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  • Title: Effects of thymoxamine and its metabolites on clonidine-induced hypotension and bradycardia in rats.
    Author: Roquebert J, Demichel P, Malek A.
    Journal: Arch Int Pharmacodyn Ther; 1982 Jul; 258(1):116-27. PubMed ID: 6215903.
    Abstract:
    Thymoxamine and deacetylthymoxamine antagonized the hypertensive response of pithed rats to noradrenaline and clonidine, probably by inhibiting vascular post-synaptic alpha-adrenoceptors. N-demethyl-deacetylthymoxamine was inactive. Clonidine-induced reduction of tachycardia caused by electrical stimulation of the cardiac sympathetic nerve in pithed rats was antagonized by phentolamine but not by prazosin, thymoxamine, deacetylthymoxamine or N-demetyldeacetylthymoxamine. This suggests that prazosin and thymoxamine and its derivatives predominantly blocked alpha 1-adrenoceptors. In pentobarbitone-anaesthetized normotensive rats, prazosin, thymoxamine and deacetylthymoxamine decreased the hypotensive effect of clonidine, but had no significant effect on clonidine-induced bradycardia. These substances antagonized clonidine-induced hypotension in rats by the probable mechanism of inhibiting central alpha 1-adrenoceptors whose characteristics are similar to peripheral receptors. These data suggest that clonidine produced a hypotensive action in pentobarbitone anaesthetized rats via alpha 1-adrenoceptors and produced bradycardia via alpha 2-adrenoceptors.
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