α1A-adrenoceptors activate glucose uptake in L6 muscle cells through a phospholipase C-, phosphatidylinositol-3 kinase-, and atypical protein kinase C-dependent pathway

The role of alpha(1)-adrenoceptor activation on glucose uptake in L6 cells was investigated. The alpha(1)-adrenoceptor agonist phenylephrine [pEC(50) (-log(10) EC50), 5.27 +/- 0.30] or cirazoline (pEC(50), 5.00 +/- 0.23) increased glucose uptake in a concentration-dependent manner, as did insulin (pEC(50), 7.16 +/- 0.21). The alpha(2)-adrenoceptor agonist clonidine was without any stimulatory effect on glucose uptake. The stimulatory effect of cirazoline was inhibited by the alpha(1)-adrenoceptor antagonist prazosin, but not by the alpha(1)-adrenoceptor antagonist propranolol. RT-PCR showed that the alpha(1A)-adrenoceptor was the sole alpha(1)-adrenoceptor subtype expressed in L6 cells. Cirazoline- or insulin-mediated glucose uptake was inhibited by the phosphatidylinositol-3 kinase inhibitor LY294002, suggesting a possible interaction between the alpha(1)-adrenoceptor and insulin pathways. Cirazoline or insulin stimulated phosphatidylinositol-3 kinase activity, but alpha(1)-adrenoceptor activation did not phosphorylate Akt. Both cirazoline- and insulin-mediated glucose uptake were inhibited by protein kinase C (PKC), phospholipase C, and p38 kinase inhibitors, but not by Erk1/2 inhibitors (despite both treatments being able to phosphorylate Erk1/2). Insulin and cirazoline were able to activate and phosphorylate p38 kinase. The phorbol ester 12-O-tetradecanoylphorbol-13- acetate and the calcium ionophore A23187 produced significant increases in glucose uptake, indicating roles for PKC and calcium in glucose uptake. Down-regulation of conventional PKC isoforms inhibited glucose uptake mediated by 12-O-tetradecanoylphorbol-13-acetate, but not by insulin or cirazoline. This study demonstrates that alpha(1)-adrenoceptors mediate increases in glucose uptake in L6 muscle cells. This effect appears to be related to activation of phospholipase C, phosphatidylinositol-3 kinase, p38 kinase, and PKC.