Abstract

The primary goal of this study was to determine the extent that selective muscarinic receptor antagonists could discriminate between the chronotropic and coronary vasoconstrictor responses to acetylcholine in isolated rat hearts perfused at constant flow rate. Bolus injections of acetylcholine caused dose-dependent decreases in heart rate and increases in perfusion pressure. The ED₅₀(95% confidence) of acetylcholine for decreasing rate was 0.463 (0.336–0.640) nmol and the dose that increased perfusion pressure by 30 mm Hg (ED_{30 mmHg↑}) was 3.19 (2.00–5.08) nmol. The M₂ selective antagonist methoctramine (3.16 μM) produced a 307-fold increase in the ED₅₀ for bradycardia but had no significant effect on the pressor response to acetylcholine. In marked contrast, the M₃ antagonist hexahydrosiladifenidol displayed a distinct preference for inhibiting coronary vasoconstrictor responses to acetylcholine. When present at 316 nM, this drug produced a 66-fold increase in the ED_{30 mmHg↑} but only a 6-fold increase in the ED₅₀for bradycardia. The M₁ selective antagonist pirenzepine (316 nM) produced a 5- to 7-fold increase in both parameters. Pretreatment with pertussis toxin (25 μg/kg, i.p.) essentially eliminated acetylcholine-evoked bradycardia although pressor responses persisted with some reduction. These observations demonstrate that cardiac and coronary vascular effects of acetylcholine can be clearly discriminated with specific muscarinic antagonists. Furthermore, they provide evidence that the M₃ receptor subtype mediates the vasoconstrictor effect of acetylcholine on resistance vessels in rat heart.