The effect of dihydro-ouabain and lithium-ions on the outward current in cardiac Purkinje fibers. Evidence for electrogenicity of active transport

Membrane currents and tension in cat ventricular muscle treated with cardiac glycosides

The effect of cardiac glycosides on membrane currents and tension in cat ventricular muscle was studied using the single sucrose gap voltage clamp method. Complete tension-voltage and current-voltage relations were obtained in five preparations before and during treatment with dihydro-ouabain (DHO, 1.7 X 10(-5)M). After 1-2 minutes of DHO, the developed tension was 15% greater than control, but there was no change in either the slow inward (calcium) current (Ica) or the level of the outward current flowing at the end of a 300-msec depolarization (Iout). After 6-8 minutes of DHO, there was a 60% increase in developed tension, a noticeable increase in resting tension, a 20% decrease in Ica, and a smaller increase in Iout. It seems possible that the reduction of Ica was due to a reduced driving force. In preparations treated with ouabain (5 X 10(-7)M, 3-5 minutes), developed tension was 45-150% greater than control with no change in Ica or Iout between -45 and + 15 mv. We conclude that the inotropic action of these cardiac glycosides is not mediated by an increase in Ica.

Editorial: Electrical activity and excitation-contraction coupling in mammalian ventricular muscle

The modern view of the mammalian ventricular action potential is that Na-ions are responsible for depolarization, the plateau is maintained by an inward Ca current and repolarization is due to the inactivation of this inward Ca current against a background outward K current. The electrical activity spreads over the surface and T-tubules of the cell and the inflow of Ca-ions, and possibly also the electrical signal, cause, a further release of Ca from internal stores, the lateral cisternae of the sarcoplasmic reticulum (SR). This released Ca, by triggering the splitting of ATP, induces contraction. The released Ca is pumped back into the longitudinal SR and, as the Ca concentration is reduced, relaxation occurs. The Ca in the longitudinal SR is transported back to the storage site in the cisternae. This completes the cycle, release, uptake, transport back to the releasing site. On the basis of this scheme, the action of various drugs is briefly discussed.

Contribution of an electrogenic sodium pump to the membrane potential in rabbit sinoatrial node cells

A study has been made of the transient hyperpolarization (K+-induced hyperpolarization) which developed following readmission of potassium after having pre-treated the rabbit sinoatrial node tissue with K+-depleted Tyrode solution for 4--5 min at 35 degrees C. Evidence is presented indicating that the K+-induced hyperpolarization results from the activity of an electrogenic sodium pump: The K+-induced hyperpolarization was inhibited by substituting Li+ for Na+ and by cooling the tissue. The amplitude of the K+-induced hyperpolarization was increased either by increasing K+ concentration in the recovery solution or by decreasing K+ concentration in the pre-treatment K+-depleted solution. By removing Cl- from the perfusates, the amplitude of the K+-induced hyperpolarization increased. In a Cl--depleted solution, the sinoatrial node cell membrane hyperpolarized by approximately 15 mV without a transient depolarization.

Temperature sensitivity of outward current in cardiac Purkinje fibers. Evidence of electrogenicity of active transport

1. In cardiac Purkinje fibers the temperature sensitivity of the membrane current flowing after 2 sec in response to depolarizing clamp steps was recorded. When the temperature was quickly lowered (30 sec) from 37 degrees C to 20 degrees C for a period of 2 min the outward current was markedly reduced. The effect was immediately reversed upon rewarming. The reduction in outward current on cooling was most pronounced between 30 degrees C and 20 degrees C. 2. In the range of anomalous rectification cooling to 20 degrees C shifted the i.v. relation in a negative direction by a constant amount of 20 nA. Outside this potential range (negative to -80 mV and positive to -45 mV) the slope conductance was reduced with a Q10 of about 1.3. 3. In the presence of dihydroouabain cooling did not further reduce the outward current in the potential range of anomalous rectification. However, negative to -80 mV and positive to -40 mV the slope conductance was reduced. The results support the view that part of the outward current is generated by an electrogenic sodium pump which is inhibited by cooling.