Validation of a direct radioimmunoassay of melatonin in the blue fox.

A direct radioimmunoassay procedure for the determination of melatonin in the blood of blue fox has been validated and applied. The assay required 50 μl of sample and standard, 100 ul of antiserum and 100 μl of (H)melatonin. After overnight incubation at 4°C the antibody bound melatonin was separated from the free hormone with dextran-coated charcoal. Following centrifugation the antibody bound (H)melatonin was determined in a beta scintillation counter. The antiserum bound 30–35 % of the (H)melatonin at a final dilution of 1:36000. The non specific binding represented less than 5% of the total radioactivity in all assays. The lowest detectable amount of melatonin was 2.6 fmol/tube, corresponding to 52.5 pmol/1. The inter-assay coefficient of variation at 178 and 510 pmol/1 was 15.6 and 8.8%, respectively. The precision profile, calculated from a 10-replicate standard curve, showed that the coefficient of variation decreased from 43% at 84 pmol/1 to 15% at 336 pmol/1, and remainded at or below 10% for concentrations exceeding 670 pmol/1. Plasma was collected from 2 male blue foxes at about hourly intervals during a 24 h period in September and assayed for melatonin. Maximum (421 pmol/1) and minimum (97 pmol/1) concentrations of the hormone were inversely related to light intensity.