Gender and sodium-ascorbate transporter isoforms determine ascorbate concentrations in mice

We evaluated the hypothesis that sodium-dependent vitamin C (ascorbate) transporters SVCT1 and SVCT2 (encoded by genes Slc23a1 and Slc23a2) regulate ascorbate concentrations in tissues of adult mice. Slc23a2+/- and Slc23a2+/+ mice were fed an ascorbate-free diet for 10-12 wk, and then segregated according to gender and genome, and were placed in groups of 3-4 in metabolic cages for 24-h urine collection. RT-PCR analysis showed that liver and kidney expressed mainly SVCT1, and brain, skeletal muscle, and spleen expressed predominantly SVCT2. Slc23a2+/- mice had low SVCT2 but normal SVCT1 messenger RNA (mRNA) levels, which did not differ between genders. Ascorbate concentrations were lower in Slc23a2+/- than Slc23a2+/+ mice in tissues where SVCT2 was the main isoform. Compared with males, females had lower ascorbate excretion and ascorbate:creatinine ratio in urine and had higher ascorbate concentrations in plasma and SVCT1-predominant tissues. SVCT2 contributed to a gender effect in spleen because males had higher spleen ascorbate concentration than females in wild-type but not in Slc23a2+/- mice. Hepatic gulonolactone oxidase mRNA and activity levels did not differ with genotype or gender, indicating no differences in ascorbate synthesis. We concluded that SVCT2 is a major determinant of ascorbate accumulation in tissues lacking SVCT1. The SVCT isoforms appear to function independently of one another because SVCT1 expression and ascorbate concentrations in SVCT1-predominant organs were not affected by SVCT2 deficiency. Additionally, lower ascorbate excretion in females may elevate the vitamin's concentrations in plasma and tissues expressing SVCT1 that, unlike SVCT2, is not saturated by plasma ascorbate concentrations.