Lipolytic response and adenyl cyclase activity of rat adipocytes as related to cell size

The number of fat cells contained in the rat epididymal fat pad was found to increase rapidly as the rats grew to a weight of about 300 g. Additional increases in cell number above this weight were minimal. By contrast, cell size, as measured by the amount of triglyceride per cell, increased linearly until the rats reached about 600 g. Glycerol release per 10(6) cells in response to norepinephrine in vitro was observed to be independent of cell size. Basal release expressed in this manner showed a slight but significant positive correlation with increasing cell size. When the rate of lipolysis was based either on the amount of triglyceride in the incubation medium, as is the usual custom, or on the cell surface area, lipolysis was inversely related to cell size. In addition to these observations on lipolysis, it was also demonstrated that norepinephrine-activated adenyl cyclase activity expressed per 10(6) cells was unaffected by cell size. This leads to the suggestion that the number of adrenergic receptors in the fat cell is fixed and is independent of the size of the cell; as the cell enlarges, these receptors are merely distributed over a greater surface area.