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1 Department of Pharmacology, Ohio State University College of Medicine, Columbus, Ohio
Guinea-pig and rat hearts were perfused via the aorta at constant flow with 10-7 M digoxin-H3 for periods up to 64 min. The temperature was 28°C. The hearts were then perfused for 8 min with nonradioactive medium and the whole tissue and the subcellular fraction digoxin contents were measured. The arteriovenous digoxin difference in this system was zero after 2 to 4 min of perfusion. The heart tissue digoxin content, however, continued to rise throughout the 64 min of perfusion, particularly during the period from 4 to 32 min. The concentration of digoxin also rose in the nuclear, mitochondrial and microsomal fractions with time, the fastest rise and the greatest eventual concentrations being found in the microsomal fractions. Incubation of heart homogenates with digoxin failed to show similar accumulation by the particulate fractions. Rat and guinea-pig hearts were found to differ substantially when perfused with digoxin under the same conditions. Rat hearts demonstrated a reduced total digoxin uptake, higher ratios of supernate to pellet digoxin content and markedly lower digoxin concentration in all the particulate fractions. Preliminary studies of the nature of the digoxin binding to the microsomal fraction from guinea-pig hearts indicated that disruption of hydrogen bonding forces or of the tertiary structure of the microsomal protein was necessary before the digoxin was freely extractable. No binding to macromolecular structures could be demonstrated for the digoxin that was contained in the supernatant fraction of heart tissue. It was concluded that the microsomal fraction of heart tissue, which accumulates digoxin over a time course comparable to that required for the development of inotropic responses, may contain the cardiac digitalis receptors.
Submitted on July 27, 1967
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