Comparison of the Lipolytic Effects of Norepinephrine and BRL 37344 in Rat Brown and White Adipocytes

Abstract

The lipolytic effects of norepinephrine (a non‐selective β‐agonist) and BRL 37344 (a selective β3‐agonist) were compared in isolated rat brown and white adipocytes. Norepinephrine and BRL 37344 maximally stimulated lipolysis in brown and white adipocytes, approximately 10 times above basal values. However, adipocyte sensitivity for BRL 37344 was greater than that for norepinephrine, particularly in brown adipocytes [the EC50 values (nM) for BRL 37344 and norepinephrine were 5 ± 1 and 103 ± 31 in brown adipocytes (P <0.01) versus 56 ± 9 and 124 ± 17 in white adipocytes (P <0.05), respectively]. On the other hand, the lipolytic effects of norepinephrine were totally blocked by 20–40 times superior concentrations of propranolol or bupranolol in brown as well as in white adipocytes. In contrast, the lipolytic effects of BRL 37344 were fully inhibited by concentrations of propranolol or bupranolol that were 200–1000 superior to the β3 agonist concentration. The results demonstrate that: (1) the (β3‐agonist BRL 37344 is as effective as norepinephrine for maximally stimulating lipolysis in rat brown and white adipocytes, (2) both adipocyte types are more sensitive to the lipolytic effects of BRL 37344 than to those of norepinephrine, (3) although bupranolol is a better antagonist than propranolol on BRL 37344‐stimulated lipolysis, it cannot be considered as a specific β3‐antagonist, (4) brown adipocytes are 10 times more sensitive than white adipocytes to the lipolytic effects of BRL 37344, suggesting an important role of β3‐receptors in brown adipose tissue. 1994 North American Association for the Study of Obesity (NAASO)