Characterization of the electrogenic sodium pump in cardiac Purkinje fibres

Abstract

SUMMARY1. The Na pump is examined in sheep cardiac Purkinje fibres using a two microelectrode voltage clamp technique.2. After reducing the external K concentration, [K]O, to zero for 2 min or more, subsequent addition of an 'activator cation' (known to activate the Na pump in other preparations) produces a transient increase of outward current. This outward current transient is abolished by 10-5 M-strophanthidin (cf. Gadsby & Cranefield, 1979a).3. It is concluded that this transient increase of outward current is a result of a transient stimulation of the sodium pump by the raised [Na]1 following exposure to 0-K0. Although this current transient may reflect the activity of an electrogenic Na pump, it is difficult to use K as the activator cation to establish this point. This is due to the extracellular K depletion that occurs during Na pump reactivation and the subsequent change that this K depletion produces in the current-voltage relationship of the Purkinje fibre.4. Rbo or Cso have been used instead of Ko to -reactivate the Na pump when examining the transient increase of outward current. On adding either of these cations after exposing a preparation to a solution without such 'activator cations', the outward current transient is relatively voltage independent over a wide range of potentials (-90 to + 10 mV). It is concluded that, following the addition of Rbo or Cs0, the transient increase of outward current is a direct measure of the transient increase of the electrogenic Na pump current. Ritchie, 1968), it is shown that the decay rate constant of the electrogenic Na pump current transient is a good measure of the degree of activation of the external site of the Na pump. At a given concentration of activator cation, Rbo produces a greater activation of the Na pump than does Cso. The Ko.5 for Rbo is 6-3 mm and for Cso is 14-2 mM. Lio activates the Na pump more weakly than Rbo and Cs0.6. The coupling ratio of the Na pump is shown to be independent of Rbo or Cso Gadsby & Cranefield (1979 a), the coupling ratio is independent of Nai over the range considered.7. The Q10 for the electrogenic Na pump current transient varies between 1P6 and 2-3 over the range of temperature 26-46 'C. 8. A maximum Na pump current of about 0-78 tA cm-2 is obtained. Assuming a coupling ratio of 3Na/2K, the rate of Na ion transport into the cell is estimated to be about 23 p-mole cm-2 sec-1. Assuming a Na pump turnover of 150 sec1, we estimate that there are about 1000 Na pump sites per jum2 of cell surface.

Potential dependency of the electrogenic Na+-pump current in bullfrog atrial muscles.

“…moid curve. The electrogenic Na+-pump currents induced by Rb+, Cs, and Li+ at the concentration of 5 mM were approximately 100, 42, and 11 % of the currents induced by Kt These results are comparable qualitatively with those reported by RANG and RITCHIE (1968) in the nerve fiber and by EISNER and LEDERER (1980) in the sheep Purkinje fiber.…”

“…The Vmax for Rb+ was almost the same as the Vmax for K+ but the Vmax for Cs~ or Li+ was somewhat lower. A similar finding has been reported for the squid axon (BAKER et al, 1969), frog skeletal muscle (BEAUGE,1975), and sheep Purkinje fiber (EISNER and LEDERER, 1980). Vol.…”

“…To prepare solutions with the alkali metal cations (K+, Rb+, Cs, Lit), the chloride salt of the alkali metal cation was added to the K+-free Ringer solution and the NaCI concentration was reduced to maintain the tonicity (cf. EISNER and LEDERER, 1980). The pH of all the solutions was 7.4+0.2.…”

“…On the other hand, the Na+-pump activity appears to be independent of membrane potential in a variety of tissues including squid giant axon (HODGKIN and KEYNES, 1955;BRINLEY and MULLINS, 1974), Anisodoris giant neurone (MARMOR, 1971), human red blood cells (COTTERRELL and WHITTAM,1971), frog skeletal muscle (BEAUGE and SJODIN, 1976), and sheep Purkinje fiber (EISNER and LEDERER, 1980). TAHARA et al (1973) suggested that the activity of the Nat-pump is directly dependent upon membrane potential in frog skeletal muscle.…”

“…The reason for using these alkali metal cations, instead of K+, was to minimize or eliminate the alteration of passive ionic current due to "depletion," which could be caused by the activation of the Nat-pump (cf. EISNER and LEDERER, 1980). METHODS Strips (0.3-0.5 x 4 mm in size) of quiescent muscle fiber bundles, excised from the atrium of bullfrog (liana catesbeiana) heart were used.…”