Characterization of pigment aggregating α2‐adrenoceptors of fish melanophores by use of different agonists after partial irreversible receptor inactivation

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Abstract
1 The affinity for, and the intrinsic efficacy on, postsynaptic melanosome aggregating α2-adrenoceptors of fish melanophores was studied for B-HT 920, clonidine, medetomidine, noradrenaline, phenylephrine and UK-14,304. Investigations were carried out by evaluating the effects of progressive, irreversible inactivation of the α2-adrenoceptors by benextramine. 2 The double reciprocal plots of equieffective concentrations of B-HT 920, clonidine, noradrenaline and phenylephrine were linear, which indicated that these compounds exerted their effects, mainly, through interaction with one receptor site. 3 The affinity for the α2-adrenoceptor selective agonist B-HT 920, was found to be about 1000 times higher than the affinity for the α1-adrenoceptor selective agonist phenylephrine. 4 The corresponding plot of equieffective concentrations of medetomidine was not linear, which may indicate that this imidazole compound exerted its effect through more than one receptor site. However, when phenoxybenzamine was used in place of the more selective benextramine, a linear relationship was obtained.

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