Inhibitory effect of aluminium on calcium absorption in small intestine of rats with different thyroid hormone status

To analyse the influence of thyroid status on the effect of aluminium (Al) upon intestinal calcium (Ca) absorption, adult male Wistar rats with experimentally altered thyroid hormones circulating levels, were orally treated (o.g.) with 0 (control), or 50 mg elemental Al (as chloride)/kg body weight (b.w.) per day, for a 14 d period. Hyper- and hypo-thyroid conditions were respectively achieved by means of administration of either sodium levothyroxine (50 μg/kg b.w. per day, o.g.) or methimazole, a thyroxine synthesis inhibitor (1 mg/kg b.w. per day, o.g.). In duodenum–jejunum segments, in vitro mucosa-to-serosa ⁴⁵Ca flux (JCa_ms) and kinetics of ⁴⁵Ca uptake in isolated enterocytes, were determined. In serum, concentrations of thyroxine (T4) and triiodothyronine (T3) were measured by chemiluminescent enzyme immunoassay. Unlike non-Al-treated rats, JCa_ms of Al-exposed rats decreased as serum levels of T4 and T3 increased, showing a significant inverse correlation in both cases (T4: r² = 0.414, P = 0.024; T3: r² = 0.443, P = 0.018). Enterocytes isolated from rats treated with Al plus thyroxine showed a reduction of both maximum Ca uptake (4.86 ± 0.44 vs. 6.85 ± 1.04 nmol Ca/mg protein, P < 0.05) and K_m (0.84 ± 0.18 vs. 1.05 ± 0.36 mM, P < 0.05) when compared to control. The observed variability in the Al effect on Ca transport with thyroid status of rats could be reflecting a negative interaction of Al with thyroid hormone action mechanisms on intestinal Ca absorption, which would take place mainly at Ca entry into enterocyte from lumen.