 complex also interacts with members of the Wiskott-Aldrich syndrome protein (WASP) [8] family - Scar1 [9] [10] and WASP itself [11]. We interfered with the targeting of the Arp2/3 complex to Listeria by using carboxy-terminal fragments of Scar1 that bind the Arp2/3 complex [11]. These fragments completely blocked actin tail formation and motility of Listeria, both in mouse brain extract and in Ptk2 cells overexpressing Scar1 constructs. In both systems, Listeria could initiate actin cloud formation, but tail formation was blocked. Full motility in vitro was restored by adding purified Arp2/3 complex. We conclude that the Arp2/3 complex is a host-cell factor essential for the actin-based motility of L. monocytogenes, suggesting that it plays a pivotal role in regulating the actin cytoskeleton.

Scar, a WASp-related protein, activates nucleation of actin filaments by the Arp2/3 complex

The Arp2/3 complex, a stable assembly of two actin-related proteins (Arp2 and Arp3) with five other subunits, caps the pointed end of actin filaments and nucleates actin polymerization with low efficiency. WASp and Scar are two similar proteins that bind the p21 subunit of the Arp2/3 complex, but their effect on the nucleation activity of the complex was not known. We report that full-length, recombinant human Scar protein, as well as N-terminally truncated Scar proteins, enhance nucleation by the Arp2/3 complex. By themselves, these proteins either have no effect or inhibit actin polymerization. The actin monomer-binding W domain and the p21-binding A domain from the C terminus of Scar are both required to activate Arp2/3 complex. A proline-rich domain in the middle of Scar enhances the activity of the W and A domains. Preincubating Scar and Arp2/3 complex with actin filaments overcomes the initial lag in polymerization, suggesting that efficient nucleation by the Arp2/3 complex requires assembly on the side of a preexisting filament-a dendritic nucleation mechanism. The Arp2/3 complex with full-length Scar, Scar containing P, W, and A domains, or Scar containing W and A domains overcomes inhibition of nucleation by the actin monomer-binding protein profilin, giving active nucleation over a low background of spontaneous nucleation. These results show that Scar and, likely, related proteins, such as the Cdc42 targets WASp and N-WASp, are endogenous activators of actin polymerization by the Arp2/3 complex.

Glucocorticoids and acidosis stimulate protein and amino acid catabolism in vivo

We have shown that chronic metabolic acidosis in awake rats accelerates whole body protein turnover using stochastic modeling and a continuous infusion of L-[1-13C] leucine. To delineate the role that glucocorticoids play in mediating these catabolic responses, we measured protein turnover in awake, chronically catheterized, adrenalectomized rats in the presence or absence of glucocorticoids and/or a NH4Cl feeding regimen which induced chronic metabolic acidosis. In adrenalectomized rats receiving no glucocorticoids there was no statistical difference in amino acid oxidation, protein degradation or synthesis whether or not the rats had acidosis. In contrast, chronically acidotic, adrenalectomized rats receiving glucocorticoids demonstrated accelerated whole body protein turnover with a 84% increase in amino acid oxidation and a 26% increase in protein degradation, compared to rats not receiving glucocorticoids or those given the same dose of glucocorticoids but without acidosis. We conclude that metabolic acidosis accelerates amino acid oxidation and protein degradation in vivo, and that glucocorticoids are necessary but not sufficient to mediate the catabolic effects of metabolic acidosis.

Metabolic acidosis stimulates muscle protein degradation by activating the adenosine triphosphate-dependent pathway involving ubiquitin and proteasomes

Metabolic acidosis often leads to loss of body protein due mainly to accelerated protein breakdown in muscle. To identify which proteolytic pathway is activated, we measured protein degradation in incubated epitrochlearis muscles from acidotic (NH4Cl-treated) and pair-fed rats under conditions that block different proteolytic systems. Inhibiting lysosomal and calcium-activated proteases did not reduce the acidosis-induced increase in muscle proteolysis. However, when ATP production was also blocked, proteolysis fell to the same low level in muscles of acidotic and control rats. Acidosis, therefore, stimulates selectively an ATP-dependent, nonlysosomal, proteolytic process. We also examined whether the activated pathway involves ubiquitin and proteasomes (multicatalytic proteinases). Acidosis was associated with a 2.5- to 4-fold increase in ubiquitin mRNA in muscle. There was no increase in muscle heat shock protein 70 mRNA or in kidney ubiquitin mRNA, suggesting specificity of the response. Ubiquitin mRNA in muscle returned to control levels within 24 h after cessation of acidosis. mRNA for subunits of the proteasome (C2 and C3) in muscle were also increased 4-fold and 2.5-fold, respectively, with acidosis; mRNA for cathepsin B did not change. These results are consistent with, but do not prove that acidosis stimulates muscle proteolysis by activating the ATP-ubiquitin-proteasome-dependent, proteolytic pathway.

Metabolic responses to nephrosis: effect of a low-protein diet

To determine whether dietary protein restriction (LPD) causes protein catabolism in adriamycin nephrosis, nephrotic and control rats were paired by weight and gavage fed an 8.5% protein diet for 3 days (protocol 1) or 12 days (protocol 2). Fasting whole body protein turnover was then measured using a constant infusion of L-[1-14C]leucine. After 3 days of LPD, proteinuria decreased slightly and body weight did not change in either group. In contrast, leucine oxidation and urinary urea nitrogen excretion in nephrotic rats decreased by 18% and 37%, respectively (P < or = 0.05). After 12 days of LPD, weight loss did not differ between groups. In contrast to protocol 1, proteinuria decreased by 45% in nephrotic rats fed LPD for 12 days, and leucine oxidation rats increased to the level of control rats. Rates of whole body protein synthesis (PS) and degradation (PD) did not differ between nephrotic and control rats receiving LPD for 3 or 12 days, but were significantly lower than rates measured in rats fed 22% protein. We conclude that 1) proteinuria stimulates protein conservation even when dietary protein intake is restricted; 2) the decrease in amino acid oxidation was dependent on moderate proteinuria, since prolonged LPD ameliorated nephrosis and leucine oxidation rates increased to control levels; and 3) since weight loss and rates of whole body PS and PD in nephrotic and control animals were indistinguishable, moderate proteinuria did not increase protein catabolism.

Metabolic consequences of uremia: extending the concept of adaptive responses to protein metabolism

An early response to metabolic acidosis is an increase in the degradation of muscle protein to provide the nitrogen needed to increase glutamine production so the kidney can excrete acid. In patients with renal insufficiency, this process may represent an example of a trade-off adaptation to uremia. It requires a hormone (glucocorticoids) and the metabolic response is maladaptive because the inability of the damaged kidney to maintain acid-base balance results in loss of muscle protein. Studies of cultured cells and rats and humans with normal kidneys demonstrate that acidosis stimulates the degradation of both amino acids and protein, which would block the normal adaptive responses to a low-protein diet (ie, to reduce the degradation of essential amino acids and protein). Evidence from studies in rats and humans with chronic uremia show that acidosis is a major stimulus for catabolism. The mechanism includes stimulation of specific pathways for the degradation of protein and amino acids. Since other catabolic conditions (eg, starvation) appear to stimulate the same pathways, understanding the mechanism in acidosis could be applicable to other conditions. Thus, the loss of lean body mass in uremia appears to be a consequence of a normal metabolic response that persists until acidosis is corrected.

Mechanisms of adaptation to proteinuria in adriamycin nephrosis

To evaluate the impact of urinary protein losses on whole body protein turnover (WBPT) independent of acidosis or uremia, we utilized a model of unilateral adriamycin nephrosis. Control rats were matched by weight to nephrotic rats and pair fed 22% protein chow for 14-18 days; urinary urea nitrogen (UUN) was measured on day 12, and leucine turnover measurement was performed on the final day. Growth rates of nephrotic and pair-fed control rats did not differ during the first 2 wk of pair feeding; thereafter, a small difference in growth could be detected. Despite an identical intake of dietary protein, UUN excretion was 29% less in the nephrotic rats (P < or = 0.02). Fasting whole body protein synthesis and degradation did not differ between nephrotic and control rats; in contrast, leucine oxidation decreased by 21% in nephrosis (P < 0.05). On the basis of near normal growth and normal rates of WBPT, we conclude that nephrotic rats fed ad libitum can adapt to the stress of continuous protein losses. A reduction in amino acid oxidation and UUN excretion were the primary mechanisms responsible for protein conservation in experimental nephrosis.

Chronic metabolic acidosis accelerates whole body proteolysis and oxidation in awake rats

Previous work has documented an acceleration of proteolysis and branched-chain amino acid oxidation when muscles from rats with chronic metabolic acidosis were incubated in vitro. The present study examines the impact of chronic metabolic acidosis on whole body amino acid turnover and oxidation in chronically catheterized awake male Sprague-Dawley rats using stochastic modeling and a primed continuous infusion of L-[1-14C] leucine. Whole body protein turnover was accelerated by acidosis as reflected in a 70% increase in proteolysis and a 55% increase in protein synthesis. Amino acid oxidation was increased 145% in rats with chronic metabolic acidosis relative to control rats receiving diets identical in protein and calories based on a reciprocal pool model and plasma alpha-ketoisocaproate specific radioactivity. These changes were accompanied by a 104% increase in liver branched-chain ketoacid dehydrogenase (BCKAD) activity in rats with acidosis, similar to previously documented increases in skeletal muscle BCKAD activity caused by acidosis. In contrast, kidney BCKAD activity was decreased 38% by acidosis, illustrating the tissue-specificity of the changes that were present. We conclude that chronic metabolic acidosis accelerates whole body protein turnover and affects the reincorporation of amino acid into body proteins by accelerating amino acid oxidation.

Leucine-induced amino acid antagonism in rats: muscle valine metabolism and growth impairment

The deleterious effects of branched-chain amino acid (BCAA) antagonism caused by excess dietary leucine include growth depression and subnormal valine and isoleucine pools. To investigate mechanisms causing these changes, rats were gavage-fed low-protein (9%) diets with or without BCAA supplements, and the metabolism of another BCAA (valine) was measured in incubated rat epitrochlearis muscles. A 10% leucine supplement (HL-10) inhibited growth; growth remained subnormal even when 2.6% isoleucine and 2.4% valine (HLIV-10) were added to the diet. Valine decarboxylation in muscle increased 170-270% in rats fed the HL-10 or HLIV-10 diets, but was still markedly lower than we previously found in muscle of rats fed a 14% protein diet. Valine incorporation into muscle protein as an estimate of protein synthesis was unaffected by any of the BCAA supplements. When a lower (4%) concentration of leucine (without or with 0.16% isoleucine and 0.16% valine) was studied, growth was also suppressed but only if rats had not been preconditioned to 9% protein. Although increased BCAA decarboxylation in muscle caused by excess dietary leucine contributes to low valine and isoleucine pools, abnormal growth appears to be independent of low valine and isoleucine levels and is not reflected in suppression of valine incorporation into muscle protein.

Review: mechanisms for abnormal protein metabolism in uremia

Loss of protein stores, reflected by negative nitrogen balance and accelerated accumulation if nitrogenous breakdown products, is an important factor in the morbidity of chronic renal failure and the high mortality rate of acute renal failure. Low protein intake intensifies the suppressed protein synthesis that results from impaired insulin-stimulated protein anabolism. The metabolic acidosis of uremia contributes to tissue loss, both by increasing muscle protein degradation, and by raising the requirements for essential amino acids. Correcting metabolic acidosis improves the nitrogen balance and reduces tissue wasting. It is important to ensure adequate nutrient intakes, rather than the low protein diet often prescribed to slow loss of renal function.

Abnormal cation transport in uremia. Mechanisms in adipocytes and skeletal muscle from uremic rats

The cause of the abnormal active cation transport in erythrocytes of some uremic patients is unknown. In isolated adipocytes and skeletal muscle from chronically uremic chronic renal failure rats, basal sodium pump activity was decreased by 36 and 30%, and intracellular sodium was increased by 90 and 50%, respectively, compared with pair-fed control rats; insulin-stimulated sodium pump activity was preserved in both tissues. Lower basal NaK-ATPase activity in adipocytes was due to a proportionate decline in [3H]ouabain binding, while in muscle, [3H]ouabain binding was not changed, indicating that the NaK-ATPase turnover rate was decreased. Normal muscle, but not normal adipocytes, acquired defective Na pump activity when incubated in uremic sera. Thus, the mechanism for defective active cation transport in CRF is multifactorial and tissue specific. Sodium-dependent amino acid transport in adipocytes closely paralleled diminished Na pump activity (r = 0.91), indicating the importance of this defect to abnormal cellular metabolism in uremia.

Acidosis, not azotemia, stimulates branched-chain, amino acid catabolism in uremic rats

To investigate branched-chain, amino acid metabolism (BCAA) in muscle in chronic renal failure (CRF), we studied rats with moderately severe uremia (PUN 110 approximately mg/dl) and spontaneous metabolic acidosis (bicarbonate, 19 +/- 1 mEq/liter). Plasma BCAA levels in CRF compared to pair-fed control rats were approximately 15% lower and muscle valine was 93 microM lower (P less than 0.05). BCAA metabolism was measured in incubated epitrochlearis muscles using L-[1-14C]valine or L-[1-14C]leucine in the presence and absence of insulin. BCAA decarboxylation was increased (P less than 0.05) and insulin-stimulated BCAA incorporation into protein was blunted (P less than 0.05) by CRF. Since we have found that metabolic acidosis, by itself, stimulates muscle branched-chain, ketoacid dehydrogenase activity, another group of CRF and control rats was given NaHCO3 which corrected the acidosis, but not the azotemia. BCAA decarboxylation in muscle was reduced in CRF rats given NaHCO3, and this was reflected in increased plasma and muscle BCAA concentrations. We conclude that in CRF, chronic metabolic acidosis stimulates BCAA decarboxylation in skeletal muscle and this could contribute to the reduced intra- and extracellular concentrations of BCAA. Correction of acidosis should be a goal of therapy in CRF, especially when dietary regimens restrict intake of BCAA.

Branched-chain amino acid metabolism in rat muscle: abnormal regulation in acidosis

Branched-chain amino acid (BCAA) metabolism is frequently abnormal in pathological conditions accompanied by chronic metabolic acidosis. To study how metabolic acidosis affects BCAA metabolism in muscle, rats were gavage fed a 14% protein diet with or without 4 mmol NH4Cl X 100 g body wt-1 X day-1. Epitrochlearis muscles were incubated with L-[1-14C]-valine and L-[1-14C]leucine, and rates of decarboxylation, net transamination, and incorporation into muscle protein were measured. Plasma and muscle BCAA levels were lower (P less than 0.05) in acidotic rats. Rates of valine and leucine decarboxylation and net transamination were higher (P less than 0.05) in muscles from acidotic rats; these differences were associated with a 79% increase in the total activity of branched-chain alpha-keto acid dehydrogenase and a 146% increase in the activated form of the enzyme. We conclude that acidosis affects the regulation of BCAA metabolism by enhancing flux through the transaminase and by directly stimulating oxidative catabolism through activation of branched-chain alpha-keto acid dehydrogenase.

Mechanisms for defects in muscle protein metabolism in rats with chronic uremia. Influence of metabolic acidosis

Chronic renal failure (CRF) is associated with metabolic acidosis and abnormal muscle protein metabolism. As we have shown that acidosis by itself stimulates muscle protein degradation by a glucocorticoid-dependent mechanism, we assessed the contribution of acidosis to changes in muscle protein turnover in CRF. A stable model of uremia was achieved in partially nephrectomized rats (plasma urea nitrogen, 100-120 mg/dl, blood bicarbonate less than 21 meq/liter). CRF rats excreted 22% more nitrogen than pair-fed controls (P less than 0.005), so muscle protein synthesis and degradation were measured in perfused hindquarters. CRF rats had a 90% increase in net protein degradation (P less than 0.001); this was corrected by dietary bicarbonate. Correction of acidosis did not reduce the elevated corticosterone excretion rate of CRF rats, nor did it improve a second defect in muscle protein turnover, a 34% lower rate of insulin-stimulated protein synthesis. Thus, abnormal nitrogen production in CRF is due to accelerated muscle proteolysis caused by acidosis and an acidosis-independent inhibition of insulin-stimulated muscle protein synthesis.

Metabolic acidosis stimulates protein degradation in rat muscle by a glucocorticoid-dependent mechanism

Metabolic acidosis is associated with enhanced renal ammonia-genesis which is regulated, in part, by glucocorticoids. The interaction between glucocorticoids and chronic metabolic acidosis on nitrogen utilization and muscle protein metabolism is unknown. In rats pair-fed by gavage, we found that chronic acidosis stunted growth and caused a 43% increase in urinary nitrogen and an 87% increase in urinary corticosterone. Net protein degradation in incubated epitrochlearis muscles from chronically acidotic rats was stimulated at all concentrations of insulin from 0 to 10(4) microU/ml. This effect of acidosis persisted despite supplementation of the media with amino acids with or without insulin, indomethacin, and inhibitors of lysosomal thiol cathepsins. Acidosis did not change protein synthesis; hence, the increase in net protein degradation was caused by stimulation of proteolysis. Acidosis did not increase glutamine production in muscle. The protein catabolic effect of acidosis required glucocorticoids; protein degradation was stimulated in muscle of acidotic, adrenalectomized rats only if they were treated with dexamethasone. Moreover, when nonacidotic animals were given 3 micrograms/100 g of body weight dexamethasone twice a day, muscle protein degradation was increased if the muscles were simply incubated in acidified media. We conclude that chronic metabolic acidosis depresses nitrogen utilization and increases glucocorticoid production. The combination of increased glucocorticoids and acidosis stimulates muscle proteolysis but does not affect protein synthesis. These changes in muscle protein metabolism may play a role in the defense against acidosis by providing amino acid nitrogen to support the glutamine production necessary for renal ammoniagenesis.

Specific defects in insulin-mediated muscle metabolism in acute uremia

The mechanisms underlying the abnormal insulin-mediated muscle glucose metabolism occurring in acute uremia (ARF) have not been identified. To characterize the defects, insulin dose-response curves for glucose uptake, glycogen synthesis, glucose oxidation, glycolysis, and lactate release were measured in incubated rat epitrochlearis muscles. ARF did not affect insulin sensitivity, but decreased the responsiveness to insulin of glucose uptake, glycogen synthesis, and glucose oxidation. Glycogen synthesis was subnormal at all levels of insulin and at the maximal insulin concentration; it was 54% lower in muscles of ARF compared to control rats. This inhibition of glycogen synthesis in ARF could be caused by a 23% decrease in the total activity of muscle glycogen synthase and the percentage of enzyme in the activated form. Glycogen phosphorylase activity was unchanged by ARF. ARF also increased the ratio of muscle lactate release to glucose uptake at concentrations of insulin from 10 to 10(4) microU/ml. In the absence and presence of insulin, muscle protein degradation was increased by ARF. In individual muscles incubated with insulin, the rate of proteolysis was correlated with the ratio of lactate release to glucose uptake (r = + 0.82; P less than 0.01). From the insulin dose-response relationships and changes in enzyme activities, we conclude that ARF increases protein degradation in muscle and causes abnormal insulin-mediated glucose metabolism. The abnormalities in glucose metabolism are caused by changes in post-receptor events.