Abstract
Trendelenburg and Goebel 1 demonstrated that serum from thyro-parathyroidectomized cats, when used as a nutrient fluid for the isolated heart of the frog, diminished the amplitude of contraction of the heart, as compared with serum from normal cats. They attributed this effect to a reduction in the total calcium of the serum, with a corresponding decrease in ionized calcium, but were unable to rule out the possibility of the formation of an un-ionized compound of calcium, presumably by combination with an organic acid.
Two of the authors have shown that the sensitivity of the frog's heart to changes in the concentration of calcium is specific for ionized calcium, bound calcium being inert with respect to the preparation, and have utilized this property of the heart for direct quantitative estimations of Ca++ concentrations in biological fluids. 2 They have also shown 3 that the ionization of calcium in the protein-containing fluids of the human body follows, as a first approximation, a simple mass-law relationship, expressed by the equation
from which it is possible to calculate Ca++ concentrations, from analysis for total calcium and total protein, with a degree of accuracy at least as great as that of direct observation by the frog heart method.
Using the methods of direct observation and of calculation of Ca++ concentrations, the authors have studied the influence of thyro-parathyroidectomy and of the administration of the parathyroid hormone upon the ionization of calcium in the serum of the cat. The cat was chosen as the experimental animal because its serum was found to be especially favorable for observations by the frog heart method, which is not the case with the dog, and because the state of calcium in its serum was found to be accurately described by the mass-law relationship above referred to, which is not the case with the rabbit.
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