The ‘sodium pump lag’ revisited

The ‘sodium pump lag’ revisited

J ~Mol Cell Cardiol 15, 647451 ( 1983 ) EDITORIAL The ‘Sodium Pump In 1963 J. Walter Woodbury published a general discussion on the relation o...

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J ~Mol Cell

Cardiol

15, 647451

( 1983 )

EDITORIAL

The

‘Sodium

Pump

In 1963 J. Walter Woodbury published a general discussion on the relation of ion transport mechanism to excitatory events [32]. In the light of a number of recent studies on the relationship among sodium (Na), potassium (K) and calcium (Ca) exchange in the heart it now seems appropriate, twenty years later, to once again draw attention to the Woodbury paper and to the concept of the ‘Na pump lag’ which grew from it. The concept that excitable tissue would require time to adapt its Na transport rate to changes in frequency of excitation [32] and that the most important controller of Na pump activity was probably intracellular sodium, [Na]i, [25] led me [17] to consider the possibility that increased [Na]r might be primary to the treppe or force staircase response of heart muscle described by Bowditch in 1871 [4]. The original concept [19] was developed as follows : An increase of heart rate will increase cellular Na influx in proportion to the magnitude of the increased rate. For example, an increase from 60 to 120 beats/min will essentially double Na influx/min. 24Na and 42K flux studies [18, 291 showed that a loss of cellular K occurred over a period of minutes after an abrupt increment in heart rate and that during this time cellular Na exchange was increasing. The loss of K indicated a gain of cellular Na. The period during which K loss occurred was approximately the same period required for force redevelopment to achieve a steady value at the increased frequency. Therefore a period of some minutes was required for the rate of cellular Na efflux to match the increased Na influx rate. This delay in * Supported by USPHS 0022-2828/83/100647+05 M.C.C.

Grant $03.00/O

ROI

HL28539-01

Lag’

Revisited*

reaching a new steady-state, during which [NaJt was increasing, was termed the ‘Na pump lag’ [ 17, 191. (Lag is defined as ‘retarded in attaining maximum value’Webster’s Collegiate Dictionary.) 45Ca uptake studies [17] showed increased cellular uptake over the same period during which [Na]i was presumed to be rising and this increased Ca was proposed to be responsible for the positive force staircase [19]. It is to be emphasized that the concept of the lag should imply no inherent deficiency in the Na-pump mechanism-only that the sensitivity of the feedback system ([Na]i regulates its own pumping) requires that relatively large changes (few mmolar) of [Na]t occur to produce significant changes in pump rate. The early study by Conn and Wood [8] in dog heart indicated that steadystate cellular Na did, indeed, increase with heart rate. At rates in the range of 50 to 70/min cell Na was measured at 7.5 to 8.0 mmol/kg; at rates of 140/min cell Na was in the 17 to 19 mmol/kg range. These values were calculated by subtraction of mannitol space from total isotopically lahelled Na space and were, given this technique, approximate. The recent development of ion selective microelectrodes (ISME) has permitted direct and more accurate measurements of cellular Na activity (arNa) on line in ventricular muscle from guinea-pig, rabbit and sheep [7]. Stimulation produced a rate and time-dependent elevation of aiNa. Rates as low as 0.2 Hz produced a measurable increase over the quiescent muscle and faster rates elevated aiNa by over 3Oq&. Philipson and Nishimoto [27] have shown, in isolated sarcolemmal vesicles, from rabbit

and the Castera 0

Foundation.

1983 Academic

Press Inc.

(London)

Limited Ic

618

G. A. Langer

heart, that Na-pump rate is essentially linearly related to [Na]t over the measured range of aiNa (5 to 14 mM) in ventricular tissue. According to the relation defined by Philipson and Nishimoto at an arNa of 6 mM the Na pump would be functioning at of its saturable capacity. A doubling of 30:;, stimulation rate would require that pumping be increased to about SO%:, capacity which would require an aiNa of 11 mM-a gain of 5 mM at steady-state at the higher frequency. When the proposal’ for the Na pump lag and its possible role in regulation of transsarcolemmal Ca movement was first made in 1965 [I71 there was no experimental evidence available which demonstrated a link between intracellular Na and transsarcolemma1 Ca exchange. However, over the past 15 years, abundant documentation of a Na-Ca exchange system in the heart has accumulated [ZZ]. In 1968 Reuter and Seitz [31] found evidence for a membrane carrier system involving Na and Ca. They demonstrated that Ca efflux rate was sensitive to the concentration gradient for Na across the sarcolemma. In 1969, Baker et al. [I] demonstrated, in the internally perfused squid axon, that coupled Na-Ca exchange could occur in either direction-i.e., Na outward coupled to Ca inward movement or vice-versa. A series of studies in sarcolemmal vesicles derived from heart muscle clearly documented the Na-Ca exchange system in vitro. Reeves and Sutko [28] showed, in membrane vesicles isolated from rabbit ventricle, that the vesicles accumulated Ca when an outwardly directed Na gradient was established in the absence of another energy source, e.g., ATP. A potassium (K) gradient had little effect and if the Na gradient was depleted by addition of Na, Ca accumulated in the vesicles was rapidly discharged. Following this initial work a number of studies [Z, 5, 26, 291 clearly demonstrated that the Na-Ca exchange system was electrogenie, i.e., that more than 2Na+ (probably 3) were carried for each Ca2+. The fact that the carrier produces a net charge movement makes it sensitive to changes in transsarcolemmal membrane potential. The re-

versa1 potential (Vn) exchanger is defined by: VR =

nVNs

for

the

charged

~ 2Vci,

n -- 2

where 71 = the Na/Ca coupling ratio and VN~ and Vca are the equilibrium potentials. If n = 3, Vn is between - 15 and -30 mV dependent upon the values assigned for V’N~ and Vca, Therefore the carrier would move net positive charge inward during diastole when V, is negative to VR and outward during systole when V, is positive to VR. In terms of Ca movement the effect of increased Nat would be to augment Ca in&x during systole and depress efflux during diastolea net movement of Ca which would be expected to produce a positive inotropic result. A recent study [16] using ISME for measurement of intracellular Ca activity (a*Ca) in sheep Purkinje tissue, related atCa and force development to stimulation frequency between 0.2 and 3 Hz. atCa increased by 74:; as force increased by 88:!;,. It was calculated, on the basis of previous measurements of a*Ca [7] and assuming a Na/Ca coupling ratio of 3/l, that an increase of atNa from 8.2 at rest to 10.5 mM at 3 Hz would double arCa. The ratio of cellular Na ion activity and concentration (apparent activity coefficient) has been estimated at approximately 0.3 so that the 2.3 mM increase in atNa would represent a 7.7 mM increase in concentration within the cell. The studies reviewed are consistent with the proposal that Na-pump lag is the basis for the Bowditch staircase response to increased frequency of stimulation. If such is the case then heart muscle from a species that does not demonstrate a positive force staircase response should not show evidence of a Na-pump lag. Adult rat ventricle shows either no or a negative staircase upon increased rate of stimulation. Blesa et al. [3], using 4sK effluent analysis, found no evidence for net cellular K loss (indicating no cellular Na gain) in ventricle from adult rat upon increase of stimulation frequency in the range between 15 and 150 beats/min, over which range the staircase was negative. By contrast

Editorial

recent studies of frog ventricle [24], sheep Purkinje tissue [7), guinea-pig ventricle [9] and rabbit atrium [15] show clear evidence of a Na-pump lag upon increased stimulation frequency. All of these tissues demonstrate a positive staircase response. The concept of Na-pump lag has been applied to the inotropic action of digitalis [ZO]. The original concept was proposed by Repke 1301 and further developed by Langer [.?I]. Though the digitalis glycosidcs interact with other systems (see below) it is accepted that they inhibit Na-K ATPase, the controlling enzyme of the Na-K pump in cell membranes. Such inhibition causes a rise in Nat and this has now been documented in a number of studies using ISMEs to monitor arNa [9, 10, 231. Contractile force, when measured, increased as a4Ya increased and is consistent with increased Nat producing augmented Ca uptake via the Na-Ca exchanger. In the case of digitalis the ‘Napump lag’ is induced at a fixed stimulation rate by inhibition of the Na-K pump rather than through increased demand on the pump secondary to increased stimulation rate-as proposed for the production of the force staircase. The basic mechanism, via the Na-C:a exchanger responding to increased Nai is, however, proposed to be the same. The above proposal for the basis of glycoside inotropy seemed inconsistent with evidence that low doses of glycoside (lo-$ to 10--R M) .rtimulated the Na-K pump but still increased force [12, 231. This apparent paradox may be explained by the demonstration of two glycoside-sensitive sites. One set of sites is associated with Na-K. pump inhibition at higher glycoside concentrations and would be implicated in the Na-pump lag mechanism. The other sites may involve interaction with a catecholamine system. Hougen, et al. [14] found, in guinea-pig left atria, that 10-s isoproterenol stimulated Na-K activity. Low concentration of ouabain (3 :*% lo-$ M) also caused a significant increase in pump activity, but at concentrations greater than 10e8 the pump was consistently inhibited. Beta-blockade with propranolol blocked both the stimulatory effects of low dose ouabain and isoproterenol. If the atria were depleted of endogenous catecholamine by administration of reserpine

ti-f!f

or 6-hydroxydopamine, then 3 x IO-$ vA ouabain produced only a decrease in pump activity. Therefore low dose glycoside seems capable of producing an increase in free catecholamine levels (probably by inhibition of uptake of catechol in sympathetic nerve terminals by interacting with Na-K ATPase of the neuronal membrane) which will both stimulate the pump and produce positive inotropy. Higher doses inhibit the pump, increase Nat and, as proposed, stimulate increased Ca uptake via the Na-(:a exchanger. As the Na pump lag has been presented the mechanism is visualized to be based at the sarcolemmal membrane, leading to modification of transsarcolemmal Ca flux. An alternative might be that, rather than the sarcolemma, the sarcoplasmic reticulum (SR) is the primary structure involved. Following an increase of rate, increased Ca influx would occur over time-i.e., no increase per cardiac cycle across the sarcolemma but with more cardiac cycles per minute Ca influx/minute This might increase the would increase. steady-state level of Ca in the SR which would lead to increased release [II] and the production of a positive force staircase. The involvement of the intracellular SR, is, however, not supported by the prominent force staircase developed by frog ventricle, a tissue noted for its sparse SR and dependency on transsarcolemmal Ca flux for contractile activation [II]. Chesnais et al. [6], in their study on the frog ventricle staircase, concluded that the staircase ‘is a result of progressively increasing calcium influx per beat rather than a beat-by-beat augmentation of an intracellular calcium pool which contributes to activation’. Therefore, if the staircase has a basis common to all species in which it is demonstrated, it is unlikely that the SR plays a primary role in this phenomenon. Similarly the digitalis glycosides have no effect on SR membrane and this structure cannot, therefore, be implicated in the primary effect of the drug in the production of positive inotropy. In summary, recent experimental evidence appears, in my opinion, to give support to the Na pump lag hypothesis proposed almost twenty years ago. T!Te discovery and functional de6niG.m of the Na-Ca exchange

650

G. A. Langer

system has been most important. Further advances in this area will depend upon

isolation of the exchange system so that it can be studied at the molecular level. G. A. Langer Departments of Medicine and PhysioloD and the American Heart Association, Greater Los Angeles [email protected] Cardiovascular Research Laboratories, UCLA School of Medicine, Center for the Health Sciences, Los Angeles, CAL 90024, USA

References 1

P. F., BLAUSTEIN, M. P., HODGKIN, A. L., STEINHARDT, R. A. The influence of calcium on sodium in squid axons. J Physiol [Lond] 208,431458 (1969). BERS, D. M., PHILIPSON, K. D., NISHIMOTO, A. Y. Sodium-calcium exchange and sidedness of isolated cardiac sarcolemmal vesicles. Biochim Biophys Acta 601, 358-371 (1980). BLESA, E. S., LANCER, G. A., BRADY, A. J., SERENA, S. D. Potassium exchange in rat ventricular myocardium: its relation to rate of stimulation. Am J Physiol219, 747-754 (1970). BOWDITCH, H. P. Uber die Eigenthiimlichkeiten der Reizbarkeit, welche die Muskelfasern des Herzens zcigen. Arch Physiol Leipzig 6, 139-176 (1871). CARONI, P., REINLIB, L., CARAFOLI, E. Charge movements during Na+/Ca2+ exchange in heart sarcolemmal vesicles. Proc Nat1 Sci USA 77, 6354-6358 (1980). CHESNAIS, J. M., KAVALER, F., ANDERSON, T. W., CORABOEUP, E. Staircase in frog ventricular muscle. Its dependence on membrane excitation and extracellular ionic composition. Circ Res 43, 917-925 (1978). COMEN, C. J., FOZZARD, H. A., SHEU, S. S. Increase in intracellular sodium ion activity during stimulation in mammalian cardiac muscle. Circ Res 50,651-662 (-1982). CONN, H. L., JR, WOOD, J. C. Sodium exchange and distribution in the isolated heart of the normal dog. Am J Physiol 197, 631-636 (1959). DAUT, J. The role of intracellular sodium ions in the regulation of cardiac contractility. J Mel Cell Cardiol 14, 189-192 (1982). ELLIS, D. The effects of external cations and ouabain on the intracellular sodium activity of sheep heart Purkinje fibers. J Physiol 273, 21 l-240 (1977). FABIATO, A., FABIATO, F. Calcium-induced release of calcium from the sarcoplasmic reticulum of skinned cells from adult human, dog, cat, rabbit, rat and frog hearts and from fetal and new born rat ventricles. AnnNY Acad-Sci307,491-522 (1978). GODFRAIND, T., GHYSEL-BURTON, J. Binding sites related to ouabain-induced stimulation or inhibition of the sodium pump. Nature 265, 165-166 (1977). GODFRAIND, T., GHYSEL-BURTON, J., DEPOVER, A. Dihydroouabain is an antagonist of ouabain inotropic action. Nature 299, 824-826 (1982). HOUGEN, T. J., SPICER, N., SMITH, T. W. Stimulation of monovalent cation active transport by low concentrations of cardiac glycosides: role of catecholamines. J Clin Invest 68, 1207-1214 (1981). KUNZE, D. Rate-dependent changes in extracellular potassium in the rabbit atrium. Circ Res 41, 122-127 (1977). LADO, M. G., SHEU, S.-S., FOZZARD, H. A. Changes in intracellular Car+ activity with stimulation in sheep cardiac Purkinje strands. Am J Physio1243, H 133-H 137 ( 1982). LANCER, G. A. Calcium exchange in dog ventricular muscle: relation to frequency of contraction and maintenance of contractility. Circ Res 17, 78-90 (1965). LANGER, G. A. Sodium exchange in dog ventricular muscle. Relation to frequency of contraction and its possible role in the control of myocardial contractility. J Gen Physiol 50, 1221-1239 (1967). LANCER, G. A. Ion fluxes in cardiac excitation and contraction and their relation to myocardial contractility. Physiol Rev 48, 708-757 (1968). LANBER, G. A., SERENA, S. D. Effects of strophanthidin upon contraction and ion exchange in rabbit ventricular myocardium: relation to control of active state. J Mol Cell Cardiol 1, 65-90 (1970). LANCER, G. A. The intrinsic control of myocardial contraction-ionic factors. N Engl J Med 285, 1065-1071 (1971). LANCER, G. A. Sodium-calcium exchange in the heart. Annu Rev Physiol44,435-449 (1982). LEE, C. O., KANG, D. H., SOKOL, J. H., LEE, K. S. Relation between intracellular Na ion activity and tension in sheep cardiac Purkinje fibers exposed to dihydro-ouabain. Biophys J 29, 3 1.5-330 (1980). BAKER,

efflux

2 3 4 5 6 7 8 9 10 II

12 13 14 15 16 17 18 19 20 21 22 23

Editorial 24 25 26 27 28 29 30 31 32

hi

MARTIN, C., MOKAD, M. Activity induced potassium accumulation and its uptake in frog ventricular muscle. J Physiol [Lond] 328, 205-227 (1982). OPIT, L. J., CHARNOCK, J. S. A molecular model for a sodium pump. Nature 208, 471-474 (1965). PHILIPSON, K. D., NISHIMOTO, A. Y. Nat-Cal+ exchange is affected by membrane potential in cardiac sarcolemmal vesicles. J Biol Chem 255, 6880-6882 (1980). PHILIPSON, K. D., NISHIMOTO, A. Y. ATP-dependent Na+ transport in cardiac sarcolemmal vesicles. Biwhim Biophys Acta (In press). REEVES, J, P., SUTKO, J. L. Sodium-calcium exchange in cardiac membrane vesicles. Proc Nat1 hcad Sci USA 76, 590-594 (1979). REEVES, J. P., SUTKO, J. L. Sodium-calcium exchange activity generates a current in cardiac membrane vesicles. Science 208, 1461-1464 (1980). REPKE, K. Uber den biochemischen Wirkingsmodus von Digitalis. Klin Wochenschr 42, 1577165 (1964). REUTER, H., SEITZ, N. The dependence of calcium emux from cardiac muscle on temperature and external ion composition. J Physiol [Lond] 195, 45-70 (1968). WOODBURY, J. W. Interrelationships between ion transport mechanisms and excitatory events. Fed Proc 22. 31-35 (1963).

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