Lack of beta 3-adrenergic effect on lipolysis in human subcutaneous adipose tissue

Human adipocytes are responsive to both alpha- (antilipolytic) and beta-(lipolytic) actions of catecholamine agonists. These effects are mediated by G proteins which interact with adenylcyclase beta 1- and beta 2-adrenoreceptor (AR) subtypes are identified in human adipose tissue. The existence of a third beta-adrenergic (beta 3) receptor has been recently proposed. The beta 3-AR has been reported to mediate catecholamine-induced cAMP accumulation and to be more responsive in this regard than either the beta 1- or the beta 2-AR. To examine the possibility that a beta 3-AR plays a significant role in the control of catecholamine-stimulated lipolysis in human adipose tissue, we used a sensitive in vitro measure of lipolysis. Abdominal and gluteal sc adipose tissue samples from 14 adults were incubated with isoproterenol, a nonspecific beta-AR agonist which activates adenylyl cyclase via all three beta-AR subtypes, in the presence of varying concentrations of a beta-AR antagonist, alprenolol, which blocks the beta 1- and beta 2-AR, but not the beta 3-AR. In tissue from both abdominal and gluteal sites, alprenolol (10(-3) M) completely abolished the lipolytic response to isoproterenol (10(-6) M). Alprenolol (10(-3) M) alone reduced basal lipolysis by 10.2 +/- 2.2% (SEM) in abdominal tissue (P < 0.01), and 7.1 +/- 2.7% in gluteal tissue (P < 0.05). In the presence of alprenolol (10(-3) M), isoproterenol (10(-6) M) had no demonstrable lipolytic effect. The probability of incorrect acceptance of the null hypothesis that isoproterenol had no effect on lipolysis in alprenolol-blocked tissue is P beta < 0.05 for both depots. As an additional control, pooled inguinal adipose tissue from adult male Sprague-Dawley rats was incubated with the same concentrations of alprenolol. Rat adipose tissue has been previously shown to express the beta 3-adrenoreceptor and lipolysis was increased in rat adipose tissue in the presence of alprenolol 10(-4) M and 10(-5) M. We conclude that beta 3-ARs, if present in human adipocyte plasma membranes, do not significantly affect rates of lipolysis.