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

Abstract

Summary 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. Zusammenfassung Isotopen‐Verdünnungsmethode zur Bestimmung der endogenen fäkalen Exkretion und wahren Absorption von Calcium bei 45Ca markierten Ratten In einem Stoffwechselversuch sollten die methodischen Aspekte der Isotopen‐Verdünnungsmcthode zur Bestimmung der endogenen fäkalen Exkretion und wahren Absorption von Calcium bei der Laborratte geklärt werden. Hierzu wurde 32 wachsenden, männlichen SD‐Ratten eine 45Ca‐Lösung i.m. injiziert und je 8 Tiere am Tag 6, 9, 12 und 14 nach der Injektion getötet. Während des Experiments wurde einheitlich eine semisynthetische Diät mit 0,43% Ca (als CaCO3) restriktiv gefüttert. Am ersten Tag nach der 45Ca‐Injektion betrug die fäkale und renale 45Ca Exkretion 10‰ bzw. 3‰ der injizierten Aktivität. Innerhalb der nächsten 2 Tage sank die tägliche 45Ca‐Exkretion auf 3‰ (Kot) bzw. 0,5‰ (Harn) und nahm im weiteren Versuchsverlauf kontinuierlich ab. Die Verteilung der täglichen 45Ca‐Ausscheidung zwischen Kot und Harn war nach einer Woche Zeitabstand zur 45Ca‐Injektion...