title: Apparent unmasking of [3H]GDP binding in rat brown-fat mitochondria is due to mitochondrial swelling.
abstract: The presence of and biochemical background for the so-called 'unmasking' phenomenon in rat brown-fat mitochondria was investigated (i.e. the apparent increase in [3H]GDP binding to the 'uncoupling' protein thermogenin, without a concomitant increase in the amount of the protein). It was found that an unmasking could be observed both 1 h after norepinephrine injection and after 1 h cold stress, provided that the rats were preacclimated to 28 degrees C. The unmasking could be observed both when a filtration method and when a centrifugation method for determination of [3H]GDP-binding capacity were used; however, the absolute values were higher with the filtration method. Based on observations of slower cytochrome-c oxidase sedimentation during centrifugation, the possibility that the matrix volume of brown-fat mitochondria isolated from warm-acclimated animals was smaller than that of cold-stressed animals was investigated with 3H2O. The cold stress increased the matrix volume from being nearly non-existent to about 1 microliter/mg. A preswelling procedure in an ionic medium could similarly increase the matrix volume in mitochondria from warm-acclimated animals but was without significant effect in the already swollen mitochondria from cold-stressed animals or from animals adapted to a lower temperature. In mitochondria from warm-acclimated animals, the ionic preswelling procedure was fully able to increase the apparent amount of GDP binding to that observed in mitochondria from cold-stressed animals, but it was practically without effect on GDP binding in mitochondria from cold-stressed animals or from animals acclimated to a lower temperature. It is concluded that the apparent 'unmasking' phenomenon, observed when the tissue is less activated than in normal control situations, is not (as hitherto anticipated) due to a specific change in thermogenin as such, but is a reflection of a general mitochondrial phenomenon.
mesh_terms: D002001:Adipose Tissue, Brown / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D003080:Cold Temperature; D005260:Female; D006150:Guanine Nucleotides / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism*; D008933:Mitochondrial Swelling / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats
chemical_list: D006150:Guanine Nucleotides; D006153:Guanosine Diphosphate; D009638:Norepinephrine