Muscarinic cholinergic receptors modulate vagal cardiac responses in man

Respiratory sinus arrhythmia (peak-valley P-P interval changes during controlled breathing) and carotid baroreceptor-cardiac reflex responses (provoked by neck suction) were studied before and after low (0.725 microgram/kg) or high (0.043 mg/kg) dose atropine sulfate infusions in 22 healthy young adults. There was a highly significant (r = 0.78, P = 0.002) correlation between resting respiratory sinus arrhythmia and baroreflex responses. Low dose atropine increased the magnitude of sinus arrhythmia and baroreflex responses. Large dose atropine reduced sinus arrhythmia and baroreflex responses in an exponential fashion. Our results link respiratory sinus arrhythmia with baroreflex responsiveness and provide inferential evidence that vagal cardiac efferent activity is modulated by inhibitory cholinergic receptors. We speculate that blockade of these receptors by low doses of atropine amplifies vagal motoneuron responses to incoming baroreceptor information, and thereby increases average maximum and minimum levels of vagal cardiac outflow during breathing.