Biological potency and radioimmunoassay of canine calcitonin

Calcitonin (CT) is a major calcitropic hormone. Because of low cross reacti vity of canine CT (cCT) in radioimmunoassays (RIA) developed for other spec ies, a homologous RIA is needed. Synthesis of cCT allowed study of its biol ogic potency using a rat bioassay and its plasma half-life in dogs. The ava ilability of cCT also made possible the development of a homologous RIA for measurement of basal and stimulated plasma CT concentrations in dogs. The biologic potency of the synthesized cCT in rats is 24 IU/mg of peptide, whi ch is low in comparison with the 4,000 IU/mg of the salmon CT standard. In the dog, an even lower potency of 4.4 IU/mg of cCT was found. Measurement o f the disappearance of iv-injected radioiodinated or nonradioiodinated cCT revealed a short biologic half-life of less than 3 min, followed by a long half-life of 20 min. A polyclonal antiserum against synthetic cCT was raise d in a goat. Using a final antiserum dilution of 1:12,000 and I-125-labeled synthetic cCT, the RIA had a detection limit of 6.5 ng/l, The antibody did not crossreact with standard human CT and had <0.1% cross reactivity with porcine CT. For measurement of plasma cCT concentrations, an extraction pro cedure was developed using ethanol. Dilutions of synthetic cCT and canine p lasma extracts revealed parallelism over a wide range of concentrations. Si ze exclusion chromatography of canine plasma extracts on Biogel P-10 reveal ed a single cCT peak at the same position as [I-125]-cCT, showing that ther e was little interference by other proteins or cCT prohormone. Basal plasma CT concentrations were 12-80 ng/l, and there was an 8- and 20-fold increas e after calcium (1 and 2.5 mg/kg body weight) bolus infusion. (C) 1999 Else vier Science Inc. All rights reserved.