Depletion and accumulation of potassium in the extracellular clefts of cardiac Purkinje fibers during voltage clamp hyperpolarization and depolarization

SUMMARY1. The single sucrose voltage clamp technique was used to control the membrane potential of strips of frog ventricular muscle and to measure the membrane current. The extracellular K accumulation was estimated from the after-potential observed after the release of the voltage clamp.2. Comparing the time course of the membrane current to the time course of the development of the after-potential at different membrane potentials, it was found that all slow current changes are related to changes in the K current across the membrane.3. Based on measurements of membrane current and the after-potential, the total membrane current was separated into two fractions: (a) the K current which gives rise to K accumulation and (b) the residual membrane current which is unrelated to K accumulation. The current-voltage relation for the residual membrane current is linear or slightly inwardly-rectifying. Residual current is zero at the resting potential and increases to about 1 /tA/cm2 at -20 mV.4. The measured membrane currents and after-potentials indicate qualitative differences between the K currents which dominate below and above -20 mV. More negative to -20 mV the after-potential develops rapidly while at potentials positive to -20 mV the after-potential develops with some delay.5. The current dominating below -20 mV is inwardly-rectifying. The currentvoltage relation has a maximum (about 2 #A/cm2) and a region with marked negative slope conductance. The outward current in the region of negative slope conductance is increased with increasing [K]O. 6. A model for the inwardly rectifying K current is described. The model accurately reproduces the shape of the measured current-voltage relations and their modification by alterations in the extracellular K concentration. The model is also compatible with the observation that all slow current changes below -20 mV are directly related to K accumulation.7. The K current which dominates at potentials positive to -20 mV is activated by a potential and time dependent process which is unrelated to extracellular K accumulation.8. Q10 for the magnitude of the inwardly rectifying K current is about 1-35 while the Q1o for the rate of increase of the time dependent K current is about 3-4.L. CLEEMANN AND M. MORAD 9. Cs blocks the inwardly rectifying K current but has little effect on the time dependent K current.10. The changes in the action potential duration caused by increasing the extracellular K concentration or addition of Cs to the perfusate can be explained by the effect of K and Cs on the inwardly rectifying K current.

Section: Discussionmentioning

confidence: 99%

Potassium currents in frog ventricular muscle: evidence from voltage clamp currents and extracellular K accumulation.

Cleemann¹,

Morad²

1979

“…Depletion currents are characterized by a decreasing inward current, the time-dependent component being greater the greater or longer the applied potential gradient (Baumgarten, Isenberg, McDonald & TenEick, 1977). The examples in Fig.…”

Section: -2 291mentioning

confidence: 99%

Induction and removal of inward‐going rectification in sheep cardiac Purkinje fibres

Carmeliet

1982

SUMMARY1. In sheep cardiac Purkinje fibres superfused with K-free, Na-free medium, the membrane potential can be stable either at a low negative level (-50 mV) or at a high negative level (-100 mV). The mechanism underlying the existence of these two stable potential levels was investigated using the two-micro-electrode voltage-clamp technique.2. By applying a voltage clamp of a certain duration at an appropriate level the membrane potential could be shifted from one stable level to the other. The shift was observed in Cl-free medium, excluding a redistribution of Cl as a possible explanation.3. Currents during and following a voltage step and their change with amplitude and duration of the voltage step could not be explained on the basis of depletion or accumulation of K ions in the narrow extracellular clefts.4. Instantaneous currents determined from the high negative resting level showed a high conductance and a pronounced inward rectification, while measurements from the low negative resting level indicated a low conductance and absence of inward rectification. The steady-state current-voltage relation was dependent on the holding potential and showed memory or hysteresis.5. Estimation of the conductance by superimposed short voltage-clamp pulses showed an increase in conductance during a hyperpolarizing clamp from the low negative level and a decrease in conductance during a depolarizing clamp from the high negative level. The time-dependent current during a hyperpolarizing clamp from the low negative level reversed direction at a potential level corresponding to EK, assuming a cleft K concentration of about 1 mm. In the presence of 0-1 mM-Ba the time-dependent current was abolished.6. The results suggest that the shift between the two stable levels is due to a time-dependent conductance change in the K inward rectifier channel, iK1. The existence of memory excludes activation or de-activation only depending on the voltage gradient. Interaction of extracellular K ions with a site in the membrane is proposed as the activating mechanism.

“…For the sake of comparison, Fig. 5D shows an analogous experiment in a sheep Purkinje fibre (Baumgarten, Isenberg, McDonald & Ten Eick, 1977 experiments. In 2 mM-K0, the ratio was difficult to determine because both id and io were small.…”

Section: Rabbit Purkinje Fibre Membrane Currentsmentioning

confidence: 99%

Electrical properties associated with wide intercellular clefts in rabbit Purkinje fibres.

Colatsky¹,

Tsien²

1979

SUMMARY1. Rabbit Purkinje fibres were studied using micro-electrode recordings of electrical activity or a two-micro-electrode voltage clamp. Previous morphological work had suggested that these preparations offer structural advantages for the analysis of ionic permeability mechanisms.2. Viable preparations could be obtained consistently by exposure to a K glutamate Tyrode solution during excision and recovery. In NaCl Tyrode solution, the action potential showed a large overshoot and fully developed plateau, but no pacemaker depolarization at negative potentials.3. The passive electrical properties were consistent with morphological evidence for the accessibility of cleft membranes within the cell bundle. Electrotonic responses to intracellular current steps showed the behaviour expected for a simple leaky capacitative cable. Capacitative current transients under voltage clamp were changed very little by an eightfold reduction in the external solution conductivity.4. Slow current changes attributable to K depletion were small compared to those found in other cardiac preparations. The amount of depletion was close to that predicted by a cleft model which assumed free K diffusion in 1 ,sm clefts. 5. Step depolarizations over the plateau range of potentials evoked a slow inward current which was resistant to tetrodotoxin but blocked by D600. 6. Strong depolarizations to potentials near 0 mV elicited a transient outward current and a slowly activating late outward current. Both components resembled currents found in sheep or calf Purkinje fibres.7. These experiments support previous interpretations of slow plateau currents in terms of genuine permeability changes. The rabbit Purkinje fibre may allow various ionic channels to be studied with relatively little interference from radial non-uniformities in membrane potential or ion concentration.