Sodium-potassium pump current in rabbit sino-atrial node cells

1. The Na+-K+ pump current (I-p) was studied in sino-atrial (SA) node cells of rabbits using the whole-cell patch-clamp technique. 2. With 50 mM Na+ in the pipette solution ([Na+](pip)), changing the external K+ concentration ([K+](0)) from 0 to 5.4 mM caused the holding current to shift in an outward direction and reach a new steady state. The current-voltage relationships obtained by subtraction of current traces recorded at 0 mM K-0(+) from those recorded at 5.4 mM K-0(+) revealed time-independent and voltage-dependent characteristics. The external K+-induced current was completely blocked by external application of 10 mu M ouabain, indicating the existence of I-p in SA node cells of rabbit heart. 3. I-p increased as [K+](0) increased. With 30 mM Na-pip(+), I-p at 0 mV was activated by [K+](0) with non-linear least-squares fit parameters for the Hill equation of K-0.5 of 1.4 mM and a Hill coefficient (n(H)) of 1.2 (n = 7). 4. The cation dependence of the K+ site of the Na+-K+ pump was examined using various monovalent cations. The sequence was K+ greater than or equal to Rb+ > Cs+ > > > Li+. 5. I-p at 0 mV also increased as [Na+](pip) was increased from 10 to 150 mM at 5.4 mM K-0(+), with a K-0.5 value of 14 mM and a n(H) of 1.3 (n = 54). 6. I-p at 0 mV was reduced by lowering the temperature from 37 to 25 degrees C with 30 mM Na-pip(+) and 5.4 mM K-0(+). The temperature coefficient (Q(10)) for I-p was 2.1 (n = 27). 7. With 10 mM Na-pip(+) and 5.4 mM K-0(+), the half-activation voltage of I-p was -52 +/- 16 mV and the current at this voltage wax 22.5 +/- 3.5 pA (n = 10), indicating that I-p contributes significantly to the background outward current during the normal pacemaker potential of SA node cells.