KINETICS OF CALCIUM-ABSORPTION IN THE SMALL AND LARGE-INTESTINE OF THE FROG RANA-PIPIENS

Absorption of Ca-45(2+) was studied in the small and large intestine of the adult male frog Rana pipiens through an in situ intestinal ligated-loop preparation. With a graded series of luminal calcium concentrations, Ca-45 uptake into small intestine in March animals is by passive diffusion alone, while in April through June Ca-45 uptake in duodenum and jejunoileum follows a curvilinear function that shows a saturable and a nonsaturable component. The duodenal nonsaturable component had a diffusion constant (K-d) of 0.73 +/- .09 mL/g/hr. The saturable component followed Michaelis-Menten kinetics, with a saturable flux (J(max)) of 23.9 +/- 2.5 mu mol/g/h and half-saturation constant (K-m) of 9.5 +/- 2.1 mM Ca2+. Jejunoileum also exhibited nonsaturable and saturable components with a K-d of 0.65 +/- 0.09 mL/g/h. Colon exhibited only a passive diffusion uptake pattern with a K-d of 0.40 +/- 0.03 mL/g/h, which is significantly lower than that of the small intestine. After 500 mu g of vitamin D-3, duodenal J(max) was elevated to 34.6 +/- 5.3 mu mol/g/h, while jejunoileum was unchanged at a J(max) of 19.2 +/- 1.7 mu mol/g/h, and colon maintained a K-d of 0.45 +/- 0.03 mL/g/h. Passive diffusion is present in all intestinal segments, with evidence of a low-binding-capacity active transport system in the proximal segment most apparent during April through June. A putative calcium-binding protein may be functionally similar to that of homeotherms; however, the kinetics suggest that it may be structurally dissimilar.