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Biomedical subjects

M Conrad

Publications and source records attributed to M Conrad.

At least 91 records · Page 5Linked to original sources
PubMed1985

Neuron generator potentials evoked by intracellular injection of cyclic nucleotides and mechanical distension.

Depolarization of neuron membrane was shown to occur during cAMP injection by means of both iontophoresis and pressure. Distension of the neuron by means of blowing with large volumes of solution without cAMP can produce reversible responses. The time course can be made similar to that of the cAMP effect and the responses can be repeated many times in the same neuron. The effect of cAMP can be differentiated from the mechanical response by injecting small volumes of concentrated solutions. The delay in this depolarization response to cAMP is only 0.1-0.3 s. The calculated time of diffusion from the electrode tip to the membrane is about 10 times greater. The similarity of cAMP and mechanical responses and the short delay of the cAMP effect suggests that a cAMP effect may be mediated by mechanical signals in the cytoskeleton.

Animals↗
PubMed1985

Evolve III: a discrete events model of an evolutionary ecosystem.

Evolve III is a discrete events model of an evolutionary ecosystem. The model includes three levels of organization: population, organism and genetic structure. Each of these components was modeled independently, so that selective replacement of subsystems can be used to create families of models capable of testing alternative hypotheses about the real system. To demonstrate the use of the model we describe an experiment on the relationship between adaptability of populations and the variability of the environment. Populations cultured in a constant environment usually dominated those cultured in a variable environment when both were placed in a variable environment at an early stage of development, whereas the opposite is the case at later stages of development. This agrees with experiments on laboratory microcosms and lends credence to the potential predictive value of the model.

Biological Evolution↗
PubMed1985

Evolve II: a computer model of an evolving ecosystem.

The Evolve II program is a model of an ecosystem in which organisms are allowed to evolve. Organisms are subject to a changeable environment and competition from other organisms for a limited food supply. The gene structure may change through mutation. A feature of Evolve II is that the magnitude of phenotypic change resulting from mutation is itself a property of the gene. The system was studied under a number of environmental variation schemes. We report three significant findings. Two species (lineages with distinctly different survival strategies) evolved and coexisted in the same environmental conditions. Organisms developed a resistance to phenotypic change in response to mutation in slowly varying environments. However, traits which favor survival of the individual at the expense of reproduction could in some cases undergo phenotypic change in response to mutation despite the fact that this did not favor the survival of the offspring. This demonstrates that gene structures can evolve which are advantageous from the standpoint of the lineage, but not advantageous from the standpoint of individual offspring.

Aging↗
PubMed1984

Effects of captopril on pulmonary haemodynamics.

The effects of oral captopril on pulmonary haemodynamics were studied in two groups of 6 patients, one of subjects with chronic respiratory failure (PaO2 52 +/- 5.1 mmHg, PaCO2 54 +/- 2.1 mmHg), and the others with chronic heart failure and high plasma renin activity. Two potential mechanisms of its actions were assessed, namely inhibition of hypoxic vasoconstriction and inhibition of the possible effects of angiotensin II on the pulmonary circulation. There were significant (p less than 0.05) decreases in mean arterial pressure, pulmonary wedge pressure and in systemic arterial resistance associated with improvement in cardiac index in both groups. In the chronic respiratory failure group there was no change in blood gases, mean pulmonary arterial pressure or pulmonary vascular resistance. An increase in driving pressure (p less than 0.05) indicated that captopril had had no effect on pulmonary haemodynamics. In chronic heart failure, mean pulmonary arterial pressure and pulmonary capillary wedge pressure were decreased by a similar extent, so that driving pressure and pulmonary vascular resistance were not changed. It is concluded that oral captopril did not inhibit hypoxic vasoconstriction, and that it modified pulmonary haemodynamics in chronic heart failure patients with high renin activity only as a consequence of reduction in systemic afterload.

Adult↗
PubMed1983

Evidence that natural selection acts on silent mutation.

Analysis of nucleic acid sequence data of mammalian hemoglobin, yeast cytochrome c, and human interferon reveals strong biases in favor of specific codons. These biases do not appear to dissipate over time, suggesting that an indirect form of selection acts on silent mutations. The data are compatible with the "bootstrapping" hypothesis that silent mutations which alter the rate of evolution can hitchhike with traits whose appearance they facilitate. Selection involving modulating effects of codon usage on gene expression may also be involved, but the data appear to exclude simple maximization of gene expression.

Animals↗
PubMed1983

[Chronic obstructive bronchopneumopathies as manifestations of a Gougerot-Sjögren syndrome. Apropos of 2 cases].

The authors report two cases of Sjögren's syndrome presenting as chronic obstructive bronchopulmonary disease. They stress the need to consider the possibility of Sjögren's syndrome when confronted with chronic obstructive lung disease in a woman in her fifties, without history of bronchopulmonary disease or smoking, unexposed to occupational dust and without ENT infection or gastro-oesophageal reflux.

Aged↗
PubMed1982

Bootstrapping model of the origin of life.

The origin of life is analyzed in terms of the self-facilitating aspect of evolution. According to the self-facilitation (or bootstrap) principle the structure of biological systems becomes increasingly suited to effective evolutionary search through the process of evolution. The principle may be extended to primitive collections of polymers with catalytic properties. The origin of the code may be based on a form of bootstrapping evolution. Once a primitive code appeared it could become more sophisticated through a multi-coding mechanism together with classical Darwinian mechanisms. The bootstrapping principle is also formulated in terms of the fluctuation-instability framework of Prigogine, Nicolis, and Babloyantz. Primitive collections of polymers accumulate evolution-enhancing redundancies which tend to reduce the extent to which they change when destabilized by fluctuation, but which increase the chances that these changes will lead to new predominant regimes. We show that the bootstrapping of evolutionary amenability is accompanied by the accumulation of a thermodynamic load, that is, a free energy cost which reduces the mechanistic efficiency. The bootstrapping effect suggests that the origin of life is most fruitfully approached as a long process during which the capacity to evolve facilitates itself in a step by step fashion rather than as a series of low probability events.

Genetic Code↗
PubMed1982

Natural selection and the evolution of neutralism.

The efficiency of evolutionary search increases as the density of acceptable proteins in a protein space increases. Populations caught in regions whose density is too low to support evolution can be pulled into high density regions by hitchhiking selection. As they move into such regions, the action of natural selection becomes more effective, yet these populations will satisfy conditions which lead to predictions made by neutral, so-called non-Darwinian models.

Amino Acid Sequence↗
PubMed1982

Microinjection of cyclic nucleotides provides evidence for a diffusional mechanism of intraneuronal control.

Cyclic 3',5'-AMP and cyclic 3',5'-GMP injected into large neurons of the snail Helix lucorum altered neuron activity. The effect of cAMP is usually depolarizing and that of cGMP hyperpolarizing. The results are specific for 3',5'-cyclic nucleotides. The experiments support the hypothesis that reaction-diffusion processes involving cyclic nucleotides form the basis of an intraneuronal system of information processing.

Animals↗
PubMed1981

Algorithmic specification as a technique for computing with informal biological models.

Conceptual biological models can sometimes be usefully expressed in algorithmic form. Models expressed in this way are often capable of capturing more aspects of the complete system than could be captured by other modeling approaches, though in general each aspect is captured in a highly simplified way. Two examples are given. The first involves algorithmic specification of the theory of evolution. The second involves a recently implemented computational model of the brain. Work with such models has, to some extent, the style of experimental work. It often suggests interesting problems or exposes subtle features of the system whose importance has been overlooked.

Biological Evolution↗
PubMed1980

Computational illustration of the bootstrap effect.

A constructive computational model is used to show that even under poor case assumptions genetic systems accumulate features which increase their amenability to evolution. These features decrease the fitness of the individual organism, but hitchhike along with the fitness-increasing traits whose probability of appearance they increase.

Biological Evolution↗