Animal studies demonstrated that there exists a dissociation in sympathetic nervous system (SNS)vs. adrenal medullary response (AM) to combine stressors (fasting and cold exposure). It has been hypothesized that the reciprocal change in these components of the sympatho-adrenal system (SAS) reflect a role of redundancy for the AM. Human studies pertaining to glucose conterregulation have clearly established a precedence for the AM having a role of redundancy in regards to preventing hypoglycemia. Taken together, this research suggests that the dissociation in the SAS that occurs under conditions of fasting and cold will likely occur in humans. The major objective of this project is to determine if their exists in humans a similar dissociation of the NS response vs. the AM response to combined stressors as has been shown to occur in animals. It is hypothesized.that using tracer methodology, i.e., NE kinetics, to assess SNS function will facilitate characterizing the dissociation and/or redundancy between the SNS and AM that is postulated to occur in conjunction with the stress of fasting combined with exercise and cold exposure. The results of this project 'will provide a basis for further assessment of possible mediators/stimuli that are responsible for the varied response between these two components of the sympathoadrenal system. Subsequent to investigating these issues, this project will be doing pioneer work in measuring NE kinetics during conditions of fasting and cold exposure, (and reaffirming its use for steady state exercise). Having a better understanding of the nature of the dissociation of the SNS and AM has potential use in studying obesity, caloric restriction, overfeeding, cardiovascular disease, hypoxia, glucose homeostasis, and exercise training.
The specific aims of the project are (1) to determine the existence and/or nature of the dissociation of the SNS vs. AM in response to combined fasting and exercise; (2) to determine the existence and/or nature of the dissociation of the SNS vs. AM in response to combined fasting and cold exposure; (3) provide original data regarding SNS activity as determined via NE kinetics in response to fasting; (4) provide original data regarding SNS activity as determined via NE kinetics in response to cold exposure (5) confirm previous findings regarding SNS activity as determined via NE kinetics in response to moderate exercise. The research design appropriate to assessing the response of multiple factors in relation to a baseline value is that of a multifactorial design with a covariate, with the baseline response acting as the covariate. Fasting, exercise, and cold exposure will serve as the independent variables and the response to these conditions will be compared across treatments using ANCOVA. Additionally, correlations will be determined between SAS related parameters, e.g., NE RA and dependent variables, e.g., energy expenditure, in an effort to identify potential mediating factors in the hypothesized dissociation/redundancy between SNS and AM under these conditions.
