Atrial inotropic responses to brief vagal stimuli: frequency-force interactions.

The effect of brief vagal stimulus burst on atrial contractile force was assessed in paced- and unpaced-heart preparations by recording the changes in pressure (AP) generated in a balloon in the right atrial appendage or by a miniature strain gauge on the atrium. One and 3 stimuli per vagal burst depressed AP up to 62 and 90%, respectively. The response varied with the time in the cardiac cycle at which the stimulus was given. Identical stimuli produced a greater inotropic depression at longer cardiac cycle durations. The differences between the magnitudes of the AP responses at two different mean heart rates were relatively constant when the stimulus intensity was increased from 1 to 99 stimuli per burst. A low-level stimulus caused a greater depression in contractile force when the heart rate was allowed to change than with the identical stimulus when the heart was paced, although this difference in paced- and unpaced-heart responses tended to disappear when the stimulus intensity (burst width) was increased. The composite frequency-force relationship was relatively flat over normal heart periods, but fell off by 15-20% at the extremes. The present results show an unexpected interaction between the vagal inotropic effect and the frequency-force relationship such that a given vagal stimulus causes a greater depression of atrial contractile force at lower heart rates.