CIRCULATING LEVELS OF ANGIOTENSIN-I MEASURED BY RADIOIMMUNOASSAY IN HYPERTENSIVE SUBJECTS

Abstract

The development of a sensitive, specific Al [angiotensin] antiserum led to the establishment of a radioimmunoassay capable of detecting 7.5 pg of Al/ml of plasma. Due to its sensitivity this assay permits the measurement of circulating levels of Al. Previously Al assays required either an incubation step at 37.degree. C to allow renin to catalyze the formation of sufficient Al or an extraction procedure to concentrate sufficient peptide to make measurements feasible. The sensitivity of this assay depends upon the availability of very pure trace, a method is described for preparing monoiodinated 125I-Al of specific activity > 1000 .mu.Ci/.mu.g. To demonstrate the versatility and sensitivity of this assay, changes in circulating Al levels in response to physiologic stimuli were measured. Blood samples were obtained from the inferior vena cava below the renal veins of 88 subjects in the supine and upright positions. Values ranged from 12-1990 pg/ml of plasma. In 85 of the 88 subjects the Al level rose during an upright tilt, the mean for the group increasing from 220-385 pg/ml of plasma. In 3 subjects the samples were drawn simultaneously from the inferior vena cava and a peripheral artery and/or vein. The amounts of Al in all 3 sampling locations were essentially the same. Essential hypertension patients (11) underwent an infusion of 1.5 l of normal saline, and circulating Al levels were determined before and 120 and 150 min after the start of the infusion. Al levels were suppressed in the 17 patients the mean for the group at 0, 120 and 150 min being 177, 55 and 50 pg/ml of plasma, respectively. Circulating Al correlated well (r [correlation coefficient] = 0.87009) with plasma renin activity in 226 samples from the renal veins and inferior vena cava from individuals with hypertension of various etiologies.