Intestinal calcium absorption during hyperinsulinemic euglycemic glucose clamp in healthy humans

Summary

The influence of postprandial-like plasma insulin levels on intestinal calcium absorption (CaA) was studied in 9 health men. On separate occasions, they received either an i.v. infusion of 40 mU/m² minute synthetic human insulin as well as a variable glucose infusion in order to clamp the plasma glucose at the baseline level (=glucose clamp), or insulin- and glucose-free vehicle infusions (=vehicle). During these infusions, an oral load containing 326 mg Ca in the form of Ca chloride was administered and CaA was determined thereafter with a⁴⁷Ca/⁸⁵Sr double tracer method. During glucose clamp, mean plasma insulin was 172 ±(1 SEM) 10 as compared to 6±1 μU/ml during vehicle infusions. During the clamp, 3-hour cumulative CaA rose significantly by 14% as compared to vehicle (39.2±2.5 vs. 34.4±2%,P<0.02). At the same time, serum potassium and phosphorus dropped significantly, whereas serum parathyroid hormone (PTH) and 1,25(OH)₂D levels were unchanged as compared to vehicle. The urinary excretions of potassium, sodium, and inorganic phosphorus as well as the urinary specific activity of⁴⁷Ca, dropped significantly during glucose clamp, whereas the urinary excretion of cAMP was unchanged as compared to vehicle. The results suggest that, under the conditions of euglycemic hyper-insulinemic clamp, insulin stimulates CaA of healthy humans in a PTH- and 1,25(OH)₂D-independent manner. Insulin may thus possibly be regarded as a factor participating in the regulation of CaA in humans.