title: Thyroid hormones stimulate calcium transport systems in rat intestine.
abstract: 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.
authors.affiliation: Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh-160 012, India.
authors.forename: Vivek
authors.initials: V
authors.lastname: Kumar
authors.forename: Rajendra
authors.initials: R
authors.lastname: Prasad
mesh_terms: D000818:Animals; D001692:Biological Transport; D001835:Body Weight; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D002149:Energy Intake; D006980:Hyperthyroidism / Q000378:metabolism*; D007037:Hypothyroidism / Q000378:metabolism*; D066298:In Vitro Techniques; D007413:Intestinal Mucosa / Q000502:physiology; D007421:Intestine, Small / Q000502:physiology*; D007700:Kinetics; D008297:Male; D008871:Microvilli / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013816:Thermodynamics; D013961:Thyroid Gland / Q000502:physiology*; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
chemical_list: D014284:Triiodothyronine; D000242:Cyclic AMP; D000252:Calcium-Transporting ATPases; D013974:Thyroxine; D002118:Calcium