Overview
A technique to facilitate stable isotope tracer studies of Cu metabolism in small animals has been developed. The two stable isotopes of copper are naturally present in high abundance and the sensitivity of detection of either isotope when used as a tracer is therefore limited. The practicability of reducing the abundance of 65Cu in rats by feeding a diet containing highly enriched 63Cu was assessed. The aim was to increase the sensitivity with which isotope enrichment in rat tissues could be detected following injection of a 65Cu tracer. Male rats were fed a diet containing 63Cu from weaning up to 100 days post-weaning, and liver and muscle tissues were obtained at logarithmic time intervals. Isotope ratios of extracted and purified Cu were determined using thermal ionization mass spectrometry. The rat tissues showed a rapid rate of enrichment with 63Cu over the initial 32 days. The enrichment rate subsequently diminished in an asymptotic manner up to day 100 when the % abundance of 63Cu in the liver was 98.01%, as compared to 98.11% in the diet. Significant improvements in the sensitivity of enrichment detection on administration of a 65Cu spike were predicted at 100 days and above. In a second study, three groups of rats were given a diet enriched with 63Cu for 70 days and a further three groups received a non-enriched diet. Two of the three groups on each diet were injected with a spike of either 9 or 90 I~g 65Cu on day 70, and the liver Cu isotope ratios were analyzed 24 hours after tracer administration. Although, due to biological variation in the rate of tissue 63Cu enrichment, no significant enhancement of detection sensitivity was found at 70 days, the data from the first study shows that very significant advantages would be achieved by 100 days. The technique of artificial enrichment with 63Cu offers a way of extending the duration of a tracer study and reducing the amount of 65Cu spike necessary for the study of Cu metabolism.