Control of the Na-Ca exchanger in isolated heart cells. I. Induction of Na-Na exchange in sodium-loaded cells by intracellular calcium.

Abstract

Isolated adult rat heart cells in suspension were loaded with sodium by incubation with ouabain in the absence of calcium for 30 minutes. Addition of low levels of calcium induced accelerated rates of sodium influx and efflux, as measured with 22Na. The magnitude of calcium-induced 22Na efflux was 50-fold greater than the net rate of calcium uptake and required extracellular sodium, but not extracellular calcium, once some calcium was taken up. Calcium did not induce 8'Rb efflux. The accelerated rate of 22Na efflux was prevented by verapamil, but verapamil was ineffective when added after calcium. Addition of EGTA after calcium reversed the effect of calcium, but only after incubation. Dichlorobenzamil, unlike verapamil, both prevented and reversed the induction of sodium fluxes by calcium. We conclude 1) that intracellular calcium induces Na-Na exchange through the Na-Ca exchanger in sodium-loaded cells exposed to calcium; and 2) that Na-Na exchange can be activated by calcium that enters the cell through calcium channels. We propose that this Na-Na exchange reflects the intrinsic activity of the Na-Ca exchanger. (Circulation Research 1991;69:1506-1513 T he Na-Ca exchanger is an active component of the sarcolemma in heartl-5 and nerve.6-10 Although the participation of the exchanger in excitation-contraction coupling in heart has long been postulated, its role has remained controversial. There is mounting evidence that the exchanger does have a major role in calcium efflux11-14 and perhaps also in calcium influx during excitation.1516 To elucidate this role it is important that the properties of the exchanger in heart be well understood. To date, most of the detailed studies on Na-Ca exchange in intact tissue have been done with squid axon, where both internal and external solutes can be controlled. Two significant properties revealed by these studies are that the exchanger is controlled by ATP7 and by intracellular calcium.1017 The exchanger in heart is likewise controlled by ATP18 and by intracellular calcium.19-21 In squid axon, Na-Na exchange activated by intracellular calcium has also been demonstrated.10These authors showed that Nai-Na0 exchange and Nai-Ca0 exchange had a similar affinity for activation by intracellular calcium and concluded that the Na-Na exchange was a mode of operation of the Na-Ca exchanger. We show here that a similar Na-Na exchange mode exists for the Na-Ca exchanger in heart and that this activity can be induced by calcium that enters the cell through calcium channels. We propose that this Na-Na exchange activity is indicative of the extent of activation of the Na-Ca exchanger by intracellular calcium. The significance of this result goes beyond a comparison between heart and nerve. In the following article we demonstrate that in normal cells at rest there is a near absence of intracellular calcium-dependent Na-Na exchange activity through the exchanger, but such activity can be induced by electrical stimulation. On the basis of our conclusions here, this implies that, at rest, the...

Synergistic interactions between Ca2+ entries through L‐type Ca2+ channels and Na+–Ca2+ exchanger in normal and failing rat heart

“…One possibility is that the crosstalk between LCCs and NCX is mediated by Ca 2+ . Haworth et al (1991) and Matsuoka et al (1995) have shown that NCX can be activated by Ca 2+ entry through LCCs, presumably by binding to the cytosolic, catalytic Ca 2+ ‐activation site on the exchanger. According to several recent studies, the affinity of the cytosolic Ca 2+ activation sites is in the range of 300–600 n m (Levitsky et al 1994; Reeves & Condrescu, 2003) (although some studies reported lower values, e.g.…”

“…Additionally, there may be interplay between different Ca 2+ ‐entry pathways in initiation of SR Ca 2+ release. For example, it has been proposed that Ca 2+ entry through LCCs and reverse‐mode NCX interact synergistically to activate RyR channels in the SR (Haworth et al 1991; Litwin et al 1998; Viatchenko‐Karpinski & Györke, 2001). Integration of Ca 2+ ‐input signals from different entry pathways could provide an important mechanism for regulation of cardiac EC coupling and contractility.…”

Ca 2+ Sparks in Rabbit Ventricular Myocytes Evoked by Action Potentials

“…42 Ca 2ϩ influx through the exchanger is increased by that through LCCs. 43,44 Therefore, when a brief opening of an LCC cannot produce sufficient accumulation of Ca 2ϩ to trigger sparks specially at positive potentials (eg, APs), it is possible that Ca 2ϩ entry through LCCs activates Ca 2ϩ entry by Na ϩ -Ca 2ϩ exchanger and these trigger a spark in concert when neither of them can do so alone. However, Ca 2ϩ influx through LCCs is still essential to trigger sparks.…”

Na+currents are required for efficient excitation-contraction coupling in rabbit ventricular myocytes: a possible contribution of neuronal Na+channels

“…This embodies the classical ideas of Ca 2+ ‐induced Ca 2+ release proposed by Fabiato (1983), as well as the original evidence by London & Krueger (1986) that Ca 2+ currents ( I Ca ) are responsible for triggering Ca 2+ release from the terminal cisternae of the SR. Although it is widely held that I Ca is the principal trigger for SR Ca 2+ release in cardiac cells, there are studies suggesting that the Na + –Ca 2+ exchanger (NCX) may also be involved in this process (Haworth et al 1991; Nuss & Houser, 1992; Kohomoto et al 1994; Levi et al 1994 b ; Vites & Wasserstrom, 1996 a , b ; Wasserstrom & Vites, 1996; Litwin et al 1998).…”