Further characterisation of the inotropic effect of a bufodienolide glycoside -- an endogenous ouabain like compound

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Abstract
A ouabain like compound obtained from toad skin and plasma and identified to be a steroidal bufodienolide glycoside was found to displace ouabain from its binding site and to inhibit Na+-K+ATPase, and have positive inotropic effects on cardiac muscle. The ionic and rate dependence of this positive inotropy was studied in the frog atrium. The effect was dependent on extracellular potassium, sodium, and calcium concentrations and on the rate of stimulation, which is similar to the properties of cardiac glycosides. Occasionally, the compound gave transient or even negative inotropic responses, as do the glycosides. The action potential configuration was also affected by the compound in the same complex pattern as is that of cardiac glycosides. It is concluded that the endogenous bufodienolide compound has the same physiological effects as cardiac glycosides. Since the same compound is present in toad plasma it may serve as an intrinsic humoral regulator of cardiac contractility. This study is the first detailed characterisation of the cardioactive properties of this compound.