Positive inotropic effect of porcine left ventricular extract on canine ventricular muscle

1 We previously isolated an extract from porcine left ventricle that possessed digitalis‐like properties such as inhibition of cardiac and kidney Na + , K + ‐ATPase, displacement of [ 3 H]‐ouabain from its binding sites and cross reactivity with digoxin antibodies. The extract also had a positive inotropic effect on the guinea‐pig heart. 2 In the present study the positive inotropic response of the extract was characterized in canine right ventricular trabeculae. Maximum inotropic response (501 ± 20%) was produced by 300 μl and the half maximal increase occurred with 125 μl of the extract. 3 Ouabagenin produced aftercontractions in rapidly paced trabeculae. Equipotent and even greater amounts of the extract did not produce aftercontractions. 4 The extract increased the amplitude of the delayed component (P2) of biphasic contractions produced by replacing about 92–96% of the external Ca with Sr. A smaller increase in the size of the early component (P1) was also seen. 5 The extract decreased post‐rest potentiation after rest for 30 s and 2 min. After 8 min of rest, post‐rest potentiation was converted to post‐rest depression. 6 The extract (20 μl) produced a decrease in the amplitude of the post‐rest rapid cooling contracture (RCC) at all rest intervals. The steady state RCC, although greater than that in the control muscle, was increased to a lesser extent than the size of the steady state electrically driven contractions. 7 It is suggested that the extract from porcine left ventricle produces a positive inotropic response by increasing the trans‐sarcolemmal influx of Ca. It also has additional effect(s) on the sarcoplasmic reticulum in that it may facilitate the loss of Ca from the sarcoplasmic reticulum and/or inhibit the uptake of Ca by the organelle.