A Biological Method for the Estimation of Calcium Ion Concentration*

The sensitivity of the frog’s heart to changes in the calcium content of its nutrient fluid has long been known. Straub (1) commented that this sensitivity could be made use of to titrate calcium concentrations. Trendelenburg and Goebel (2) used it to demonstrate the reduction in calcium in the serum of the cat in parathyroid tetany. Klinke (3), in his comprehensive review, mentions the frog’s heart as a possible biological indicator of calcium ion concentrations, but expresses doubt, based partly on the work of Giinther and Heubner (4), as to whether its contraction is affected by calcium in the ionized form alone. The use made of the frog’s heart by Clark, Percival, and Stewart (5) in a study of calcium and citrate mixtures is referred to in detail in the accompanying paper (6). The method here described is the result of an attempt to utilize this sensitivity of the frog’s heart, in a strictly quantitative manner, for the estimation of calcium ion concentrations in certain saline solutions and biological fluids. The validity of the method is based upon the following assumptions or demonstrations : 1. That all of the calcium in the solutions used as standards of reference is present in ionized form, even when these solutions are supersaturated with respect to CaC03. Since all Ca++ observations are expressed in terms of the standard solutions, any error due to this assumption would have a corresponding effect upon the Ca++ concentrations estimated by the method.