Impaired cardiac muscarinic receptor function in dogs with heart failure

Prior physiological studies have suggested that parasympathetic control is altered in heart failure. The goal of our studies was to investigate the influence of heart failure on the muscarinic receptor, and its coupling to adenylate cyclase. Ligand binding studies using [3H]quinuclidinyl benzilate and enriched left ventricular (LV) sarcolemma, demonstrated that muscarinic receptor density in heart failure declined 36% from a control of 5.6 +/- 0.6 pmol/mg, with no change in antagonist affinity. However, agonist competition studies with both carbachol and oxotremorine showed that it was a loss of high affinity agonist binding sites in the sarcolemma from failing LV that accounted for this difference. The functional efficacy of the muscarinic receptor was also examined. When 1 microM methacholine was added to 0.1 mM GTP and 0.1 mM isoproterenol, adenylate cyclase stimulated activity was inhibited by 15% in normal LV but only 5% in LV sarcolemma from animals with heart failure even when the reduced adenylate cyclase in these heart failure animals was taken into account. Even at 100-fold greater concentrations of methacholine, significantly less inhibition of adenylate cyclase activity was observed in LV failure as compared with normal LV sarcolemma. Levels of the GTP-inhibitory protein known to couple the muscarinic receptor to adenylate cyclase, as measured with pertussis toxin labeling, were not depressed in LV failure. Thus, the inhibitory pathway regulating LV adenylate cyclase activity is defective in heart failure. The decrease in muscarinic receptor density, and in particular the specific loss of the high affinity agonist binding component of this receptor population, appears to be the major factor underlying this abnormality.