Familial hypobetalipoproteinemia caused by a mutation in the apolipoprotein B gene that results in a truncated species of apolipoprotein B (B-31). A unique mutation that helps to define the portion of the apolipoprotein B molecule required for the formation of buoyant, triglyceride-rich lipoproteins.

Apolipoprotein B-100 has a crucial structural role in the formation of VLDL and LDL. Familial hypobetalipoproteinemia, a syndrome in which the concentration of LDL cholesterol in plasma is abnormally low, can be caused by mutations in the apo B gene that prevent the translation of a full-length apo B-100 molecule. Prior studies have revealed that truncated species of apo B [e.g., apo B-37 (1728 amino acids), apo B-46 (2057 amino acids)] can occasionally be identified in the plasma of subjects with familial hypobetalipoproteinemia; in each of these cases, the truncated apo B species has been a prominent protein component of VLDL. In this report, we describe a kindred with hypobetalipoproteinemia in which the plasma of four affected heterozygotes contained a unique truncated apo B species, apo B-31. Apolipoprotein B-31 is caused by the deletion of a single nucleotide in the apo B gene, and it is predicted to contain 1425 amino acids. Apolipoprotein B-31 is the shortest of the mutant apo B species to be identified in the plasma of a subject with hypobetalipoproteinemia. In contrast to longer truncated apo B species, apo B-31 was undetectable in the VLDL and the LDL; however, it was present in the HDL fraction and the lipoprotein-deficient fraction of plasma. The density distribution of apo B-31 in the plasma suggests the possibility that the amino-terminal 1425 amino acids of apo B-100 are sufficient to permit the formation and secretion of small, dense lipoproteins but are inadequate to support the formation of the more lipid-rich VLDL and LDL particles.

Simultaneous measurement of percentage free thyroxine and triiodothyronine: comparison of equilibrium dialysis and Sephadex chromatography.

An equilibrium dialysis technique was used to measure simultaneously the proportion of free thyroxine (%FT4) and free 3,4,5'-triiodothyronine (%FT3) in sera from patients with diverse states of thyroid function and abnormal levels of plasma T4-binding proteins. In general, the correlation between %FT4 and %FT3 values was excellent in the entire group of patients studied. Studies were also conducted to ascertain whether Sephadex columns could be employed to obtain simultaneous measures of plasma binding of T4 and T3. Mixtures of diluted serum and 125I-T4 and 131I-T3 were applied to columns of Sephadex in order to separate "bound" and "free" fractions. The values for percent free T4 and T3 yielded by the Sephadex process (%FT4S and %FT3S), although far greater numerically, correlated closely with 5FT4 and %FT3 measured directly by equilibrium dialysis. When %FT4 and %FT3 were multiplied by their respective serum concentrations, the resulting free T4 and free T3 indices provided good separation of hyperthyroid and hypothyroid groups from the euthyroid group. As in the dialysis method, %FT4S closely correlated with %FT3S.

Shortening and [Ca2+] dynamics of left ventricular myocytes isolated from exercise-trained rats.

The effects of run endurance training and fura 2 loading on the contractile function and Ca2+ regulation of rat left ventricular myocytes were examined. In myocytes not loaded with fura 2, the maximal extent of myocyte shortening was reduced with training under our pacing conditions [0.5 Hz at 2.0 and 0.75 mM external Ca2+ concentration ([Ca2+]o)], although training had no effect on the temporal characteristics. The "light" loading of myocytes with fura 2 markedly suppressed (approximately 50%) maximal shortening in the sedentary and trained groups, although the temporal characteristics of myocyte shortening were significantly prolonged in the trained group. No discernible differences in the dynamic characteristics of the intracellular Ca2+ concentration ([Ca2+]) transient were detected at 2.0 mM [Ca2+]o, although peak [Ca2+] and rate of [Ca2+] rise during caffeine contracture were greater in the trained state at 0.75 mM [Ca2+]o. We conclude that training induced a diminished myocyte contractile function under the conditions studied here and a more effective coupling of inward Ca2+ current to sarcoplasmic reticulum Ca2+ release at low [Ca2+]o, and that fura 2 and its loading vehicle DMSO significantly alter the intrinsic characteristics of myocyte contractile function and Ca2+ regulation.

Effects of chronic run training on Na+-dependent Ca2+ efflux from rat left ventricular myocytes.

The effects of endurance run training on Na+-dependent Ca2+ regulation in rat left ventricular myocytes were examined. Myocytes were isolated from sedentary and trained rats and loaded with fura 2. Contractile dynamics and fluorescence ratio transients were recorded during electrical pacing at 0.5 Hz, 2 mM extracellular Ca2+ concentration, and 29 degreesC. Resting and peak cytosolic Ca2+ concentration ([Ca2+]c) did not change with exercise training. However, resting and peak [Ca2+]c increased significantly in both groups during 5 min of continuous pacing, although diastolic [Ca2+]c in the trained group was less susceptible to this elevation of intracellular Ca2+. Run training also significantly reduced the rate of [Ca2+]c decay during relaxation. Myocytes were then exposed to 10 mM caffeine in the absence of external Na+ or Ca2+ to trigger sarcoplasmic reticular Ca2+ release and to suppress cellular Ca2+ efflux. This maneuver elicited an elevated steady-state [Ca2+]c. External Na+ was then added, and the rate of [Ca2+]c clearance was determined. Run training significantly reduced the rate of Na+-dependent clearance of [Ca2+]c during the caffeine-induced contractures. These data demonstrate that the removal of cytosolic Ca2+ was depressed with exercise training under these experimental conditions and may be specifically reflective of a training-induced decrease in the rate of cytosolic Ca2+ removal via Na+/Ca2+ exchange and/or in the amount of Ca2+ moved across the sarcolemma during a contraction.

Renal hypertension prevents run training modification of cardiomyocyte diastolic Ca2+ regulation in male rats.

The combined effects of endurance run training and renal hypertension on cytosolic Ca2+ concentration ([Ca2+]c) dynamics and Na+-dependent Ca2+ regulation in rat left ventricular cardiomyocytes were examined. Male Fischer 344 rats underwent stenosis of the left renal artery [hypertensive (Ht), n = 18] or a sham operation [normotensive (Nt), n = 20]. One-half of the rats from each group were treadmill trained for >16 wk. Cardiomyocyte fura 2 fluorescence ratio transients were recorded for 7 min during electrical pacing at 0.5 Hz, 2 mM extracellular Ca2+ concentration, and 29 degrees C. The rate of [Ca2+]c decline was not changed by run training in the Nt group but was reduced in the Ht group. At 7 min, cardiomyocytes were exposed to 10 mM caffeine in the absence of Na+ and Ca2+, which triggered sarcoplasmic reticular Ca2+ release and suppressed Ca2+ efflux via Na+/Ca2+ exchanger. External Na+ was then added, and Na+-dependent Ca2+ efflux rate was recorded. Treadmill training significantly enhanced Na+-dependent Ca2+ efflux rate under these conditions in the Nt group but not in the Ht group. These data provide evidence that renal hypertension prevents the normal run training-induced modifications in diastolic [Ca2+]c regulation mechanisms, including Na+/Ca2+ exchanger.

Estimation of mechanical properties of cortical bone by computed tomography.

It is difficult to assess from conventional x-rays the amount of loading that a bone can tolerate. The question therefore was asked whether the mechanical properties of cortical bone could be estimated by using a computed tomography (CT) system typically employed in the clinical setting. In vitro cross sectional diaphyseal scans of adult human tibiae were made using a GE 9800 scanner and linear attenuation coefficients determined in several regions of the central cross sections. Samples from the mid-diaphyses of these tibiae were harvested, tested in three-point bending to failure, and mechanical properties as well as density and ash fraction determined. The respective relationships between CT measurements, mechanical properties, and physical properties were calculated using regression analysis. In addition, a solid calibration phantom (tricalciumphosphate) was scanned to evaluate the variability of CT measurements. The physical parameters measured in this study were found to be comparable with data from other authors but correlations were moderate to weak. Linear regression revealed the following correlation coefficients with CT data: r = 0.55 (Young's modulus), r = 0.50 (strength), r = 0.65 (apparent density) and r = 0.46 (ash fraction). The correlation coefficients of these regressions for both linear and power fits were not significantly different. A high linear correlation (r = 0.99) was found between the chamber densities and the measured attenuation coefficients, but accuracy varied between 2 and 6%. The small range of specimen mechanical properties as well as the limitations inherent with the methods employed may explain these results. We conclude that clinical equipment as used in this study is not sufficient to accurately estimate the mechanical properties of cortical bone.

Resistive Liquid-Vapor Surface Sensors for Liquid Nitrogen and Hydrogen.

Ten resistance thermometers were tested as point sensors for detecting the liquid-vapor interface in liquid nitrogen and liquid hydrogen. Test results showed that most could be made to detect the liquid surface and lead orientation can be important. A silicon resistive sensor had the fastest response and produced the greatest signal change.

Controlling absenteeism can help curb hospitals' costs.

Absenteeism involves costs associated with sick pay, overtime pay, decreased employee productivity, and less effective patient care. Hospital management and supervisors must minimize absenteeism and its costs by thoroughly recording and evaluating employees' attendance patterns, counseling and disciplining employees when necessary, and applying policies on attendance with consistency.

Tracheal volume deformation in a developmental rabbit model.

Alterations in tracheal volume were evaluated in a developmental in vivo rabbit model during and following the application of continuous positive pressure (CPP). Sequential volume changes were recorded during 60 min of CPP to a bypassed tracheal segment. The distal trachea was utilized for spontaneous ventilation. CPP of 10 cm H2O was applied to three developmental groups: group I, 5 term newborn pups; group II, 5, 7-day-old pups, and group III, 5 adult female rabbits of 18 +/- 6 months old. Changes in tracheal volume were measured by a micropipette system. Following 60 min of CPP, tracheal volumes increased (p less than 0.01) by 39.8% in group I; 36.1% in group II and 5.1% in group III. During a recovery period (60 min), return towards initial resting volume was observed in all groups, though maximal persistent volume deformation was observed in groups I and II. Thus, these data indicate tracheal barotrauma in the form of persistent dimensional deformation at early stages of development in an in vivo rabbit model.

A mathematical model of cardiocyte Ca(2+) dynamics with a novel representation of sarcoplasmic reticular Ca(2+) control.

Cardiac contraction and relaxation dynamics result from a set of simultaneously interacting Ca(2+) regulatory mechanisms. In this study, cardiocyte Ca(2+) dynamics were modeled using a set of six differential equations that were based on theories, equations, and parameters described in previous studies. Among the unique features of the model was the inclusion of bidirectional modulatory interplay between the sarcoplasmic reticular Ca(2+) release channel (SRRC) and calsequestrin (CSQ) in the SR lumen, where CSQ acted as a dynamic rather than simple Ca(2+) buffer, and acted as a Ca(2+) sensor in the SR lumen as well. The inclusion of this control mechanism was central in overcoming a number of assumptions that would otherwise have to be made about SRRC kinetics, SR Ca(2+) release rates, and SR Ca(2+) release termination when the SR lumen is assumed to act as a simple, buffered Ca(2+) sink. The model was sufficient to reproduce a graded Ca(2+)-induced Ca(2+) release (CICR) response, CICR with high gain, and a system with reasonable stability. As constructed, the model successfully replicated the results of several previously published experiments that dealt with the Ca(2+) dependence of the SRRC (, J. Gen. Physiol. 85:247-289), the refractoriness of the SRRC (, Am. J. Physiol. 270:C148-C159), the SR Ca(2+) load dependence of SR Ca(2+) release (, Am. J. Physiol. 268:C1313-C1329;, J. Biol. Chem. 267:20850-20856), SR Ca(2+) leak (, J. Physiol. (Lond.). 474:463-471;, Biophys. J. 68:2015-2022), SR Ca(2+) load regulation by leak and uptake (, J. Gen. Physiol. 111:491-504), the effect of Ca(2+) trigger duration on SR Ca(2+) release (, Am. J. Physiol. 258:C944-C954), the apparent relationship that exists between sarcoplasmic and sarcoplasmic reticular calcium concentrations (, Biophys. J. 73:1524-1531), and a variety of contraction frequency-dependent alterations in sarcoplasmic [Ca(2+)] dynamics that are normally observed in the laboratory, including rest potentiation, a negative frequency-[Ca(2+)] relationship, and extrasystolic potentiation. Furthermore, under the condition of a simulated Ca(2+) overload, an alternans-like state was produced. In summary, the current model of cardiocyte Ca(2+) dynamics provides an integrated theoretical framework of fundamental cellular Ca(2+) regulatory processes that is sufficient to predict a broad array of observable experimental outcomes.

Monozygotic twins with Klinefelter's syndrome discordant for systemic lupus erythematosus and symptomatic myasthenia gravis.

Monozygotic twins with Klinefelter's syndrome were evaluated for two distinct illnesses. One subject had clinical and serologic evidence of systemic lupus erythematosus and no symptoms of muscle weakness. His identical twin had typical symptoms and laboratory evidence of myasthenia gravis. Antibodies to acetylcholine receptors were present in both subjects. These patients are discussed in relation to genetic, hormonal, and immunologic mechanisms involved in the pathogenesis of these two disorders.

Binding of thyroid hormones and their analogues to thyroxine-binding globulin in human serum.

The present study was undertaken to study the binding of several thyroid hormones and structurally related compounds to human serum thyroxine-binding alpha-globulin (TBG). The source of TBG was normal human serum diluted 1:100 in 0.035 M barbital buffer, pH 7.4. In the binding assays, 125I-thyroxine, unlabeled thyroxine, and diluted serum were incubated for 20 h at 37 degrees in Plexiglas equilibrium dialysis units. Two orders of binding sites were discerned: a high affinity, low capacity binding site with an affinity constant of approximately 2.5 X 10(9) M-1, and a low affinity, very high capacity binding site with an affinity constant of less than 10(6) M-1. Studies with purified TBG, serum deficient in TBG, and purified human serum albumin indicated that the high affinity site represented binding to TBG and the low affinity site represented binging to albumin. The ability of several groups of thyroid hormone analogues to bind to TBG was then investigated. As a result of these studies, the following structural features of thyroid hormones were found to be important for optimal binding activity: (a) the L-alanine side chain conformation, (b) the presence of a 4'-hydroxyl group, (c) the presence of two substituents in the inner and outer rings (positions 3, 5, 3', and 5'), and (d) the presence of either bromines or iodines in the inner ring and iodines in the outer ring. Of lesser importance was the presence of an oxygen atom in the ether position.

Chronic run training suppresses alpha-adrenergic response of rat cardiomyocytes and isovolumic left ventricle.

The effects of endurance run training on alpha-adrenergic responsiveness of rat left ventricle (LV) were examined in cardiomyocytes and isovolumic LV. Female Sprague-Dawley rats were sedentary (Sed) or trained (Tr) for >20 wk by treadmill running. Cardiomyocyte shortening and fura 2 fluorescence ratio were recorded before and during 5-min exposure to 5 microM phenylephrine (PE) while paced at 0.5 Hz in 2 mM extracellular Ca2+ concentration at 29 degrees C. Cardiomyocyte shortening and shortening velocity increased with PE, and these effects were more pronounced in the Sed group. The rate of cytosolic Ca2+ concentration removal was reduced by PE in the Sed cardiomyocytes, but was unaffected in the Tr. Isovolumic LV pressure was recorded immediately before and during 5-min perfusion with 5 microM PE during pacing at 280 beats/min and 37 degrees C, and positive inotropy due to PE was more pronounced in the Sed than in the Tr. These data demonstrated that the effects of alpha-adrenergic stimulation on myocardial positive inotropy and calcium regulation were reduced in this rat model of run training at both the cellular and whole organ levels.

Four new mutations in the apolipoprotein B gene causing hypobetalipoproteinemia, including two different frameshift mutations that yield truncated apolipoprotein B proteins of identical length.

Familial hypobetalipoproteinemia can be caused by mutations in the apolipoprotein (apo)B gene that interfere with the translation of a full-length apoB molecule. Frequently, a truncated apoB molecule can be detected in the plasma lipoproteins of affected subjects. In this report, we characterize four different apoB gene mutations causing hypobetalipoproteinemia that are associated with the synthesis of truncated apoB proteins. Two of the mutations are nonsense mutations caused by single nucleotide substitutions; these mutations are associated with the production of apoB-32.5 (1473 amino acids) and apoB-82 (3733 amino acids). The other two mutations are single nucleotide deletions (of apoB cDNA nucleotides 7295 and 7359, respectively). The altered reading frames created by these different frameshift mutations terminated with the same stop codon, and both therefore yielded a truncated protein of identical size: apoB-52.8 (2395 amino acids). The two apoB-52.8 proteins differ, however, in the number of novel carboxyl-terminal amino acids introduced by the frameshift. The buoyant density of lipoproteins containing the truncated apoBs was inversely related to the length of the truncated apoB. ApoB-32.5 was present only in high density lipoproteins (HDL) and the d > 1.21 g/ml fraction, whereas apoB-82 was present almost exclusively in very low density lipoproteins (VLDL). ApoB-52.8 was present primarily in VLDL, intermediate density lipoproteins (IDL), and low density lipoproteins (LDL); trace amounts were observed in the HDL.