Volume of distribution of radioactive chloride in dogs; comparisons with sodium, bromide and inulin spaces.

I N THIS study two of the radioactive isotopes of chlorine, Cl36 (half life 400,000 years) and C138 (half life 38 minutes) have been used in an effort to establish more definitively than has been done previously the relationships, in the dog, of the chloride space to the volumes of distribution of sodium, bromide and inulin. The first measurements of the radioactive chloride space in dogs were made by Winkler, Elkington and Eisenman (I) who reported that C138 distributed through approximately 25 per cent of the body weight compared to 28 per cent for Na? Burch, Ray and Threefoot (2), using C136, reported volumes of distribution varying from 32 to 38 per cent in five young dogs. Following the observation that the bromide distribution in the plasma and tissues was proportional to that of chloride (3, 4), several workers have used the dilution of stable bromide as a measure of the chloride space. Weir (s), Gaudino, Schwartz and Levitt (6), Brodie, Brand and Leshin (7) have reported bromide spaces of 30 to 32 per cent body weight. Berger, Dunning, Steele and Brodie (8) and Gaudino (6) measured volumes of distribution of bromide that were approximately 150 per cent of simultaneously determined inulin spaces. Emphasizing direct and simultaneous comparisons in three groups of experiments, the relationships of the radioactive chloride space to those of radioactive sodium (Na24), radioactive bromide (Br82) and inulin were observed. The radioactive chloride and the substance with which it was being compared were injected simultaneously and the volumes of distribution were calculated from plasma concentrations determined from the same blood sample. As will be described, both the shortlived and the long-lived isotopes of chlorine are easily combined with Na24 and Br82 in double tracer experiments. These double isotope experiments are simple to carry out and more consistently accurate comparisons of the dilutions of two agents are obtained than when chemical methods are used.