The influence of progressive hyperinsulinemia on duodenal calcium absorption in the rat.

Using an in situ loop technique, we studied the influence of moderate hyperinsulinemia on the bidirectional fluxes [lumen-to-plasma (LP), plasma-to-lumen (PL)] and on the net absorption of calcium (Ca) in the duodenum of the rat. Under hyperinsulinemic euglycemic clamp conditions, three different steady-state plasma insulin levels (66 +/- 16, 187 +/- 13, 263 +/- 24 microU/mL) were maintained by intravenous (IV) infusion of either 20, 40, or 60 mU/h of insulin. LP flux and net Ca absorption (CaA) increased significantly under all, while the PL flux was not changed by any of the three insulin doses. Under 40 and 60 mU/h of IV insulin, the individual plasma insulin levels and LP fluxes were positively and significantly correlated, suggesting a dose-dependent action of insulin on duodenal CaA. No significant changes were seen in plasma somatostatinlike immunoreactivity (SLI), parathyroid hormone (PTH), and serum Ca, while serum inorganic phosphate (Pi) and 1,25(OH)2 vitamin D levels fell with increasing doses of IV insulin. It is concluded for the rat that physiologic degrees of hyperinsulinemia enhance duodenal CaA by an as yet unknown mechanism, this action seems to be independent of PTH and, the role of 1,25(OH)2 vitamin D in this context is poorly understood.