Interaction of brief sympathetic stimuli and heart period on atrial contractile force

We studied the dynamic interaction between the independent effects of sympathetic stimuli and heart period on atrial contractile force with three experiments: 1) we measured the inotropic response to brief sympathetic stimuli when the heart was paced; this represented the pure sympathetic effect on contractile force. 2) We measured the responses to identical stimuli when the heart was not paced; this represented the effect of the combined influences. 3) We measured the contractile responses to the identical sequences of changing cycle lengths that were recorded (expt 2) but in the absence of sympathetic activity; this represented the pure effect of changing cardiac cycle length on contractile force. We subtracted the paced from the unpaced responses. This difference represented the linear elimination of the pure sympathetic effect from the combined responses. We then compared this difference with the responses to changing cycle lengths that were measured in the absence of sympathetic activity (expt 3). The values were not equal from 2.5 through 22.5 s after the stimulus of experiment 1 of the protocol above. We performed a similar comparison after expressing the results in the frequency domain. The values were significantly different for most of the frequency spectrum where the greatest power was concentrated. We therefore conclude that there is a significant interaction between the effects of sympathetic activity and changing heart periods, and this interaction determines the resultant inotropic state on any specific beat.