Distinction between mechanisms underlying alpha 1- and beta-adrenergic respiratory stimulation in brown fat cells.
Experimental conditions are described for selective alpha 1- and beta-adrenergic stimulation of the respiration of brown fat cells. The dual agonist norepinephrine was unsuitable as a selective alpha 1-agonist, since unacceptably high concentrations of propranolol were needed to abolish the beta-response. Phenylephrine at 50 microM, in the presence of 5 microM dl-propranolol, was shown to lead to a maximal, selective alpha 1-stimulation, whereas maximal, selective beta-stimulation was achieved with 1 microM isoproterenol in the presence of 5 microM prazosin. The mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was able to further increase respiration that was already maximally alpha 1-stimulated, but when added before the alpha 1-stimulation, FCCP totally abolished the response. In contrast, FCCP had no effect on the beta-stimulated response. Similarly, oligomycin (an inhibitor of mitochondrial ATP synthesis) inhibited alpha 1-respiration but had a much smaller effect on beta-respiration. Ouabain (an inhibitor of the Na+-K+-ATPase) halved alpha 1-respiration but only induced a small inhibition of beta-respiration. It is concluded that only a small fraction of thermogenesis from beta-adrenergic processes is due to oxidative phosphorylation, whereas alpha 1-respiration is largely due to the oxygen cost of mitochondrial ATP synthesis, and a large fraction of this ATP is apparently used for the restoration of ion gradients.
D002001:Adipose Tissue, Brown / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D001696:Biomechanical Phenomena; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D006224:Cricetinae; D008647:Mesocricetus; D008928:Mitochondria / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D010101:Oxygen Consumption / Q000187:drug effects*; D013268:Stimulation, Chemical; D013566:Sympathomimetics / Q000494:pharmacology*
D013566:Sympathomimetics; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D010042:Ouabain