Muscarinic receptor subtypes, β-adrenoceptors and cAMP in the tracheal smooth muscle of conventional and double-muscled calves

The total muscarinic (M₁ + M₂ + M₃) and -adrenergic receptors in the tracheal smooth muscle of conventional and double-muscled calves were identified and characterized with the non-specific antagonists [³H]quinuclidinyl benzilate ([³H]QNB) and [³H]dihydroalprenolol ([³H]DHA) respectively.Although the quantity of -adrenoceptors in double-muscled calves was 25% lower (p<0.05) than in conventional calves (B _max=327±89 fmol/mg protein), adenylate cyclase assays indicated that the basal adenylate cyclase activity and the (–)-isopropylnoradrenaline (ISO)- and sodium fluoride (NaF)-stimulated values were not significantly different between these calves. However, the density of muscarinic receptors in double-muscled calves was 40% higher (p<0.01) than in conventional calves (B _max=2955±625 fmol/mg protein). Subtypes of muscarinic receptors were studied with [³H]telenzepine (M₁-receptors), [³H]AF-DX 384 (M₂-receptors) and [³H]4DAMP (M₁ and M₃-receptors). It was found that in both double-muscled and conventional calves about 40% of the receptors were of the M₃-subtype, the remaining 60% being M₂-receptors.From these results, it is suggested that inflammation of the respiratory tract in double-muscled calves may be complicated by an imbalance between the cholinergic bronchoconstrictor and the -adrenergic bronchodilator components of the autonomic nervous system.