The fixation of radioactive digitoxin by isolated hearts.

The nature of digitoxin fixation in isolated hearts was studied with C14 labelled digitoxin. Fixation wvas greatest in the early stages of perfusion. The amount of drug fixed was measured directly. A considerable percentage cf the digitoxin in cardiac muscle was changed to other substances. These substances are more firmly bound to the heart than digitoxin. T HE preparation of pure, randomly labelled , radioactive digitoxin in this lab-oratory' has made available a sensitive tracer technic particularly suited to studying the fate of digitoxin in isolated organs and intact animals. In contrast to other methods of assay, this new technic enables one to follow the unchanged drug as well as metabolic products formed after interaction with body tissues. The extreme sensitivity permits experiments with digitoxin in amounts which are not lethal to the intact animal. Furthermore, in perfusion experiments on isolated hearts, the concentration of labeled digitoxin is low. The work of previous investigators has not settled the problem of digitoxin fixation in cardiac muscle. Conclusions drawn were only by inference since the results depended on bio-logic assay experiments. With the radioactive technic, direct measurement of digitoxin uptake can be carried out on homogenates of hearts previously perfused with radioactive solutions. The percentage of conversion to other substances can also be determined by this direct method. Furthermore, studying the radioactivr-ity of the perfusion fluids of isolated heart preparations enables one to judge the rapidity of drug fixation by heart muscle. With these possibilities in mind we studied the fixation of radioactive digitoxin by isolated hearts of rats, guinea pigs, rabbits and cats. 100 METHODS A small amount of radioactive digitoxin with a specific activity of 350 counts per minute per ug. was supplied in a stock solution containing 50gg. of drug per cc. of 95 per cent ethanol. For perfusion, digitoxin was diluted with Ringer-Locke solution to a concentration of 0.2 ,gg. per cc. The hearts were perfused by the Langendorff2 technic through a cannula inserted into the aorta. A two-way stopcock above the cannula permitted rapid change from control to radioactive solutions. After stabilization of the rate and rhythm, the hearts were perfused with radioactive digi-toxin and six successive 25 cc. samples of per-fusate were collected. Control specimens were drawn from the side arm of the cannula. Immediately following perfusion, a 10 per cent water homogenate of the hearts was made and 1 cc. volumes of homogenate and perfusion …