Inhibition of isoproterenol-induced lipolysis in rat inguinal adipocytes in vitro by physiological melatonin via a receptor-mediated mechanism

Because the pineal hormone melatonin has been implicated in Meting adiposit y in rats and fatty acid transport in certain rat tumor models, we tested w hether melatonin regulates lipolysis in a normal cell system in vitro. Adip ocytes were isolated from the inguinal fat pads (i.e. sc fat) of Sprague Da wley male rats during mid-light phase. Lipolysis was stimulated with isopro terenol (3 muM), and cells were incubated for 4 h in the presence or absenc e of a physiological circulating concentration of melatonin (I nm). Lipolys is was measured by determining the amount of glycerol present in the incuba tion buffer, expressed as nmol glycerol/mg cellular fatty acid. We observed a 20- to 30-fold stimulation of basal lipolysis by isoproterenol, and this stimulation was inhibited 50-70% by melatonin. Melatonin exhibited this ef fect over a wide range of concentrations tested (100 pM-1 muM) with an IC50 of approximately 500 pM. The effect by melatonin (1 nm) was completely blo cked by pertussis toxin (50 ng/ml), by 8-bromo-cAMP (10 nm), and by the mel atonin receptor antagonist S-20928 (1 nm). These results suggest that the a ntilipolytic effect occurs through one of the G(i) protein-coupled melatoni n receptors because we have shown that both the mt(1) (Mel 1a) and MT2 (Mel 1b) melatonin receptors are expressed in inguinal adipocytes. Melatonin in hibition of lipolysis was not observed in adipocytes isolated from rat epid idymal fat pads (i.e. visceral fat), even though these cells also express b oth the mt, and MT2 receptors. The results indicate that physiological circ ulating concentrations of melatonin inhibit isoproterenol-induced lipolysis in rat adipocytes via a G protein-coupled melatonin receptor-mediated sign al transduction pathway in a site-specific manner.