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

Calcium plays a significant role in a number of processes like muscle contraction, gene expression, synaptic plasticity, signal transduction etc. but the significance of calcium oscillation is not yet completely understood in most of the cell types. A number of investigators have reported the oscillatory behavior of calcium due to intracellular concentration of inositol 1,4,5-trisphosphate (IP3). In this paper, an attempt has been made to study the oscillations induced in calcium due to dynamically changing membrane potential with special relevance to sodium pump. A mathematical model is developed which incorporates nearly all important and necessary biophysical components like L-type calcium channel, sodium channel, potassium channel, cytosolic buffers, calcium pump, sodium–calcium exchanger (NCX), sodium–potassium ATPase (sodium pump), and dynamic membrane potential. These channels have realistic gating mechanism and emulate the gating mechanism proposed in the famous paper of Hodgkin and Huxley.1 The model leads to an initial value problem involving system of non-linear ordinary differential equations. A numerical solution has been obtained using Gear's method. The numerical results have been used to study the effect of sodium pump over the frequency of Ca 2+ oscillation. At lower and higher sodium pump current densities the Ca 2+ oscillation frequency is observed to be 278 Hz and 225 Hz respectively.

Download Full-text

Sodium-pump potentials and currents in guinea-pig ventricular muscles and myocytes

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y99-030 ◽

1999 ◽

Vol 77 (5) ◽

pp. 339-349 ◽

Cited By ~ 1

Author(s):

Yuji Kasamaki ◽

An Chi Guo ◽

Lesya M Shuba ◽

Toshitsugu Ogura ◽

Terence F McDonald

Keyword(s):

Guinea Pig ◽

Voltage Clamp ◽

Cardiac Glycosides ◽

Sodium Pump ◽

Pump Current ◽

Papillary Muscles ◽

Maximal Concentration ◽

Sodium Pump Current ◽

Reproducible Manner

When guinea-pig papillary muscles were depolarized to ca. -30 mV by superfusion with K+-free Tyrode's solution supplemented with Ba2+, Ni2+, and D600, addition of Cs+ transiently hyperpolarized the membrane in a reproducible manner. The size of the hyperpolarization (pump potential) depended on the duration of the preceding K+-free exposure; peak amplitudes (Epmax) elicited by 10 mM Cs+ after 5-, 10-, and 15-min K+-free exposures were 12.9, 17.7, and 23.2 mV, respectively. Pump potentials were unaffected by external Cl- but suppressed by cardiac glycosides, hyperosmotic conditions, and low-Na+ solution. Using Epmax as an indicator of Na+ pump activation, the half-maximal concentration for activation by Cs+ was 12-16.3 mM. At 6 mM, Cs+ was three times less potent than Rb+ or K+ and five times more potent than Li+. From these findings, and correlative voltage-clamp data from myocytes, we calculate that (i) a pump current of 7.8 nA/cm2 generates an Epmax of 1 mV and (ii) resting pump current in normally polarized muscle (~0.16 µA/cm2) is five times smaller than previously estimated.Key words: sodium pump, cesium, rubidium, sodium pump current.

Download Full-text

Effects of ouabain on background and voltage-dependent currents in canine colonic myocytes

AJP Cell Physiology ◽

10.1152/ajpcell.1990.259.3.c402 ◽

1990 ◽

Vol 259 (3) ◽

pp. C402-C408 ◽

Cited By ~ 23

Author(s):

E. P. Burke ◽

K. M. Sanders

Keyword(s):

Membrane Potential ◽

Potential Gradient ◽

Circular Muscle ◽

Input Resistance ◽

Pump Current ◽

Muscle Layer ◽

Membrane Properties ◽

Resting Potential ◽

Voltage Dependent ◽

In Cells

Previous studies have suggested that the membrane potential gradient across the circular muscle layer of the canine proximal colon is due to a gradient in the contribution of the Na(+)-K(+)-ATPase. Cells at the submucosal border generate approximately 35 mV of pump potential, whereas at the myenteric border the pump contributes very little to resting potential. Results from experiments in intact muscles in which the pump is blocked are somewhat difficult to interpret because of possible effects of pump inhibitors on membrane conductances. Therefore, we studied isolated colonic myocytes to test the effects of ouabain on passive membrane properties and voltage-dependent currents. Ouabain (10(-5) M) depolarized cells and decreased input resistance from 0.487 +/- 0.060 to 0.292 +/- 0.040 G omega. The decrease in resistance was attributed to an increase in K+ conductance. Studies were also performed to measure the ouabain-dependent current. At 37 degrees C, in cells dialyzed with 19 mM intracellular Na+ concentration [( Na+]i), ouabain caused an inward current averaging 71.06 +/- 7.49 pA, which was attributed to blockade of pump current. At 24 degrees C or in cells dialyzed with low [Na+]i (11 mM), ouabain caused little change in holding current. With the input resistance of colonic cells, pump current appears capable of generating at least 35 mV. Thus an electrogenic Na+ pump could contribute significantly to membrane potential.

Download Full-text

Electric current generated by squid giant axon sodium pump: external K and internal ADP effects

AJP Cell Physiology ◽

10.1152/ajpcell.1978.235.1.c63 ◽

1978 ◽

Vol 235 (1) ◽

pp. C63-C68 ◽

Cited By ~ 44

Author(s):

R. F. Abercrombie ◽

P. de Weer

Keyword(s):

Sodium Pump ◽

Giant Axon ◽

Pump Current ◽

Membrane Resistance ◽

Extracellular Potassium ◽

Sodium Efflux ◽

Steroid Sensitive ◽

External Potassium ◽

Loligo Pealei ◽

External K

The operation of the sodium pump of giant axons of the squid, Loligo pealei, has been studied simultaneously in two independent ways: 1) by measuring sodium efflux with 22Na, and 2) by calculating the transmembrane current generated by the pump from measurements of membrane resistance and digitalis-sensitive membrane potential. In normal, untreated axons, the effect of increasing the external potassium concentration on both sodium efflux and pump current is similar, which suggests that Na:K pump stoichiometry remains relatively constant in the range of 0-20 mM external K. The data are compatible with a 3:2 Na:K ratio. In axons whose intracellular ADP level has been elevated by injection of L-arginine, a large, electrically silent, cardiotonic steroid-sensitive sodium efflux takes place in the absence of external potassium; this suggests that pump-mediated Na:Na exchange is 1:1 or electroneutral. Finally, elevation of external potassium levels causes the appearance, in high-ADP axons, of electrogenic pumping, with little effect on sodium efflux; hence, in contrast to what is seen in normal (low-ADP) axons, the charge translocated, per sodium ion extruded, increases sharply with increasing extracellular potassium levels.

Download Full-text