Studies of the Effect of Lysine on the Absorption of Radiocalcium and Radiostrontium by the Rat

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Abstract
The action of lysine in increasing the absorption of a single dose of Ca45 and Sr85 by fasted rats was most pronounced in the ileum. It was shown that lysine will exert an effect only if present in the same ligated segment as the dose of Ca45 and Sr85, and if the pH of the dosing solution is lower than 9.7 (the isoelectric point for lysine). Its effect upon absorption from the ileum was evident within one hour after dosing. The supplementation of a basal diet containing sufficient lysine to meet the needs of the rat for this essential amino acid with three different levels of lysine did not significantly increase the net retentions of stable calcium, Ca45, or Sr85 during a 14-day balance period. Similarly, an addition of 2.09% of lysine to a low-calcium diet did not influence the absorption of Sr85. Possible reasons for the inability of lysine supplementation to increase calcium and strontium absorption in balance trials, as compared with the marked increases obtained in single dose studies with fasted rats, have been investigated. In single, oral dose studies, it was found that 1.25 mmole of lysine will almost double the deposition of Ca45 and Sr85 in the femur when only 10 mg of Cacl3 are present in the dose. The increase was only 26% in the case of Sr85, and 38% in the case of Ca45, however, when the level of CaCl2 in the dose was raised to 100 mg. The presence of either glutamic or aspartic acid markedly reduced the enhancement of absorption of Sr85 and Ca45 produced by lysine. Glutamic acid was also shown to have a similar suppressing effect on the action of lactose. A possible chelation between the glutamic acid and calcium has been postulated to account for this suppression. The presence of casein and starch also largely suppressed the action of lysine.