Mechanisms of calcium transport in human colonic basolateral membrane vesicles.

Human colon has been suggested to play an important role in calcium absorption especially after extensive disease or resection of the small intestine. We have previously demonstrated the presence of a carrier-mediated calcium uptake mechanism in the human colonic luminal membrane vesicles. Current studies were, therefore, undertaken to investigate the mechanism(s) of calcium exit across the basolateral membrane domain of the human colon. Human colonic basolateral membrane vesicles (BLMVs) were isolated and purified from mucosal scrapings of organ donor colons, utilizing a technique developed in our laboratory. 45Ca uptake was measured by a rapid filtration technique. 45Ca uptake represented transport into the intravesicular space as evidenced by an osmolarity study and by the demonstration of Ca2' efflux from calcium preloaded vesicles by Ca2+ ionophore A23187. Calcium uptake was stimulated by Mg2+ ATP. The kinetic parameters for ATP-dependent Ca2+ uptake revealed saturation kinetics with Michaelis constant (Km) of 0.22 +/- 0.04 microM and a maximum rate of uptake (Vmax) of 0.38 +/- 0.12 nmol/mg protein/min. The Km of ATP concentration required for half maximal Ca2+ uptake was 0.39 +/- 0.04 mM. ATP-stimulated calcium uptake into these vesicles was further stimulated in the presence of calmodulin and was inhibited by calmodulin antagonist, trifluoperazine. Uptake of 45Ca into BLMVs was markedly inhibited by cis-Na+ but was significantly stimulated by trans-Na+ (40-50% stimulation). Our results demonstrate the presence of a Mg2+/ATP-dependent calmodulin-regulated Ca2+ transport system and a Na+-Ca2+ exchange process in the human colonic basolateral membranes.