Applications of microdialysis in studies of the adrenergic regulation of lipolysis in human skeletal muscle and adipose tissue

Lipolysis, i.e., hydrolysis of triaeylglycerol to free fatty acids and glycerol, is a central process in lipid metabolism, and the regulation of lipid metabolism is altered in several common disorders, such as diabetes, obesity and atherosclerosis. Lipolysis is regulated in a complex way, catecholamines and insulin are important hormones, acting stimulatory and inhibitory on lipid mobilization, respectively. Both catecholamines and insulin are vasoactive, which implies that effects of the hormones on local blood flow have to be considered in studies of lipolysis regulation. Microdialysis is a novel technique that allows continuous sampling and manipulation of the interstitial space of a number of tissues including muscle and adipose tissue. It is possible to simultaneously monitor metabolic substrate concentrations and relative changes in local blood flow. The aim of this thesis was to focus on methodological and physiological aspects of the in vivo adrenergic regulation of lipolysis in human skeletal muscle and adipose tissue.

The influence of blood flow on adipose tissue glycerol levels was investigated in human subcutaneous adipose tissue in situ (Paper 1). The microdialysis technique proved beneficial studying pharmacological interference with lipolysis, and it was found that stimulation of blood flow under certain conditions may decrease the level of glycerol in the extracellular space of adipose tissue although the mobilization of glycerol from fat cells to this compartment is increased. Hence, changes in blood flow and glycerol should be considered together when adipose tissue lipolysis is investigated by microdialysis. Although it is well established in several mammalian species that ß-adrenoceptors play a major role in regulating lipolysis and thermogenesis in adipose tissue, the functional existence and role of this receptor subtype in man has been controversial. In paper II, we demonstrate the presence of a functional ß-adrenoceptor in vivo in man and, moreover, the co-existence with ß,- and ß-adrenoceptors, indicating a role in lipolysis regulation.

The combined use of the atraumatic microdialysis technique and easy accessible adipose tissue is an attractive model system for pharmacological in vivo studies in humans. This setting was used to investigate the synergistic effect of forskolin, a direct activator of the catalytic subunit of adenylyl cyclase, on ß-adrenoceptor-mediated stimulation of lipolysis and blood flow in human adipose tissue in vivo (Paper III). It was found that local administration of forskolin and isoprenaline induce lipolysis and increase blood flow and that the effect of isoprenaline is potentiated by forskolin in vivo in man.

It has been previously shown that the antilipolytic effect of insulin is primarily mediated by activation of phosphodiesterase 3B (PDE3B) in adipocytes. Various PDE inhibitors were applied locally to the abdominal subcutaneous fat as well as the gastrocnemius skeletal muscles during a hyperinsulinemic euglycemic condition (Paper IV). We report that insulin inhibits lipolysis in adipose tissue and skeletal muscle by various PDEs, suggesting a unique metabolic role of muscle lipolysis.

In paper V, the relative differences in glycerol and lactate concentrations in muscle and adipose tissue were investigated. In addition, methodological issues regarding substrate recovery and microdialysis catheter contamination were addressed. A substantial mobilization of glycerol and lactate was recognized in both adipose tissue and skeletal muscle at rest. Glycerol and lactate production are influenced by hyperinsulinemia and hypoglycemia in both tissues. Adipose tissue appears to be the major site ot glycerol production, whereas skeletal muscle and fat may be equally important for lactate production.

In conclusion, microdialysis is a valuable tool with exceptional advantages for in vivo metabolic studies in skeletal muscle and adipose tissue, and more so when variations in local blood flow is accounted for. A new ß-adrenoceptor was functionally demonstrated in vivo and the synergistic effect of a direct adenylyl cyclase activator on 13-adrenoceptor-mediated stimulation of lipolysis and blood flow was elucidated. Moreover, we suggest a unique regulation of lipolysis in skeletal muscle and report on the absolute concentrations of glycerol and lactate in human skeletal muscle and adipose tissue.