Nonlinear summation of depressor effects of carotid sinus pressure changes and aortic nerve stimulation in the rabbit.

We studied the interaction between the carotid sinus and aortic arch baroreflexes' control of arterial pressure in 13 rabbits anesthetized with urethane. The carotid sinus baroreceptor region was isolated to control carotid sinus pressure. The aortic nerve was transected cervically and stimulated with 4-V electrical pulses at 5 Hz and 0.5-msec duration, which caused a 20-30 mm Hg decrease in arterial pressure. Several protocols were used to analyze the interaction. In the first, we compared the combined effect of the stimulation of a unilateral aortic nerve and a simultaneous increase of bilateral carotid sinus pressures from 40 to 70-80 mm Hg to the sum of these effects measured separately. The combined effect was always larger than the sum of the separate effects. In the second protocol, we varied carotid sinus pressures between 40 and 140 mm Hg and, at each carotid sinus pressure, measured the decrease in arterial pressure caused by unilateral aortic nerve stimulation. The fall in arterial pressure was always greater in the middle range of carotid sinus pressure (70-90 mm Hg) than in the lower or higher range. In the third protocol, we increased the intensity of aortic nerve stimulation from 2 to 4 V while raising carotid sinus pressure from 90 to 110 mm Hg. At this increased tonic input level, the combined effect was always smaller than the sum of the separate effects. Thus, we observed a facilitatory summation when the input variables changed from subthreshold to physiological level, but an inhibitory summation when the input stimulus intensity increased from a physiological to a supraphysiological level. We conclude that interaction between the two reflexes depends in part upon the initial level of the input intensity to the receptors.