α_1A-Adrenoceptors activate mTOR signalling and glucose uptake in cardiomyocytes

The capacity of G protein-coupled receptors to modulate mechanistic target of rapamycin (mTOR) activity is a newly emerging paradigm with the potential to link cell surface receptors with cell survival. Cardiomyocyte viability is linked to signalling pathways involving Akt and mTOR, as well as increased glucose uptake and utilization. Our aim was to determine whether the α_1A-adrenoceptor (AR) couples to these protective pathways, and increased glucose uptake. We characterised α_1A-AR signalling in CHO-K1 cells co-expressing the human α_1A-AR and GLUT4 (CHOα_1AGLUT4myc) and in neonatal rat ventricular cardiomyocytes (NRVM), and measured glucose uptake, intracellular Ca²⁺ mobilization, and phosphorylation of mTOR, Akt, 5' adenosine monophosphate-activated kinase (AMPK) and S6 ribosomal protein (S6rp). In both systems, noradrenaline and the α_1A-AR selective agonist A61603 stimulated glucose uptake by parallel pathways involving mTOR and AMPK, whereas another α₁-AR agonist oxymetazoline increased glucose uptake predominantly by mTOR. All agonists promoted phosphorylation of mTOR at Ser2448 and Ser2481, indicating activation of both mTORC1 and mTORC2, but did not increase Akt phosphorylation. In CHOα_1AGLUT4myc cells, siRNA directed against rictor but not raptor suppressed α_1A-AR mediated glucose uptake. We have thus identified mTORC2 as a key component in glucose uptake stimulated by α_1A-AR agonists. Our findings identify a novel link between the α_1A-AR, mTORC2 and glucose uptake, that have been implicated separately in cardiomyocyte survival. Our studies provide an improved framework for examining the utility of α_1A-AR selective agonists as tools in the treatment of cardiac dysfunction.