Radioimmunoassay of Angiotensin II in Peripheral Venous Plasma: Effects of Diet and Posture and Application to Measurement of Renal Venous Renin Ratio*

A radioimmunoassay specific for angiotensin II was used to determine the activity of the renin-angiotensin system in peripheral venous plasma of normal subjects. The range for 18 upright normal subjects on uncontrolled diet was 5–46 pg/ml (mean 23± 11 SD). In two subjects measurements were made twice daily, at 8.00 a.m. following overnight recumbency and at 11.00 a.m. following three hours of upright posture, before and during seven days of dietary sodium restriction to 10 mEq per day. Levels rose progressively over the first five days and then fluctuated, possibly indicating achievement of sodium balance. The upright values were consistently higher than the recumbent values. These changes resemble those previously described for renin and aldosterone under the same conditions. In the assessment of hypertensive patients for possible renovascular hypertension, excellent correlation was found between renal venous renin activity ratios measured by bioassay and by radioimmunoassay of angiotensin generated in vitro. In this particular situation, with identical treatment of samples collected simultaneously, measurement of the smaller component (angiotensin II) appears to be as useful as measurement of the larger component (angiotensin I).