Sodium-calcium ion exchange in cardiac membrane vesicles.

Abstract

Membrane vesicles isolated from rabbit ventricular tissue rapidly accumulated Ca2+ when an outwardly directed Na+ gradient was formed across the vesicle membrane. Vesicles loaded internally with K+ showed only 10% of the Ca2+ uptake activity observed with Na+-loaded vesicles. Dissipation of the Na+ gradient with the monovalent cation exchange ionophores nigericin or narasin caused a rapid decline in Ca2+ uptake activity. The Ca2+-ionophore A23187 inhibited Ca2+ uptake by Na+4loaded vesicles and enhanced the rate of Ca2+ loss from the vesicles after uptake. Efflux of preaccumulated Ca2+ from the vesicles was stimulated 30-fold by the presence of 50 mM Na+ in the external medium. Na+-dependent uptake and efflux of Ca2+ were both inhibited by La3+. The results indicate that cardiac membrane vesicles exhibit Na+-Ca2+ exchange activity. Fractionation of the vesicles by density gradient centrifugation revealed a close correspondence between Na+-Ca2+ exchange activity and specific ouabain-binding activity among the various fractions. This relationship suggests that the observed Na+-Ca2+ exchange activity derives from the sarcolemmal membranes within the vesicle preparation.Na+-Ca2+ exchange is a process whereby transmembrane movements of Ca2+ are coupled directly to reciprocal movements of Na+. This process has been demonstrated in a variety of excitable tissues (1-3), as well as in dog erythrocytes (4). In cardiac muscle, attention has been focused on Na+-Ca2+ exchange as a possible entry mechanism for contractile Ca2+ on a beat-to-beat basis (5), as a mechanism for extrusion of Ca2+ from the cell (6), and as a mediator of the inotropic effects of cardiac glycoside administration and changes in stimulation frequency (1,5,(7)(8)(9).Thus far, the study of Na+-Ca2+ exchange in cardiac tissue has been based on mechanical responses (10-12) or isotopic flux data (3, 6, 9, 13) obtained with intact muscle preparations. The pioneering work of Kaback and colleagues (14,15) showed that transport phenomena could be studied by using sealed membrane vesicles, a system in which the conditions on either side of the membrane can be manipulated at will. Vesicles derived from cardiac sarcolemma have been used to study the enzymatic activity of transport ATPases (16)(17)(18)(19), and several investigators have recently reported that sarcolemmal membrane vesicles exhibit ATP-dependent transport of Ca2+ (20)(21)(22)(23)(24).The present report shows that cardiac membrane vesicles exhibit Na+-Ca2+ exchange activity. The data demonstrate that transmembrane Ca2+ movements in either direction are markedly stimulated by generating an oppositely directed gradient of Na+. These Na+-Ca2+ interactions appear to be associated with the sarcolemmal membranes within the preparation. MATERIALS AND METHODSPreparation of Membrane Vesicles. Ventricular tissue from male New Zealand rabbits (1.4-2.0 kg) was finely minced with scissors in 3-4 vol of ice-cold 0.3 M sucrose/5 mM MgSO4/10 mM imidazole-HCI (pH 7.0). Several different homogenization procedu...

Sodium/calcium exchange in smooth-muscle microsomal fractions

“…The Na+ concentration producing half-maximal depression of Ca2 + uptake (intersection with the abscissa) was 28 mM. This value is close to that reported by Reeves & Sutko (1979) for Na+/Ca2+ exchange in cardiac sarcolemma (half-maximal inhibition at 16mM-NaClo).…”

“…The Ca2 + concentration at which halfmaximal Ca2+ accumulation was observed (Ko.5) was 50OM for aortic microsomal fraction and 18pgM for intestinal smooth-muscle microsomal fraction. Similar K0.5 values have been reported for the Na+/Ca2+ exchange system in cardiac vesicles by Pitts & Okhuysen (1980) (26iM) and by Reeves & Sutko (1979) (18pM), and the half-maximal value of Na+-dependent Ca2+ uptake was obtained at 7upM-Ca2 + in myometrium plasma-membrane vesicles (Grover et al, 1983).…”

“…The characteristics of the Na+-driven Ca2 + transport identified in intestinal and vascular smooth-muscle microsomal fractions do not appear different from Na+/Ca2 + exchange systems described in brain, cardiac and uterine plasmamembrane vesicles (Gill et al, 1981;Reeves & Sutko, 1979;Grover et al, 1981Grover et al, , 1983. The coexistence of a Na+/Ca2 + exchange system with the ATP-dependent Ca2 + pump in smooth-muscle plasmalemmal vesicles is consistent with Brading's (1979) suggestion that the smooth-muscle cell might possess the two Ca-extrusion mechanisms.…”

“…A Na+/Ca2 + exchange mechanism has been clearly demonstrated in plasma-membrane fractions from heart (Reeves & Sutko, 1979;), brain (Gill et al, 1981) and myometrium (Grover et al, 1981). The purpose of the present work was to study the influence of Na + on Ca2 + movements Vol.…”

The receptor function of the Na+, K+-activated adenosine triphosphatase system

“…Although the chemical nature of the Na+: Ca2+ exchanger is not yet known, the functional demonstration of its presence in cardiac sarcolemmal vesicles (Reeves & Sutko, 1979;Pitts, 1979;Caroni et al, 1980;Miyamoto & Racker, 1980;Philipson & Nishimoto, 1980), and coreconstitution of the brain Na+:Ca2+ exchanger and Na,K-ATPase in liposomes (Eckert & Grosse, 1982) gives hope for its early isolation and characterization.…”