Serotonin (5-hydroxytryptamine), a novel regulator of glucose transport in rat skeletal muscle

Abstract

In this study we show that serotonin (5-hydroxytryptamine (5-HT)) causes a rapid stimulation in glucose uptake by ~50% in both L6 myotubes and isolated rat skeletal muscle. This activation is mediated via the 5-HT(2A) receptor, which is expressed in L6, rat, and human skeletal muscle. In L6 cells, expression of the 5-HT(2A) receptor is developmentally regulated based on the finding that receptor abundance increases by over 3-fold during differentiation from myoblasts to myotubes. Stimulation of the 5-HT(2A) receptor using methylserotonin (mHT), a selective 5-HT(2A) agonist, increased muscle glucose uptake in a manner similar to that seen in response to 5-HT. The agonist-mediated stimulation in glucose uptake was attributable to an increase in the plasma membrane content of GLUT1, GLUT3, and GLUT4. The stimulatory effects of 5-HT and m-HT were suppressed in the presence of submicromolar concentrations of ketanserin (a selective 5-HT(2A) antagonist) providing further evidence that the increase in glucose uptake was specifically mediated via the 5-HT(2A) receptor. Treatment of L6 cells with insulin resulted in tyrosine phosphorylation of IRS1, increased cellular production of phosphatidylinositol 3,4,5-phosphate and a 41-fold activation in protein kinase B (PKB/Akt) activity. In contrast, m-HT did not modulate IRS1, phosphoinositide 3-kinase, or PKB activity. The present results indicate that rat and human skeletal muscle both express the 5-HT(2A) receptor and that 5-HT and specific 5-HT(2A) agonists can rapidly stimulate glucose uptake in skeletal muscle by a mechanism which does not depend upon components that participate in the insulin signaling pathway.