The dependence of sodium pump current on internal Na concentration and membrane potential in cardioballs from sheep Purkinje fibres

Abstract

The effect of various intracellular Na concentrations (CiNa) and membrane potentials on the Na pump current (Ip) was studied in isolated, cultured sheep cardiac Purkinje cells ('cardioballs'). Ip was identified as cardiac steroid sensitive current. The dependence of Ip on CiNa was investigated at a membrane potential of -40 mV by means of whole-cell recording from cardioballs internally perfused with media containing various Na concentrations. Internal perfusion with a Na free solution abolished Ip. The amplitude of Ip as a function of CiNa displayed saturation kinetics. Half maximal activation of Ip occurred at a CiNa of about 9 mM. The maximal Ip density was estimated to be 1.1 microA/cm2. The potential dependence of Ip was studied by conventional whole-cell recording under various ionic conditions. Generally Ip displayed little voltage dependence at membrane potentials positive to -20 mV. Ip declined at more negative potentials. The pump cycle probably includes only one voltage sensitive step. The potential dependence of Ip was more pronounced at lower CiNa or lower concentrations of the external pump activator Cs+. The findings are in line with the idea that increasingly steeper ionic gradients against which the cations are pumped strengthen the voltage dependence of Ip in the potential range studied. Other factors probably affecting the pump current-voltage (Ip-V) relation are discussed. The results suggest that Ip varies during electrical activity.

Sodium‐‐potassium pump current in rabbit sino‐atrial node cells.

“…In previous studies using multicellular preparations the values reported were around 5-7 mM in canine Purkinje fibres and rabbit AV node (Eisner & Lederer, 1980;Kurachi et al 1981 i a -r those obtained in the present experiments: 1-9 mm in cultured chick heart cells (Stimers, Shigeto & Lieberman, 1990b), 1'50 + 0-32 mm in guinea-pig myocytes ) and 1-9 mm in rabbit Purkinje myocytes (Bielen, Glitsch & Verdonck, 1991). In a more recent study on rat ventricular myocytes, Kinard, Liu, Liu & Stimers (1994) , 9 mm in cultured sheep Purkinje cells (Glitsch et al 1989) and 20 mm in sheep Purkinje fibres (Sejersted, Wasserstrom & Fozzard, 1988 (Eisner & Lederer, 1979;Kurachi et al 1981;Bielen et al 1991). These findings are in good agreement with the present findings, coincide with Eisenman's sequence 4 and are similar to that for the SA node background inward current (Hagiwara et al 1992).…”

“…found that the mean Ip density at +40 mV was 1-1 + 0-1 pA pF-' at 50 mm Nepip in guinea-pig ventricular cells. Glitsch, Krahn & Pusch (1989) found that Ip density at +20 mV with 10 mm Nepip was 0-87 + 0-35 uA cm-2 in sheep Purkinje fibres. This value is in good accord with our data in SA node cells (0-80 + 0-24 pA pF-').…”

Intracellular Na + Concentration Is Elevated in Heart Failure But Na/K Pump Function Is Unchanged

“…18 The K m found here compares well with values derived from Na/K pump current measurements in cardiac cells in the presence of intracellular K ϩ or Cs ϩ . 20,27,28 The most important result regarding the Na/K pump is that under physiological conditions (ie, appropriate resting [Na ϩ ] i ), the rate of Na ϩ extrusion by the pump is higher in HF (Table and Figure 2C). Higher resting Na/K pump current (Ϸ2-fold) has also been reported for dogs with chronic atrioventricular block.…”

Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis.

“…As in the first control the membrane current at very negative voltages Nas more iniward than in DHO-containing solution, whereas the current showed little difference at other clamp potentials. Immediately after the second control the patch pipette medium (for intracellular perfusion) was changed to a solution (Glitsch et al 1989). Figure 3C shows the mean p-V curve from six cells studied at increased sarcolemmal Na+ and Cs+ gradients The voltage dependence of Ko.5 under both conditions is plotted in Fig.…”

“…The pipettes were fixed in a holder designed for intracellular perfusion with various solutions. The holder was originally described by Soejima & Noma (1984) and was modified as reported earlier (Glitsch et al 1989). This 'run-dow\n' of I, has already been described for cardioballs (Glitsch & Tappe, 1993a).…”