Some electrocardiographic responses of isolated embryonic chick hearts to digitoxigenin and selected thiocardenolides

Naturally occurring cardiac glycosides, such as digitalis, are well known for their therapeutic effects in the treatment of congestive heart failure (Goodman and Gilman, 1970). Administration of these glycosides elicits an increase in the force of myocardial contraction known as the positive inotropic effect which indicates increased efficiency of the myocardium (Marks, .1964) . A cardiac glycoside is a combination of an aglycona with one to four sugar molecules on carbon 3 (see Figure 1A). Physiological activity is imparted by the aglycone or genin portion of the molecule (Hoch, 1961). Its basic structure (see Figure IB) consists of a steroid nucleus with an alpha, beta-unsaturated lactone ring at carbon 17 (Goodman and Gilman, 1970). The lactone ring has been shown to be essential for cardiac activity of the glycosides and aglycones (Meyers, et al. , 1968). Addition of a sugar moiety to the aglycone imparts greater solubility in water, facilitates membrane transport and increases potency and duration of glycoside activity but does not otherwise affect cardioactivity (Drill, 1971).