This project is a study of the digitalis-receptor and of its biosynthesis in the heart. In the first phase of the project the digitalis-receptor will be isolated from the purified cardiac sarcolemma of a digitalis-sensitive (guinea pig) and a digitalis-insensitive species (rat). Three procedures will be tried for the isolation of the receptor: (1) Affinity labeling of the receptor in the particulate membranes and subsequent isolation with polyacrylamide gel electrophoresis. Tritiated, oxidized ouabain, digoxin or digitoxin will be bound covalently to the receptor as the affinity label. (2) Solubilization of the receptor with detergents and fractionation by rush-precipitation, gel-filtration and ion-exchange chromatography. (3) Affinity chromatography of the solubilized receptor on agarose to which an oxidized digitalis glycoside was attached. The structure of the purified receptor will be investigated to answer the following questions: (a) What is the gross chemical structure of the receptor? (b) What is the relationship between the digitalis-receptor and the Na ion-pump? (c) Which factors determine the affinity of the receptor for digitalis. In the second phase of the project the biosynthesis of the digitalis-receptor in the heart will be investigated. The turnover of the receptor will be estimated by single and double pulse labeling with radioactive amino acids. The effect of hormones and drugs (including digitalis derivatives) on the turnover will be studied to elucidate the physiological regulation and possible pharmacological modification of the turnover. The structure and the biosynthesis of the receptor will be investigated both in digitalis-sensitive and insensitive species to find the cause of this biological difference.