Measurement of plasma renin activity in the freshwater turtle

Components of the renin angiotensin system have been identified in many nonmammalian vertebrates. However, in many of these animals, including reptiles, the physiological functions and importance of the system remain unclear. To aid in the study of the system in a reptile we modified a commercially available radioimmunoassay (RIA) kit containing antibody against human angiotensin I (ANG I) for use in the freshwater turtle, Pseudemys scripta. Cross-reactivity between anti-human ANG I antibodies (Rainen Angiotensin I RIA Kit, New England Nuclear) and turtle ANG I was demonstrated. Cross-reactivity with the antibody in two other human ANG I assay kits (Travenol-Genentech and Biotecx) was very limited. Blood for assay was collected from conscious turtles in EDTA, centrifuged, and the plasma frozen at -20 degrees. Turtle ANG I was generated by incubation at 0.5 ml plasma at pH 5.5 for 2 hr at 30 degrees with addition of dimercaprol and 8-hydroxyquinoline. Angiotensin generation increased with temperature and with generation time. The recovery of turtle ANG I added to turtle plasma prior to incubation was 92-97%. The assay procedure was used to measure plasma renin activity (ng/ml/hr incubation) from unstimulated turtles.