RESUMO
A histophysical method has been adapted to determine the thermotropic phase transitions of adrenocortical lipid droplets using a polarizing microscope equipped with a cold/hot stage. Cryosections of freshly-removed, unfixed adrenals, derived from control (untreated), and 14 days ACTH-treated rats were examined. The lipid droplets in the zona fasciculata and zona reticularis of the untreated rats were birefringent at room temperature (22 degrees C). The birefringence of zona glomerulosa lipids selectively increased in the temperature range from -10 to -15 degrees C. In cryosections prepared from ACTH-treated rats, thermotropic phase transitions of the lipid droplets appeared at a temperature range between -30 and -40 degrees C in each cortical zone. The chemical analysis of the isolated lipids revealed that the relative amount of triglycerides in the zona fasciculata lipids increased, while that of free and esterified cholesterol decreased after chronic ACTH treatment. Present data suggest that the increased fluidity of lipid droplets promotes lipid mobilization in response to the enhanced demand of the chronically stimulated adrenocortical cells. Viscosity-dependent mobilization of free cholesterol from lipid droplets is not a rate-limiting process in adrenal steroidogenesis, but rather may represent an important control of the availability of precursor from lipid stores.
Assuntos
Córtex Suprarrenal/metabolismo , Hormônio Adrenocorticotrópico/farmacologia , Metabolismo dos Lipídeos , Córtex Suprarrenal/efeitos dos fármacos , Animais , Birrefringência , Cromatografia em Camada Fina , Masculino , Microscopia de Polarização , Ratos , Ratos Endogâmicos , Valores de Referência , Zona Fasciculada/citologia , Zona Fasciculada/efeitos dos fármacos , Zona Fasciculada/metabolismo , Zona Reticular/citologia , Zona Reticular/efeitos dos fármacos , Zona Reticular/metabolismoRESUMO
The accuracy of distinguishing amino-terminal signal peptides from cytosolic proteins has been improved to 95% by combining a neural network classifier with von Heijne's statistical prediction, the latter is itself 85-90% reliable. The network processed not the cleavage site, but amino-terminal 20-residue segments by the 'tiling' algorithm. Concordant positive predictions of both methods led to the safe identification of 496 novel signal peptides from the Protein Identification Resources.
