The goal of this project is to investigate the impairment of thermogenesis in brown adipose tissue (BAT) in the senescent rat. Nonshivering thermogenesis in BAT is an important source of heat to warm an animal after exposure to the cold, especially in the cold-adapted rat. There are four specific aims to this proposal. (1) To determine if the cold-induced gene expression of BAT uncoupling protein (UCP) is mediated by other than beta-3-adrenergic receptors (beta-3ARs) in senescent rats. The population of beta-ARs in BAT consists mostly of the atypical beta-3AR subtype and this receptor mediates the induction of BAT UCP in young rats.However, cold-induced thermogenesis is preserved in senescent rats to a greater extent than is beta-3AR stimulated thermogenesis, suggesting another pathway may be mediating thermogenesis in older rats. The first goal will be to examine and compare the time-course of cold-stimulated and beta-3-agonist-stimulated gene expression of UCP by assessment of UCP mRNA and the level of UCP by immunoreactivity in rats of 6-, 12- and 24-months. (2) To determine if thermogenesis is mediated by beta-1ARs or alpha-1-adrenergic receptors (alpha-1 ARs) rather than beta-3ARs in senescent rats. The applicants' data indicate that alpha-1 ARs are more important for the thermogenic response in old compared to young rats. The second goal will be to examine and compare thermogenesis (O2 consumption and mitochondrial GDP binding), the gene expression of UCP and the level of UCP following stimulation with the endogenous agonist, norepinephrine (NE); the beta-3-agonist, CGP-1277A; the alpha-1-agonist, phenylephrine; the beta-1 selective agonist, tazolol; as well as post receptor agents such as forskolin and the cAMP analog, Sp-cAMPS in rats of three ages. (3) To determine if beta-3ARs are desensitized by beta-agonists. It has been suggested that beta- 3ARs may not be desensitized by catecholamines in the same manner as beta-1 or beta-2ARs. The applicants propose to assess desensitization of beta-3 and beta-1 signal transduction by assessing receptor number, adenylate cyclase activity, and the gene expression of beta-3 and beta-1ARs by assessing beta-3 and beta-1 mRNA in BAT following NE, CGP-12177A and tazolol administration in rats of three ages. (4) To determine if cold adaptation or chronic drug treatment can restore the BAT thermogenic response to beta-3-agonists in aged rats. The synthesis of UCP (and therefore the capacity for thermogenesis) can be increased by drugs (beta-agonists) or physiologically (cold exposure). In addition, some data suggest that beta-agonist treatment can up regulate beta- 3ARs. The fourth goal of this proposal is to restore the reduced beta- 3AR-mediated thermogenic response with age by cold adaptation or chronic administration of CGP-12177A and/or phenylephrine. Thermogenesis (body temperature and oxygen consumption) will be examined and correlated with UCP gene expression, GDP binding, and beta-AR signal transduction in control and chronically drug-treated and cold-adapted rats of two ages.
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