Concentration of Calcium Ions in Biological Fluids

The sensitivity of the frog's heart to changes in the calcium content of a nutrient fluid has long been known^{1, 2}. Utilization of this property for the estimation of calcium ions in biological fluids has, however, not been generally considered feasible, because considerable doubt has existed 3 as to whether the known sensitivity is to calcium ions or to total calcium. The method here presented is based upon (1) the demonstration, simultaneously reported, that the reaction of the frog's heart is specific for calcium ions, bound calcium being without effect, and (2) elimination or control of other factors which may influence the behavior of the heart.

The method consists essentially of recording the amplitude of the contraction of the frog's heart, suspended from a modified Straub cannula inserted through the aorta into the ventricle. After taking certain precautions to standardize the procedure, the concentration of calcium ions in the unknown solution or biological fluid is taken to be that of the known solution which produces an amplitude of contraction most nearly matching that produced by the unknown. A satisfactory preparation is sensitive to changes of 0.1 millimol per liter of calcium ions (0.4 mg. per 100 cc.) or less, so that the theoretical accuracy of the method is to within± 0.1 mM per liter.

The method is applicable to such solutions or biological fluids as will sustain the action of the frog's heart without toxic effects. It can be used in artificially prepared salt solutions of widely varying composition, and has been found suitable for serum, plasma, cere-brospinal fluid, chest fluid, and ascitic fluid of human origin, and for serum and cerebrospinal fluid of the dog. It has been found unnecessary to dilute these fluids, the only precaution being to make the known solutions isotonic with the unknowns, rather than with frog's blood.