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 共查询到10条相似文献,搜索用时 93 毫秒
1.
Identification of Na-Ca exchange current in single cardiac myocytes   总被引:18,自引:0,他引:18  
S Mechmann  L Pott 《Nature》1986,319(6054):597-599
In cardiac muscle the exchange of intracellular Ca2+ for extracellular Na+ is an important transport mechanism for regulation of the intracellular free Ca2+ concentration [( Ca]i) and hence the contractile strength of the heart. Due to its stoichiometry of greater than or equal to 3:1 Na+/Ca2+ (refs 3,5), Na-Ca exchange is supposed to generate a current across the cell membrane. It is thought that such a current may contribute to cardiac action potential and physiological or pathological pacemaker activity. Although the occurrence of Na-Ca exchange is well documented, a membrane current generated by this transport has not been identified unequivocally. Previous attempts to detect such a current in multicellular preparations, for example, by measuring small current differences after varying the extracellular ionic composition, although providing evidence, did not rule out other possible interpretations. Here we demonstrate that a transient rise in [Ca]i caused by release of Ca from sarcoplasmic reticulum (SR) generates a membrane current in cardiac myocytes. The dependence of this current on the transmembrane gradients for Na+ and Ca2+ and on membrane potential meets the criteria for a current produced by electrogenic Na-Ca exchange. Cyclic activation of this current by release of Ca from the SR can cause maintained spontaneous activity, suggesting that Na-Ca exchange contributes to certain forms of cardiac pacemaking.  相似文献   

2.
Electrogenic Na-Ca exchange in retinal rod outer segment   总被引:7,自引:0,他引:7  
K W Yau  K Nakatani 《Nature》1984,311(5987):661-663
Previous work has suggested that a Na-Ca exchanger may have a key role in visual transduction in retinal rods. This exchanger is thought to maintain a low internal free Ca2+ concentration in darkness and to contribute to the rod's recovery after light by removing any internally released Ca2+. Little else is known about this transport mechanism in rods. We describe here an inward membrane current recorded from single isolated rods which appears to be associated with such external Na+-dependent Ca2+ efflux activity. External Na+, but not Li+, could generate this current; high external K+ inhibited it while small amounts of La3+ (10 microM) completely abolished it. The exchanger can also transport Sr2+, but not Ba2+ or other divalent cations. The exchange ratio was estimated to be 3Na+:1Ca2+. As well as demonstrating clearly the Na-Ca exchanger in the rod outer segment, our experiments also cast serious doubt on the commonly held view that light simply releases internal Ca2+ to bind to and block the light-sensitive conductance.  相似文献   

3.
Kang TM  Hilgemann DW 《Nature》2004,427(6974):544-548
The cardiac Na+/Ca2+ exchanger (NCX1; ref. 2) is a bi-directional Ca2+ transporter that contributes to the electrical activity of the heart. When, and if, Ca2+ is exported or imported depends on the Na+/Ca2+ exchange ratio. Whereas a ratio of 3:1 (Na+:Ca2+) has been indicated by Ca2+ flux equilibrium studies, a ratio closer to 4:1 has been indicated by exchange current reversal potentials. Here we show, using an ion-selective electrode technique to quantify ion fluxes in giant patches, that ion flux ratios are approximately 3.2 for maximal transport in either direction. With Na+ and Ca2+ on both sides of the membrane, net current and Ca2+ flux can reverse at different membrane potentials, and inward current can be generated in the absence of cytoplasmic Ca2+, but not Na+. We propose that NCX1 can transport not only 1 Ca2+ or 3 Na+ ions, but also 1 Ca2+ with 1 Na+ ion at a low rate. Therefore, in addition to the major 3:1 transport mode, import of 1 Na+ with 1 Ca2+ defines a Na+-conducting mode that exports 1 Ca2+, and an electroneutral Ca2+ influx mode that exports 3 Na+. The two minor transport modes can potentially determine resting free Ca2+ and background inward current in heart.  相似文献   

4.
D W Hilgemann  D A Nicoll  K D Philipson 《Nature》1991,352(6337):715-718
Na+/Ca2+ exchange is electrogenic and moves one net positive charge per cycle. Although the cardiac exchanger has a three-to-one Na+/Ca2+ stoichiometry, details of the reaction cycle are not well defined. Here we associate Na+ translocation by the cardiac exchanger with positive charge movement in giant membrane patches from cardiac myocytes and oocytes expressing the cloned cardiac Na+/Ca2+ exchanger. The charge movements are initiated by step increments of the cytoplasmic Na+ concentration in the absence of Ca2+. Giant patches from control oocytes lack both steady-state Na+/Ca2+ exchange current (INaCa) and Na(+)-induced charge movements. Charge movements indicate about 400 exchangers per micron 2 in guinea-pig sarcolemma. Fully activated INaCa densities (20-30 microA cm-2) indicate maximum turnover rates of 5,000 s-1. As has been predicted for consecutive exchange models, the apparent ion affinities of steady state INaCa increase as the counterion concentrations are decreased. Consistent with an electroneutral Ca2+ translocation, we find that voltage dependence of INaCa in both directions is lost as Ca2+ concentration is decreased. The principal electrogenic step seems to be at the extracellular end of the Na+ translocation pathway.  相似文献   

5.
E Niggli  W J Lederer 《Nature》1991,349(6310):621-624
The sodium-calcium exchanger is critical in the normal functioning of many cells. In heart muscle, it is the principal way by which the cells keep the concentration of intracellular calcium low, pumping out the Ca2+ that enters the cytosol through L-type Ca2+ channels. The exchanger may also contribute to the triggering of Ca2+ release during voltage-activated excitation-contraction coupling in heart. Time resolved examination of the conformational changes of macromolecules in living cells has so far been largely restricted to ion-channel proteins whose gating is voltage-dependent. We have now directly measured electrical currents arising from the molecular rearrangements of the sarcolemmal Na-Ca exchanger. Changes in the conformation of the exchanger protein were activated by a rapid increase in the intracellular calcium concentration produced by flash photolysis of caged calcium in voltage-clamped heart cells. Two components of membrane current were produced, reflecting a calcium-dependent conformational change of the transporter proteins and net transport of ions by the exchanger. The properties of these components provide evidence that the Na-Ca exchanger protein undergoes two consecutive membrane-crossing molecular transitions that each move charge, and that there are at least 250 exchangers per micron 2 turning over up to 2,500 times per second.  相似文献   

6.
L M Crespo  C J Grantham  M B Cannell 《Nature》1990,345(6276):618-621
Compelling evidence has existed for more than a decade for a sodium/calcium (Na-Ca) exchange mechanism in the surface membrane of mammalian heart muscle cells which exchanges about three sodium ions for each calcium ion. Although it is known that cardiac muscle contraction is regulated by a transient increase in intracellular calcium ([Ca2+]i) triggered by the action potential, the contribution of the Na-Ca exchanger to the [Ca2+]i transient and to calcium extrusion during rest is unclear. To clarify these questions, changes in [Ca2+]i were measured with indo-1 in single cardiac myocytes which were voltage clamped and dialysed with a physiological level of sodium. We find that Ca entry through the Na-Ca exchanger is too slow to affect markedly the rate of rise of the normal [Ca2+]i transient. On repolarization, Ca extrusion by the exchanger causes [Ca2+]i to decline with a time constant of 0.5 s at -80 mV. The rate of decline can be slowed e-fold with a 77-mV depolarization. Calcium extrusion by the exchanger can account for about 15% of the rate of decline of the [Ca2+]i transient (the remainder being calcium resequestration by the sarcoplasmic reticulum (SR]. The ability of the cell to extrude calcium was greatly reduced on inhibiting the exchanger by removing external sodium, which itself led to an increase in resting [Ca2+]i. This finding is in contrast to the suggestion that calcium extrusion at rest is mediated mainly by a sarcolemmal Ca-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   

7.
Calcium is transported across the surface membrane of both nerve and muscle by a Na+-dependent mechanism, usually termed the Na:Ca exchange. It is well established from experiments on rod outer segments that one net positive charge enters the cell for every Ca2+ ion extruded by the exchange, which is generally interpreted to imply an exchange stoichiometry of 3 Na+:1 Ca2+. We have measured the currents associated with the operation of the exchange in both forward and reversed modes in isolated rod outer segments and we find that the reversed mode, in which Ca2+ enters the cell in exchange for Na+, depends strongly on the presence of external K+. The ability of changes in external K+ concentration ([K+]o) to perturb the equilibrium level of [Ca2+]i indicates that K+ is co-transported with calcium. From an examination of the relative changes of [Ca2+]o, [Na+]o, [K+]o and membrane potential required to maintain the exchange at equilibrium, we conclude that the exchange stoichiometry is 4 Na+:1 Ca2+, 1 K+ and we propose that the exchange should be renamed the Na:Ca, K exchange. Harnessing the outward K+ gradient should allow the exchange to maintain a Ca2+ efflux down to levels of internal [Ca2+] that are considerably lower than would be possible with a 3 Na+:1 Ca2+ exchange.  相似文献   

8.
Effects of ATP and vanadate on calcium efflux from barnacle muscle fibres   总被引:3,自引:0,他引:3  
M T Nelson  M P Blaustein 《Nature》1981,289(5795):314-316
Calcium ions carry the inward current during depolarization of barnacle muscle fibres and are involved in the contraction process. Intracellular ionized calcium ([Ca2+]i) in barnacle muscle, as in other cells, is kept at a very low concentration, against a large electrochemical gradient. This large gradient is maintained by Ca2+ extrusion mechanisms. When [Ca2+]i is below the contraction threshold, Ca2+ efflux from giant barnacle muscle fibres is, largely, both ATP dependent and external Na+ (Na+0) dependent (see also refs 5,6). When [Ca2+]i is raised to the level expected during muscle contraction (2-5 muM), most of the Ca2+ efflux from perfused fibres is Na0 dependent; as in squid axons, this Na+0-dependent Ca2+ efflux is ATP independent. Orthovanadate is an inhibitor of (Na+ + K+) ATPase and the red cell Ca2+-ATpase. We report here that vanadate inhibits ATP-promoted, Na+0-dependent Ca2+ efflux from barnacle muscle fibres perfused with low [Ca2+]i (0.2-0.5 microM), but has little effect on the Na+0-dependent, ATP-independent Ca2+ efflux from fibres with a high [Ca]i (2-5 microM). Nevertheless, ATP depletion or vanadate treatment of high [Ca2+]i fibres causes an approximately 50-fold increase of Ca2+ efflux into Ca2+-containing lithium seawater. These results demonstrate that both vanadate and ATP affect Ca2+ extrusion, including the Na+0-dependent Ca2+ efflux (Na-Ca exchange), in barnacle muscle.  相似文献   

9.
We have studied transmembrane La3+ movement in rat ventricular myocytes for the first time by using the whole-cell patch-clamp recording mode. La3+ (0.01-5.0 mmol/L) could not bring out inward currents through the L-type calcium channel in rat ventricular myocytes, while it could enter the cells by the same way carried by 1μmol/L ionomycin. When the outward Na+ concentration gradient is formed, La3+ can enter the cells via Na-Ca exchange, and the exchange currentsincrease with the increase of external La3+ concentrations. But compared with Na-Ca exchange currents in the same concentration, the former is only 14%-38% of the latter. The patch-clamp experiment indicates that La3+ normally can not enter ventricular myocytes through L-type calcium channel, but it can enter the cells via Na-Ca exchange.  相似文献   

10.
Oscillations of intracellular Ca2+ in mammalian cardiac muscle   总被引:2,自引:0,他引:2  
C H Orchard  D A Eisner  D G Allen 《Nature》1983,304(5928):735-738
Contraction of cardiac muscle depends on a transient rise of intracellular calcium concentration ([Ca2+]i) which is initiated by the action potential. It has, however, also been suggested that [Ca2+]i can fluctuate in the absence of changes in membrane potential. The evidence for this is indirect and comes from observations of (1) fluctuations of contractile force in intact cells, (2) spontaneous cellular movements, and (3) spontaneous contractions in cells which have been skinned to remove the surface membrane. The fluctuations in force are particularly prominent when the cell is Ca2+-loaded, and have been attributed to a Ca2+-induced Ca2+ release from the sarcoplasmic reticulum. In these conditions of Ca2+-loading the normal cardiac contraction is followed by an aftercontraction which has been attributed to the synchronization of the fluctuations. The rise of [Ca2+]i which is thought to underlie the aftercontraction also produces a transient inward current. This current, which probably results from a Ca2+-activated nonspecific cation conductance, has been implicated in the genesis of various cardiac arrhythmias. However, despite the potential importance of such fluctuations of [Ca2+]i their existence has, so far, only been inferred from tension measurements. Here we present direct measurements of such oscillations of [Ca2+]i.  相似文献   

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