Immunochemical analysis of rabbit antibodies against angiotensin II

Blood clearance rates of angiotensin II and its metabolites in sheep: presence of immunoreactive fragments in arterial blood

1. The blood clearance rates of exogenous (des-1-Asp)-angiotensin II and (des-1-Asp-2-Arg)-angiotensin II calculated from the arterial blood level during steady state infusion in conscious sodium-replete sheep were found to be 125 litre/h (s.d. = 27, n = 18) and 188 litre/h (s.d. = 23, n = 18), respectively. These blood clearance rates were in the same order as values for angiotensin II previously reported. 2. Using descending paper chromatography, immunoreactive fragments of angiotensin II in arterial blood extracts were adequately separated from angiotensin II. Quantitative determination of angiotensin II by direct immunoassay on whole blood extract using recovery tracer to correct for losses during extraction were on the average 31% (n = 9) higher than those obtained after the same blood extract had been subjected to chromatographic separation. 3. The blood clearance rates of exogenous 1-Asp-5-Ile-angiotensin II calculated from infusion rate/blood angiotensin II concentration before and after chromatographic separation were significantly different, being 103 litre/h (s.d. = 16, n = 18) and 134 litre/h (s.d. = 25, n = 9), respectively (P less than 0.001;t-test, 16 d.f.). 4. Our data demonstrate the presence of immunoreactive fragments of angiotensin II in arterial blood of sheep during steady-state infusion and indicate that direct immunoassay on arterial blood extracts without adequate separation may result in artificially higher values of angiotensin II in blood and consequently false value for its clearance rate.

Angiotensin II and its heptapeptide (2-8), hexapeptide (3-8), and pentapeptide (4-8) metabolites in arterial and venous blood of man

We made separate measurements of angiotensin II (A II) and of its immunoreactive metabolites (2-8 heptapeptide, 3-8 hexapeptide, and 4-8 pentapeptide) in arterial and venous plasma from subjects with widely different plasma levels of the peptides. A II and its three metabolites were extracted from blood, separated by paper chromatography, and measured by radioimmunoassay using an A II antiserum which had a 100% cross-reaction with each metabolite. In contrast to results of previous studies, A II was found to predominate in both arterial (60-100%) and venous (55-100%) blood. The biologically active 2-8 heptapeptide metabolite accounted for only 10% of the activity in arterial plasma. Radioimmunoassay of venous plasma extracts using an A II antiserum which had a low cross-reaction with the 3-8 hexapeptide and the 4-8 pentapeptide confirmed the results obtained using the antiserum which had a 100% cross-reaction with the metabolites. We conclude that radioimmunoassay methods for measuring A II in venous blood may be more accurate and relevant than has previously been recognized. The small difference between A II concentrations in arterial and venous plasma suggests further that there may be significant conversion of angiotensin I (A I) to A II in the limb vasculature.

Blood pressure and plasma angiotensin II concentration after renal artery constriction and angiotensin infusion in the dog. (5-Isoleucine)angiotensin II and its breakdown fragments in dog blood

We measured arterial plasma angiotensin II concentration, renal blood flow, and arterial blood pressure in six conscious dogs during intravenous infusion of angiotensin II (5, 10, and 20 ng/kg per min). The same measurements were made on a different occasion in the same six animals, while they were conscious, before and during constriction of a main renal artery. Arterial blood pressure and plasma angiotensin II rose and renal blood flow decreased in both experiments. The similarity of regressions for plasma angiotensin II concentration and arterial blood pressure in the two experiments strongly suggests that the rise of circulating angiotensin II after renal artery constriction is sufficient to account for the hypertension by its direct pressor action. As discussed, a different mechanism seems likely to be involved in the later stages of renal hypertension. Angiotensin II is more likely to be in the 5-isoleucine form than in the 5-valine form in the dog. In contrast to the rat, plasma concentrations of the heptapeptide (angiotensin III), hexapeptide, and pentapeptide fragments of angiotensin II are low in the dog.

Substrate requirements for angiotensin I conversion in vivo and in vitro

The substrate requirements for angiotensin I-converting enzyme were studied in vivo in the dog lung and in vitro in plasma, using angiotensin I (AI), 5- D -Ile-AI, 7- D -Pro-AI, and 8-D-Phe-AI which had been synthesized by the solid-phase technique. All peptides were inactive in the rabbit aortic strip preparation. None of the D-amino acid-substituted peptides gave a pressor response in the anesthetized dog; none showed significant immunologic cross-reactivity with anti-AI serum or antiangiotensin II serum. Each peptide was labeled with 125 I, and the monoiodinated species was isolated. The iodinated peptides were incubated with diluted dog plasma or injected into the right ventricle of intact anesthetized dogs. In vitro, 125 I-AI, 125 I-5-D-IIe-AI, and 125 I-7-D-Pro-AI were converted to angiotensin II (AII). 8-D-Phe-AI was not converted. In vivo, 15 seconds after injecting 125 I-AI, 125 I-5-D-IIe-AI, or 125 I-7-D-Pro-AI into the right ventricle, 125 I-AJI accounted for 70, 60, and 45%, respectively, of the radioactive material in aortic blood. These results and our previous observations on the importance of the C-terminal sequence of AI for conversion indicate that the enzymatic binding sites for AI-converting enzyme in vivo and in vitro extend from position 10 to position 8 but not to position 7. D-amino acid substitution at positions 7 and 5 abolishes biologic and immunologic activity without interfering with conversion.

Effect of rabbit antibodies against angiotensin-II on the pressor response to angiotensin-II and renal hypertension in the rat

1. In the intact or nephrectomized rat, the pressor responses to synthetic angiotensin-II-amide, beta-angiotensin-II and native "rat angiotensin" could be inhibited by intravenous injection of rabbit serum containing antibodies against synthetic angiotensin-II-amide.2. The pressor responses to vasopressin or noradrenaline were not influenced by anti-angiotensin serum.3. A single injection of anti-angiotensin serum inhibited the pressor responses to angiotensin-II-amide for a period of 2.5 hr.4. Intravenous infusion or daily intraperitoneal injection of anti-angiotensin serum did not influence blood pressure levels in renal hypertensive rats.