Background

Angiotensin II is the biologically active product of the renin-angiotensin system.

The octapeptide angiotensin II (molecular weight 1046) is the strongest physiological vasoconstrictor known. From a large protein precursor (pre-proangiontesinogen) synthesized in the liver it is liberated in a series of proteolytic steps catalyzed by enzymes from various tissues. Angiotensin II is very short-lived in the plasma: Once generated from angiotensin I, it is degraded further into physiologically inactive peptides by various plasma peptidases, at a plasma half life of less than a minute.

Since the generation of angiotensin II from angiotensinogen via angiotensin I is strongly affected by changes of the renin activity, all external factors influencing renin activity are to be carefully considered: renin activity is elevated during pregnancy, after sodium depletion, in upright position, and under the influence of a range of drugs, e.g. oral contraceptives, adrenalin, antihypertensive vasodilatators, diuretics, high doses of spironalactone and progesterone. Factors decreasing renin activity are: horizontal position, increased sodium uptake, a-methyl-DOPA, L-DOPA, propranolol, reserpin, clonidin and old age. Renin activity is also subject to a diurnal rhythm with peak values in the morning.

The angiotensin II radioimmunoassay is useful in the study of hypertension monitoring and treatment.

Technical information

After extraction of the plasma samples, angiotensin II is assayed by a competitive radioimmunoassay. This radioimmunoassay is using a rabbit anti-angiotensin II antiserum and a radio-iodinated angiotensin II tracer. Bound and free phases are separated by a second antibody bound to solid phase particles, followed by a centrifugation step. The radioactivity in the bound fractions is measured and a typical standard curve can be generated.