1. A new column-chromatographic method is described for the simple and reproducible determination of the concentration of [des-Asp1] angiotensin II (angiotensin III) in rat plasma.

2. The method uses the different abilities of the ion-exchange resins Dowex 1 (X8) and Bio Rex 70 to bind angiotensin II and angiotensin III. Under the conditions used, Bio Rex 70 binds only angiotensin III. Angiotensin II and its hexapeptide metabolite [des-Asp1,des-Arg2]angiotensin II pass the resin with the effluent. Dowex 1 (X8) binds angiotensin II and the hexapeptide metabolite, whereas it does not extract angiotensin III. It does not separate angiotensin II from the hexapeptide. Therefore the sum of both peptides is expressed as angiotensin II-like activity. The ratio of the concentrations, angiotensin II/hexapeptide, was 5:1.

3. In normal rats the plasma concentration of angiotensin III was 20 fmol/ml (sd 15; n = 8), and angiotensin II-like activity was 60 fmol/ml (sd 35; n = 8).

4. The β-sympathomimetic amine isoprenaline caused a time- and dose-dependent increase in plasma angiotensin III and angiotensin II-like activities.

5. Under the conditions studied angiotensin III contributed approximately 25% to the total amount of angiotensins in plasma.

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