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Evaluation of Vitamin B-6 Status and Function of Rats Fed Excess Pyridoxine¹

Monica C. Schaeffer, David A. Sampson, James H. Skala, Dorothy W. Gietzen^* and Robert E. Grier

Western Human Nutrition Research Center, U.S. Department of Agriculture/Agricultural Research Service, San Francisco, CA 94129 ^* Department of Physiological Sciences, University of California, Davis, CA 95616 Department of Pediatrics, Medical School, The University of Texas, Health Science Center at Houston, Houston, TX 77030

We compared the vitamin B-6 status of 12-wk-old rats (n = 12) fed excess (1400 mg/kg diet) or the recommended level (7 mg/kg diet, control) of pyridoxine (PN) hydrochloride to test if excess vitamin B-6 would cause tissue depletion of pyridoxal phosphate (PLP), the active coenzyme form of vitamin B-6. Plasma PLP, tryptophan-load test results, food intake, and tissue and body weights were not different at wk 6. Red blood cell endogenous alanine aminotransferase activity and PLP concentration were elevated (P < 0.01) in rats fed 1400 mg PN·HCl/kg diet. In contrast, PLP concentration in muscle was significantly lower (P = 0.01) in rats fed excess vitamin B-6 (9.7 ± 0.8 nmol/g, mean ± SEM) than in controls (14.9 ± 1.4). PLP concentration in other tissues, including plasma, was not affected. In rats fed excess vitamin B-6, pyridoxal was increased in all tissues examined (P < 0.05), and total vitamin B-6 was increased in plasma, red blood cells and kidneys (P < 0.05). Total glycogen phosphorylase (a + b) activity in the gastrocnemius was not affected, but phosphorylase a activity was increased in rats fed excess vitamin B-6 (P = 0.025). Concentrations of dopamine and metabolites in the caudate nucleus of the basal ganglia were not affected. A transient, but significant, elevation in acoustic startle response, a central nervous system reflex, was observed in rats fed excess vitamin B-6. The depletion in muscle PLP could not have been predicted by either plasma or red blood cell PLP concentration, although the latter did reflect vitamin B-6 intake. Decreased muscle PLP and increased phorphorylase a activity associated with excess intake of vitamin B-6 in the rat may have important consequences for the nutritional status of the organism.

KEY WORDS: • vitamin B-6 vitamers • pyridoxal phosphate • pyridoxine toxicity • behavior • glycogen phosphorylase • neurotransmitters • rat

¹ Some of these results were published in FASEB J. 2: A438, 1988, abs. #828, and FASEB J. 3: A668, 1989, abs. #2565.

Manuscript received 3 August 1988. Revision accepted 25 April 1989.