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Biomedical subjects

W A Clark

Publications and source records attributed to W A Clark.

At least 37 records · Page 2Linked to original sources
PubMed1995

Demographic foundations of political empowerment in multiminority cities.

As U.S. cities accommodate increasing ethnic and racial diversity, political choices may unify or divide their local populations. Those choices pull communities toward two different modes of pluralism: traditional "melting pot" assimilation or a complex mosaic of racial and ethnic assertiveness. Central to this issue is equity and empowerment, which may be accentuated by minority populations' size, structure, and spatial concentration. We examine two potential modes of local empowerment: "dominance," whereby each group is the majority of voters in single election districts (reinforcing separative tendencies), and "influence," whereby a group gains "influential minority" status in several districts (reinforcing unifying tendencies).

Acculturation↗
PubMed1994

Myosin heavy chain synthesis is independently regulated in hypertrophy and atrophy of isolated adult cardiac myocytes.

Hypertrophy of isolated adult feline cardiac muscle cells may be induced in culture by either alpha- or beta-adrenergic agonists. However, it has been shown previously that each of these agonists activate different subsets of immediate-early response genes and have different effects on expression of "fetal" protein isoforms and stimulation of protein synthesis. Moreover, in adult feline heart cells, beta-adrenergic agonists, such as isoproterenol, activate sustained synchronous beating and sarcomeric reorganization while alpha-adrenergic agonists, such as phenylephrine, do not. The objective of the present study was to determine whether these differences in proximal signaling events converged in a common signal pathway during activation of contractile protein synthesis. By direct comparisons of actin and myosin heavy chain (HC) synthesis and accumulation following isoproterenol and phenylephrine, it was determined that both agonists stimulate a coordinated accumulation of these proteins during cardiomyocyte growth. However, each agonist stimulated a very different program of contractile protein synthesis. During phenylephrine-induced hypertrophy, actin and myosin HC syntheses were rapidly and coordinately activated and continuously maintained at rates 10-25% greater than untreated cultures. The pattern of myosin HC synthesis following isoproterenol was very much more complex with periods during which it was as much as 40% greater or 25% less than in control cultures. Furthermore, there was no correlation between rates of actin and myosin HC synthesis following isoproterenol. It was concluded that actin and myosin HC syntheses and accumulation were regulated independently and in a very different manner following isoproterenol or phenylephrine. Since protein accumulation was not correlated with synthesis rates during development of hypertrophy, it was also concluded that post-translational mechanisms played a significant role in the maintenance of contractile protein stoichiometry during beta-adrenergic/beating-induced hypertrophy. Myosin HC synthesis also appeared to be independently regulated during cardiomyocyte atrophy induced by the calcium channel blocker nifedipine. Unlike the case in hypertrophy, however, protein balance was not maintained in nifedipine, and the depression of myosin HC synthesis and loss of myosin HC content were much greater than in the case of other contractile proteins.

Actins↗
PubMed1993

Evidence for post-translational kinetic compartmentation of protein turnover pools in isolated adult cardiac myocytes.

The kinetics of protein metabolism were evaluated in isolated adult feline cardiomyocytes maintained in long term cell culture. The results of these studies suggested that, rather than individual proteins entering a kinetically homogeneous pool, most newly synthesized proteins were segregated into different kinetic compartments with different probabilities of being degraded. Evidence for this conclusion was developed from two types of experiment. The first line of evidence was derived from the kinetics of labeled amino acid incorporation into protein when labeling was conducted over very short or long periods (4 h to 32 days). The observed rates of isotope incorporation over different periods did not fit the expected pattern for a homogeneous pool. A close approximation of observed rates of label incorporation over both short and long labeling intervals could, however, be fit to results predicted using a two-compartment model. When the results of long term equilibration labeling studies were also evaluated relative to short and long term labeling ratios, then it was also possible to identify a unique set of parameters for a two-compartment model which could account for label incorporation in both types of experiment. From this analysis it was estimated that the fast kinetic compartment represented a steady-state level of 10% of total cellular protein with a mean half-life of 21.9 h (ks = 75.9% d-1). The slow kinetic compartment comprised the remaining 90% of protein with a mean t1/2 equal to 15.6 days (ks = 4.4% d-1). Both the observed and predicted equilibration rate of this two compartment mixture was best fit by a single exponential function with an equilibration rate of 5.48% d-1. Based on this outcome, it was also predicted that mixing of nascent and long lived proteins could be followed using a protocol of long and short term labeling in different isotopes followed by a chase period without label. By tracking the isotope ratios in total proteins and SDS-polyacrylamide gel electrophoresis separated proteins, as well as in counts released into the culture medium during the chase period it was determined that nascent proteins were preferentially degraded for a period of at least 48 h following synthesis. Up to 20% of nascent proteins in both the total protein compartment as well as individual proteins, such as myosin heavy chain, were preferentially degraded prior to achieving a state of homogeneous mixing with long lived proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Actinin↗
PubMed1993

Stress activation of mesocorticolimbic dopamine neurons: effects of a glycine/NMDA receptor antagonist.

Restraint of brief duration causes a metabolic activation of mesocortical and some mesolimbic dopaminergic systems with little effect on the nigrostriatal system. We have examined the ability of an antagonist of the allosteric glycine site of the N-methyl-D-aspartate receptor complex to block the stress-induced response in dopamine utilization. Thirty minutes of restraint stress elevated dopamine metabolism, as measured by the ratio between 3,4-dihydroxyphenylacetic acid (DOPAC) and dopamine, in both the medial prefrontal cortex and nucleus accumbens. An antagonist for the glycine/N-methyl-D-aspartate receptor complex, 1-hydroxy-3-aminopyrrolidone-2 ((+)-HA-966), given systemically or injected into the ventral tegmental area, prevents the stress-induced increase in dopamine metabolism in the prefrontal cortex without altering the response in the nucleus accumbens. Similarly, systemic administration of the non-competitive antagonist for the N-methyl-D-aspartate receptor, dizocilpine ((+)-MK-801), blocked the stress-induced rise in dopamine metabolism in the medial prefrontal cortex but not the nucleus accumbens. The negative enantiomer of HA-966 did not produce a selective antagonism of the stress-induced dopamine metabolism in the medial prefrontal cortex. These results support previous work which suggest the mesocortical and mesoaccumbens dopamine neurons respond to excitatory input through different glutamate receptor mechanisms. Additionally, the specific blockade of the stress-induced change in dopamine metabolism in the medial prefrontal cortex by a glycine antagonist implies a role for such an antagonist in treatment of disease states which may involve disruptions of N-methyl-D-aspartate receptor function.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1993

Contractile activity and cell-cell contact regulate myofibrillar organization in cultured cardiac myocytes.

Adult feline ventricular myocytes cultured on a laminin-coated substratum reestablish intercellular junctions, yet disassemble their myofibrils. Immunofluorescence microscopy reveals that these non-beating heart cells lack vinculin-positive focal adhesions; moreover, intercellular junctions are also devoid of vinculin. When these quiescent myocytes are stimulated to contract with the beta-adrenergic agonist, isoproterenol, extensive vinculin-positive focal adhesions and intercellular junctions emerge. If solitary myocytes are stimulated to beat, an elaborate series of vinculin-positive focal adhesions develop which appear to parallel the reassembly of myofibrils. In cultures where neighboring myocytes reestablish cell-cell contact, myofibrils appear to reassemble from the fascia adherens rather than focal contacts. Activation of beating is accompanied by a significant reduction in the rate of total and cytoskeletal protein synthesis; in fact, myofibrillar reassembly, redevelopment of focal adhesions and fascia adherens junctions require no protein synthesis for at least 24 h, implying the existence of an assembly competent pool of cytoskeletal proteins. Maturation of the fasciae adherens and the appearance of vinculin within Z-line/costameres, does require de novo synthesis of new cytoskeletal proteins. Changes in cytoskeletal protein turnover appear dependent on beta agonist-induced cAMP production, but myofibrillar reassembly is a cAMP-independent event. Such observations suggest that mechanical forces, in the guise of contractile activity, regulate vinculin distribution and myofibrillar order in cultured adult feline heart cells.

Actins↗
PubMed1993

Cultured adult cardiac myocytes maintain protein synthetic capacity of intact adult hearts.

Previous studies have shown that the rates of protein synthesis observed in embryonic and neonatal heart cells in culture are as much as nine times greater than the rates of synthesis observed in the intact adult heart either in situ or in isolated perfusion studies. This study addressed whether adult cardiomyocytes in long-term culture maintain the protein synthetic capacity of the adult myocardium or, rather, whether the protein synthetic capacity expands or falls as adult cardiac myocytes progress in culture. Protein synthesis was evaluated in isolated adult feline cardiomyocytes maintained in serum and insulin-supplemented medium for up to 53 days in vitro. With the use of both pulse- and saturation-labeling techniques it was determined that the rate of protein synthesis in adult cardiomyocytes was maintained at a level very close to that observed in the intact heart for over 1 mo in culture. Saturation-labeling studies indicate a fractional rate of protein synthesis at 6.1%/day and an absolute synthesis rate of 1,300 nmol leucine incorporated.g protein-1.h-1. Pulse-labeling studies revealed an initial increase in protein synthesis rates during adaptation to culture and a further increase after activation of beating and cellular hypertrophy.

Aging↗
PubMed1993

Regulation of hypertrophy and atrophy in cultured adult heart cells.

Mechanical loading and alpha-adrenergic receptor stimulation have both been shown to induce hypertrophy in isolated neonatal heart cells. The present study examined the effects of adrenergic hormones and contractile activity on the hypertrophic response in isolated adult feline cardiomyocytes maintained for more than 14 days in insulin- and serum-supplemented medium. Measurements of the hypertrophic response included cell size, total protein content, myosin heavy chain content, and the time course of activation of increased protein synthesis. Reactivation of the "fetal" gene program was evaluated by secretion of atrial natriuretic factor (ANF) into the medium. Significant myocyte hypertrophy was induced in both quiescent myocytes treated with alpha 1-adrenergic agonists and in beating myocytes treated with beta-adrenergic agonists. However, there were both quantitative and qualitative differences in the response to each type of stimulation. alpha-Adrenergic agonists promoted an increase in cell size, protein content, and ANF secretion but not myofibrillar reorganization, which was observed only in beating myocytes. In contrast to results reported for neonatal heart cells, determinants of hypertrophy in beating myocytes exceeded those in nonbeating alpha 1-adrenergic agonist-treated heart cells in every parameter examined. In addition, in the case of both beating and alpha-adrenergic stimulation, there were marked time-dependent variations in rates of protein synthesis over the interval of 4 hours to 7 days of treatment with each type of stimulus. Differences were also encountered in correlations between rates of protein synthesis and protein accumulation over this interval. The effect of beating was particularly important both to the reorganization of myofibrillar structure and the metabolism of myosin heavy chain. In cultures in which beating was inhibited with the calcium channel antagonist nifedipine, the loss of myosin heavy chain was significantly greater than that of total protein.

Animals↗
PubMed1992

Effects of 6-hydroxydopamine lesions of the prefrontal cortex on tyrosine hydroxylase activity in mesolimbic and nigrostriatal dopamine systems.

The effects of prefrontal cortical dopamine depletion on subcortical dopamine function in the rat were examined. 6-Hydroxydopamine lesions of the dopaminergic innervation of the prefrontal cortex did not alter concentrations of dopamine or its metabolite 3,4-dihydroxyphenylacetic acid in either the striatum or nucleus accumbens. Similarly, the activity of the catecholamine biosynthetic enzyme tyrosine hydroxylase in the striatal complex was not changed in animals with prefrontal cortical lesions. Animals sustaining neurotoxic lesions of the prefrontal cortex were challenged with haloperidol in order to activate submaximally tyrosine hydroxylase activity. The magnitude of the haloperidol-induced increase in enzyme activity in the nucleus accumbens was significantly greater in lesioned subjects than in control animals. These data suggest that lesions of the prefrontal cortical dopamine innervation do not result in significant alterations in basal dopaminergic function in the striatal complex. However, lesions of the dopaminergic innervation of the prefrontal cortex significantly increase the responsiveness of mesolimbic dopamine afferents to pharmacological challenge.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1992

beta Adrenergic receptor desensitisation may serve a cardioprotective role.

OBJECTIVE: The aim was to test the hypothesis that beta adrenoceptor desensitisation is a form of cardioprotection whereby the myocardium is protected from the injurious effects of excessive adrenergic stimulation by isoprenaline. METHODS: A new sensitive and specific method of identifying cardiac myocyte necrosis with monoclonal antimyosin was employed. Antibody labelled necrotic cardiomyocytes from male Sprague-Dawley rats (190-300 g) were identified by immunofluorescence. Homologous beta adrenoceptor desensitisation was induced by 9 d pretreatment with isoprenaline infusion (20 mg.kg-1.d-1), and heterologous desensitisation by treatment with 0.15% propylthiouracil in the diet for six weeks. The effects of isoprenaline induced cardiomyocyte injury in these animals were compared with those in control rats. RESULTS: In euthyroid control rats, treatment with the beta adrenergic agonist, isoprenaline, produced prolonged tachycardia and hypotension, and a significant amount of cardiac myocyte necrosis subendocardially. When the same isoprenaline challenge was given to rats pretreated for 9 d with an isoprenaline infusion, the haemodynamic response was markedly attenuated and very little myocyte necrosis was noted. In the model of heterologous desensitisation occurring in hypothyroidism, isoprenaline challenge produced markedly diminished haemodynamic response and little cardiac myocyte necrosis. CONCLUSIONS: In both homologous and heterologous models of beta adrenoceptor desensitisation, the susceptibility of the myocardium to isoprenaline induced cytotoxicity is markedly reduced.

Adrenergic beta-Antagonists↗
PubMed1992

Comparing cross-sectional and longitudinal analyses of residential mobility and migration.

"A reevaluation of cross-sectional versus longitudinal models of residential mobility...casts doubt on the study by Davies and Pickles in 1985 in which it was argued that cross-sectional analysis is inadequate.... With the use of data from the [U.S.] Panel Study of Income Dynamics (PSID), the conclusion of this reevaluation is that longitudinal analysis increases our understanding of the processes and provides a richer understanding of migration and mobility but does not undermine the work that has been produced from cross-sectional analysis."

Americas↗
PubMed1992

Residential preferences and residential choices in a multiethnic context.

A study of the expressed preferences of four different ethnic groups in the Los Angeles metropolitan area shows strong desires for own-race combinations in the ethnicity of neighborhoods that individuals say they would choose when seeking a new residence. The results also show that Anglos are not the only group to practice "avoidance" of other racial/ethnic neighborhoods, although avoidance behavior by Anglos is the strongest. Because the issues of racial composition are socially sensitive, additional tests examined the relationship of preferences to behavior. Although many behaviors generally follow expressed preferences, members of households who expressed "no preference" also were found to largely choose own race neighborhoods. The results of this study suggest that the expressed preference for own race/own ethnicity, in combination with short-distance local moves, is likely to maintain present patterns of separation in U.S. metropolitan areas.

Black or African American↗
PubMed1991

Subcellular compartmentalization of myosin isoforms in embryonic chick heart ventricle myocytes during cytokinesis.

Embryonic chick heart ventricle myocytes retain the ability to alternate between proliferation and functional differentiation. A cytoplasmic isoform of myosin is present in cleavage furrows of various nonmuscle cells during cytokinesis, whereas one or more of the cardiac myosin isoforms are localized in sarcomeres of beating cardiomyocytes. Antibodies were employed to reveal the subcellular localizations of cytoplasmic and cardiac myosin isoforms in embryonic chick ventricle cardiomyocytes during cytokinesis. Monoclonal anticytoplasmic myosin antibodies were prepared against myosin purified from brains of 1-day-posthatched chickens and shown to react with chick brain myosin heavy chain by Western blots and/or ELISA tests. One monoclonal antibrain myosin antibody also cross-reacted with chick cardiac myosin but not with skeletal or smooth muscle myosins. Two antichick cardiac myosin monoclonal antibodies and one antichick skeletal myosin polyclonal antibody that cross-reacts with cardiac myosin were employed to identify cardiac sarcomeric myosin. Cells were isolated from day 8 embryonic chick heart ventricles, enriched for myocytes, grown in vitro for 3 days, and then examined by immunofluorescence techniques. Monoclonal antibodies against cytoplasmic myosin preferentially localized in the cleavage furrows of both cardiofibroblasts and cardiomyocytes in all stages of cytokinesis. In contrast, antibodies that recognize cardiac myosin were distributed throughout cardiomyocytes during early stages of cytokinesis, but became progressively excluded from the furrow area during middle and late stages of cytokinesis. These data suggest that in cells that contain both cytoplasmic and sarcomeric myosin isoforms, only cytoplasmic myosin isoforms are mobilized to from the contractile ring for cytokinesis.

Animals↗
PubMed1991

Cell shape and organization of the contractile apparatus in cultured adult cardiac myocytes.

The isolation and culture of adult cardiac myocytes has proved to be an ideal model system to explore myocardial biology at the cellular level. A major criticism of this model, however, has been that organ-specific characteristics such as cell shape and subcellular structural organization cannot be retained in vitro for prolonged periods of time. Encasing freshly isolated myocytes in a matrix of calcium alginate enables one to maintain the rod-like, three-dimensional (3D) shape of the cultured myocyte. Such preparations more closely resemble their in vivo counterparts with respect to the organization of the contractile apparatus, the transverse tubular system and the sarcoplasmic reticulum than do heart cells cultured on a two-dimensional (2D) plastic surface. Stereologic measurements reveal that myofibrillar volume density (VvMYF) decreases in both non-beating preparations over a 2-week interval, but VvMYF is conserved in cells cultured in an alginate matrix when compared to those myocytes maintained on a laminin-coated substratum. The present observations suggest that in the absence of contractile function myofibrillar atrophy appears responsible for the decline in VvMYF in alginate (3D) preparations, whereas atrophy and subcellular remodelling probably mediate the myofibrillar loss and reorganization that develops when adult heart cells are cultured on a 2D surface.

Alginates↗
PubMed1991

Autoimmune determinants of rheumatic carditis: localization of epitopes in human cardiac myosin.

Rheumatic carditis is a sequela of group A streptococcal throat infection. Although the pathogenic mechanisms which lead to heart damage in acute rheumatic fever (ARF) are not well understood, autoimmune processes have been implicated, involving molecular mimicry between streptococci and the human heart. We have studied the immunological cross-reactions between the group A Streptococcus and human heart to understand their molecular and immunological basis. Human and mouse monoclonal antibodies (mAb) and affinity-purified anti-myosin antibodies from acute rheumatic fever sera were characterized and shown to cross-react with group A streptococcal M protein and myosin. Studies of proteolytic fragments of human cardiac myosin identified sites of cross-reactivity in the rod region of the myosin heavy chain. Murine monoclonal antibodies cross-reactive with streptococcal M protein and myosin recognized epitopes located in the S2 and light meromyosin (LMM) subfragments of the heavy chain. None of the cross-reactive monoclonal antibodies recognized the S1 subfragment. One broadly cross-reactive monoclonal antibody was highly cytotoxic for heart cells in vitro and reactive with the LMM fragment. The data suggest that the cross-reactive epitopes recognized by these antibodies are conformational, dependent upon their alpha-helical structures, and potentially damaging to host tissues.

Antigens, Bacterial↗
PubMed1991

Hypertrophy of isolated adult feline heart cells following beta-adrenergic-induced beating.

Catecholamine-induced beating and myocardial hypertrophy were evaluated in isolated adult feline cardiomyocytes maintained in culture for up to 30 days. Adult feline cardiomyocytes were used in this study because they displayed several unique characteristics that facilitated assessment of factors regulating cardiomyocyte hypertrophy in vitro. These characteristics included the following. 1) A single heart provides a high yield of 20-40 x 10(6) calcium-tolerant rod-shaped myocytes. 2) In culture, isolated adult feline cardiomyocytes maintain a stable population of differentiated myocytes that could be maintained without the dramatic loss of cell number, DNA content, or cell structure seen in adult rat cardiomyocyte cultures. 3) Cultured feline cardiomyocytes remained quiescent in culture unless appropriately stimulated to begin beating. 4) Sustained regular beating activity could be readily initiated up to 3 wk in culture by addition of 1 x 10(-5) M isoproterenol, other beta-adrenergic agonists, or agents known to elevate adenosine 3',5'-cyclic monophosphate. Beating could be maintained indefinitely in the presence of isoproterenol, but ceased upon removal of isoproterenol from the medium. Initiation of beating in 7-day-old cultures resulted in a profound restructuring of cardiomyocyte morphology compared with quiescent cultures. Beating heart cells were 66% larger with increased protein content, and they had significantly greater development of striated myofibrillar structure than quiescent myocytes at the same age in culture. We conclude that maintenance of an organized myofibrillar structure in cultured adult cardiac myocytes requires activation of intrinsic beating. Cardiomyocyte hypertrophy also develops following beta-adrenergic activation of beating, but it is unclear whether beating per se is required for inducing hypertrophy in isolated adult cardiomyocytes in vitro.

1-Methyl-3-isobutylxanthine↗
PubMed1991

Residential preferences and neighborhood racial segregation: a test of the Schelling segregation model.

The debate over the role of the forces that create the patterns of residential separation has identified neighborhood preferences as one of the explanatory variables, but although we possess some empirical data on the nature of neighborhood racial preferences, the theoretical contributions have received only limited empirical evaluation. Among the theoretical statements, Schelling's model of the effects of small differences in preferences on residential patterns has provided a basic building block in our understanding of preferences, choices, and patterns. Several recent surveys of residential preferences provide the data with which to evaluate the underpinnings of the Schelling model. The preference/tolerance schedules that are derived from the data have a different functional form from that suggested by Schelling, but confirm the view that stable integrated equilibria are unlikely.

Black or African American↗
PubMed1990

Prefrontal cortical dopamine depletion enhances the responsiveness of mesolimbic dopamine neurons to stress.

Mild footshock stress results in the metabolic activation of the prefrontal cortical dopamine (DA) innervation, but does not augment DA utilization in mesolimbic areas (such as the nucleus accumbens septi, NAS) or the striatum. However, increases in either the intensity or duration of footshock stress increase DA utilization in the subcortical sites. DA afferents to the prefrontal cortex (PFC) hold corticofugal projection neurons under tonic inhibition. Previous data suggest that removal of these corticofugal glutamatergic neurons from tonic DA inhibition results in a transsynaptic alteration in the NAS, such that the DA innervation of the NAS is rendered hyperresponsive to certain perturbations. We therefore examined the effects of stress on subcortical DA systems in rats previously subjected to 6-hydroxydopamine lesions of the PFC DA innervation. Mild footshock stress resulted in an increase in concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the PFC, but not NAS or striatum, of sham-lesioned animals. Footshock resulted in a significant increase in the concentration of DOPAC in the nucleus accumbens of animals sustaining PFC lesions two weeks previously. The PFC lesion did not result in a stress-induced increase in DA release in the striatum. These results suggest that disruption of the PFC DA innervation results in an enhanced responsiveness of the mesolimbic DA innervation to stress. These data may help explain the stress-elicited exacerbation of the psychotic process in schizophrenia.

3,4-Dihydroxyphenylacetic Acid↗