Pathogenic Mechanisms

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The blood flow from visceral fat depot is drained via the portal vein to the liver, in contrast to other fat depots that are drained to the systemic circulation. Visceral adipose tissue has a higher turnover rate of fat, in both men and women, than other adipose tissue depots (11). Both lipid accumulation, by the action of lipoprotein lipase (LPL), and the lipolytic response to catecholamines are elevated (11-13). The increased lipolytic activity of visceral fat combined with its anatomical localization means that the liver is exposed to higher concentrations of free fatty acids (FFA) than any other organ. FFA have important influence on the liver metabolism. Increased levels of FFA attenuates the hepatic clearance of insulin from the pancreas and enhances the gluconeogenesis and the secretion of very low density lipoproteins (VLDL) from the liver (14-17). Therefore, with enlarged visceral adipose tissue depots, as in visceral obesity, these effects of FFA on the liver would be expected to cause peripheral hyperinsulinemia, hyperglycemia, and elevated levels of VLDL, all of which are known to be important risk factors for NIDDM and arteriosclerosis.

The lipolytic process is mainly regulated by catecholamines in human adipose tissue (18). Furthermore, visceral adipocytes have a higher density of lipolytic ^-adrenergic receptors than other fat cells, mediating lipolysis by the action of norepinephrine (19). The density of glucocorticoid as well as androgen receptors is also higher (12,20). The effect of cortisol is mainly to increase visceral fat mass by increasing the expression of LPL (21,22), while testosterone has the capacity to decrease fat accumulation by inhibiting LPL (23,24) and enhancing lipolysis by the increasing the expression of P-adren-ergic receptors (25,26). In addition to these intrinsic characteristics of the visceral adipocytes the surroundings of these cells are different from other adipocytes. Blood flow is higher than in other adipose tissues (27), which is of fundamental importance for both lipid uptake and mobilization, and in addition visceral adipose tissue contains more catecholamines and catecholaminergic nerves than other adipose tissues (28).

Taken together this means that visceral adipose tissue has a unique metabolic capability, hormone receptor density, blood flow and innervation to form a metabolic center of adipose tissue. This is of considerable and potential importance in view of the effects of FFA on the hepatic regulation of metabolism.

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