Comparison of intestinal discrimination between calcium 47, strontium 85, and barium 133

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Abstract
A mathematical treatment of data is described for comparing the fractional rates of Sr and Ca movement in gastrointestinal absorption. This is done by assuming that the ratio of various rate constants of absorption for the two elements is a constant which we have called the instantaneous intestinal strontium-calcium discrimination factor and designated as K. It was observed that this K value averaged 0.72 in studies on strontium and calcium absorption by the entire gastrointestinal tract of rats under in vivo conditions. This was in good agreement with a similar value for the reabsorption of these alkaline earths by mammalian kidney, reported by others as 0.7. The comparative absorption of barium could not be characterized by this mathematical model. Any treatment that alters the over-all comparative absorption of strontium and calcium can do so in two ways: by changing the absorption of both alkaline earths proportionally, and by changing the comparative rate constants of strontium and calcium transport. These two effects can be experimentally delineated. In the present study both lactose and lysine, which are known to alter Sr-Ca discrimination, were found to do so by increasing absorption and not by changing the rate constants.

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