Changes in gastric sodium-iodide symporter (NIS) activity are associated with differences in thyroid gland sensitivity to perchlorate during metamorphosis

- Stage 55 tadpoles are less sensitive than earlier stage tadpoles to perchlorate.
- Perchlorate blocks I− uptake in many non-thyroid tissues in tadpoles.
- Perchlorate sensitive I− uptake was greatest in stomach.
- Perchlorate-sensitive I− uptake in stomach was greater at stage 55 than stage 49.

We investigated stage-dependent changes in sensitivity of the thyroid gland to perchlorate during development of African clawed frog tadpoles (Xenopus laevis) in relation to non-thyroidal iodide transporting tissues. Perchlorate-induced increases in thyroid follicle cell size and colloid depletion were blunted when exposures began at Nieuwkoop-Faber (NF) stage 55 compared to when exposures began at NF stages 49 or 1-10. To determine if the development of other iodide transporting tissues may contribute to this difference we first examined which tissues expressed transcripts for the sodium dependent iodide symporter (NIS). RT-PCR analysis revealed that NIS was expressed in stomach and small intestine in addition to the thyroid gland of X. laevis tadpoles. NIS mRNA was not detected in lung, kidney, skin, gill, muscle, heart or liver. Perchlorate sensitive 125I uptake was found in stomach, lung, kidney, gill, and small intestine but not muscle, liver, or heart. Perchlorate-sensitive 125I uptake by stomach was 6-10 times greater than in any other non-thyroidal tissue in tadpoles. While NF stage 49 tadpoles exhibited perchlorate-sensitive uptake in stomach it was roughly 4-fold less than that observed in NF stage 55 tadpoles. Although abundance of NIS gene transcripts was greater in stomachs from NF stage 55 compared to NF stage 49 tadpoles this difference was not statistically significant. We conclude that gastric iodide uptake increases between NF stages 49 and 55, possibly due to post-translational changes in NIS glycosylation or trafficking within gastric mucosal cells. These developmental changes in gastric NIS gene expression may affect iodide availability to the thyroid gland.