The secretion of <tex-math>${}^{85}{\rm Sr}$</tex-math> and <tex-math>${}^{47}{\rm Ca}$</tex-math> from the vascular space into the intestine was determined in rats as a function of time during both feeding and fasting. <tex-math>${}^{85}{\rm Sr}$</tex-math> and <tex-math>${}^{47}{\rm Ca}$</tex-math> were injected intravenously, and the rats were sacrificed after 1/2, 1, 2, 4, and 24 hours. The gastrointestinal radioisotope content was measured in six separate segments. During feeding, slightly more <tex-math>${}^{47}{\rm Ca}$</tex-math> than <tex-math>${}^{85}{\rm Sr}$</tex-math>, 5.1% vs 4.6% of the dose, respectively, was present at 1/2 hour in the gastrointestinal tract. At 4 hours the <tex-math>${}^{85}{\rm Sr}$</tex-math> in the intestine rose to 7.0%, while <tex-math>${}^{47}{\rm Ca}$</tex-math> had increased only very slightly. Between 4 and 24 hours the excretion of both isotopes via the intestine increased, but the increase in <tex-math>${}^{85}{\rm Sr}$</tex-math> excretion was greater than that of <tex-math>${}^{47}{\rm Ca}$</tex-math>, the values being 11% and 8.6% of the dose at 24 hours, respectively. Fasting decreased the secretion of both isotopes into the intestine at all time intervals, and this effect was more marked for <tex-math>${}^{47}{\rm Ca}$</tex-math> than for <tex-math>${}^{85}{\rm Sr}$</tex-math>. At 24 hours, during fasting, the intestinal excretion of <tex-math>${}^{85}{\rm Sr}$</tex-math> was less than half of the feeding level, while the intestinal excretion of <tex-math>${}^{47}{\rm Ca}$</tex-math> was less than one-third of that during feeding, resulting in a <tex-math>${}^{85}{\rm Sr}/{}^{47}{\rm Ca}$</tex-math> ratio of 2.0 during fasting vs 1.3 during feeding.

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