The process linking excitation to contraction in cardiac muscle is complex, possibly involving two or more coupling mechanisms acting on as many as five separate Ca compartments. To understand the process fully, it is necessary to be able to separate these individual processes through the use of various physiologic or pharmacologic inhibitors. However, inhibitors of excitation-contraction coupling have not been fully characterized. This research proposes to examine more fully the nature of the negative inotropic actions of several clinically useful agents. These drugs will be investigated in a variety of animal species having apparently different mechanisms of excitation-contraction coupling to determine common sites and mechanisms of action. The research will utilize standard isometric recording techniques and Na-free or Na-poor solutions to examine the actions of these drugs on Ca:Na exchange processes and on electrogenic Ca current. In addition, the role of the sarcolemma as a site of drug action will be differentiated from that of the intracellular organelles through the use of mechanically disaggregated hyperpermeable muscle fragments. This method permits the study of the inotropic and subcellular actions of drugs in the absence of a functioning sarcolemma. At a minimum, this research will provide new insights into the actions of several clinically useful drugs. If the research is maximally informative, it will also provide new information about the processes of excitation-contraction coupling in cardiac muscle, eventually leading to a deeper understanding of the process of heart failure.