Enhanced negative chronotropy by inhibitory receptors in transgenic heart overexpressing β2-adrenoceptors

Transgenic (TG) mice overexpressing β₂-adrenoceptors (AR) in the heart have enhanced β-adrenergic activity. Since the degree of β-adrenergic activation influences the negative chronotropic control of heart rate (HR), we studied the inhibitory effect of cholinergic and purinergic stimulation on HR in TG and wild-type (WT) control mice. Bradycardia in response to vagal nerve stimulation and administration of acetylcholine or adenosine was studied in anesthetised animals and perfused hearts. Basal HR was significantly higher in TG than WT mice (P<0.01). Electrical stimulation of vagal nerves (1–32 Hz) induced a Hz-dependent reduction in HR and the response was more pronounced in TG than WT groups (P<0.01). In perfused hearts, HR reduction by acetylcholine (ACh) was more pronounced with EC₅₀ 110-fold lower in TG than WT hearts. Adenosine-induced bradycardia, which was abolished by a P₁ antagonist, was more pronounced in TG hearts. After pre-treatment with pertussis toxin (PT, 100 μg/kg), bradycardia by vagal nerve stimulation or ACh remained unchanged in WT, but markedly inhibited in TG hearts (both P<0.01). Conversely, inhibiting guanylyl cyclase with LY83583 (30 μM) or nitric oxide synthase with l-NMMA (100 μM) attenuated HR reduction by vagal nerve stimulation in WT but not in TG hearts. Immunobloting assay showed similar G_iα2 abundance in TG and WT hearts. Thus, cardiac overexpression of β₂AR with high β-adrenergic activity leads to hypersensitivity of inhibitory receptors controlling HR due to increase in activity of PT-sensitive G-proteins.