Increased calcium absorption in nephrolithiasis explained by uptake studies in ileal brush border membrane vesicles

Area of Science:

• Gastroenterology and Nephrology
• Mineral Metabolism
• Urolithiasis Research

Background:

• Recurrent calcium renal stone formers exhibit elevated urinary calcium and oxalate.
• This is linked to impaired citrate absorption, affecting calcium homeostasis.
• The precise mechanism of citrate's influence on intestinal calcium uptake requires elucidation.

Purpose of the Study:

• To investigate the mechanism of citrate-induced calcium absorption in the gut.
• To determine the roles of citrate and phosphate in regulating intestinal calcium uptake.
• To clarify the implications for patients with recurrent calcium nephrolithiasis.

Main Methods:

• Utilized guinea pig ileal brush border membrane vesicles (BBMVs) as an in vitro model.
• Assessed calcium uptake kinetics, including Michaelis-Menten parameters (Km and Vmax).
• Examined the effects of citrate and phosphate, individually and combined, on calcium absorption.

Main Results:

• Calcium absorption in BBMVs demonstrated concentration-dependent, single-mechanism kinetics.
• Citrate and phosphate inhibited maximal calcium uptake; their combination normalized absorption.
• Citrate's inhibition is attributed to free ions; phosphate mitigates this by affecting ionized calcium and preventing precipitation.

Conclusions:

• Citrate enhances intestinal calcium absorption via direct interaction in the gut lumen.
• Phosphate counteracts citrate's effect, normalizing calcium absorption.
• Altered citrate absorption significantly impacts intestinal calcium handling, explaining increased urinary calcium in stone formers.