Pharmacological characterization of the activity of endogenous inotropic factor from porcine left ventricle.

We report some of the unique pharmacological properties of a semipurified endogenous inotropic factor (EIF) present in the extract of the porcine left ventricle. EIF produced the following effects: (a) increase in isometric contractile force developed by electrically driven canine right ventricular trabecula, reaching a maximum with 60-100 microliters/ml concentration; (b) inhibition of Na-pump activity in canine portal vein; (c) no digitalis-like cardiac toxicity, e.g., increased diastolic tension or spontaneous diastolic mechanical oscillatory activity, despite inhibition of the sodium pump; (d) a small increase in sarcoplasmic reticular Ca release from the heart but a large increase in transsarcolemmal Ca influx as seen in biphasic contractions, an action similar to that produced by digitalis-like substances; and (e) prolongation of the action potential duration and refractory period of the canine isolated trabeculae. This latter action may confer a unique antiarrhythmic property to EIF.