We estimated the overall open-loop gain of the rapidly acting arterial pressure control system without surgically or pharmacologically opening the reflex loop in rabbits. Animals anesthetized by intravenous injection of Nembutal (27.5 mg X kg-1 body weight) and heparinized were bled by 2 ml X kg-1 body weight within 1-2 s through a catheter inserted into the aortic arch. Arterial pressure change after the quick mild hemorrhage was monitored via a catheter placed in the right subclavian artery for more than 2 min. The overall open-loop gain of the lumped arterial pressure control system determined from the mean arterial pressure response to the hemorrhage was 7.3. It was almost constant over the range between 12.9 and 22.2 kPa systemic pressure.

The purpose of the reported experiments was to examine whether the vagally mediated baroreflex system (V-system) increases its feedback gain with time after sectioning of the carotid sinus nerves (CSN). In 10 dogs anesthetized with Nembutal, we determined the overall open-loop gain (G) of the rapidly acting arterial pressure control system. G was assessed as (delta API/delta APs)-1, where delta API and delta APs represent the immediate and steady-state decreases in arterial pressure at the aortic arch following a fast step-wise reduction in blood volume. delta API, delta APs and GINTACT in the intact condition (30th min before sectioning of the CSN) were -1.8 +/- 0.1 kPa, -0.20 +/- 0.01 kPa and 8.4 +/- 0.3 (mean +/- SE), respectively. The mean values of delta API, delta APs and G after sectioning of the CSN (Gv), first averaged within individual dogs and then averaged for ten dogs, were -2.53 +/- 0.07 kPa, -1.1 +/- 0.05 kPa, and 1.5 +/- 0.1, respectively. Gv did not change with time over about 4 h after sectioning of the CSN. It is concluded that the V-system cannot augment its ability to restore arterial pressure in compensation for the lost function of the carotid sinus baroreflex system over 4 h after sectioning of the CSN in the anesthetized dog.