EFFECTS OF ERGOTAMINE AND DIHYDROERGOTAMINE ON 5-HYDROXYTRYPTAMINE-2ARECEPTORS IN THE ISOLATED RAT AORTA

Ergotamine contracted isolated rat aorta rings with an intrinsic activ ity of 50% of that of 5-hydroxytryptamine (5-HT, 0.1 mmol/l). Dihydroe rgotamine did not contract the tissue, but insurmountably blocked cont raction in response to ergotamine and 5-HT. The 5-HT2A receptor antago nist, ketanserin (0.1 mu mol/l), inhibited ergotamine (pK(B) 8.0) and 5-HT (pK(B) 8.1) induced contractions. These results indicate that in the rat aorta ergotamine is a partial 5-HT2A receptor agonist, whilst dihydroergotamine is an insurmountable 5-HT2A receptor antagonist. The present data could explain why ergotamine displays more cardiovascula r and uterotonic side effects than dihydroergotamine.