Dynamics of cardiac period responses after prolonged vagal stimulation in the dog

In 12 dogs anesthetised with α-chloralose and urethane, and β-adrenergic blocked with propranolol, the cervical vagi were stimulated for 60 seconds with supramaximal constant current pulses at frequencies between 2.0 and 10 Hz. The time course of the recovery of the cardiac period response, after cessation of vagal stimulation, was analyzed using nonlinear curve fitting techniques. It was found that the recovery phase could be reliably fitted with a function consisting of the sum of up to three exponential terms. The first term has a moderate rate constant of 0.2260±0.0112 S⁻¹ (SE) and is independent of vagal stimulus frequency and the preceding bradycardia. We propose that this term is associated with a recovery from the bradycardia with a rate constant indicative of the reduction of acetylchloline at the pacemaker caused by hydrolysis and washout. The second term has a small rate constant of 0.0499±0.0014 s⁻¹ (SE) and a negative gain. This term describes the time course of the post vagal tachycardia observed at cessation of stimulation. It is also independent of stimulus frequency and the preceding bradycardia. The third term has a large and variable rate constant (range: 0.247–8.01 s⁻¹) and becomes increasingly dominant when the preceding bradycardia is large. We propose that this third component arises from a rapid return of the pacemaker focus to the dominant location prior to vagal stimulation. The mathematical characterisation of this component is important to permit the accurate derivation of the time courses of the remaining two components. Curvilinear relationships exist between the gain parameters for each of the exponential terms and the preceding bradycardia.