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Biomedical subjects

A P Martin

Publications and source records attributed to A P Martin.

At least 19 recordsLinked to original sources
PubMed1992

Rates of mitochondrial DNA evolution in sharks are slow compared with mammals.

The rate of mitochondrial DNA (mtDNA) evolution has been carefully calibrated only in primates. Similarity between the primate calibration and rates estimated for other vertebrates has led to widespread assumption of a constant molecular clock in vertebrates even though this has never been rigorously tested. We report here the examination of mtDNA sequence variation for 13 species of sharks from two orders that are well represented in the fossil record to test the constancy hypothesis. Nucleotide substitution rates in the cytochrome b and cytochrome oxidase I genes in sharks are seven- to eightfold slower than in primates or ungulates. This difference in substitution rate cannot be explained by nucleotide composition bias, codon-usage bias, selection, or choice of genes sequenced, and was confirmed by comparing species recently separated by the rise of the Isthmus of Panama. Such differences in mtDNA substitution rates among taxa indicate that it is inappropriate to use a calibration for one group to estimate divergence times or demographic parameters for another group. High-resolution studies of molecular evolutionary rates require taxon-specific calibrations.

Animals↗
PubMed1991

Anatomical barriers in endodontics.

Endodontic therapy can be challenging at the diagnostic and technical levels. The more one can advance one's accomplishments, the more that would-be barriers cease to be so. Difficult aspects of tooth isolation and assessment of root integrity can be rendered less problematic by some relatively simple methods. Gaining a clear awareness of the internal layout of a coronal chamber and any possible inclusions can be assisted by use of a modified endodontic explorer. It can be usefully employed to resolve with certainty the common dilemma of whether openings at the base of a molar or maxillary premolar coronal access cavity are exposures in the pulp chamber roof or orifices in the chamber floor. Locating and then gaining full working length entry into partly calcified canals, even when they are not radiographically discernible, remains one of the difficult tasks in endodontic therapy, but methods can be implemented that maximize their successful negotiation and management. The use of fine engine-driven reamers, but absolutely restricted to a reciprocating action handpiece, is described.

Dental Cavity Preparation↗
PubMed1990

Effect of aging on the oxidative phosphorylation pathway.

The proposed study was undertaken to investigate the effect of aging on control of the oxidative phosphorylation pathway. Flux control coefficients for adenine nucleotide translocase and cytochrome c oxidase were determined using the procedure of Groen et al. [J. Biol. Chem., 257 (1982) 137-144]. Hepatic mitochondrial fractions from Fischer 344 rats were isolated from control (average age 6.5 months), and aged (average age 27.3 months) groups. No aging-related changes in the extent of control of respiration by the oxidase were obtained, however, differences were observed for the translocase. For the control group of animals, the greatest regulation occurred at 80-85% maximal respiratory rates, and declined at higher rates. For the aged group, a similar flux control coefficient was obtained at 80-85% respiration, but was maintained as respiration increased to maximal rates. It is proposed that changes in the flux control coefficients at maximal respiratory rates are associated with an aging-related decrease in translocase activity. Evaluation of translocase content revealed no significant differences between the two groups supporting the concept that the decreased activity was not due to decreased content. During the course of these experiments, it also became apparent that there was a significant aging-related decrease in the rate of succinate oxidation providing an adequate supply of ADP was present. No significant changes in respiratory rates, or RCR, were evident at suboptimal concentrations of ADP as reported previously from this laboratory [Vorbeck, M.L. et al., Arch. Biochem. Biophys., 214 (1982) 67-79]. Since similar decreases in respiration were obtained upon addition of an uncoupler, the aging-related changes in respiration are attributed to differences at the level of the electron transport system, including its associated reactions. The aging-related differences in respiratory rates, and extent of control of respiration, were both observed under conditions of maximal stimulation of respiration. This suggests an inability of mitochondria from aged animals to respond to the increased demands of oxidation. Basic to these differences may be the lipid-membrane associated changes seen during aging.

Adenosine Diphosphate↗
PubMed1989

Apical sealing.

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Chloroform↗
PubMed1986

Root resorption.

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Humans↗
PubMed1985

Influence of Ca2+ on the plasma membrane potential and electrogenic uptake of glycine by myeloma cells. Involvement of a Ca2+-activated K+ channel.

The involvement of Ca2+-activated K+ channels in the regulation of the plasma membrane potential and electrogenic uptake of glycine in SP 2/0-AG14 lymphocytes was investigated using the potentiometric indicator 3,3'-diethylthiodicarbocyanine iodide. The resting membrane potential was estimated to be -57 +/- 6 mV (n = 4), a value similar to that of normal lymphocytes. The magnitude of the membrane potential and the electrogenic uptake of glycine were dependent on the extracellular K+ concentration, [K+]o, and were significantly enhanced by exogenous calcium. The apparent Vmax of Na+-dependent glycine uptake was doubled in the presence of calcium, whereas the K0.5 was not affected. Ouabain had no influence on the membrane potential under the conditions employed. Additional criteria used to demonstrate the presence of Ca2+-activated K+ channels included the following: (1) addition of EGTA to calcium supplemented cells elicited a rapid depolarization of the membrane potential that was dependent on [K+]o; (2) the calmodulin antagonist, trifluoperazine, depolarized the membrane potential in a dose-dependent and saturable manner with an IC50 of 9.4 microM; and (3) cells treated with the Ca2+-activated K+ channel antagonist, quinine, demonstrated an elevated membrane potential and depressed electrogenic glycine uptake. Results from the present study provide evidence for Ca2+-activated K+ channels in SP 2/0-AG14 lymphocytes, and that their involvement regulates the plasma membrane potential and thereby the electrogenic uptake of Na+-dependent amino acids.

Animals↗
PubMed1979

Serum high-density lipoprotein cholesterol, glucose tolerance and other variables in obese black adolescent girls.

Obesity is very common in South Africal Black girls aged 18 - 20 years. Severely obese White women have greatly reduced values for serum high-density lipoprotein (HDL) cholesterol. In a series of Black girls studied in rural and urban areas, mean values for HDL cholesterol were not significantly different in the obese compared with the non-obese groups. There were also no significant differences between obese and non-obese groups in mean serum cholesterol, triglyceride, uric acid, and blood glucose at fasting and 1 hour after a glucose drink. The same finding prevailed regarding blood pressures. All values for blood components were biologically more favourable in Black than in White adolescent girls (reported values). Mean sugar intakes of the obese were not significantly lower. Evidently, the metabolic and other disabilities linked with obesity in young White women have no counterpart in obese Black adolescent girls.

Adolescent↗
PubMed1975

Membrane phospholipid metabolism in the isoproterenol-induced cardiomyopathy of the rat.

Within 60 minutes after treatment with low doses of IPR (5 mg/kg), relative specific activity of cardiolipin and sphingomyelin significantly increased in the heart. Other phospholipids of the heart were not significantly changed. Relative specific activity of cardiolipin increased much later in liver and salivary gland. Activity of CTP:PA cytidyl transferase increases 40% at 1 hour following treatment with IPR. No significant change was observed in the respiratory control ratio of heart mitochondria for 2 hours following injection with IPR; however, a significant decrease was observed 24 hours after IPR.

Animals↗