True versus immunoreactive angiotensin II in human plasma

Area of Science:

• Cardiovascular Physiology
• Endocrinology
• Analytical Chemistry

Background:

• The renin-angiotensin system plays a crucial role in blood pressure regulation.
• Accurate measurement of specific angiotensin peptides is essential for understanding its physiological and pathological roles.
• Existing methods for measuring angiotensin II may lack specificity, potentially confounding results.

Purpose of the Study:

• To develop and validate a sensitive and specific method for quantifying angiotensin-(1-8)octapeptide in human plasma.
• To assess the behavior of angiotensin-(1-8)octapeptide and immunoreactive angiotensin II during converting-enzyme inhibition.
• To evaluate the clinical utility of this method in hypertensive patients treated with enalapril.

Main Methods:

• Plasma peptide extraction using reversible adsorption to bonded-phase silica.
• Isolation of angiotensin-(1-8)octapeptide via isocratic reversed-phase high-performance liquid chromatography (HPLC).
• Quantification by radioimmunoassay (RIA); assessment of recovery, precision, and detection limits.

Main Results:

• The developed HPLC-RIA method demonstrated high extraction recovery (99%) and overall recovery (80%) with good precision.
• Normal subjects had baseline angiotensin-(1-8)octapeptide levels of 4.2 fmol/ml, increasing with furosemide and standing.
• Converting-enzyme inhibition led to the complete disappearance of angiotensin-(1-8)octapeptide, while immunoreactive angiotensin II decreased only by 44%.

Conclusions:

• The validated HPLC-RIA method specifically and sensitively measures angiotensin-(1-8)octapeptide in plasma.
• True angiotensin II (angiotensin-(1-8)octapeptide) is effectively eliminated by converting-enzyme inhibition.
• This method provides a more accurate assessment of the renin-angiotensin system activity compared to less specific assays.