Characterization of the β-adrenoceptor subtype involved in mediation of glucose transport in L6 cells

1. The receptor that mediates the increase in glucose transport (GT) in response to β-adrenoceptor (β-AR) agonists was characterized in the rat skeletal muscle cell line L6, using the 2-deoxy-[³H]-D-glucose assay. 2. The β₃-AR agonist BRL37344 (pEC₅₀=6.89±0.21), the β-AR agonist isoprenaline (pEC₅₀=8.99±0.24) and the β₂-AR agonist zinterol (pEC₅₀=9.74±0.15) increased GT as did insulin (pEC₅₀=6.93±0.15). The highly selective β₃-AR agonist CL316243 only weakly stimulated GT. 3. The pK_B values calculated from the shift of the pEC₅₀ values of the agonists in the presence of the β₁-AR selective antagonist CGP 20712A or the β₃-AR selective antagonist SR 59230A were not indicative of activation of β₁- or β₃-ARs. Only (-)-propranolol and the β₂-AR selective antagonist ICI 118551 caused marked rightward shifts of CR curves to isoprenaline (pK_B=10.2±0.2 and 9.6±0.3), zinterol (pK_B=9.0±0.1 and 9.4±0.3) and BRL 37344 (pK_B=9.4±0.3 and 8.4±.2), indicating participation of β₂-ARs. 4. The pharmacological analysis was supported by reverse transcription and polymerase chain reaction analysis of L6 mRNA, which showed high levels of expression of β₂-AR but not β₁- or β₃- AR in these cells. 5. Forskolin and dibutyryl cyclic AMP produced negligible increases in GT while the phosphatidylinositol-3 kinase inhibitor, wortmannin, significantly decreased both insulin- and zinterol-stimulated GT, suggesting a possible interaction between the insulin and β²-AR pathways. 6. This study demonstrates that β₂-ARs mediate the increase in GT in L6 cells to β-AR agonists, including the β₃-AR selective agonist BRL 37344. This effect does not appear to be directly related to increases in cyclic AMP but requires P13K.