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 o f the adult male frog Rana pipiens through an in situ intestinal ligat ed-loop preparation. With a graded series of luminal calcium concentra tions, Ca-45 uptake into small intestine in March animals is by passiv e diffusion alone, while in April through June Ca-45 uptake in duodenu m and jejunoileum follows a curvilinear function that shows a saturabl e and a nonsaturable component. The duodenal nonsaturable component ha d a diffusion constant (K-d) of 0.73 +/- .09 mL/g/hr. The saturable co mponent followed Michaelis-Menten kinetics, with a saturable flux (J(m ax)) 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 satur able components with a K-d of 0.65 +/- 0.09 mL/g/h. Colon exhibited on ly 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. Aft er 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. P assive diffusion is present in all intestinal segments, with evidence of a low-binding-capacity active transport system in the proximal segm ent most apparent during April through June. A putative calcium-bindin g protein may be functionally similar to that of homeotherms; however, the kinetics suggest that it may be structurally dissimilar.