Suppression of insulin-stimulated glucose transport in L6 myocytes by calcitonin gene-related peptide.

The binding of calcitonin gene-related peptide (CGRP) to L6 myocytes, the coupling of this receptor to adenylyl cyclase and the resultant effects on insulin-stimulated 2-deoxyglucose uptake were examined. L6 cells express specific binding sites for CGRP. Binding of human [125I]CGRP was inhibited by rat CGRP with an IC50 of approximately 10(-9) M. Synthetic human calcitonin at concentrations up to 10(-6) M had no effect on the binding of CGRP, suggesting that L6 cells express CGRP receptors, rather than calcitonin receptors which are also capable of binding CGRP. The CGRP receptor appeared to be coupled to adenylyl cyclase. Concentrations of CGRP greater than 3 x 10(-9) M increased the cellular content of cAMP. At 3 x 10(-8) M, CGRP increased cAMP 500-fold. CGRP at 10(-10) M and above suppressed the stimulation of 2-deoxyglucose uptake by insulin. Acute incubation of L6 cells with insulin stimulated 2-deoxyglucose uptake 1.6-fold, which was inhibited up to 70% by CGRP. Our results demonstrate that the specific binding of CGRP to L6 cells causes large increase in the cellular content of cAMP - and inhibition of insulin-stimulated 2-deoxyglucose uptake, but the differences in the dose-response curves suggest that the suppression of insulin action by CGRP cannot be solely explained by the increase in cAMP.