β3-adrenergic receptors are responsible for the adrenergic inhibition of insulin-stimulated glucose transport in rat adipocytes

Abstract

The inhibition of insulin-stimulated glucose transport by isoprenaline, a mixed β-adrenergic-receptor (AR) agonist, is well documented in rat adipocytes. Since it has been described that rat adipocytes possess not only β1 and β2- but also β3-ARs, the influence of various subtype-selective β-AR agonists and antagonists on 2-deoxyglucose (2-DG) transport was assessed in order to characterize the β-AR subtype involved in the adrenergic counter-regulation of the insulin effect. The stimulation of 2-DG transport by insulin was-counteracted, in a dose-dependent manner, by all the β-AR agonists tested, and the magnitude of the inhibition followed the rank order: BRL 37344 > isoprenaline = noradrenaline >> dobutamine = procaterol. The same rank order of potency was obtained for lipolysis activation. This is not in accordance with the pharmacological definition of a β1- or a β2-adrenergic effect, but agrees with the pharmacological pattern of a β3-adrenergic effect. The inhibitory effect of the β3-agonist BRL 37344 on insulin-stimulated 2-DG transport was not reversed by either the selective β1-antagonist ICI 89406 or the β2-antagonist ICI 118551. In addition, neither of these β-antagonists was able to block the isoprenaline and noradrenaline effects, supporting major β3-adrenoceptor-subtype involvement in the adrenergic inhibition of insulin-stimulated 2-DG transport. Like isoprenaline, BRL 37344 inhibited (60% inhibition) insulin-stimulated glucose transport only when adenosine deaminase was present in the assay. Furthermore, the maximal inhibitory effects of isoprenaline and BRL 37344 were not additive, and were both dependent on albumin concentration in the incubation medium: they increased when the albumin concentration decreased in the medium from 3.5 to 1%. To conclude, the similarities between isoprenaline and BRL 37344 action on insulin-stimulated 2-DG transport, the poor efficacy of the β1-/β2-agonists and the lack of effect of selective β1- and β2-antagonists are compelling arguments to support the important role of β3-adrenoceptors in the adrenergic inhibition of glucose transport in rat adipocytes.