Rapid resetting of aortic nerves in conscious rabbits.

The effect of increased arterial pressure on aortic depressor nerve activity was studied in the conscious rabbit. Aortic baroreceptor resetting was observed following 15 min of sustained pressure elevation. At 15 min, there was a significant increase in the threshold arterial pressure for aortic nerve activity, but peak nerve activity did not change. This resulted in an increase in the slope of the pressure-nerve activity relationship. Therefore, except for peak nerve activity, aortic nerve activity was reduced at all pressures following 15 min of sustained pressure elevation. At 30 min, peak nerve activity also increased, resulting in a parallel shift in the pressure-activity curve. The early increase in slope of the pressure-activity relationship may be due to an early resetting of low threshold fibers with a high pressure differential between their threshold pressure and the sustained pressure elevation used to induce resetting. A late resetting of high threshold fibers with a low pressure differential between their threshold pressures and the pressure used to cause resetting of baroreceptors results in a parallel shift in the curve observed at 30 min. These data obtained from multiunit recordings provide unexpected evidence about baroreceptor resetting, which is not apparent from single-unit recordings.