Complex trait architecture: the pleiotropic model revisited.

There is currently much debate about how much the genetic heritability of complex traits is due to very rare alleles. This issue is important because it determines sampling strategies for genetic association studies. Several recent theoretical papers based on a pleiotropic model for trait evolution suggest that it is possible that a large proportion of the genetic variance could be explained by rare alleles. This model assumes that mutations with a large effect on fitness also tend to have large positive or negative effects on phenotypic traits. We show that conclusions based on standard diffusion results are generally applicable to simulations of whole genomes with overlapping generations in a finite population, although the variance contribution of rare alleles is somewhat smaller than theoretical predictions. We show that under many scenarios the pleiotropic model predicts trait distributions that are unrealistically leptokurtic. We argue that this imposes a limit on the relationship between fitness and trait effects.

99mTc-HL91: "hot spot" detection of ischemic myocardium in vivo by gamma camera imaging.

BACKGROUND: 99mTc-HL91 is a new hypoxia imaging agent that demonstrates increased uptake and retention in globally hypoxic myocardium in vitro. The purpose of this study was to determine whether 99mTc-HL91 could detect regional ischemia in vivo by gamma camera imaging. METHODS AND RESULTS: Eight open-chest dogs with left circumflex (LCx) stenoses were studied. Injection of 5 mCi of 99mTc-HL91 and microspheres was followed by imaging over 4 hours. Heart slices were imaged, then stained with triphenyltetrazolium chloride (TTC), and tissues were well-counted. TTC staining demonstrated no injury. Mean LCx blood flow was 0.32+/-0.04 mL x min(-1) x g(-1), and mean left anterior descending coronary artery (LAD) flow was 0.96+/-0.02 mL x min(-1) x g(-1) (ratio, 0.33). "Hot spots" were detected in 8 of 8 experiments in vivo within 60 minutes and improved over 4 hours. Region of interest analysis of LCx/LAD activity ratios demonstrated significant increases within 30 minutes (final ratio, 3.0; P<0.05). LCx and LAD washout curves demonstrated significant differences within 15 minutes. Washout curves were biexponential over 1 hour, followed by linear retention from 1 to 4 hours. Four-hour fractional retention was 0.12 for LAD and 0.44 for LCx (P<0.01). Myocardial flow versus tracer uptake demonstrated 2 phases: phase 1 (flow, 0.05 to 0.7 mL x min(-1) x g(-1)) had an inverse linear correlation (r= -0.80); phase 2, (flow, >0.7 mL x min(-1) x g(-1)) had no correlation. Ischemic heart/liver ratios remained near 1.0 for 4 hours. CONCLUSIONS: 99mTc-HL91 positively identifies regional myocardial ischemia in a canine model using 99mTc imaging. Quantitative techniques allowed identification of ischemic myocardium within 15 minutes of tracer administration.

Mechanism of uptake of technetium-tetrofosmin. I: Uptake into isolated adult rat ventricular myocytes and subcellular localization.

BACKGROUND: 99mTc-labeled tetrofosmin is a new myocardial imaging agent that gives stable heart uptake. However, little is known about the mechanism of uptake in heart tissue. METHODS AND RESULTS: Uptake of 99mTc-labeled tetrofosmin has been examined in isolated adult rat ventricular myocytes. The time course of uptake, efflux rate, and the effect of metabolic and cation channel inhibitors have been assessed. The subcellular localization of radioactivity in ex vivo rat heart tissue was examined by differential centrifugation of ventricular homogenate. Uptake into rat myocytes was found to be rapid and plateaued at approximately 1.5 pmol/10(6) cells/nmol extracellular Tc-labeled tetrofosmin after 60 minutes of incubation. Uptake was temperature dependent but independent of extracellular Tc-labeled tetrofosmin concentration. Uptake at 30 minutes was inhibited by the metabolic inhibitors iodoacetic acid acid and 2,4-dinitrophenol protein but was not affected by cation channel inhibitors. Cells previously incubated with 99mTc-labeled tetrofosmin and then placed into fresh medium were found to have a slow efflux of activity; after 1 hour, 65% of activity was still cell associated. The localization of radioactivity in subcellular fractions indicated that the majority of activity was recovered with the cytosol. However, examination of the distribution of two mitochondrial enzymes indicated that this may have been artifactual. Use of carbonyl cyanide m-chlorophenyl-hydrazone or oligomycin to perturb mitochondrial membrane potential decreased or increased recovery in the mitochondrial fraction, respectively. CONCLUSIONS: 99mTc-labeled tetrofosmin uptake by myocytes is by a metabolism-dependent process that does not involve cation channel transport. The most likely mechanism for this is by potential driven diffusion of the lipophilic cation across the sarcolemmal and mitochondrial membranes.