Clenbuterol-Stimulated Glucose Uptake Activates both GS and GI Pathways through Β2-Adrenoceptor in Mouse Isolated Soleus Muscle

Abstract

β2-adrenoceptors activated by adrenaline can also couple to both Gs and Gi proteins. The former is associated with an increase in cAMP to illicit the effect of the catecholamine. In the later, β2-AR induces PKA-catalysed phosphorylation of the receptor, which intends couples to Gi, at high concentration. We proposed that, clenbuterol which stimulates glucose uptake at low concentra-tion and inhibits it at high concentration might have identical signalling pathway as adrenaline. Mouse isolated muscles were pre-incubated in flasks containing 3 ml of Krebs-Henseleit Bicar-bonate buffer. After 120 min of pre-incubation, with the appropriate concentration of PTX the muscles were transferred to another incubation flask containing 3 ml of the same buffer and 0.3mCi 2-deoxy[1-14C]glucose containing varying concentrations of adrenaline, clenbuterol or 1nM insulin with or without PTX concentrations of 100ng/ml for adrenaline and clenbuterol and 0.1 or 1.0ng/ml for insulin, or 1M acetylcholine for 45 minutes. Adrenaline stimulated glu-cose uptake in isolated mouse soleus muscle at low concentration (10-10M) and inhibited it at high concentration (10-5M). The effect of the lower concentration was mediated through the β2-AR coupling to the Gs protein and to the Gi protein at high concentration. Similarly, clenbuterol stimulated glucose uptake at lower concentration (10-11M) is mediated through the β2-AR cou-pling to the Gs protein and to the Gi protein at the higher concentration (10-7M). These effects of high concentration of adrenaline and clenbuterol were supported by the fact that 100ng/ml PTX relieved their inhibitory effects. The higher concentration effect of clenbuterol was additionally supported by the fact that, 1M acetylcholine, relieved the inhibitory effect.

Keywords: PTX, Pertusis toxin, G-proteins, Guanine nucleotide binding proteins, β-AR, beta adrenoceptor, M2, muscarinic receptors