Influence of chloride, potassium, and tetraethylammonium on the early outward current of sheep cardiac Purkinje fibers

In voltage clamp studies of cardiac Purkinje fibers, a large early outward current is consistently observed during depolarizations to voltages more positive than -20 mV. After the outward peak of the current, the total membrane current declines slowly. Dudel et al. (1967. Pfluegers Arch. Eur. J. Physiol. 294:197--212) reduced the extracellular chloride concentration and found that the outward peak and the decline of the current were abolished. They concluded that the total membrane current at these voltages was largely determined by a time- and voltage-dependent change in the membrane chloride conductance. We reinvestigated the chloride sensitivity of this current, taking care to minimize possible sources of error. When the extracellular chloride concentration was reduced to 8.6% of control, the principal effect was a 20% decrease in the peak amplitude of the outward current. This implies that the membrane chloride conductance is not the major determinant of the total current at these voltages. The reversal potential of current tails obtained after a short conditioning depolarization was not changed by alterations in the extracellular chloride or potassium concentrations. We suspect that the tail currents contain both inward and outward components, and that the apparent reversal potential of the net tail current largely reflects the kinetics of the outward component, so that this experiment does not rule out potassium as a possible charge carrier. The possibility that potassium carries much of the early outward current was further investigated using tetraethylammonium, which blocks potassium currents in nerve and skeletal muscle. This drug substantially reduced the early outward current, which suggests that much of the early outward current is carried by potassium ions.     

An odorant-suppressed Cl– conductance in lobster olfactory receptor cells

Odorants evoke an outward current in cultured lobster olfactory receptor neurons voltage clamped at -60 mV. The reversal potential of the outward current is independent of the reversal potential of potassium, but shifts with imposed changes in the reversal potential of chloride. The slope of the current-voltage relationship is negative, suggesting that the current is mediated by the odorant suppressing a steady-state conductance. Anthracene-9-carboxylic acid, a specific chloride channel blocker, reversibly inhibits the steady-state conductance. Local application of odorants to the outer dendrites evokes a hyperpolarizing receptor potential in lobster olfactory receptor neurons current-clamped at -70 mV in situ. Consistent with the current characterized in the cultured cells, hyperpolarizing receptor potentials in some cells are voltage sensitive, blocked by anthracene-9-carboxylic acid and associated with a decrease in membrane conductance. These results support the hypothesis that odorants suppress a steady-state chloride conductance in lobster olfactory receptor neurons. Evidence that the chloride conductance can coexist with a 4-aminopyridine-blockable potassium conductance reported earlier in these cells suggests that two distinct mechanisms can mediate odorant-evoked inhibition in lobster olfactory receptor neurons.     

Gating of delayed rectification in acutely isolated canine cardiac Purkinje myocytes. Evidence for a single voltage-gated conductance.

Studies of time-dependent, plateau outward current (delayed rectification) in the heart are complicated by the accumulation and depletion of K+ ions in intercellular clefts. To minimize this problem, we studied delayed rectification in acutely isolated (enzymic solution, gentle agitation) canine cardiac Purkinje myocytes using the single microelectrode voltage-clamp technique. We found a sigmoidal voltage-dependence for activation of outward plateau current, with maximal activation occurring at potentials near -10 mV. The activation and deactivation of plateau outward current was adequately described as the sum of a fast and slow exponential component. A comparison of the time course of activation of plateau outward current and the "envelope" of tail currents suggests that a single voltage-gated conductance with one open and two closed states can account for delayed rectification in Purkinje myocytes. These results differ from those previously obtained with intact sheep Purkinje fibers in which two time-dependent conductances were postulated to account for delayed rectification (Noble, D., and R. W. Tsien, 1969, J. Physiol. (Lond.), 200:205-231).     

Calcium- and voltage-activated plateau currents of cardiac Purkinje fibers

We have used the two-microelectrode voltage-clamp technique to investigate the components of membrane current that contribute to the formation of the early part of the plateau phase of the action potential of calf cardiac Purkinje fibers. 3,4-Diaminopyridine (50 microM) reduced the net transient outward current elicited by depolarizations to potentials positive to -30 mV but had no consistent effect on contraction. We attribute this effect to the blockade of a voltage-activated transient potassium current component. Ryanodine (1 microM), an inhibitor of sarcoplasmic reticulum calcium release and intracellular calcium oscillations in Purkinje fibers (Sutko, J.L., and J.L. Kenyon. 1983. Journal of General Physiology. 82:385-404), had complex effects on membrane currents as it abolished phasic contractions. At early times during a depolarization (5-30 ms), ryanodine reduced the net outward current. We attribute this effect to the loss of a component of calcium-activated potassium current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. At later times during a depolarization (50-200 ms), ryanodine increased the net outward current. This effect was not seen in low-sodium solutions and we could not observe a reversal potential over a voltage range of -100 to +75 mV. These data suggest that the effect of ryanodine on the late membrane current is attributable to the loss of sodium-calcium exchange current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. Neither effect of ryanodine was dependent on chloride ions, which suggests that chloride ions do not carry the ryanodine-sensitive current components. Strontium (2.7 mM replacing calcium) and caffeine (10 mM), two other treatments that interfere with sarcoplasmic reticulum function, had effects in common with ryanodine. This supports the hypothesis that the effects of ryanodine may be attributed to the inhibition of sarcoplasmic reticulum calcium release.     

Depletion and accumulation of potassium in the extracellular clefts of cardiac Purkinje fibers during voltage clamp hyperpolarization and depolarization: experiments in sodium-free bathing media

Voltage clamp hyperpolarization and depolarization result in currents consistent with depletion and accumulation of potassium in the extracellular clefts o cardiac Purkinje fibers exposed to sodium-free solutions. Upon hyperpolarization, an inward current that decreased with time (id) was observed. The time course of tail currents could not be explained by a conductance exhibiting voltage-dependent kinetics. The effect of exposure to cesium, changes in bathing media potassium concentration and osmolarity, and the behavior of membrane potential after hyperpolarizing pulses are all consistent with depletion of potassium upon hyperpolarization. A declining outward current was observed upon depolarization. Increasing the bathing media potassium concentration reduced the magnitude of this current. After voltage clamp depolarizations, membrane potential transiently became more positive. These findings suggest that accumulation of potassium occurs upon depolarization. The results indicate that changes in ionic driving force may be easily and rapidly induced. Consequently, conclusions based on the assumption that driving force remains constant during the course of a voltage step may be in error.     

4-Aminopyridine and the early outward current of sheep cardiac Purkinje fibers

We have studied the effects of the potassium-blocking agent 4-aminopyridine (4-AP) on the action potential and membrane currents of the sheep cardiac Purkinje fiber. 4-AP slowed the rate of phase 1 repolarization and shifted the plateau of the action potential to less negative potentials. In the presence of 4-AP, the substitution of sodium methylsulfate or methanesulfonate for the NaCl of Tyrode's solution further slowed the rate of phase 1 repolarization, even though chloride replacement has no effect on the untreated preparation. In voltage clamp experiments, 4-AP rapidly and reversibly reduced the early peak of outward current that is seen when the Purkinje fiber membrane is voltage-clamped to potentials positive to -20 mV. In addition, 4-AP reduced the steady outward current seen at the end of clamp steps positive to -40 mV. 4-AP did not appear to change the slow inward current observed over the range of -60 to -40 mV, nor did it greatly change the current tails that have been used as a measure of the slow inward conductance at more positive potentials. 4-AP did not block the inward rectifying potassium currents, IK1 and IK2. A phasic outward current component that was insensitive to 4-AP was reduced by chloride replacement. We conclude that the early outward current has two components: a chloride-sensitive component plus a 4-AP-sensitive component. Since a portion of the steady-state current was sensitive to 4-AP, the early outward current either does not fully inactivate or 4-AP blocks a component of time-independent background current.     

On The Mechanism of Spontaneous Impulse Generation in the Pacemaker of the Heart

Rhythmic activity in Purkinje fibers of sheep and in fibers of the rabbit sinus can be produced or enhanced when a constant depolarizing current is applied. When extracellular calcium is reduced successively, the required current strength is less, and eventually spontaneous beating occurs. These effects are believed due to an increase in steady-state sodium conductance. A significant hyperpolarization occurs in fibers of the rabbit sinus bathed in a sodium-free medium, suggesting an appreciable sodium conductance of the "resting" membrane. During diastole, there occurs a voltage-dependent and, to a smaller extent, time-dependent reduction in potassium conductance, and a pacemaker potential occurs as a result of a large resting sodium conductance. It is postulated that the mechanism underlying the spontaneous heart beat is a high resting sodium current in pacemaker tissue which acts as the generator of the heart beat when, after a regenerative repolarization, the decrease in potassium conductance during diastole reestablishes the condition of threshold.     

Properties of potassium currents in Purkinje cells of failing human hearts

Cardiac Purkinje fibers play an important role in cardiac arrhythmias, but no information is available about ionic currents in human cardiac Purkinje cells (PCs). PCs and midmyocardial ventricular myocytes (VMs) were isolated from explanted human hearts. K(+) currents were evaluated at 37 degrees C with whole cell patch clamp. PCs had clear inward rectifier K(+) current (I(K1)), with a density not significantly different from VMs between -110 and -20 mV. A Cs(+)-sensitive, time-dependent hyperpolarization-activated current was measurable negative to -60 mV. Transient outward current (I(to)) density was smaller, but end pulse sustained current (I(sus)) was larger, in PCs vs. VMs. I(to) recovery was substantially slower in PCs, leading to strong frequency dependence. Unlike VM I(to), which was unaffected by 10 mM tetraethylammonium, Purkinje I(to) was strongly inhibited by tetraethylammonium, and Purkinje I(to) was 10-fold more sensitive to 4-aminopyridine than VM. PC I(sus) was also reduced strongly by 10 mM tetraethylammonium. In conclusion, human PCs demonstrate a prominent I(K1), a time-dependent hyperpolarization-activated current, and an I(to) with pharmacological sensitivity and recovery kinetics different from those in the atrium or ventricle and compatible with a different molecular basis.     

Slow inactivation of a tetrodotoxin-sensitive current in canine cardiac Purkinje fibers.

We used the two-microelectrode voltage clamp technique and tetrodotoxin (TTX) to investigate the possible occurrence of slow inactivation of sodium channels in canine cardiac Purkinje fibers under physiologic conditions. The increase in net outward current during prolonged (5-20 s) step depolarizations (range -70 to +5 mV) following the application of TTX is time dependent, being maximal immediately following depolarization, and declining thereafter towards a steady value. To eliminate the possibility that this time-dependent current was due to inadequate voltage control of these multicellular preparations early during square clamp pulses, we also used slowly depolarizing voltage clamp ramps (range 5-100 mV/s) to ensure control of membrane potential. TTX-sensitive current also was observed with these voltage ramps; the time dependence of this current was demonstrated by the reduction of the peak current magnitude as the ramp speed was reduced. Reducing the holding potential within the voltage range of sodium channel inactivation also decreased the TTX-sensitive current observed with identical speed ramps. These results suggest that the TTX-sensitive time-dependent current is a direct measure of slow inactivation of canine cardiac sodium channels. This current may play an important role in modulating the action potential duration.     

Time-dependent outward current in guinea pig ventricular myocytes. Gating kinetics of the delayed rectifier

Several conflicting models have been used to characterize the gating behavior of the cardiac delayed rectifier. In this study, whole-cell delayed rectifier currents were measured in voltage-clamped guinea pig ventricular myocytes, and a minimal model which reproduced the observed kinetic behavior was identified. First, whole-cell potassium currents between -10 and +70 mV were recorded using external solutions designed to eliminate Na and Ca currents and two components of time-dependent outward current were found. One component was a La3(+)-sensitive current which inactivated and resembled the transient outward current described in other cell types; single-channel observations confirmed the presence of a transient outward current in these guinea pig ventricular cells (gamma = 9.9 pS, [K]o = 4.5 mM). Analysis of envelopes of tail amplitudes demonstrated that this component was absent in solutions containing 30-100 microM La3+. The remaining time-dependent current, IK, activated with a sigmoidal time course that was well-characterized by three time constants. Nonlinear least-squares fits of a four-state Markovian chain model (closed - closed - closed - open) to IK activation were therefore compared to other models previously used to characterize IK gating: n2 and n4 Hodgkin-Huxley models and a Markovian chain model with only two closed states. In each case the four-state model was significantly better (P less than 0.05). The failure of the Hodgkin-Huxley models to adequately describe the macroscopic current indicates that identical and independent gating particles should not be assumed for this K channel. The voltage-dependent terms describing the rate constants for the four-state model were then derived using a global fitting approach for IK data obtained over a wide range of potentials (-80 to +70 mV). The fit was significantly improved by including a term representing the membrane dipole forces (P less than 0.01). The resulting rate constants predicted long single-channel openings (greater than 1 s) at voltages greater than 0 mV. In cell-attached patches, single delayed rectifier channels which had a mean chord conductance of 5.4 pS at +60 mV ([K]o = 4.5 mM) were recorded for brief periods. These channels exhibited behavior predicted by the four-state model: long openings and latency distributions with delayed peaks. These results suggest that the cardiac delayed rectifier undergoes at least two major transitions between closed states before opening upon depolarization.     

Increased voltage-gated potassium conductance during interleukin 2- stimulated proliferation of a mouse helper T lymphocyte clone

Recent work has demonstrated the presence of voltage-gated potassium channels in human peripheral blood T lymphocytes (Matteson, R., and C. Deutsch, 1984, Nature (Lond.), 307:468-471; DeCoursey T. E., T. G. Chandy, S. Gupta, and M. D. Cahalan, 1984, Nature (Lond.), 307:465-468) and a murine cytolytic T-cell clone (Fukushima, Y., S. Hagiwara, and M. Henkart, 1984, J. Physiol., 351:645-656). Using the whole cell patch clamp, we have found a potassium conductance with similar properties in a murine noncytolytic T lymphocyte clone, L2. Under voltage clamp, a step from a holding potential of -70 mV to +50 mV produces an average outward current of 100-150 pA in "quiescent" L2 cells at the end of their weekly maintenance cycle. When these cells are stimulated with human recombinant interleukin 2 (rIL2, 100 U/ml), they grow in size and initiate DNA synthesis at approximately 24 h. Potassium conductance is increased as early as 8 h after stimulation with rIL2 and rises to a level 3-4 times that of excipient controls by 24 h. The level remains elevated through 72 h, but as the cells begin to leave the cell cycle at 72-96 h, the conductance decreases quickly to a value only slightly higher than the initial one. Quinine, a blocker of this conductance, markedly reduces the rate at which L2 cells traverse the cell cycle, while also reducing the rate of stimulated protein synthesis. The regulation of potassium conductance in L2 cells during rIL2-stimulated proliferation suggests that potassium channel function may play a role in support of the proliferative response.     

Pharmacological and biophysical properties of swelling-activated chloride channel in mouse cardiac myocytes

The present study was designed to observe the properties of swelling-activated chloride channel (ICl.swell) in mouse cardiac myocytes using patch clamp techniques. In whole-cell recordings, hypotonic solution activated a chloride current that exhibited outward rectification, weak voltage-dependent inactivation, and anion selectivity with permeability sequence of I- > Br- > Cl-. The current was sensitive to Cl- channel blockers tamoxifen, NPPB and DIDS. In single-channel recordings, cell swelling activated a single channel current which showed outward rectification with open probability of 0.76 +/- 0.08 and conductance of 38.1 +/- 2.5 pS at +100 mV under [Cl-] symmetrical condition. I-V relation revealed the reversal potential as expected for a Cl(-)-selective channel. These results suggested that in mouse cardiac myocytes, swelling-activated, outward rectifying chloride channel with a single channel conductance of 38.1 +/- 2.5 pS (at +100 mV under [Cl-] symmetrical condition) underlies the volume regulatory Cl- channel.     

A study of the molecular sources of nonideal osmotic pressure of bovine serum albumin solutions as a function of pH.

Two types of the late Na channels, burst and background, were studied in Purkinje and ventricular cells. In the whole-cell configuration, steady-state Na currents were recorded at potentials (-70 to -80 mV) close to the normal cell resting potential. The question of the contribution of late Na channels to this background Na conductance was investigated. During depolarization, burst Na channels were active for periods (up to approximately 5 s), which exceeded the action potential duration. However, they eventually closed without reopening, indicating the presence of slow and complete inactivation. When, at the moment of burst channel opening, the potential was switched to -80 mV, the channel closed quickly without reopening. We conclude that the burst Na channels cannot contribute significantly to the background Na conductance. Background Na channels undergo incomplete inactivation. After a step depolarization, their activity decreased in time, approaching a steady-state level. Background Na channel openings could be recorded at constant potentials in the range from -120 to 0 mV. After step depolarizations to potentials near -70 mV and more negative, a significant fraction of Na current was carried by the background Na channels. Analysis of the background channel behavior revealed that their gating properties are qualitatively different from those of the early Na channels. We suggest that background Na channels represent a special type of Na channel that can play an important role in the initiation of cardiac action potential and in the TTX-sensitive background Na conductance.     

Voltage-dependent ionic currents in taste receptor cells of the larval tiger salamander

Voltage-dependent membrane currents of cells dissociated from tongues of larval tiger salamanders (Ambystoma tigrinum) were studied using whole-cell and single-channel patch-clamp techniques. Nongustatory epithelial cells displayed only passive membrane properties. Cells dissociated from taste buds, presumed to be gustatory receptor cells, generated both inward and outward currents in response to depolarizing voltage steps from a holding potential of -60 or -80 mV. Almost all taste cells displayed a transient inward current that activated at -30 mV, reached a peak between 0 and +10 mV and rapidly inactivated. This inward current was blocked by tetrodotoxin (TTX) or by substitution of choline for Na+ in the bath solution, indicating that it was a Na+ current. Approximately 60% of the taste cells also displayed a sustained inward current which activated slowly at about -30 mV and reached a peak at 0 to +10 mV. The amplitude of the slow inward current was larger when Ca2+ was replaced by Ba2+ and it was blocked by bath applied CO2+, indicating it was a Ca2+ current. Delayed outward K+ currents were observed in all taste cells although in about 10% of the cells, they were small and activated only at voltages more depolarized than +10 mV. Normally, K+ currents activated at -40 mV and usually showed some inactivation during a 25-ms voltage step. The inactivating component of outward current was not observed at holding potentials more depolarized -40 mV. The outward currents were blocked by tetraethylammonium chloride (TEA) and BaCl2 in the bath or by substitution of Cs+ for K+ in the pipette solution. Both transient and noninactivating components of outward current were partially suppressed by CO2+, suggesting the presence of a Ca2(+)-activated K+ current component. Single-channel currents were recorded in cell-attached and outside-out patches of taste cell membranes. Two types of K+ channels were partially characterized, one having a mean unitary conductance of 21 pS, and the other, a conductance of 148 pS. These experiments demonstrate that tiger salamander taste cells have a variety of voltage- and ion-dependent currents including Na+ currents, Ca2+ currents and three types of K+ currents. One or more of these conductances may be modulated either directly by taste stimuli or indirectly by stimulus-regulated second messenger systems to give rise to stimulus-activated receptor potentials. Others may play a role in modulation of neurotransmitter release at synapses with taste nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)     

The pacemaker current in cardiac Purkinje myocytes

It is generally assumed that in cardiac Purkinje fibers the hyperpolarization activated inward current i(f) underlies the pacemaker potential. Because some findings are at odds with this interpretation, we used the whole cell patch clamp method to study the currents in the voltage range of diastolic depolarization in single canine Purkinje myocytes, a preparation where many confounding limitations can be avoided. In Tyrode solution ([K+]o = 5.4 mM), hyperpolarizing steps from Vh = -50 mV resulted in a time-dependent inwardly increasing current in the voltage range of diastolic depolarization. This time- dependent current (iKdd) appeared around -60 mV and reversed near EK. Small superimposed hyperpolarizing steps (5 mV) applied during the voltage clamp step showed that the slope conductance decreases during the development of this time-dependent current. Decreasing [K+]o from 5.4 to 2.7 mM shifted the reversal potential to a more negative value, near the corresponding EK. Increasing [K+]o to 10.8 mM almost abolished iKdd. Cs+ (2 mM) markedly reduced or blocked the time-dependent current at potentials positive and negative to EK. Ba2+ (4 mM) abolished the time-dependent current in its usual range of potentials and unmasked another time-dependent current (presumably i(f)) with a threshold of approximately -90 mV (> 20 mV negative to that of the time-dependent current in Tyrode solution). During more negative steps, i(f) increased in size and did not reverse. During i(f) the slope conductance measured with small (8-10 mV) superimposed clamp steps increased. High [K+]o (10.8 mM) markedly increased and Cs+ (2 mM) blocked i(f). We conclude that: (a) in the absence of Ba2+, a time-dependent current does reverse near EK and its reversal is unrelated to K+ depletion; (b) the slope conductance of that time-dependent current decreases in the absence of K+ depletion at potentials positive to EK where inactivation of iK1 is unlikely to occur. (c) Ba2+ blocks this time-dependent current and unmasks another time-dependent current (i(f)) with a more negative (> 20 mV) threshold and no reversal at more negative values; (d) Cs+ blocks both time-dependent currents recorded in the absence and presence of Ba2+. The data suggest that in the diastolic range of potentials in Purkinje myocytes there is a voltage- and time-dependent K+ current (iKdd) that can be separated from the hyperpolarization- activated inward current i(f).     

Potassium currents in cultured cells of the rat retinal pigment epithelium

Whole-cell currents were investigated in cultured rat retinal pigment epithelial (RPE) cells. Two voltage-dependent conductances were discriminated. First, at potentials more positive than −30 mV, a time-dependent outward current was activated. Inhibition by Ba²⁺ (10 mM) and 4-aminopyridine (10 mM) indicated that this current was carried by potassium ions. This current showed no inactivation during 5 sec depolarizations. Second, an inward current, sensitive to Ba²⁺ (10 mM) and 4-aminopyridine (10 mM), was activated at potentials more negative than — 70 mV. Under extra- and intracellular potassium-free conditions, both currents disappeared. In summary, cultured rat RPE cells expressed one potassium conductance similar to the delayed rectifier and one similar to the inward rectifier. The delayed rectifier expressed characteristics comparable with those known in mammalian species and different from those in non-mammalian species.     

Outward currents in olfactory receptor neurons activated by odorants and by elevation of cyclic AMP

We studied the outward currents elicited by an odorous compound, isoamyl acetate, in isolated olfactory receptor neurons of the grass frog under whole-cell perforated-patch voltage-clamp recording. Odorant-induced outward currents were relatively rare, occurring in about 16% of the responding cells. Responses had smaller amplitudes and shorter time courses when compared to the more commonly found odorant-induced inward currents. There was a high correlation between odorant-induced outward current and responses evoked by either 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate, a membrane-permeant cyclic adenosine monophosphate analog, or 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor. The outward current responses to all three substances increased in amplitude when the membrane potential was more negative than -60 mV and decreased in amplitude when the membrane potential was more positive. Responses were still present when the potential was held at -100 mV, indicating that the responses are not the result of a potassium conductance. Removal of external calcium from the perfusion medium abolished the outward currents. Our results indicate that the odorant-induced outward current is a calcium-dependent event that may be mediated by cyclic adenosine monophosphate.     

Spontaneous action potentials and resting potential shifts in fertilized eggs of the tunicate Clavelina

Electrical activity in the fertilized egg of the tunicate Clavelina was studied with microelectrode recording and voltage clamp techniques. The resting potential could assume either of two stable values (approximately ?70 or ?30 mV) and could be shifted between these values by direct current stimulation. Spontaneous shifts between two stable resting potentials were also seen. Egg cells produced action potentials spontaneously and in response to depolarizing stimuli. Inward currents were carried by both Na and Ca ions and a prominent outward potassium current was seen with depolarization to voltages above ?15 mV. The steady-state current-voltage relationship (I–V curve) of the membrane showed two voltages where the net membrane current equaled zero: approximately ?35 and ?70 mV. Between these two voltages, membrane current was inward and carried by noninactivating Na and Ca currents. Inward rectification, which was blocked by external Rb, occurred at voltages below ?70 mV. The voltage dependence of inward rectification is thought by the authors to be important for establishing the more negative resting potential; it is also thought the presence of inward current which does not inactivate completely at voltages more negative than about ?20 mV is an important determinant of the more depolarized resting potential.     

A voltage-gated chloride conductance in rat cultured astrocytes

Large voltage-dependent outward currents are recorded with the whole-cell patch-clamp technique from rat cultured astrocytes under conditions where an outward movement of potassium ions is excluded (either by blockage of the potassium channels pharmacologically or by replacement of the internal potassium by the impermeant large organic cation N-methyl-(+)-glucamine). The current, which is activated at potentials more positive than -40 to -50 mV, is normally carried by an inward movement of chloride ions. Its reversal potential is the same as the chloride equilibrium potential. With depolarization to +60 mV (for 225 ms) little or no inactivation of the current occurs: with depolarizations to +90 to +110 mV a time-dependent decay is seen. The current, which is often not marked immediately after formation of the whole-cell clamp, generally increases over a period of a few minutes to a maximum (after which it usually declines), as if some as yet unknown intracellular factor keeping the channels closed were being washed away from the membrane. The time course of this phenomenon is not affected by changing of the internal free calcium concentration (from 10(-8)M to 10(-6)M) or by an intracellular mixture of cyclic AMP (1 mM), ATP (4 mM) and Mg+ (2 mM). The conductance is slightly increased when the chloride of the bathing medium is replaced by bromide; is much reduced on replacement by methylsulphate, sulphate, isethionate, or acetate; and is virtually abolished on replacement by the large anion gluconate. The outward current is inhibited by the disulphonate stilbenes DIDS and SITS; this blocking action was initially partly reversible, although never completely so. It is suggested that the chloride conductance plays a role in the spatial buffering of potassium by astrocytes.     

Electrogenic sodium extrusion in cardiac Purkinje fibers

Thin canine cardiac Purkinje fibers in a fast flow chamber were exposed to K-free fluid for 15 s to 6 min to initiate "sodium loading," then returned to K-containing fluid to stimulate the sodium pump. The electrophysiological effects of enhanced pump activity may result from extracellular K depletion caused by enhanced cellular uptake of K or from an increase in the current generated as a result of unequal pumped movements of Na and K, or from both. The effects of pump stimulation were therefore studied under three conditions in which lowering the external K concentration ([K]0) causes changes opposite to those expected from an increase in pump current. First, the resting potential of Purkinje fibers may have either a "high" value of a "low" (less negative) value: at the low level of potential, experimental reduction of [K]0 causes depolarization, whereas an increase in pump current should cause hyperpolarization. Second, in regularly stimulated Purkinje fibers, lowering [K]0 prolongs the action potential, whereas an increase in outward pump current should shorten it. Finally, lowering [K]0 enhances spontaneous "pacemaker" activity in Purkinje fibers, whereas an increase in outward pump current should reduce or abolish spontaneous activity. Under all three conditions, we find that the effects of temporary stimulation of the sodium pump are those expected from a transient increase in outward pump current, not those expected from K depletion.