Abstract
The aim of the present functional study was to assess the role of β3-adrenoceptors in the light of recent findings suggesting the existence of a putative fourth β-adrenoceptor in adipose and heart tissue. The effect of the non-conventional β3-adrenoceptor partial agonist CGP12177A is resistant to the effect of the β3-adrenoceptor antagonist SR59230A. Under isotonic conditions in circular muscle strips of human distal colon, the concentration–effect relationship of CGP12177A and SR59104A (β3-adrenoceptor agonists), alone and in the presence of CGP20712A (β1-adrenoceptor antagonist) ICI118551 (β2-adrenoceptor antagonist) and SR59230A, all 0.1 μmwas studied. CGP12177A concentration-dependently relaxed circular muscle strips (pEC50=6.16±0.05). This effect was left unchanged by β1-/β2-adrenoceptor blockade, but antagonised by SR59230A (pA2=8.12±0.02). SR59104A concentration-dependently relaxed circular muscle strips (pEC50=5.43±0.01), an effect that was not significantly affected by pretreatment with CGP20712A and ICI118551, but competitively antagonised by SR59230A (pKB=7.89). Isoprenaline-induced relaxations were antagonised by propranolol with a low pA2value (7.76±0.16). These results provide further evidence for the presence of functional β3-adrenoceptors in the human colon, but do not support a role for an atypical β-adrenoceptor distinct from the β3-subtype.
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