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Angiotensin was infused into intact, conscious rabbits to produce a continuous, moderate elevation of arterial pressure. The reflex cardiac rate response was studied. Within two hours of initial drop in heart rate, average time 37 minutes for 13 rabbits, the heart rate began to rise. The rates of rise were between 1.2 and 6.0 per cent of the initial drop per minute. Thus, in the presence of elevated arterial pressure, the cardiac reflex mechanism can adjust so that heart rate reaches control values within an average time of one hour in most animals. One additional animal showed no tendency for adjustment in over seven hours of infusion. The time between start of heart rate rise and restoration of 50 per cent of the initial drop was directly related to the sensitivity of the initial reflex response, i.e., heart rate drop per mm. Hg increase in diastolic pressure. Most of the animals developed a postinfusion tachycardia which was directly related to the amount of increase in heart rate attained by the end of the infusion. We conclude that in the presence of continuously elevated arterial pressure, the reflex cardiac slowing mechanism can adjust within a relatively short time, and the evidence that this represents a resetting of the mechanism within the central nervous system is discussed.

Adjustment of Reflex Bradycardia to Elevated Arterial Pressure in Conscious Rabbits