Isotope-dilution technique for determination of endogenous faecal excretion and true absorption of calcium in ⁴⁵ Ca labelled rats

A balance study was conducted to clarify the methodical conditions for the determination of endogenous faecal excretion and true absorption of calcium by the isotope-dilution technique in the laboratory rat. For this purpose 32 growing male Sprague-Dawley rats were injected intramuscularly with ⁴⁵Ca. At day 6, 9, 12 and 14 after the ⁴⁵Ca injection eight animals were killed. During the experiment one semisynthetic diet containing 0.43% Ca as CaCO₃ was fed to all the animals in restricted amounts. At day 1 the faecal and renal ⁴⁵Ca excretion was 10‰ and 3‰ of injected ⁴⁵Ca activity. Within the next 2 days the daily ⁴⁵Ca excretion dropped to 3‰ (faeces) and 0.5‰ (urine) and then decreased steadily. The distribution of daily ⁴⁵Ca excretion between faeces and urine was stable after 1 week elapsed time from the ⁴⁵Ca injection (88% faeces; 12% urine). The specific ⁴⁵Ca activities of dissected tissues, urine and faeces declined during the experiment. From day 9 after the ⁴⁵Ca injection the change in specific ⁴⁵Ca activities was proportional to that of faeces in the case of blood plasma, muscle, liver, spleen and testes with the best conformity found in blood plasma. For kidney, brain, pancreas, small intestine, skin (including hair), femur and urine no proportionality to faeces was found indicating that these tissues (fluids) are not suitable as a reference source for the isotope-dilution method. The estimates of endogenous faecal Ca excretion based on blood plasma, muscle, liver, spleen and testes as reference source were similar among tissues and constant with respect to time for day 9, 12 and 14 after the ⁴⁵Ca injection. The most precise estimates were found in the case of blood plasma (endogenous Ca: 16% of total faecal Ca; true absorption: 85% of Ca intake). In conclusion, the endogenous faecal Ca can be estimated by the isotope-dilution method. Blood plasma is the most suitable reference source. The lag time between the parenteral ⁴⁵Ca injection and the onset of faecal ⁴⁵Ca monitoring should be at least 1 week.