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Biomedical subjects

I Walker

Publications and source records attributed to I Walker.

At least 37 records · Page 2Linked to original sources
PubMed1990

The pattern of population growth as a function of redundancy and repair.

A basic model of hierarchical structure, expressed by simple, linear differential equations, shows that the pattern of population growth is essentially determined by conditions of redundancy in the sub-structure of individuals. There does not exist any possible combination between growth rate and accident rate that could balance population numbers and/or the level of redundancy within the population; all possible combinations either lead to extinction or to positive population growth with a decline of the fraction of individuals with redundant substructure. Declining populations, however, can be held fluctuating between certain limits by periodic phases of sub-unit repair. These results are particularly pertinent to the population dynamics of diploid (polyploid) organisms.

DNA Repair↗
PubMed1988

Measurement of the transfer of the nitrogen moiety of intestinal lumen glutamic acid in man after oral ingestion of l-[15N]glutamic acid.

1. The measurement of the intestinal metabolism of the nitrogen moiety of glutamic acid has been investigated by oral ingestion of l-[15N]glutamic acid and sampling of arterialized blood. 2. Measurements have been made in six normal adults weighing an average of 72.8 kg ingesting 100 mg of l-[15N]glutamic acid after an overnight fast. 3. Measurement of the enrichment of arterial glutamic acid, glutamine and alanine was by gas chromatography-mass spectrometry. Isotopic enrichment of the amino acids was followed for 150 min after the ingestion of the amino acid. 4. Arterialized venous blood amino acid concentrations, measured by h.p.l.c., demonstrated no significant changes during the course of the experiment. 5. From the observed appearance of label in arterialized glutamic acid, alanine and glutamine, little luminal glutamic acid reaches the extracellular pool. The majority of the administered nitrogen label appears in the arterial alanine and glutamine components.

Administration, Oral↗
PubMed1987

High-dose cytosine arabinoside as the initial treatment of poor-risk patients with acute nonlymphocytic leukemia: a Leukemia Intergroup Study.

Sixty-seven patients with newly diagnosed acute nonlymphocytic leukemia (ANLL) who were considered to be poor candidates for treatment with cytosine arabinoside (ara-C)/anthracycline antibiotic therapy were treated with high-dose ara-C (HDara-C) remission induction therapy. Thirty-four of the 67 patients had a hematologic disorder before developing acute leukemia or had a history of exposure to marrow toxins, 23 patients were greater than 70 years old, and 10 patients had medical problems that were felt to be a contraindication to therapy with an anthracycline antibiotic. Forty-two percent of patients entered complete remission (CR), whereas 22% failed to enter remission because of persistent leukemia. Treatment was associated with substantial toxicity varying from nausea and vomiting to irreversible cerebellar toxicity. Thirty-four percent of patients died during therapy. Poor performance status, a low serum albumin, and a low platelet count were associated with death during remission induction therapy, whereas a high pretherapy leukemic cell mass and a large number of residual leukemic cells in the marrow after six days of therapy were associated with treatment failure due to persistent leukemia.

Acute Disease↗
PubMed1986

High-dose cytosine arabinoside in the treatment of preleukemic disorders: a leukemia intergroup study.

Fifteen patients with myelodysplastic/myeloproliferative disorders were treated with high-dose cytosine arabinoside therapy. While severe toxicity was produced in every patient, only two of the 15 patients entered complete remission and two achieved partial remission status. The therapeutic responses were confined to patients who had severe myelofibrosis of apparently recent onset.

Adult↗
PubMed1985

Treatment of patients with acute nonlymphocytic leukemia in relapse: a Leukemia Intergroup study.

Forty patients with acute nonlymphocytic leukemia (ANLL) in first relapse were treated at eight member institutions of the Leukemia Intergroup with a 10-day continuous intravenous infusion of cytosine arabinoside and an anthracycline antibiotic administered on days 1, 2, and 3. Twenty of the 40 patients achieved a complete response. Seven of the patients who did not enter remission were drug-resistant failures, while 13 patients failed to enter remission for reasons other than persistent leukemia. Pretreatment parameters such as age, presence of infection, platelet count, and liver function tests were important predictors of survival. The percent bone marrow cellularity, the percent circulating abnormal (leukemic) cells, and the height of the white blood cell count prior to treatment were helpful in distinguishing patients who would enter remission from those who would not enter remission because of persistent leukemia.

Adolescent↗
PubMed1983

Complex-irreversibility and evolution.

Both, irreversibility and evolution, imply order in time. It is argued that the only possible concept of time is a 'system-specific time', and that order in time is convertible into order in space and vice-versa. While life-less, complex systems are irreversible because of their complexity and, hence, not repeatable, living systems are reproduced by irreversible copy-reproduction and by coding. This mode of reproduction results of necessity in an arrow of time of growth and increasing complexity with death as its antagonist, and in obligatory spatial asymmetry. This arrow of increasing organic complexity is simultaneous with, and independent of, the arrow of increasing entropy. - A generalized, organic hierarchy is proposed as the model to study higher evolution. This hierarchy reproduces itself by differential rates of reproduction of its subunits within and between the various hierarchical levels of organization. Phylogenetic change is brought about by a change in this hierarchy's specific phase pattern of growth. Continuous and discrete organization is defined, and it is shown that specific relations between continuous and discrete levels within the hierarchy result in accumulation of neutral alleles. This accumulation is due to complex-irreversibility and causes genetic stabilisation, i.e. heritability, of the species-specific morphology of organisms.

Animals↗
PubMed1983

The physical dimensions and biological meaning of the coefficients in the Volterra competition equations and their consequences for the possibility of coexistence.

Exact definitions in physical and biological terms of the coefficients in Volterra's (1926, 1931) original competition equations are indispensable for the understanding of the system. In agreement with Volterra's own, but not quite sufficient specifications, it is tried in this paper to give more precise definitions of the parameters used by Volterra. This leads to some consequences; i.a. that there does not exist a "principle of competitive exclusion". In order to allow for competitive exclusion - or for stabilization - the Volterra equations need additional biophysical specifications and mathematical modifications. Some such modifications are proposed, for instance for aggressive interference as a function of population numbers.

Animals↗
PubMed1981

The low specificity of postoperative perfusion lung scan defects.

Ventilation and perfusion lung scans were performed preoperatively and postoperatively in 169 patients and classified blindly according to preset criteria. Perfusion lung scan abnormalities were present in 25 (15%) of the preoperative scans and 42 (25%) of the postoperative scans; 16 (38%) of the 42 abnormal postoperative scans were identical to the preoperative scans. Perfusion defects indicating a "high probability" of pulmonary embolism (lobar or segmental defects) were present in 5 preoperative scans and 10 postoperative scans; the 10 postoperative scans were classified as showing "definite" (5), "possible" (1) or "no" (4) pulmonary embolism on the basis of the preoperative scan and the ventilation scan; none of the 10 patients had clinical evidence of pulmonary embolism. Venous thrombosis was present in 12 patients, including 4 of the patients whose lung scans showed definite pulmonary embolism. Thus, postoperative perfusion lung scan defects are potentially misleading even when large.

Adult↗
PubMed1981

Myositis ossificans in hemophilia.

A review of the radiographs of 60 hemophilia patients showed nine (15%) with ectopic new bone formation. Three of these patients had multiple sites of involvement. The high frequency discovered in this series contrasts with the paucity of descriptions to be found in the literature. This process of myositis ossificans affects the lower half of the body and probably represents dysplastic metaplasia developing at the site of an intramuscular hematoma when remote from bone, as well as ossification of hemorrhagic lesions related to the periosteum. In conventional radiographs anatomic localization of bone foci is difficult, but use of computed tomography permits precise identification of the affected muscle. There is negligible disability associated with this condition.

Adolescent↗
PubMed1981

Drug-induced thrombocytopenia is associated with increased binding of IgG to platelets both in vivo and in vitro.

Thrombocytopenia is a common serious adverse effect of drug treatment. A variety of in vitro diagnostic techniques to confirm the diagnosis are available, but the majority lack sufficient sensitivity to detect all cases of drug-induced thrombocytopenia. We studied 19 patients with suspected drug-induced thrombocytopenia and demonstrated that platelet-associated IgG (PAIgG) was elevated in all at the time of thrombocytopenia, and PAIgG returned to normal levels as the thrombocytopenia resolved. In the majority of patients, the platelet count rapidly returned to normal after the drug was discontinued; however, in six patients, the thrombocytopenia persisted well beyond the period of time that the offending drug would be expected to be cleared from the blood. In 13 patients, serum obtained after recovery was used to identify the drug responsible for the thrombocytopenia in an in vitro assay. In all cases, the addition of the drug historically associated with the thrombocytopenic episode was associated with an increased binding of IgG to control platelets. For uncertain reasons, the concentration of drug required to increase the in vitro binding of IgG to test platelets was often more than the concentration usually achieved in vivo. Wider application of these techniques may provide better understanding of the clinical characteristics and mechanisms responsible for drug-induce thrombocytopenia.

Adult↗
PubMed1981

The reversible binding of vinblastine to platelets: implications for therapy.

The ability of platelets to adsorb vinblastine has been used to treat patients with immune thrombocytopenia. It is hypothesized that the drug-platelet complex is coated with antibody, taken up by macrophages which are then destroyed by the drug. We gave 16 courses of vinblastine-platelets to six patients with immune thrombocytopenia. Only one patient responded, and therefore we examined possible reasons for the lack of benefit. Using 3H-vinblastine, the kinetics of vinblastine binding to platelets was studied in vitro. The binding of vinblastine to both human and rabbit platelets was identical with maximal binding occurring within 10 min at 600 microgram/ml vinblastine. Similarly, the plasma half-life of vinblastine in rabbits was close to that reported for man, and therefore, in vivo binding of vinblastine to platelets in rabbits was considered a suitable model for man. Homologous donor rabbit platelets were labeled with 51Cr alone, 51Cr plus vinblastine, or 3H-vinblastine and infused into recipient rabbits. Vinblastine had no effect on 51Cr survival, but all measureable vinblastine had left the platelets within 2 hr of the infusion. These observations suggest that delivery of the vinblastine to the macrophages depends on the platelets being phagtocytized before the drug leaves the platelets. This would be likely to occur only in those patients with severe immune thrombocytopenia. Further investigations into this treatment should be directed at methods to maintain the drug within the platelet.

Adsorption↗
PubMed1979

The mechanical properties of proteins determine the laws of evolutionary change.

The general inorganic nature of traditional selection theory (based on differential growth between any two systems) is pointed out, wherefrom it follows that this theory cannot provide explanations for the characteristics of organic evolution. Specific biophysical aspects enter with the complexity of macro-molecules: vital physical conditions for the perpetuation of the system, irrevocable extinction (= death) and random change leading to novelty, are the result of 'complexity per se'. Further biophysical properties are a direct function of the pathway along which random mutation in nucleic acids is converted into continuous protein - (specifically enzyme) - function, from there into organismic phenotype with fitness components which may, or may not, correspond to identifiable structural units in DNA. The general machine-like properties of enzymes, in that there is no additive relationship between structural (amino acid) composition and functional output, is discussed in more detail. The continuous growth functions of molecular concentrations, directed by enzyme turnover, determine simple laws of growth and morphogenesis in the organic hierarchy and thus of phenotype. Thus, the combined effect of DNA-structure and of environmental parameters (temperature, pressure, pH, etc.) on protein function determines ultimately the actual phenotype and hence, quality and intensity of genotypic selection.

Amino Acids↗
PubMed1978

The evolution of sexual reproduction as a repair mechanism. Part I. A model for self-repair and its biological implications.

The theory is presented that the sexual process is a repair mechanism which maintains redundancy within the sub-structure of hierarchical, self-reproducing organisms. In order to keep the problems within mathematically tractable limits (see Part II), a simple model is introduced: a wheel with 6 spokes, 3 of them vital and 3 redundant, symbolizes the individual (cell or organism). Random accidents destroy spokes; the wheels replicate at regular cycles and engage periodically in pairing and repair phases during which missing spokes are copy-reproduced along the intact spokes of the partner wheel. The hierarchical structure of such a system is analysed and an 'autonomous unit' is defined: this is the unit of minimal hierarchical complexity which is capable of perpetuating autonomously all higher and all lower levels of the hierarchy; this is the central unit of selection. Four basic, physical parameters are isolated which determine the essential features of any eucaryotic life cycle: 1. The number of levels of the hierarchy (unicellular, multicellular, colonial, etc.); 2. the relation between the phases of replication (asexual generations) and repair (sexual generations); 3. the duration of potential repair (haplo-diplo-phase); 4. the position of the sexual partners within the hierarchy (selfing, monecy, dioecy, reproductive individuals within colonies, etc.). The evaluation of fitness components is considered in relation to trends of reproductive patterns in evolution.

Animals↗
PubMed1978

The evolution of sexual reproduction as a repair mechanism. Part II. Mathematical treatment of the wheel model and its significance for real systems.

The dynamics of populations of self-replicating, hierarchically structured individuals, exposed to accidents which destroy their sub-units, is analyzed mathematically, specifically with regard to the roles of redundancy and sexual repair. The following points emerge from this analysis: 1. A population of individuals with redundant sub-structure has no intrinsic steady-state point; it tends to either zero or infinity depending on a critical accident rate alpha c. 2. Increased redundancy renders populations less accident prone initially, but population decline is steeper if alpha is greater than a fixed value alpha d. 3. Periodic, sexual repair at system-specific intervals prevents continuous decline and stabilizes the population insofar as it will now oscillate between two fixed population levels. 4. The stabilizing sexual interval increases with increased complexity provided this is accompanied by appropriate levels of redundancy. 5. The model closely simulates the dynamics of heterosis effects. 6. Repair fitness is a population fitness: the chance of an individual being repaired is a function of the statistical make-up of the population as a whole at the particular period. Populations living at alpha greater than alpha c either engage in sexual repair at the appropriate time or they die out. 7. The mathematical properties of the model illustrate mechanisms which possibly played a role in the evolution of a mortal soma in relation to sexual reproduction.

Animals↗
PubMed1978

Impedance plethysmography: the relationship between venous filling and sensitivity and specificity for proximal vein thrombosis.

We investigated the hypothesis that the diagnostic accuracy of impedance plethysmography (IPG) for thrombosis of the popliteal or more proximal veins increases with enhanced venous filling. Venous filling was increased by prolonging cuff occlusion and by sequential testing. IPG and vengography were performed on 169 legs with and 317 legs without proximal vein thrombosis. The sensitivity and specificity of IPG rose significantly with increased venous filling. Changes in venous filling were associated with corresponding changes in emptying in normal legs, but not those with proximal vein thrombosis, so that the regression lines relating venous filling and emptying in normal and abnormal legs diverged significantly (P less than 0.001). If the IPG sequence had been terminated after only a single 45 second occlusion time test, sensitivity would have deteriorated by 10% and specificity by 20%. These observations indicate that the accuracy of IPG can be significantly enhanced if optimal venous filling is obtained.

Evaluation Studies as Topic↗