Demonstration of an in vivo functional beta 3-adrenoceptor in man

Although it is well established in several mammalian species that beta 3-adrenoceptors play a major role in regulating lipolysis and thermogenesis in adipose tissue, the functional existence and role of this receptor subtype in man has been controversial. We investigated whether the beta 3-adrenoceptor functionally co-exists with beta 1- and beta 2-adrenoceptors in vivo in human adipose tissue. Subcutaneous abdominal adipose tissue of healthy non-obese subjects was microdialyzed with equimolar concentrations of dobutamine (selective beta 1-adrenoceptor agonist), terbutaline (selective beta 2-adrenoceptor agonist), or CGP 12177 (selective beta 3-adrenoceptor agonist). All three agents caused a rapid, sustained, concentration-dependent and significant elevation of the glycerol level in the microdialysate (lipolysis index). However, only terbutaline stimulated the nutritive blood flow in adipose tissue, as measured by an ethanol escape technique. Dobutamine and CGP 12177 was equally effective in elevating the glycerol level (maximum effect 150% above baseline). Terbutaline was significantly more effective than the other two beta-agonists (maximum effect 200% above baseline). When adipose tissue was pretreated with the beta 1/beta 2-selective adrenoceptor blocker propranolol the glycerol increasing effect of dobutamine or terbutaline was inhibited by 80-85% but the glycerol response to CGP 12177 was not influenced. It is concluded that a functional beta 3-adrenoceptor is present in vivo in man. It co-exists with beta 1- and beta 2-adrenoceptors in adipose tissue and may therefore play a role in lipolysis regulation. It appears, however, that the beta 2-adrenoceptor is the most important beta-adrenoceptor subtype for the mobilization of lipids from abdominal subcutaneous adipose tissue because of its concomitant stimulatory effect on lipolysis and blood flow.