An endogenous positive inotropic factor (EPIF) from porcine heart: its effects on sarcoplasmic reticular (SR) Ca2+ metabolism

We have isolated an endogenous positive inotropic factor (EPIF) from porcine left heart ventricular tissue, which demonstrated to have only weak digitalis-like properties including the inhibition of myocardial Na+,K(+)-ATPase. EPIF completely lacks digitalis-like toxicity such as after-contractions in larger doses. In our recent studies, we have demonstrated that EPIF produces a decrease in the amplitude of the post-rest rapid cooling contracture which indicated that EPIF may release Ca2+ from the sarcoplasmic reticulum. In the present study, the effects of EPIF were investigated on the Ca2+ uptake and release properties of SR enriched membrane vesicles from rat heart. At pH 6.8 and in the presence of oxalate, EPIF dose-dependently inhibited the ATP-dependent uptake of Ca2+ by SR vesicles. Concentrations as low as 25 ul (in 1 mL uptake medium) of EPIF caused a 45-47% reduction in the uptake of Ca2+ within 3-4 min. Increases in EPIF concentration to 50 ul/mL caused additional reduction of only 15-20% in the uptake of Ca2+. Concentrations of 25 ul/mL of EPIF had little or no effects on passive release of actively loaded Ca2+ in SR vesicles. On doubling the concentrations to 50 ul/mL EPIF, however, enhanced the release of Ca2+ by 25-28% during 1-2 min and 44-48% after 4 min of incubation of Ca2+ loaded vesicles in the release medium. Relatively smaller effects of EPIF on Ca2+ release implies that EPIF may mainly lower the uptake of Ca2+ in SR. This reduced uptake of Ca2+ may be explained by the EPIF-induced inhibition of Ca2+ pump.