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S B Sears

Publications and source records attributed to S B Sears.

9 recordsLinked to original sources
PubMed2000

Environmental tobacco smoke, cardiovascular disease, and the nonlinear dose-response hypothesis.

Two recent government reports have focused attention on the hypothesis that environmental tobacco smoke (ETS) exposure increases the risk of cardiovascular disease (CVD) in nonsmokers. The first report was published by the California Environmental Protection Agency (CAEPA) in 1997. The second report was issued in 1998 by the Scientific Committee on Tobacco and Health (SCOTH) in the United Kingdom. A meta-analysis of five large prospective epidemiology studies reports that the relative risk for actively smoking 20 cigarettes per day is 1.78. Active smoking exposes the smoker to approximately 16 times the ETS concentration, and 100- to 300-fold the total smoke dose experienced by a nonsmoker (Smith and Ogden, 1998, JAMA 280, 32-33.). Despite the much lower smoke exposure, these government reports estimate the relative risk for ischemic heart disease in ETS-exposed nonsmokers at 1.30 (CAEPA) and 1.23 (SCOTH). As an explanation for this nonlinear dose-response anomaly, platelet aggregation is proposed to be a plausible and quantitatively consistent mechanism. Herein, evidence is presented suggesting that this low-dose hypothesis is inconsistent with the biochemistry and physiology of platelets and with the literature on the cardiovascular pathology of active smoking. In addition, several important biases and confounders are ignored. These epidemiologic biases and confounders include the following: misclassification of smokers as nonsmokers; improper use of death certificates as surrogates for mortality statistics; underreporting of diabetes and hypertension in the relatives of smokers; and additional atherogenic risk factors in smoking households. Future field studies on ETS and CVD should emphasize proximal markers of risk for thrombosis in exposed nonsmokers. Proximal thrombogenic risk markers identified in field studies should be mechanistically examined under controlled exposure conditions.

Bias↗
PubMed1992

Environmental tobacco smoke: current assessment and future directions.

Scientific information on environmental tobacco smoke (ETS) is critically reviewed. Key areas addressed are: differences in chemical composition between mainstream smoke, sidestream smoke, and ETS; techniques for measurement of ETS; epidemiology; in vitro and in vivo toxicology; and chamber and field studies of perceptual or physiological effects. Questions concerning estimation of ETS exposure, suitability of various biomarkers, calculation of lifetime dose, control of confounding variables, use of meta-analysis, and the relationship between ETS concentrations and human responses all emphasize the need for additional research in order to assess potential effects of ETS on health or comfort.

Animals↗
PubMed1987

Kinetics and mechanism of L-[3H]nicotine binding to putative high affinity receptor sites in rat brain.

The properties of high affinity nicotine-binding sites in rat brain were studied by monitoring the kinetics of L-[3H]nicotine binding to whole brain membrane preparations, including both total membranes and membrane subfractions. Although nicotine appeared to bind to a single class of sites, with an apparent equilibrium dissociation constant of 2-3 nM, the binding kinetics were biphasic at all temperatures and at all nicotine concentrations tested. An initial rapid binding process, with an association rate constant of around 0.02 min-1 nM-1 at 0 degree, was followed by a slower binding process. Both the rate and the proportion of binding that occurred by the slower process were dependent on the nicotine concentration. By comparison, the kinetics of dissociation were first order at all concentrations, with a rate constant of 0.04 min-1 at 0 degree. The rates of both association and dissociation increased significantly with temperature, but there was no changed in the apparent affinity of the sites. The same results were obtained with several different membrane preparations, including whole brain membrane preparations, detergent-permeabilized membranes, P-2 fractions, and synaptosomes. The results were found to be consistent with a two-state model. Analyses based on this model indicate that the binding sites can assume two different conformations, one having a high affinity (KD = 1 nM) and the other a low affinity (KD = 150 nM) for nicotine. It was estimated that approximately 60% of the sites are in the low affinity conformation in the absence of ligand. However, the evidence suggests that nicotine binding can facilitate a shift in the conformational equilibrium, favoring the high affinity state.

Animals↗
PubMed1980

An atomic kinetic energy functional with full Weizsacker correction.

A functional is proposed for representing the electronic kinetic energy of the ground state of an N-electron atom or ion in terms of its electron density, [Formula: see text] Here T(w) is the Weizsacker quantity ((1/8))integral(nablarho.nablarho/rho)dtau and T(0) is the Thomas-Fermi quantity C(F) integral rho(5 / 3)dtau. From Hartree-Fock data on 55 neutral atoms, C = 1.412 +/- 0.033; for 1200 atoms and ions, C = 1.332 +/- 0.053. The proposed functional gives the derivative deltaT/deltarho its most important correct properties. The term T(w) is shown to give the kinetic energy of the K shell, whereas the term (C/N((1/3)))T(0) gives an incorrect statistical estimate of that energy. An alternative correction -(C/N((1/3)))T gives even better results.

Journal Article↗
PubMed1978

Quantitative assessment of free gingival grafts with and without periosteum and osseous perforation.

Forty free gingival grafts were performed on 5 adult Rhesus monkeys as follows: 10 grafts were placed on recipient sites with periosteum; 10 grafts on periosteum and perforated cortical plate; 10 grafts on bared bone; and 10 grafts on perforated bared bone. All free autografts (7 X 5 X 1 mm) were taken from the palatal mucosa and were sutured in place. Clinical evaluation and photographs were taken at 1, 2, 4, 8, 12, 16, 20 and 24 weeks postoperatively. At 24 weeks, a device was used to measure quantitatively the amount of force necessary to pull the graft 2 mm away from the underlying bone. Biopsy of two grafts of each type were taken for histologic examination. The following conclusions were drawn: 1. There was no difference in the survival or healing pattern of grafts placed on periosteum and grafts placed on bared bone.

Alveolar Process↗