An unknown endogenous inhibitor of Na/Ca exchange can enhance the cardiac muscle contractility

The cardiac sarcolemma Na/Ca exchanger is a key system for controlling the intracellular calcium levels during the excitation-contraction coupling. Here, we test the hypothesis that the heart tissue contains a putative endogenous factor having a capacity to modulate the Na/Ca exchanger and muscle contractility. The concentrated cardiac extracts inhibit the Na(i)- or Ca(i)-dependent (45)Ca uptakes in isolated cardiac sarcolemma vesicles as well as the Na(o)-dependent Ca efflux, monitored by extravesicular Ca probe fluo-3. The inhibitory activity has been purified approximately 2000-fold by normal and reversed-phase HPLC procedures. The inhibitory activity is eluted from the Sephadex G-10 in the range of 350-550 Da, suggesting that the inhibitory factor is a low-molecular-weight substance. The mass spectra analysis shows a number of signals within m/z 380-560; however, it is not clear at this moment whether these recordings represent the mass of putative inhibitory factor or irrelevant impurities. The endogenous inhibitory factor of Na/Ca exchange does not resemble the properties (HPLC retention time, mass spectra, amino acid analysis, etc.) of autoinhibitory XIP peptide. The addition of inhibitory factor to muscle strip of guinea pig ventricles induces 2- to 5-fold enhancement of isometric contractions, thereby exhibiting a strong positive inotropic effect. This effect is a dose-dependent phenomenon, which can be reversed by washing the inhibitory factor from the organ bath. Assuming a molecular weight of 350-550 Da, the effective concentrations of putative inhibitor must be <10(-6) M. Therefore, the present findings demonstrate that the mammalian heart contains a low-molecular-weight factor that can inhibit Na/Ca exchange and enhance the cardiac contractility.