O-linked carbohydrate of recombinant von Willebrand factor influences ristocetin-induced binding to platelet glycoprotein 1b.

By transfecting the full-length cDNA for human von Willebrand factor (vWf) into a line of Chinese hamster ovary cells with a defect in carbohydrate metabolism, we have prepared recombinant vWf specifically lacking O-linked carbohydrates. We have compared this under-glycosylated protein to fully glycosylated recombinant vWf with respect to several structural and binding properties. vWf deficient in O-linked glycans was synthesized, assembled into multimers, and secreted in an apparently normal manner and was not prone to degradation in the extracellular milieu. It did not differ from fully glycosylated vWf in ability to bind to heparin or to collagen type I but did interact less well with glycoprotein 1b on formalin-fixed platelets. This decreased interaction was evidenced in both a lessened overall binding to platelets and in diminished capacity to promote platelet agglutination, in the presence of ristocetin. In contrast, no difference was seen in platelet binding in the presence of botrocetin. These data indicate a possible role for O-linked carbohydrates in the vWf-glycoprotein 1b interaction promoted by ristocetin and suggest that abnormalities in carbohydrate modification might contribute to the altered ristocetin-dependent reactivity between vWf and platelets described for some variant forms of von Willebrand disease.

An explanation for minor multimer species in endothelial cell-synthesized von Willebrand factor.

Initial synthesis of von Willebrand factor (vWf) by cultured human endothelial cells proceeds by formation of a dimer of pro-vWf subunits. These subunits are found only within the cell and have an apparent molecular weight of 240,000-260,000, as measured by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels. Posttranslational modifications, including proteolytic cleavage, glycosylation, and sulfation, result in the appearance of two additional vWf subunits. The major one migrates with the subunit of plasma vWf at an apparent molecular weight of 220,000-225,000 and the other migrates more slowly than pro-vWf at an apparent molecular weight of 260,000-275,000. These subunits oligomerize to form a set of vWf multimers, which are subsequently secreted into the culture medium. We isolated individual vWf oligomer species from the agarose gel bands and show that vWf minor, or satellite, species differ from major species in subunit composition.

Endothelial-cell-specific regulation of von Willebrand factor gene expression.

In both tissue sections and cell culture, the endothelial nature of a cell is most commonly determined by demonstration of its expression of von Willebrand factor (vWf) protein and/or mRNA. Thus, the mechanism of cell-type-specific transcriptional regulation of the vWf gene is central to studying the basis of endothelial-cell-specific gene expression. In this study, deletion analyses were carried out to identify the region of the vWf gene which regulates its endothelial-cell-specific expression. A 734-bp fragment which spans the sequence from -487 to +247 relative to the transcription start site was identified as the cell-type-specific promoter. It consists of a minimal core promoter located between -90 and +22, a strong negative regulatory element located upstream of the core promoter (ca. -500 to -300), and a positive regulatory region located downstream of the core promoter in the first exon. The activity of the core promoter is not cell type specific, and the negative regulatory region is required to inhibit its activity in all cell types. The positive regulatory region relieves this inhibition only in endothelial cells and results in endothelial-cell-specific gene expression. The positive regulatory region contains sequences predicting possible SP1, GATA, and octamer binding sites. Mutations in either the SP1 or octamer sequence have no effect on transcriptional activity, while mutation in the GATA binding element totally abolishes the promoter activity. Evidence that a GATA factor is involved in this interaction is presented. Thus, the positive regulatory region with an intact GATA binding site is required to overcome the inhibitory effect of the negative regulatory element and activate vWf gene expression in an endothelial-cell-specific manner.

Echocardiographic studies of the human fetus: prenatal diagnosis of congenital heart disease and cardiac dysrhythmias.

During obstetrical ultrasound examinations, 200 M-mode and 35 real-time two-dimensional echocardiographic studies were performed on 180 fetuses of high-risk pregnancies. Fetal gestational ages ranged from 18 to 41 weeks. M-mode "sweeps" demonstrating mitral- and septal-aortic fibrous continuity were obtained in 115 studies. Paradoxic septal motion in 50 fetuses suggested relarive right ventricular volume loading. Congenital cardiac malformations were accurately diagnosed in a 34-week fetus with pulmonary atresia and hypoplastic right ventricle and in a 28-week fetus with a univentricular heart. Congenital complete atrioventricular block was diagnosed in a 28-week fetus and atrial flutter with variable atrioventricular block was diagnosed in a 38-week fetus. The use of echocardiographic studies to evaluate cardiac structure and rhythm in utero assists in counseling prospective parents and in planning postnatal management for their offspring.

Fetal echocardiography. A tool for evaluation of in utero cardiac arrhythmias and monitoring of in utero therapy: analysis of 71 patients.

Fetal echocardiographic studies were performed in 71 patients referred for evaluation of cardiac rhythm disturbances at 24 to 40 weeks' gestation. After 2-dimensional echocardiographic study of cardiac structure was performed, M-mode echocardiograms were analyzed for measurement of cardiac rate, atrioventricular contraction sequence, atrioventricular valve motion, and duration of postectopic pauses. Arrhythmias were diagnosed in 59 patients. In 34 patients with isolated ectopic beats, the arrhythmia resolved during later pregnancy in 26 or within the first 5 days of life in 8. Six patients had mild sinus bradycardia and 8 had frequent sinus pauses; all 14 had resolution of the arrhythmia during pregnancy. Sustained arrhythmias occurred in 11 patients. Deaths occurred when there was associated fetal congestive heart failure (hydrops fetalis), structural heart disease, or both. M-mode echocardiography diagnosed supraventricular tachycardia in 3 fetuses. The echocardiogram was used thereafter for monitoring transplacental digoxin therapy.

Variables that may enhance medical students' perceived preparedness for computer-based testing.

OBJECTIVE: To identify variables that may enhance medical student's preparedness for computer-based administration of the United States Medical Licensing Examination (USMLE). DESIGN: A cross-sectional survey of 301 medical students who completed a self-administered questionnaire. MEASUREMENTS: The questionnaire was designed to obtain information about students' computer resources, personal experience with computers, computer expertise, opinions about computers, experience with computer-based testing, perceived preparedness for the computer-based USMLE, and demographic variables. Variables related to students' perceived preparedness for the computer-based USMLE were identified by ordinal logistic regression. RESULTS: A significant regression model yielded four significant predictors: perceived preparedness for USMLE content (P: < 0.0001), opinions about computers (P: < 0.0012), gender (P: < 0.0001), and a gender by computer-based testing experience interaction (P: < 0. 0004). Computer resources, personal experience with computers, computer expertise, age, race, and year of medical school were not significant predictors. CONCLUSION: Students' perceived preparedness for computer-based administration of high-stakes examinations may be facilitated by preparing them for examination content, by enhancing their opinions about computers, and by increasing their computer-based testing experiences.

Influence of medical school on medical students' opinions about primary care practice.

PURPOSE: To compare first- and fourth-year medical students' opinions about primary care practice. METHOD: A cross-sectional survey was made of medical students at New York Medical College (NYMC) and East Carolina University School of Medicine (ECUSOM) over three years (1993-94, 1994-95, and 1995-96). Three consecutive classes of first-year students from both schools (n = 807), two consecutive classes of fourth-year NYMC students (n = 373), and three consecutive classes of fourth-year ECUSOM students (n = 221) were given a self-administered questionnaire about professional aspects of primary care practice. Responses to ten items about primary care practice were the independent variables in a logistic regression analysis. Career choice, categorized as primary care or non-primary care, was the dependent variable. Independent, two-tailed t-tests were employed to compare the responses of the first-year students interested in primary care careers with those of the fourth-year students interested in primary care careers. RESULTS: In all, 639 (79%) of the first-year students and 396 (67%) of the fourth-year students returned completed questionnaires. The first-year students interested in primary care careers were significantly more likely to believe that primary care practice has more prestige, has more intellectual stimulation, needs a large knowledge base, and involves work that is more important than that of non-primary care physicians, and were significantly more likely to disagree with the assertion that in primary care practice, physicians have more control over their working hours. With one exception (prestige of primary care practice), all these independent variables were significant for the fourth-year students as well. The comparison of the first- and fourth-year students indicated that the fourth-year students were significantly more likely to believe that primary care practice has more intellectual stimulation, needs a large knowledge base, and requires knowledge that non-primary care practice may not; they were also significantly more likely to disagree with the assertions that primary care practice is adequately compensated, has more prestige, and allows more control over working hours. CONCLUSION: It appears that students' positive perceptions about primary care practice may change as realistic perceptions about the professional demands on primary care physicians develop during medical school.

Moving from policy to practice in curriculum change at the University of Virginia School of Medicine, East Carolina University School of Medicine, and SUNY-Buffalo School of Medicine.

A set of formative evaluation studies from the medical schools of the University of Virginia (UVA), East Carolina University (ECU), and the State University of New York at Buffalo (SUNY-Buffalo) portrays, in qualitative and quantitative terms, evidence of achievements and obstacles to the curricular reform supported by The Robert Wood Johnson Foundation's Generalist Physician Initiative (GPI). In this paper, innovations in the under-graduate curriculum, a specific course, and instructional strategies are examined. Individual interviews of faculty and focus groups with students assessed opinions about curricular change at the University of Virginia. Questionnaires and focus groups provided information about the impact of course changes at East Carolina University. Questionnaires completed by students provided information of the effect of modifying the instructional strategies at SUNY-Buffalo. The obstacles to implementing change at the three schools included breakdowns in the faculty's understanding and support of change, lack of skills required to implement change, and weakness in coordinating and assessing planned change. Although the GPI catalyzed changes in the content and conduct of generalist education at the three schools, many lessons were learned that are applicable to other medical schools.

Academic models for practice relief, recruitment, and retention at the University of New Mexico Medical Center and East Carolina University School of Medicine.

Locum tenens programs were independently developed by the University of New Mexico Health Sciences Center and East Carolina University School of Medicine to address health care needs within their states. The programs represent distinct models to provide locum tenens services for practices in rural and medically underserved regions through collaboration with stage government representatives and agencies. Differences between programs include years of operation, days of coverage provided, types of learners and providers involved, sources of funding, costs of the programs, and extent of institutional support. Common beneficial outcomes of the programs include coverage for struggling practices, training in rural medicine for locum tenens providers, recruitment of physicians to rural practice sites, and improved relationships with program partners. Adequate funding and institutional support are essential for success of locum tenens programs.

Development and crossmodal transfer of contextual control of emergent stimulus relations.

Six normally capable adults first learned three conditional relations in each of two prospective equivalence classes via match-to-sample training with figures as conditional (sample) and discriminative (comparison) stimuli. Then one trained conditional relation in each prospective class was brought under the control of contextual stimuli, two dictated nonsense syllables. Test performances indicated the emergence of untrained conditional relations, and therefore two equivalence classes, that were conditional on the contextual stimuli. These tests involved untrained combinations of contextual stimuli and stimuli in conditional relations, suggesting that the contextual stimuli functioned independently to control conditional relations rather than forming compound stimuli with samples and comparisons in training. Next, two novel figures were made equivalent to each of the original dictated contextual stimuli by match-to-sample training and testing. On subsequent tests, all subjects demonstrated transfer of conditional control of untrained conditional relations from the original auditory contextual stimuli to equivalent visual stimuli. These outcomes further supported the conclusion that the contextual stimuli exerted true conditional control over conditional relations in the equivalence classes and were not merely elements of compound stimuli.

Can a 3-day preceptorship change first-year medical students' opinions about living and working in small towns?

BACKGROUND: Research of medical school initiatives that attempt to orient medical students toward rural medicine may facilitate development of initiatives to alleviate physician maldistribution. This study investigated the effect of a 3-day family medicine preceptorship in a small town on first-year medical students' opinions about a) living and working in small towns and b) plans to live in and practice medicine in small towns. Student feedback about the preceptorship was also examined. METHODS: Questionnaires were administered to 137 first-year medical students using a separate sample pretest-posttest design. RESULTS: Student feedback indicated that the preceptorship was a valuable learning experience, but the preceptorship did not appear to influence students' opinions about or interest in living in and working in small towns or rural areas. CONCLUSION: Brief exposure to rural medicine early in the curriculum appears to have little effect on variables that might precede practice location decisions.

Standardized examination performance and specialty choice.

BACKGROUND AND OBJECTIVES: It has been suggested that medical students who attend schools known for graduating prospective primary care physicians may enter primary care residencies, rather than non-primary care residencies, because they are unable to compete for subspecialty residencies due to poorer academic performance. This study determined if performance on standardized examinations conducted by the National Board of Medical Examiners (NBME) could differentiate between students who selected primary care and those who selected non-primary care specialties at a medical school in the southeastern United States committed to graduating primary care physicians. METHODS: We examined initial scores on NBME examinations and subsequent residency selections by 780 students over a 14-year period to determine if there were differences in the kind of residency placements of students who passed and those who failed the examinations. RESULTS: Data analysis indicated that medical students who entered primary care and non-primary care residencies were not distinguishable on the basis of standardized examination performance. CONCLUSIONS: These results may help to refute negative stereotypes about students who enter primary care residencies and about medical schools known for promoting careers in primary care.

Opto-Mechanical and Electronic Design of a Tunnel-Trap Si Radiometer.

A transmission-type light-trap silicon radiometer has been developed to hold the NIST spectral power and irradiance responsivity scales between 406 nm and 920 nm. The device is built from replaceable input apertures and tightly packed different-size silicon photodiodes. The photodiodes are positioned in a triangular shape tunnel such that beam clipping is entirely eliminated within an 8 field-of-view (FOV). A light trap is attached to the output of the radiometer to collect the transmitted radiation and to minimize the effect of ambient light. The photodiodes, selected for equal shunt resistance, are connected in parallel. The capacitance and the resultant shunt resistance of the device were measured and frequency compensations were applied in the feedback network of the photocurrent-to-voltage converter to optimize signal-, voltage-, and loop-gain characteristics. The trap radiometer can measure either dc or ac optical radiation with high sensitivity. The noise-equivalent-power of the optimized device is 47 fW in dc mode and 5.2 fW at 10 Hz chopping. The relative deviation from the cosine responsivity in irradiance mode was measured to be equal to or less than 0.02 % within 5° FOV and 0.05 % at 8° FOV. The trap-radiometer can transfer irradiance responsivities with uncertainties comparable to those of primary standard radiometers. Illuminance and irradiance meters, holding the SI units (candela, color- and radiance-temperature), will be calibrated directly against the transfer standard trap-radiometer to obtain improved accuracy in the base-units.

Stimulus equivalence instruction of fraction-decimal relations.

Stimulus control technology was applied to the instruction of fraction ratio (e.g., (1/5)) and decimal (e.g., 0.20) relations, with 7 students who demonstrated difficulty in fraction and decimal tasks. The students were trained to match pictorial representations of fractions (B comparison stimuli) to printed counterpart fraction ratios (A sample stimuli), and to match printed decimals (C comparison stimuli) to pictorial representations of counterpart quantities (B sample stimuli). Posttest performance by all participants indicated the emergence of equivalence relations between fractions represented as ratios, decimals, and pictures. Limited generalization of fraction-decimal relations was observed.