Heart muscarinic receptors (MR) and beta-adrenoceptors (BAR) belong to a large family of G-protein-coupled receptors. Although the role of catecholamines in the stress has been under keen investigation for many years, the effects of immobilization on this pair of receptors, considering their almost completely opposite actions in the heart, are not yet known. We have studied the effects of short-term immobilization (for 120 min) with different times of decapitation after the end of the immobilization period (0, 3, and 24 h) on MR, BAR (beta(1)-AR and beta(2)-AR using radioligand binding studies), and adenylyl cyclase (AC; using high-pressure liquid chromatography detection of cAMP) in different heart regions (left and right atria with or without cardiac ganglion cells [auriculae], septum, and left and right ventricles). The effects of one immobilization period were first apparent after 24 h. Stress brought about a downregulation of MR and BAR with decrease in AC activity. These effects were regionally specific and were predominantly expressed in the right atria, which is rich in ganglia cells, and in the right ventricles. Our results indicate that stressful stimuli can influence not only BAR, but MR, and that AC activity can also be affected. This finding is in good agreement with our previous hypothesis that parallel changes are possible in the number of this pair of receptors on cell membranes.

Institute of Physiology 1st Faculty of Medicine Charles University Albertov 5 128 00 Prague Czech Republic

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Multitargeting nature of muscarinic orthosteric agonists and antagonists

Beta3 adrenoceptors substitute the role of M(2) muscarinic receptor in coping with cold stress in the heart: evidence from M(2)KO mice

The detection of the non-M2 muscarinic receptor subtype in the rat heart atria and ventricles