The reflex bradycardia during brain ischemia in the rabbit.

We examined the responses of whole aortic nerve activity, aortic baroreceptor activity, heart rate (HR), and arterial pressure (AP) to brain ischemia sustained for approximately 30 s in anesthetized spontaneously breathing rabbits. The minimum values of HR observed during brain ischemia were 76 +/- 11 beats/min (mean +/- S.E., n = 14) before sectioning the bilateral aortic nerve (BAN), and 161 +/- 12 beats/min after sectioning the left aortic nerve (LAN), and 225 +/- 11 beats/min after sectioning the LAN and right aortic nerve (RAN). Averages for reflex fall in HR during BAN, LAN, and RAN activation were 140 +/- 9, 78 +/- 7, and 62 +/- 7 beats/min, respectively, by subtracting the HR fall responses to brain ischemia in the absence of aortic baroreceptor afferents from their control values. The heights of the integrated whole left and right aortic nerve activities in systole slightly increased during brain ischemia, whereas the brain ischemia remarkably increased those activities during the diastolic phases. The brain ischemia induced a hysteresis in the mean AP-aortic barorecept or activity relationship. These results suggest that the total activity of aortic nerve fibers would determine the bradycardia evoked by brain ischemia and that the difference between the relative contributions of LAN and RAN on the brain ischemia-induced reflex bradycardia would reflect the total impulse frequency of aortic myelinated and non-myelinated fibers.