Detection of immunoreactive endothelin in plasma of hemodialysis patients

Abstract

Two types of radioimmunoassay (RIA) methods for measuring endothelin (ET) in human plasma were developed. One was an extraction procedure using a Sep‐Pak C₁₈ cartridge, the other being a direct method. By the extraction method, plasma ET levels were lower than the detectable limit (7 pg/ml) in normal subjects and elevated in hemodialysis patients. The absolute values obtained via the direct method were 20‐times higher than those from extraction. Gel‐filtration experiments revealed that this discrepancy was mainly due to immunoreactive (IR‐) endothelin‐like substances of high molecular mass near 11.6 kDa (large IR‐ET). Extraction of the peptide by the C₁₈ cartridge could eliminate interference by large IR‐ET and is important in the accurate measurement of ET concentrations in plasma.