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, Ca2+ 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 Ca2+ into BBMV as well as Ca2+ 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 Ca2+ uptake into BBMV from hyper-T rats was fraternized with decrease in Michaelis Menten Constant (Km), indicating a conformational change in Ca2+ transporter. Further, this finding was supported by significant changes in transition temperature and membrane fluidity. Increased Ca2+ efflux across enterocytes was attributed to sodium-dependent Ca2+ 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 Ca2+-ATPase activity of BLMs of all groups. Kinetic studies of Na+/Ca2+ exchanger revealed that alteration in Na+-dependent Ca2+ efflux was directly associated with maximal velocity (Vmax) of exchanger among all the groups. cAMP, a potent activator of Na+/Ca2+ 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 Ca2+ influx across BBM is possibly modulated by thyroid hormones by mediating changes in membrane fluidity. Thyroid hormones activated the Na+/Ca2+ exchange in enterocytes possibly via cAMP-mediated pathway.