Effect of endothelin-1 on lipolysis in rat adipocytes

Objective: To explore the role of endothelin-1 (ET-1) on lipid metabolism, we examined the effect of ET-1 on lipolysis in rat adipocytes. Research Methods and Procedure: Adipocytes isolated from male Sprague-Dawley rats, weighing 400 to 450 grams, were incubated in Krebs-Ringer buffer with or without 10(-7) M ET-1 for various times or with various concentrations of ET-1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET,R and ET,R blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET-1-stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring, changes in intracellular cAMP levels in response to ET-1 treatment. To further explore the underlying mechanism of ET-1 action, we examined the involvement of the extracellular signal-regulated kinase (ERK)-mediated pathways. Results: Our results showed that ET-1 caused lipolysis in rat adipocytes in a time- and dose-dependent manner. BQ6107 a selective ET,R blocker, blocked this effect. The adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine, had no effect on ET-1-stimulated lipolysis. ET-I did not induce an increase in intracellular cAMP levels. In addition, ET-1-induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET-1 and insulin Suppressed ET-1-stimulated lipolysis. Discussion: These findings show that ET-1 stimulates lipolysis in rat adipocytes through the ETAR and activation of the ERK pathway. The underlying mechanism is cAMP-independent. However, this non-conventional lipolytic effect of ET-1 is inhibited by the anti-lipolytic effect of insulin.