Conscious Brs3−/y mice have a reduced response to propranolol and a normal intrinsic heart rate. Heart rate 31 to 45 min after treatment with vehicle (Veh, saline), propranolol (Pro, 5 mg/kg ip), atropine (Atro, 1 mg/kg ip), or propranolol plus atropine (Pro + Atro). WT, wild-type controls; KO, Brs3−/y mice. Data are means ± SE; n = 4–6/group. *P < 0.05, Pro or Atro or Pro + Atro vs. vehicle within genotype; †P < 0.05, WT vs. KO within treatment. Experiment is a crossover design with mice age 19–23 wk.