Mechanism of beta-adrenergic agonist-induced transmural transport of glucose in rat small intestine. Regulation of phosphorylation of SGLT1 controls the function.

The perfusion of rat small intestine with 10 microM epinephrine (Epi) or 10 microM norepinephrine resulted in significant increases in the amount of 3-O-[methyl-3H]-D-glucose transported from the mucosal to serosal side. The Epi-induced increases in glucose transport were coupled with selective increases in beta-adrenoceptor density in the mucosal membranes. Treatment with 0.1 microM okadaic acid increased glucose transport even in the absence of Epi, but that with 1 microM staurosporine or 60 microM N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide dihydrochloride completely inhibited the increases in glucose transport induced by 10 microM Epi or 10 microM dibutyryl cAMP. The maximal binding sites (Bmax) of [3H]phlorizin in brush border membrane (BBM) from tissues perfused with Epi was increased, showing increases in the binding ability of the Na+/glucose cotransporter (SGLT1) to glucose. Phosphorylation and dephosphorylation of BBM with protein kinase A (PKA) and alkaline phosphatase resulted in increases and decreases in Bmax of [3H]phlorizin, respectively. The phosphorylation state of SGLT1 immunoprecipitated from BBM incubated with [gamma-32P]ATP-Mg2+ and PKA, and the analysis of phosphoamino acids composed of SGLT1 in rats given [32P]orthophosphate indicate the presence of potential sites for PKA-mediated phosphorylation of SGLT1 at serine. These findings indicate that the regulation of phosphorylation of SGLT1 leads to an alteration of its function and results in the control of glucose transport in the rat small intestine.