Continuous measurement of aortic caliber in conscious rats. Effect of acute hypertension.

For continuously measuring the circumference of the aorta in conscious rats for up to 20 days, a new electrolytic strain gauge of high sensitivity and stability was constructed of silicone tubing filled with copper nitrate. The mean systolic and diastolic circumferences measured in 11 nonanesthetized undisturbed rats were 6.557 +/- 0.128 mm and 6.533 +/- 0.128 mm, respectively--the pulse pressure (51 mm Hg) producing an increase of 0.024 mm in aortic size (0.37% increase of the diastolic circumference). The calculated dynamic elastic modulus was 13,908 dynes/cm2. Infusions of blood, angiotensin, and noradrenaline to increase mean aortic pressure acutely by 50 mm Hg caused aortic circumference to increase by 0.59, 0.58, and 0.53%, respectively. Seven rats were subjected to acute hypertension produced by subdiaphragmatic aortic constriction after recording the control measurements. Over the period of study (6 hours) after aortic constriction, mean aortic blood pressure was increased 50 mm Hg from the control of 101 mm Hg. A mean maximal increase of 6% in aortic circumference was seen at 3 hours and a mean minimum of 0.9% at 4 hours, with an average increase of 3% for the entire 6-hour period. These changes in aortic circumference coincide with an upward displacement of about 30% in the range of activation of the aortic baroreceptors.