1. Angiotensin II together with any immunoreactive fragments of angiotensin were measured in arterial and venous plasma before and during infusion of angiotensin II amide in five normotensive subjects, both in sodium-replete and sodium-depleted states. Because the antibody used in the radioimmunoassay cross-reacted with larger angiotensin breakdown products and because these fragments are present in higher concentrations in venous than arterial plasma, samples of both were assayed. Measurements on venous plasma are referred to as immunoreactive material (IM).

2. Arterial plasma angiotensin II concentration (PAC) and venous IM were compared in two additional subjects during infusion of angiotensin II amide and of synthetic human angiotensin II.

3. Arterial PAC and venous IM increased in linear relation to the dose of angiotensin infused, similar results being obtained with angiotensin II amide and synthetic human angiotensin II.

4. Before infusion, venous IM and arterial PAC were comparable (A:V ratio = 0·99 ± 0·16, n = 29). During infusion venous IM was consistently lower than arterial PAC; with angiotensin II amide the A:V ratio was 1·72 ± 0·49 (n = 35), and with synthetic human angiotensin it was 1·63 ± 0·2 (n = 6).

5. Results of an in vitro study suggest that the formation of new angiotensin in the vein is not sufficient to explain the lower A:V ratio of PAC before infusion.

6. It is proposed that arterial PAC rather than venous IM should be measured when the relationship between blood levels and biological effects of the peptide are the object of the study, particularly during angiotensin infusion or when rapid changes in circulating angiotensin occur.

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