Depression of atrioventricular sensitivity in the dog by successive brief bursts of vagal stimulation.

Abstract

When identical, successive single vagal stimulations, separated in time (T) by 2-60 seconds, are applied in the dog, the peak change in atrioventricular conduction time for the second stimulus, deltaAV2, is less than that of the first, deltaAV1, that is, the ratio deltaAV2/deltaAV1 less than 1.0 (the mean minimum deltaAV2/deltaAV1=0.52). It was further determined that at any one given T, the actual value of deltaAV2/deltaAV1 also was a function of the first stimulus amplitude. The same dependence of deltaAV2/deltaAV1 on the first stimulus amplitude was found when the amplitude was varied either by (1) changing the number of vagal fibers stimulated for both the first and second response, but at a constant AD (the delay in the cardiac cycle at which the stimulus was given), or (2) by stimulating the same number of fibers but changing AD for both the first and second response. Also the same characteristic of deltaAV2/deltaAV1 plotted vs. T was found either (1) when both the first and second responses were produced by stimulation of the same nerve, or (2) when stimulated of the nerve from one side produced the first response and the alternate side the second response. These data suggest that the reason for this deltaAV2/deltaAV1 time-dependent effect lies in changes in receptor responsiveness or in acetylcholine inactivation, but not in mechanisms purely internal to the nerve terminal associated with synthesis, storage, or release of acetylcholine.