Lipolysis in human adipocytes, effects of cell size, age and of regional differences.

In man only catecholamines and insulin have pronounced and acute effects on lipolysis in fat cells. In obesity the effects of these hormones seem to be normal or even increased. When the rate of lipolysis is expressed per cell there is a strong association between hormonal effect and cell size. Indicating that catecholamines and insulin may be involved in the regulation of adipocyte volume. There are, however, site variations in the effects of the regulatory hormones, which may be of importance for the development of various regional forms of obesity. The lipolytic effect of catecholamines is more pronounced in the abdominal than in the femoral/gluteal subcutaneous fat depots, partly owing to an increased alpha 2-adrenergic receptor mediated antilipolytic effect of catecholamines in the latter region. This alteration in lipolysis favour accumulation of fat in the femoral/gluteal region and may be of importance for the development of the female type of obesity. Furthermore, in omental fat cells the lipolytic activity is higher than in subcutaneous fat cells owing in part to less marked insulin action and lower alpha 2-adrenergic receptor mediated antilipolytic effect of catecholamines. These alterations may cause elevation of the free fatty acid levels in the portal blood so that the handling of glucose and insulin is impaired in the liver which may be one mechanism behind the increased risk to develop cardiovascular complications in the male type of obesity. Age also influences hormone-induced lipolysis. Catecholamine resistance is observed at birth and at the latest stages of life. Insulin resistance is observed in old as compared to middle-aged subjects.