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Biochim Biophys Acta. 2003 Nov 20;1639(3):185-94.

Thyroid hormones stimulate calcium transport systems in rat intestine.

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Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh-160 012, India.


Thyroid hormone status influences calcium metabolism. To elucidate the mechanism of action of thyroid hormones on transcellular transport of calcium in rat intestine, Ca(2+) influx and efflux studies were carried out in brush border membrane vesicles (BBMV) and across the basolateral membrane (BLM) of enterocytes, respectively. Steady-state uptake of Ca(2+) into BBMV as well as Ca(2+) efflux from the BLM enterocytes was significantly increased in hyperthyroid (Hyper-T) rats and decreased in hypothyroid (Hypo-T) rats as compared to euthyroid (Eu-T) rats. Kinetic studies revealed that increase in steady state Ca(2+) uptake into BBMV from hyper-T rats was fraternized with decrease in Michaelis Menten Constant (K(m)), indicating a conformational change in Ca(2+) transporter. Further, this finding was supported by significant changes in transition temperature and membrane fluidity. Increased Ca(2+) efflux across enterocytes was attributed to sodium-dependent Ca(2+) exchange activity which was significantly higher in Hyper-T rats and lower in Hypo-T rats as compared to Eu-T rats. However, there was no change in Ca(2+)-ATPase activity of BLMs of all groups. Kinetic studies of Na(+)/Ca(2+) exchanger revealed that alteration in Na(+)-dependent Ca(2+) efflux was directly associated with maximal velocity (V(max)) of exchanger among all the groups. cAMP, a potent activator of Na(+)/Ca(2+) exchanger, was found to be significantly higher in intestinal mucosa of Hyper-T rats as compared to Eu-T rats. Therefore, the results of this study suggest that Ca(2+) influx across BBM is possibly modulated by thyroid hormones by mediating changes in membrane fluidity. Thyroid hormones activated the Na(+)/Ca(2+) exchange in enterocytes possibly via cAMP-mediated pathway.

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