The present studies investigated the mechanism of Ca2+ transport across basolateral membrane vesicles (BLMVs) prepared from human small intestine. Ca2+ uptake represented transport into the intravesicular space as evident by osmolality study and by the demonstration of Ca2+ efflux from the intravesicular space by Ca2+ ionophore A23187. Ca2+ uptake was stimulated by Mg2+-ATP. Kinetic parameters for ATP-dependent Ca2+ uptake revealed a Michaelis constant (K(m)) of 0.02 ± 0.01 μM and a maximum rate of uptake (V(max)) of 1.00 ± 0.03 nmol·mg protein-1·min-1. Ca2+ uptake in the absence of Mg2+ was inhibited by 75%. The K(m) of ATP concentration required for half-maximal Ca2+ uptake was 0.50 ± 0.1 mM. Calmodulin (10 μg/ml) increased V(max) to 1.62 ± 0.02 nmol·mg protein-1·min-1 (P < 0.001). K(m) values were 0.017 ± 0.001 μM, which was not significantly different from control values. Basolateral membranes depleted of calmodulin by EDTA osmotic shock decreased ATP-dependent Ca2+ uptake by 65%. Trifluoperazine, an anticalmodulin drug, inhibited ATP-dependent Ca2+ uptake by 50%, while no inhibition was noted in calmodulin-depleted membranes. Efflux of Ca2+ in the BLMVs was stimulated by trans-Na+. Na+-dependent Ca2+ uptake was saturable with respect to Ca2+ concentration and exhibited a KM of 0.09 ± 0.03 μM and a V(max) of 1.08 ± 0.01 nmol·mg protein-1·min-1. These results are consistent with the existence of a Na+-Ca2+ exchange system and ATP and Mg2+-dependent, calmodulin-regulated Ca2+, transport mechanism in BLMVs of human enterocytes.
|Original language||English (US)|
|Journal||American Journal of Physiology - Gastrointestinal and Liver Physiology|
|State||Published - Jan 1 1988|
ASJC Scopus subject areas
- Physiology (medical)