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Belleau B, Benfey BG, Melchiorre C: Presynaptic effect of clonidine antagonized by the tetramine disulphide, benextramine. Br J Pharmacol. 1982 Apr;75(4):617-21.
1 The presynaptic alpha-adrenoceptor blocking activity of the newly synthesized alpha-adrenoceptor blocking drug, benextramine, was evaluated in the isolated left atrium of the guinea-pig heart. 2 High-voltage stimulation increased the force of contraction of electrically driven atrial strips, presumably by releasing noradrenaline from sympathetic nerve endings. Like phentolamine, benextramine increased the effect of high-voltage stimulation, presumably by blocking presynaptic alpha-adrenoceptors. 3 Clonidine reduced the effect of high-voltage stimulation, presumably by stimulating presynaptic alpha-adrenoceptors. The inhibitory effect of clonidine was antagonized noncompetitively by benextramine and competitively by phentolamine. 4 Combined administration of benextramine and phentolamine only resulted in the competitive phentolamine antagonism. Thus phentolamine protected the presynaptic alpha-adrenoceptors against benextramine blockade. |
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