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. He re, we test the hypothesis that the heart tissue contains a putative endoge nous factor having a capacity to modulate the Na/Ca exchanger and muscle co ntractility. The concentrated cardiac extracts inhibit the Na-i- or Ca-i-de pendent Ca-45 uptakes in isolated cardiac sarcolemma vesicles as well as th e Na-o-dependent Ca efflux, monitored by extravesicular Ca probe fluo-3. Th e inhibitory activity has been purified similar to 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 fa ctor 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 mom ent whether these recordings represent the mass of putative inhibitory fact or or irrelevant impurities. The endogenous inhibitory factor of Na/Ca exch ange does not resemble the properties (HPLC retention time, mass spectra, a mino acid analysis, etc.) of autoinhibitory XIP peptide. The addition of in hibitory factor to muscle strip of guinea pig ventricles induces 2- to 5-fo ld enhancement of isometric contractions, thereby exhibiting a strong posit ive 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 t hat the mammalian heart contains a low-molecular-weight factor that can inh ibit Na/Ca exchange and enhance the cardiac contractility. (C) 2000 Academi c Press.