Opposing central and peripheral effects of atropine on parasympathetic cardiac control.

Cardiac vagal efferent (CVE) activity was recorded from fine strands of the cervical vagus of chloralose-urethane-anesthetized dogs weighing an average of 14.6 kg. In spontaneously breathing animals atropine sulfate in doses of 0.003-1.5 mg significantly increased CVE activity even when the activity was corrected for changes in blood pressure. A 50% average increase (P less than 0.001) in mean activity was observed at a dose of 0.15 mg. The increase was not abolished by vagotomy as long as the animals were allowed to breathe spontaneously. The peripheral effect of atropine was characterized by the relationship between CVE activity and measured heart period changes. A 50% peripheral blockade was achieved at a dose of 0.06 mg; a dose of 1.0 mg produced essentially complete (greater than 90%) blockade. The results quantitatively demonstrate that atropine exerts a powerful central stimulating effect on CVE activity while simultaneously blocking vagal heart rate effects at the periphery.