Muscarinic acetylcholine receptors in congestive heart failure : a receptor binding study

Abstract

The parasympathetic regulation of the heart may change in diseased states like congestive heart failure (CHF). In a functional study, a carbachol induced increased inotropic effect of muscarinic acetylcholine receptors (mAChRs) has been demonstrated in rats with CHF. The present study was undertaken to look for possible changes in mAChRs in CHF compared to normal cardiac ventricular membranes in rats.

Congestive heart failure was induced in male Wistar rats by coronary artery ligation. Corresponding Sham groups were prepared and served as control group. After 6 weeks, the ventricles from non-failing and failing hearts were isolated. In addition ventricles from normal, non-operated rats were isolated. Receptor binding experiments were performed in the corresponding myocardium.

No significant changes in the affinity of the radioligand, [3H]QNB, for the mAChRs were observed in the ventricles from CHF rats compared to Sham rats (Kd value of 0.15 ± 0.04 nmol/L in CHF rats and 0.18 ± 0.06 in Sham rats). A tendency of increase in the mAChR density in CHF rats compared to Sham rats was obtained, but the increase was not significant (Bmax of 175 ± 18 fmol/mg protein in CHF rats and 139 ± 11 fmol/mg protein in Sham). mAChRs possessed high- and low affinity binding sites for the non-selective agonist, carbachol, in the absence of guanosine trisphosphate (GTP). There were no significant differences in the affinity between normal, Sham and CHF rats. In the presence of GTP, a shift in affinity to a higher concentration of agonist was observed in normal, Sham and to a lesser extent in CHF rats. This indicated a lower sensitivity to guanine nucleotides of mAChRs in CHF rats. The mAChR subtype profile was characterised by using the subtype selective antagonists nitrocaramiphen, AF-DX 116, 4-DAMP and tropicamide. No significant changes in affinity of the chosen antagonists were observed in CHF rats compared to control group, and the receptor binding profile corresponded best to M2 mAChRs. However, other mAChR subtypes and changes in the mAChR profile in CHF rats may be present and can not be excluded because of the lack of highly selective antagonists for mAChR subtypes.