Quantification of Angiotensin Peptides

There is an endogenous renin-angiotensin system in the kidney, and angiotensin (Ang) II generated within the kidney may play an important role in the regulation of renal hemodynamics and sodium excretion (1 –6 ). Assessment of the functional significance of the intrarenal renin-angiotensin system requires accurate and sensitive measurement of the Ang peptides in renal tissue. These measurements have been problematic because of methodologic difficulties, and very variable Ang peptide levels have been reported in extracts of rat kidney (7 –7 ). Radioimmunoassay (RIA) procedures carried out on crude tissue extracts have produced inaccurate results because of interference and/or crossreactivity with biologically inactive metabolites of the Ang peptides that are present in high concentrations in tissues. Additional problems include the instability of the Ang peptides during extraction from tissue, in part because of the cleavage of peptide bonds by tissue peptidases; incomplete extraction from tissue stores; variable recoveries from solid-phase extraction on octadecylsilica gel (C₁₈ ) cartridges, which are commonly used for the preliminary clean-up of samples before high-performance liquid chromatography (HPLC), and the difficulties inherent in gradient elution techniques for HPLC (13 ). We have recently developed a simplified method for extracting Ang peptides from brain tissue (14 ).