Endogenous inotropic factor-induced endothelium-dependent relaxation of vascular smooth muscle

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.

Species differences in the effects of an endogenous inotropic factor (EIF) on the myocardium and aortic smooth muscle

Possible species differences and organ specificity in contractile or relaxation effects of an endogenous inotropic factor (EIF) isolated and purified from porcine heart left ventricle were examined in guinea pig and rat isolated atria, trabeculae and aortic smooth muscle rings. EIF demonstrated a dose-dependent increase in contractile force in guinea pig and rat atria and right ventricle trabeculae. The magnitude of the contractile effect was significantly lower in both the preparations of rat species as compared with guinea pig. In rat isolated aortic rings, EIF had no effect on the basal muscle tone. However, when rings were pre-contracted with phenylephrine EIF caused dose-dependent relaxation of the muscle. When aortic rings isolated from guinea pig were used, EIF induced a dose-dependent contraction which could be blocked by pre-treating the rings with either 2 microM phentolamine or 20 microM prazosin. When these same pre-treated rings were pre-contracted with histamine before the addition of EIF, a dose-dependent relaxation of guinea pig aortic smooth muscle rings was observed. Also depletion of catecholamines from the pre-synaptic nerve terminals innervating the aortic rings, using either 0.6 mM rauwolscine or reserpinizing (5 mg/Kg) the guinea pig 24 hours before sacrificing the animal, completely prevented EIF-induced contraction of the aortic rings. Instead EIF caused a dose dependent relaxation of the histamine pre-contracted aortic rings similar to that observed in rat aortic rings. The relaxation effect of EIF was demonstrated to be endothelium dependent and nitric oxide mediated in both species since EIF-induced relaxation could be inhibited by 2 microM L-NAME, a nitric oxide synthase inhibitor. Atropine (0.2 microM) or Indomethacin (10 microM) had no significant effect on EIF-induced relaxation of either guinea pig or rat aortic smooth muscle.