Expression of muscarinic acetylcholine receptors in turkey cardiac chambers

The aim of the present study was to characterize the specific binding sites for [N-methyl-³H]-scopolamine ([³H]-NMS), a radioligand for labeling muscarinic acetylcholine receptors (mAChRs), in membranes of four heart chambers obtained from adult male British United Turkey (BUT) Big 6 ("meat-type") and Cröllwitzer ("wild-type") turkeys. MAChR subtypes were examined by inhibiting [³H]-NMS binding with subtype selective non-labelled receptor antagonists. In all left and right atria as well as left and right ventricles of both turkey breeds, the specific [³H]-NMS binding was saturable, reversible and of high affinity (K_D range: 0.5-1.0 nM). The maximum receptor density (B_max) was not significantly different between the four cardiac chambers of BUT Big 6 turkeys, but a significant difference was found between atria and ventricles of Cröllwitzer turkeys. Moreover, significant lower B_max was found in the atria of Cröllwitzer turkeys than in the atria of BUT Big 6, while the ventricular B_max was significantly higher. In all cardiac chambers, unlabeled mAChR antagonists competed for specific [³H]-NMS binding sites in a concentration-dependent manner, suggesting the presence of the M₃ and M₂ receptor subtypes, whereby the latter was the predominant subtype. The presence of the M₁ subtype could not be excluded. In conclusion, there was a difference between BUT Big 6 ("meat-type") and Cröllwitzer ("wild-type") turkeys with regard to receptor density in heart chambers with dominant M₂ and M₃ receptor subtypes.