THE PhwCIPLES AUD APPLICATIONS QF RADI. aGTIVE TRACERS TO THE MEDICAL AND BIOLOGICAL SCIENCES GY AQAD 4 Josenh G. Hamilton, E.D. The fundamental postulate upon which rests the value of the artificial radioelements as tracers is the fact that the radioactive isotope of a stable element differs only in its property of radioactivity. The chemical and physio- logical properties of the two forms of the element, or compound, into which a radioactive element may be incorporated, are identical so long as the radiations from the radioactive isotopes are not sufficiently intense to produce physiological changes. In general, this limiting factor can be avoided in most tracer studies since the amounts of radioactive material required for studies of this character are too small to produce any chemical or physiclogical changes by the action of their radiations. Three general technics have been developed for the use of radioactive isotopes as tracers in the biological and medical sciences: First, the assimila- tion of the administered radioelement, or labelled compound, its distribution in the tissues, conversion into other compound by the body, and finally its elimination may be followed quantitatively by direct measurement of the radioactivity of the samples of tissues after the removal from the body, Second, the selective accumulation of the radioactive isotopes of a number of elements and compounds may be observed in the organs and tissues of the intact animal or human subject by measurement of the radioactivity of these structures in situ. Third, the correla- tion between the distribution of the radioactive clement or compound in the tissues to their microscopic anatomy may .e studied by the aid of photogranhic films. This last mentioned technic is usually defined as radioautography,. The first named technic is the most widely employed and is applicable to all the radio-elements, The radioactive material may be given either as a simple inorganic compound, or may be incorporated into a complex organic molecule. For examole, radio-sodium is usually administered as sodium chloride, radio-phosphorous as disodium phosphate, radioc-iodine as sodium iodide, etc. The distribution of labelled elements or compounds in the body may be followed as the total content of the administered radioactive material in the various organs, tissues and excreta. For example, the rates of conversion of administered radioelements to various complex compounds by the physiological processes of the body may be observed. Illustrative of this point is the synthesis of hemoglobin from iron, of phospholipids, nucleic acids, and nucleo-vroteins from inorganic phosphates, and of thyroxin from iodine. In studies of this character, the compound is isolated from the tissues and its radioactivity is compared with the total amount of radio-element originally administered. Complex organic compounds may be labelled for traccr studies by the inclusion of radioactive atoms in the molecules, For example, thiamin (vitamin B,) has been tagged by synthesizing it from radio-sulfur. ‘The labelled thiamin is then admrinistered and its fate in the body followed by measuring the distribution of radioactivity from the radio-sulfur in the tissues, body fluids and excreta. By this procedure its conversion in the body into other compounds may be observed. The), comparatively recent availability of the 5,000 year radioisotopes of carbon, C-", has made 1t possible to label amino acids, carbohydrates, drugs, hormones, vitemins, etc., to study their metabolic nathways and fate in the body, A second general technic makes use of the ability of many artificial radioelements to emit nenetratinge gamma rays which can pass through many centimeters of tissue without serious attenuation and which can thus be measured at some distance from the site of origin. The presence of the accumulated radio-clement in the particular organ or tissue under study is detected by placing a suitable measuring BEST COPY AVAILABLE DO’ ARCHIVE