The extent to which the sympathetic nervous system and other neuroendocrine transmitters regulate adipose tissue lipolysis during basal and stimulated conditions is unclear. Individuals with a spinal cord injury (SCI) lack direct sympathetic innervation to tissues below the level of lesion. This disruption of neural pathways affects both direct stimulation of abdominal fat tissues as well as indirect stimulation by hormones (i.e., catecholamines) of the adrenal medulla. Person's with SCI, therefore, represent a model for examining lipolysis in the absence of major sympathetic drive. This protocol examined the effects of exercise and isoproterenol stimulation on adipose tissue lipolysis in persons with and without sympathetic innervation of the adrenal medulla and adipose tissue. To test the role of direct sympathetic innervation and circulating catecholamines in regulating fat breakdown in adipose tissue, lipolysis (i.e., glycerol production was measured in situ by microdialysis in able-bodied subjects and in SCI persons at rest and during arm exercise. Seven patients with high SCI (C6-T5), 4 with low SCI (T10-L2) and 10 controls were compared. There were no group differences in basal rates of lipolysis (dialysate glycerol concentrations 30+/-5, 27+/-9 and 40+/-27 umol/l, respectively), isoproterenol stimulated lipolysis (79+/-28, 62+/-24, 93+/-87 umol/l) or during submaximal exercise (69+/-16, 75+/-40 and 97+/-70 umol/l). Plasma epinephrine and norepinephrine responses to submaximal exercise were significantly lower in SCI patients than controls. These results indicate that SCI patients maintain similar lipolytic rates and responses despite disruption of direct sympathetic innervation and lower circulating catecholamine levels. Compensatory mechanisms, such as down regulation of alpha, antilipolytic responses, may serve to maintain normal lipolytic rates in these individuals. This project is terminated as of the 1997 fiscal year.
