Calcium uptake by brush-border and basolateral membrane vesicles in chick duodenum.

Area of Science:

• Biochemistry
• Cell Biology
• Physiology

Background:

• Intestinal calcium absorption is crucial for maintaining calcium homeostasis.
• Vitamin D plays a vital role in regulating calcium absorption.
• The specific mechanisms by which vitamin D affects calcium transport across intestinal membranes are not fully elucidated.

Purpose of the Study:

• To compare the effects of 1 alpha,25-dihydroxyvitamin D3 on calcium uptake in duodenal brush-border membrane vesicles (BBMV) and basolateral membrane vesicles (BLMV).
• To investigate the kinetic parameters and localization of ATP-dependent calcium uptake.
• To determine the role of different membrane transport processes in vitamin D-mediated calcium absorption.

Main Methods:

• Isolation of BBMV and BLMV from vitamin D-deficient and treated chicks.
• Measurement of calcium uptake in isolated membrane vesicles.
• Kinetic analysis of calcium transport (Vmax and Km).
• Inhibition studies using specific agents like vanadate.

Main Results:

• 1 alpha,25-dihydroxyvitamin D3 significantly increased calcium uptake in BBMV and BLMV.
• The peak increase in BBMV uptake occurred at 12-24 hours, while BLMV uptake peaked at 6 hours.
• Kinetic analysis showed increased Vmax without changes in Km, indicating an increase in transporter number or activity.
• ATP-dependent calcium uptake was localized to the basolateral membrane and inhibited by vanadate.

Conclusions:

• 1 alpha,25-dihydroxyvitamin D3 enhances intestinal calcium absorption by modulating calcium transport at both BBMV and BLMV.
• The initiation of enhanced absorption appears to be linked to increased calcium efflux at the basolateral membrane.
• These findings provide insights into the molecular mechanisms of vitamin D-regulated calcium transport.