Comparison of isoproterenol with BRL37344 in activation of β₃-adrenoceptors to inhibit the uptake of [¹⁴C]deoxy-D-glucose and translocation of glucose transporter (GLUT4) to membrane fraction in rat adipocytes

In an attempt to understand the role of β₃-adrenoceptors in the regulation of glucose uptake, the effect of isoproterenol was compared with BRL37344 in isolated white adipocytes of the rat using [¹⁴C]deoxy-D-glucose as an indicator. In the presence of BRL37344, the specific agonist of β₃-adrenoceptors, spontaneous uptake of [¹⁴C]deoxy-D-glucose (glucose uptake) into adipocytes was markedly attenuated. Similar concentration-dependent inhibition of glucose uptake was also observed in the samples treated with isoproterenol, an agonist for all kinds of β-adrenoceptors. Action of BRL37344 was blocked by propranolol at concentrations sufficient to abolish the activity of isoproterenol. Pindolol reversed BRL37344-induced inhibition more effectively than propranolol. Moreover, unlike the action of isoproterenol, BRL37344 did not modify the insulin-stimulated glucose uptake. Translocation of glucose transporter (GLUT4) from cytosol to membrane stimulated with insulin was reduced by isoproterenol but not by BRL37344. Combination with the findings that isoproterenol prolonged the time for insulin to reach maximum stimulation of glucose uptake, leads to the conclusion that delay of insulin action by isoproterenol can be considered as one of the mechanisms for this inhibition. The results obtained suggest that BRL37344 decreased the spontaneous uptake of glucose via an activation of β₃-adrenoceptors while the insulin stimulated glucose uptake was inhibited by isoproterenol but not by BRL37344.