Differential coupling of m-cholinoceptors to Gi/Go-proteins in failing human myocardium

Muscarinic acetylcholine receptors (mAChRs) mediate their main cardiac effects via pertussis toxin-sensitive G-proteins. Physiological effects differ considerably between atrium and ventricle, and it is unknown to which extent these differences derive from selective receptor–G-protein coupling or further downstream events. We have characterized specific coupling between mAChRs and Gi/Go-protein isoforms in atrial and ventricular myocardium by agonist-dependent photoaffinity labeling with [32P]azidoanilido GTP (aaGTP) and immunoprecipitation in sarcolemmal membranes from terminally failing human hearts. The total amount of mAChRs, as determined by specific binding of [3H]QNB, was significantly higher in right-atrial (RA ± SEM, 959 ± 68 fmol/mg, n = 4) than in left-ventricular membranes (LV, 582 ± 53 fmol/mg, n = 6). Standardized immunoblots revealed that Giα-2 was the predominant subtype in both regions. A 40-kDa splice variant of Goα (Goα-1 and/or Goα-3) was almost exclusively detectable in RA. Levels of Giα-3 and a 39-kDa splice variant of Goα (Goα-2) were also higher in RA. Basal aaGTP binding was higher in RA than in LV for all Giα/Goα subtypes. The carbachol (10 μmol/l)-induced increase in aaGTP binding was significantly higher in RA than in LV for Goα-1/3 (336 ± 95% of LV, n = 4) and for Giα-3 (211 ± 83%), lower for Giα-2 (42 ± 5%), and was similar in both regions for Goα-2 (130 ± 62%). The differential coupling of mAChRs in human RA and LV suggests that the initiation of different physiological responses to mAChR stimulation starts with signal sorting at the receptor–G-protein level.