Hormone-stimulated lipolysis in isolated fat cells from "young" and "old" rats.

The biphasic nature of the lipolytic dose-response curve of epinephrine in fat cells from "young" rats (40-45 days) was confirmed. The first phase (Lipolysis I) occurred at concentrations of from 10(-7) m to 3 x 10(-6) m. The second phase (Lipolysis II) occurred at concentrations of from 10(-5) m to 3 x 10(-4) m. Insulin (0.1 mU/ml) abolished Lipolysis I and slightly augmented Lipolysis II. Higher concentrations of insulin (1.0 mU/ml) augmented Lipolysis II even further. These results may help to explain some of the conflicting reports in the literature concerning the effects of insulin on lipolysis. The dose-response curve of epinephrine using fat cells from "old" rats (14-16 months) was monophasic. Based on results with propranolol, K(+)-free media, and insulin, it was concluded that the lipolytic response in tissue from older animals corresponds to Lipolysis II in tissue from younger rats. The lipolytic response to ACTH was greatly reduced in the cells from the older rats, but the response to theophylline was unaltered.