Key result
Benextramine noncompetitively antagonized the inhibitory effect of clonidine on high-voltage stimulation in isolated guinea-pig left atria, an effect competitively protected by phentolamine.
Benextramine acts as a noncompetitive antagonist at presynaptic alpha-adrenoceptors in the guinea-pig heart, an effect that can be protected by the competitive antagonist phentolamine.
No immediate clinical implications from guinea-pig data; extends evidence for noncompetitive antagonism at cardiac presynaptic alpha-adrenoceptors.
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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Belleau et al. (1982) studied this question. Benextramine vs. Phentolamine was evaluated on Force of contraction of electrically driven atrial strips. Benextramine noncompetitively antagonized the inhibitory effect of clonidine on high-voltage stimulation in isolated guinea-pig left atria, an effect competitively protected by phentolamine.
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