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Retention of ¹⁴C Label is Lower in Old than in Young Wistar Rats after Oral Dosing with [¹⁴C]Pyridoxine

TNO-CIVO Toxicology and Nutrition Institute Zeist, Department of Nutrition, 3700 AJ Zeist, The Netherlands

Ten young (6-mo-old) and ten old (31-mo-old) male Wistar rats fed a purified diet (250 g casein and 6 mg pyridoxine·HCl per kg) from weaning were given either a single oral dose or five repeated oral doses of ¹⁴C-labeled pyridoxine. At various times after dosing animals of each age group were killed. Absorption of orally dosed [¹⁴C]pyridoxine·HCl was not found to be different between young and old rats. Total body retention of ¹⁴C label administered was modestly but significantly lower in old than in young rats. However, distribution of ¹⁴C label over various tissues and among the various B-6 vitamers was similar. No significant age-related differences were observed in the biokinetic parameters derived from urinary excretion data. Contrary to the findings for ¹⁴C label distribution, age-related differences were observed for absolute level of tissue ¹⁴C-labeled vitamers. The lower [¹⁴C]pyridoxal-5'-phosphate content in liver and muscle, and [¹⁴C]pyridoxamine-5'-phosphate content in liver, of old animals indicated an age-related difference in liver and muscle vitamin B-6 disposition. In both young and old rats, and in both liver and muscle tissue, pyridoxamine-5'-phosphate was observed to be a faster-exchanging tissue vitamin B-6 pool than pyridoxal-5'-phosphate.

KEY WORDS: • aging • rats • vitamin B-6 • pyridoxine • metabolism

¹ Current address: National Institute of Public Health and Environmental Protection (RIVM), Laboratory for Medicines and Medical Devices (LGM), The Netherlands.

² To whom correspondence and reprint requests should be addressed: TNO-CIVO Toxicology and Nutrition Institute Zeist, Department of Nutrition, P.O. Box 360, 3700 AJ Zeist, The Netherlands.

Manuscript received 9 December 1991. Revision accepted 6 March 1992.