Heart rate response to atropine. Atropine sulfate (2 mg/kg) was injected to block parasympathetic transmission in the heart, resulting in increased heart rate (HR). Wildtype, solid bar, p75-/-, hatched bar. Data are expressed as percent change within each animal, and are mean±sem, n=5-6. Average basal HR: WT 546±6, P75-/- 517±19; average HR after atropine: WT 564±6, P75-/- 539±16.