The long-term objective of this research is to understand the regulation of facultative non-shivering thermogenesis. This process is crucial for energy balance and temperature regulation, and is regulated by thyroid hormone and the sympathetic nervous system (SNS). Brown adipose tissue (BAT) is the main site of facultative thermogenesis in most mammals and Uncoupling Protein (UCP) is the key element for BAT thermogenic function. UCP synthesis is regulated by the SNS and thyroid hormone. For the full expression of this protein, T3, the active thyroid hormone, must reach a high concentration in BAT nuclear T3 receptors. This T3 is supplied by the adrenergic activation of a Type II T4 5'deiodinase present in the tissue. Based on preliminary results, our working hypothesis is that norepinephrine (NE) is the primary regulator of UCP synthesis and that T3 amplifies the response to this signal. The first Specific Aim is to confirm that increased transcription is the mechanism for the NE effect using appropriate measurements of UCP, UCP mRNA and transcription. We will then identify the intracellular mediators of the NE message and determine how they are affected by T3. For these studies, we will employ a recently developed model of basal UCP expression, hypothyroid rats in which BAT is surgically denervated. We will complement these studies by the evaluation of these responses in dispersed brown adipocytes. The second Specific Aim is to elucidate the local mechanisms which can provide sufficient T3 to allow a full thermogenic response without systemic hyperthyroidism. The central question is whether the saturation of T3 receptors during adrenergic stimulation results from intracellular compartmentation of T3 or is simply the consequence of the relative rates of local T3 production and BAT-plasma T3 exchange. Kinetic studies with tracer T4 and T3 in isolated brown adipocytes and whole animals will define the existence of compartments of T3 in the brown adipocyte, the routing of extracellular and locally produced T3 into the nucleus, and the rates of BAT T3 production and BAT-plasma T3 exchange. This research relevant to the mechanisms of action of NE, of T3, especially in BAT and similar tissues such as the pituitary and brain in which this hormone is produced locally, and to SNS-thyroid interactions. Hence these studies are also relevant to the pathophysiology or hypothyroidism, hyperthyroidism, temperate regulation, energy balance and to the problem of obesity.
