In vivo transport of calcium from healed Thiry-Vella fistulas in dogs

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Abstract
Calcium transport was studied in vivo in dogs by perfusing solutions of various calcium concentrations through healed jejunal Thiry-Vella fistulas. The method is simple, quantitative, and it avoids a number of difficulties of interpretation inherent in tracer and in vitro studies. At quite low concentrations the rate of Ca absorption was approximately proportional to Ca concentration, but at higher concentration the rate fell off continuously. Above 12.5 mm/liter the absorption rate remained constant at approximately 0.5 mm/hr. There was some variation from week to week in the same dog and between different dogs. However, in 30 runs on 10 dogs this maximum absorption rate averaged 18.5 ± 1.1 mg Ca/hr. Simultaneous administration of magnesium depressed maximum Ca absorption of 9.3 mg Ca/hr. The calcium absorption data conformed to the Michaelis-Menten equation. The approach to a maximal absorption rate with increasing Ca concentration, the Mg competition, and the conformity to the Michaelis-Menten kinetics all suggest that Ca is absorbed by a carrier system which may involve either active or facilitated transport. The method lends itself to studies of physiological factors which may affect calcium absorption.

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