Circulatory responses to onset of exercise: role of arterial and cardiac baroreflexes.

Six rabbits were exercised on a moving belt at 13 m/min for 60's. Heart rate (HR), mean arterial pressure (MAP), cardiac index (CI), and systemic vascular resistance index (SVRI) were measured. Exercise was done under the following four permutations of input from baroreceptors (B) and cardiac receptors (C): BC, both inputs present; B, only baroreceptor input (intrapericardial procaine); C, only cardiac receptor input (surgical barodenervation); 0, both inputs deleted. The reflex effects on SVRI of the two inputs were calculated as (B - 0) and (C - 0) and their interaction as (BC - 0) - [(B - 0) + (C - 0)]. The effects of baroreceptor input plus interaction on all cardiovascular variables were also calculated, as (BC - C). At rest, (B - 0) and (C - 0) each tonically depressed SVRI without interacting, and (BC - C) tonically depressed SVRI, MAP, and HR. Within 10 s of the start of exercise these tonic effects were abolished, although a small, SVRI-lowering interaction appeared. Suppression of the tonic reflex effects of arterial baroreceptor and cardiac receptor input supported systemic vascular resistance at the onset of exercise and contributed to the rise of arterial pressure.