Open‐Loop Analysis of the Renin‐Angiotensin System in the Dog

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Abstract
The loop gain and time response of the renin-angiotensin system was determined in dogs in which the cardiovascular control loops of the central nervous system were eliminated by spinal cord destruction and decapitation. Step decreases in renal artery perfusion pressure were introduced for a 30-minute period, and changes were measured in systemic arterial blood pressure, cardiac output, and renin activity. The calculated loop gain of both kidneys was −1.59 ± 0.11 (SE) and for a single kidney was −0.92±0.02 between renal perfusion pressures of 65 to 100 mm Hg. This indicates nearly a 65% and 50% compensation in pressure, respectively. Below renal perfusion pressures of 65 mm Hg, the gain decreased. The system exhibits a relatively rapid time course with maximum steady-state systemic pressure elevations occurring in an average of 20 minutes, and returning to control levels in an average of 17.8 minutes for bilateral constrictions and 31.9 minutes for single artery constrictions with contralateral nephrectomy. Cardiac outputs were not significantly changed during the constriction period. The arterial renin activity and renal renin secretion rates were consistently elevated during the constrictions. Systemic pressure changes were inhibited by infusion of angiotensin II before constriction and by the injection of angiotensin II antiserum. These results demonstrate that the systemic pressure changes were the result of the renin-angiotensin vasoconstrictor system. The results indicate that the renin-angiotensin system possesses sufficient time response and gain characteristics to participate significantly in the normal regulation of arterial pressure.

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