Faculty and Research Activities

Douglas R Robertson, Ph.D. Professor of Cell and Developmental Biology 1114 Weiskotten Hall Upstate Medical University 750 East Adams Street Syracuse, NY 13210 (315) 464-8589

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Research Abstract

Mechanisms of Amphibian Intestinal Calcium Transport.

The intestinal absorption of calcium in amphibians appears to be similar to that seen in birds and mammals; with primary absorption occurring in the proximal 1/3 or duodenal segment of the small intestine. Absorption of 45Ca2+ was studied in the small and large intestine of the adult male frog, Rana pipiens, using an in situ intestinal ligated-loop preparation. Using a graded series of luminal calcium concentrations, 45Ca uptake into small intestine in early spring animals is by passive diffusion alone; while in late spring 45Ca uptake in duodenum and jejunoileum follows a curvilinear function that shows a saturable and a nonsaturable component. The duodenal nonsaturable, passive diffusion component had a Kd of 0.71 ± .09 ml/gm/hr. The hyperbolic function, representing the saturable component followed Michaelis-Menten kinetics, with a saturable flux (Jmax) of 23.9 ± 2.5 Ïmol/gm/hr with the half-saturation constant (Km) of 9.5 ± 2.1 mM Ca2+. Distal jejunoileum also exhibited a non-saturable and saturable component where Kd = 0.65 ± .09 ml/gm/hr. Colon exhibited only a linear, passive diffusion uptake pattern with Kd = .40 ± .03 ml/gm/hr, significantly lower than small intestine. 500 Ïg vitamin D3, elevated duodenal Jmax to 34.6 ± 5.3 mol/gm/hr; while uptake in jenunoileum was unchanged at Jmax = 19.2 ± 1.7 mol/gm/hr, and colon maintained a Kd = 0.45 ± .03 ml/gm/hr. Intestinal calcium transport in the frog is primarily by passive diffusion. Response of the small intestine to vitamin D during summer and after pre-adaptation to 20 mM calcium indicates that a calcium "carrier" is present, but exhibits a low binding capacity, and is weakly down-regulated. These characteristics suggest that a putative calcium binding protein may be functionally similar to that of birds and mammals; however, its transport kinetics suggest that it may be structurally dissimilar to the calcium binding proteins of higher vertebrates. An active vitamin D-dependent "carrier" does not appear to be present in the large intestine.

Selected References

Robertson, D.R. Kinetics of calcium absorption in the small and large intestine of the frog, Rana pipiens. Physiological Zoology 68:1093-1117, 1995.

Robertson, D.R. Comparative aspects of intestinal calcium transport in fish and amphibians. Zoological Science 10:223-

This profile was last updated on 12/05/2002

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