Muscarinic acetylcholine receptors M(2) are upregulated in the atrioventricular nodal tract in horses with a high burden of second-degree atrioventricular block

BACKGROUND: Second-degree atrioventricular (AV) block at rest is very common in horses. The underlying molecular mechanisms are unexplored, but commonly attributed to high vagal tone. AIM: To assess whether AV block in horses is due to altered expression of the effectors of vagal signalling in the AV node, with specific emphasis on the muscarinic acetylcholine receptor (M(2)) and the G protein-gated inwardly rectifying K(+) (GIRK4) channel that mediates the cardiac I(K,ACh) current. METHOD: Eighteen horses with a low burden of second-degree AV block (median 8 block per 20 h, IQR: 32 per 20 h) were assigned to the control group, while 17 horses with a high burden of second-degree AV block (median: 408 block per 20 h, IQR: 1,436 per 20 h) were assigned to the AV block group. Radiotelemetry ECG recordings were performed to assess PR interval and incidence of second-degree AV block episodes at baseline and on pharmacological blockade of the autonomic nervous system (ANS). Wenckebach cycle length was measured by intracardiac pacing (n = 16). Furthermore, the expression levels of the M(2) receptor and the GIRK4 subunit of the I(KACh) channel were quantified in biopsies from the right atrium, the AV node and right ventricle using immunohistochemistry and machine learning-based automated segmentation analysis (n = 9 + 9). RESULTS: The AV block group had a significantly longer PR interval (mean ± SD, 0.40 ± 0.05 s; p < 0.001) and a longer Wenckebach cycle length (mean ± SD, 995 ± 86 ms; p = 0.007) at baseline. After blocking the ANS, all second-degree AV block episodes were abolished, and the difference in PR interval disappered (p = 0.80). The AV block group had significantly higher expression of the M(2) receptor (p = 0.02), but not the GIRK4 (p = 0.25) in the AV node compared to the control group. Both M(2) and GIRK4 were highly expressed in the AV node and less expressed in the atria and the ventricles. CONCLUSION: Here, we demonstrate the involvement of the m(2)R-I(K,ACh) pathway in underlying second-degree AV block in horses. The high expression level of the M(2) receptor may be responsible for the high burden of second-degree AV blocks seen in some horses.