Fade of the responses of the isolated, blood-perfused dog atrium to cholinergic interventions

In the isolated, blood-perfused, canine right atrium, intramural parasympathetic nerve stimulation and intra-arterial infusions of acetylcholine induced substantial negative chronotropic and inotropic responses. The responses to parasympathetic stimulation reached their maximum values quickly, and then usually faded back toward control levels over the next 1 or 2 min of stimulation. The fade of the responses at high stimulation frequencies (greater than or equal to 30 Hz) was significantly greater than that at lower frequencies. The inotropic responses to acetylcholine infusion (1 microgram/min) faded slightly but significantly, whereas the chronotropic responses did not fade at all. These results suggest that the fade of the cardiac responses to parasympathetic stimulation is mainly ascribable to a progressive reduction in the rate of acetylcholine release from the nerve endings, especially at higher stimulation frequencies. The fade of the inotropic responses was more pronounced and had a longer time course than that of the chronotropic responses. Furthermore, the fade of the inotropic responses diminished significantly as the response magnitude was augmented by an increase in stimulation voltage. Conversely, the fade of chronotropic responses was not significantly affected by this intervention. These differences in the inotropic and chronotropic responses to neural stimulation, and the occurrence of a slight fade of the inotropic response to acetylcholine infusion, suggest that in addition to the predominant prejunctional mechanism, a postjunctional phenomenon may also be partly responsible for the fade of the inotropic response to cholinergic interventions.