Ergotamine contracted isolated rat aorta rings with an intrinsic activity of 50% of that of 5-hydroxytryptamine. (5-HT, 0.1 mmol/l). Dihydroergotamine did not contract the tissue, but insurmountably blocked contraction in response to ergotamine and 5-HT. The 5-HT2A receptor antagonist, ketanserin (0.1 mumol/l), inhibited ergotamine (pKB 8.0) and 5-HT (pKB 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 cardiovascular and uterotonic side effects than dihydroergotamine.
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Kalkman et al. (2008) studied this question.
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