The inwardly rectifying potassium current of embryonic chick hepatocytes

The single channel and whole-cell properties of an inward, rectifying potassium current in cultured embryonic chick hepatocytes were studied at 20°C. In cell-attached patches, channels open upon membrane hyperpolarization and are present in about 90% of cellattached patches. With 145 mm potassium in the pipette, inward current has a slope conductance of 80 pS. The conductance is not a linear function of the external potassium concentration. Current saturates at high external potassium and has a Michaelis-Menten affinity constant of 275 mm potassium. Substitution of gluconate for chloride in the external solution has no significant effect on conductance, and the reversal potential shifts approximately 18 mV with a change in external potassium from 72.5 to 145 mm indicating potassium selectivity. Channel openings are characterized by multiple brief closures during a burst. The channel is inhibited by external cesium in a concentration-dependent manner. Block is characterized by an increased frequency of transient closures. Whole-cell dialysis with 145 mm CsCl of cells bathed in 145 mm KCl reveals time-independent inward currents that reverse at 0 mV in response to 200 msecvoltage steps. Although voltage ramps evoke currents that are 75% potassium dependent and cesium sensitive, the mean chord conductance (425 pS) indicates that less than five channels are open at any instant. We suggest that the inwardly rectifying potassium channel is partially inactivated in the dialysed hepatocyte.We thank K. Paula S. Hettiaratchi and Eunice Y. Wang for expert cell isolation and culture technique, and the Natural Sciences and Engineering Research Council of Canada for supporting this work.     

Voltage-dependent block of cardiac inward-rectifying potassium current by monovalent cations

The inward-rectifying K+ current (IK1) in cat ventricular myocytes, like inward-rectifying K+ currents in many other preparations, exhibited a negative slope conductance region at hyperpolarized membrane potentials that was time-dependent. This was evident as an inactivation of inward current elicited by hyperpolarizing voltage-clamp pulses resulting in a negative slope region of the steady-state current-voltage relationship at potentials negative to -140 mV. Removing extracellular Na+ prevented the development of the negative slope in this voltage region, suggesting that Na+ can block IK1 channels in a time- and voltage-dependent manner. The time and voltage dependence of Cs+-induced block of IK1 was also examined. Cs+ blocked inward current in a manner similar to that of Na+, but the former was much more potent. The fraction of current blocked by Cs+ in the presence of Na+ was reduced in a time- and voltage-dependent manner, which suggested that these blocking ions compete for a common or at least similar site of action. In the absence of Na+, inactivation of IK1 could also be induced by both Cs+ and Li+. However, Li+ was less potent than Na+ in this respect. Calculation of the voltage sensitivity of current block by each of these ions suggests that the mechanism of block by each is similar.     

Voltage-dependent open-state inactivation of cardiac sodium channels: gating current studies with Anthopleurin-A toxin

The gating charge and voltage dependence of the open state to the inactivated state (O-->I) transition was measured for the voltage- dependent mammalian cardiac Na channel. Using the site 3 toxin, Anthopleurin-A (Ap-A), which selectively modifies the O-->I transition (see Hanck, D. A., and M. F. Sheets. 1995. Journal of General Physiology. 106:601-616), we studied Na channel gating currents (Ig) in voltage-clamped single canine cardiac Purkinje cells at approximately 12 degrees C. Comparison of Ig recorded in response to step depolarizations before and after modification by Ap-A toxin showed that toxin-modified gating currents decayed faster and had decreased initial amplitudes. The predominate change in the charge-voltage (Q-V) relationship was a reduction in gating charge at positive potentials such that Qmax was reduced by 33%, and the difference between charge measured in Ap-A toxin and in control represented the gating charge associated with Na channels undergoing inactivation by O-->I. By comparing the time course of channel activation (represented by the gating charge measured in Ap-A toxin) and gating charge associated with the O-->I transition (difference between control and Ap-A charge), the influence of activation on the time course of inactivation could be accounted for and the inherent voltage dependence of the O-->I transition determined. The O-->I transition for cardiac Na channels had a valence of 0.75 e-. The total charge of the cardiac voltage-gated Na channel was estimated to be 5 e-. Because charge is concentrated near the opening transition for this isoform of the channel, the time constant of the O-->I transition at 0 mV could also be estimated (0.53 ms, approximately 12 degrees C). Prediction of the mean channel open time-voltage relationship based upon the magnitude and valence of the O- ->C and O-->I rate constants from INa and Ig data matched data previously reported from single Na channel studies in heart at the same temperature.     

Inward rectifier potassium current in the frog embryonic skeletal myocytes

Pharmacological and kinetic properties of the inward rectifier potassium current Iir the frog embryonic skeletal myocytes were found to be identical to those of adult frog skeletal muscle fibres. The data obtained suggest that the Iir plays the main role in maintaining the myocytes resting membrane potential (RMP) when chloride conductance is insignificant. Changes of the integral conductance Gir and the RMP values correlated with the T-system development. The inward rectifier K+ channels, from the early stages of the muscle seem to be located in the T-tubule membranes.     

Effect of leptin and insulin on chick embryonic muscle cells and hepatocytes

In the present study we used the primary cultures of chick embryonic muscle and liver cells as a model for potential mutual combination effects of leptin and insulin, respectively. The influence of both homones on the proliferation and protein synthesis was dose-dependent and related to the age of embryos from which the cells were isolated. Leptin (10 and 100 ng/well) increased the proliferation (estimated by DNA content and incorporation of labeled thymidine into DNA) and protein synthesis (determined by incorporation of labeled leucine into proteins) of muscle cells. The effect of leptin and insulin in muscle cells was similar. In younger embryo (11-day-old) the lower dose of leptin was more effective than the higher one compared to the insulin effect. Mutual effects of leptin and insulin were neither additive nor synergistic and were equivalent to the effects of individual hormones. In hepatocytes the influence of leptin was dependent on the age at which the cells were isolated (11- and 19-day-old embryos). The presence of insulin neither potentiated nor inhibited the effect of leptin.     

Identification of the hyperpolarization-activated inward current in young embryonic chick heart myocytes

Whole-cell voltage-clamp experiments were performed to examine the underlying currents flowing during the pacemaker potential of spontaneously-beating embryonic chick ventricles. The holding potential was -30 mV. Long-duration (3 s) hyperpolarizing pulses were applied to -40 to -120 mV, in increments of 10 mV. A marked hyperpolarization-activated inward current (If) was produced. In cells from 3-day-old hearts, the threshold potential for the inward current was -50 to -60 mV. In 17-day-old cells, there was almost no If current. At -120 mV, the inward current was -93.8 +/- 6.3 pA (n = 5) in 3-day-old cells and -15.7 +/- 2.8 pA (n = 5) in 17-day-old cells. The average capacitances were 10.1 +/- 2.0 pF (n = 17) in 3-day-old cells, and 6.9 +/- 1.2 pF (n = 14) in 17-day-old cells. The reduction of If paralleled the decrease in spontaneous activity. In the presence of 3 mM CsCl, the inward current was blocked completely, and the tail current was reduced. In addition, 3 mM CsCl depressed the spontaneous action potentials and had a negative chronotropic effect. These results indicate that the hyperpolarization-activated inward If current exists in young embryonic chick heart cells, and decreases during development. This If current may contribute somewhat to the electrogenesis of the pacemaker potential.     

Development of the fast sodium current in early embryonic chick heart cells

Summary Single ventricle cells were dissociated from the hearts of two-, theree-, four-, or seven-day-old chick embryos, and were maintained in vitro for an additional 6 to 28 hr. Rounded 13 to 18 m cells with input capacitance of 5 to 10 pF were selected for analysis of fast sodium current (I _Na). Voltage dependence, and kinetics ofI _Na were applied with patch electrodes in the wholecell clamp configuration.I _Na was present in over half of the 2d, and all 3d, 4d and 7d cells selected. The current showed no systematic differences in activation kinetics, voltage dependence, or tetrodotoxin (TTX) sensitivity with age or culture condition, Between the 2d and 7d stages, the rate of current inactivation doubled an channel density increased about eighfold. At all stages tested,I _Na was blocked by TTX at a half-effective concentration of 0.5 to 1.0 nM. We conclude that the lack of Na dependence of the action potential upstroke on the second day of development results from the relatively depolarized level of the diastolic potential, and failure to activate the small available excitatory na current. The change from Ca to Na dependence of the upstroke during the third to the seventh day of incubation results partly from the negative shift of the diastolic potential during this period, and in part from the increase in available Na conductance.     

Development of the fast sodium current in early embryonic chick heart cells

Single ventricle cells were dissociated from the hearts of two-, three-, four- or seven-day-old chick embryos, and were maintained in vitro for an additional 6 to 28 hr. Rounded 13 to 18 micron cells with input capacitance of 5 to 10 pF were selected for analysis of fast sodium current (INa). Voltage command protocols designed to investigate the magnitude, voltage dependence, and kinetics of INa were applied with patch electrodes in the whole-cell clamp configuration. INa was present in over half of the 2d, and all 3d, 4d and 7d cells selected. The current showed no systematic differences in activation kinetics, voltage dependence, or tetrodotoxin (TTX) sensitivity with age or culture conditions. Between the 2d and 7d stages, the rate of current inactivation doubled and channel density increased about eightfold. At all stages tested, INa was blocked by TTX at a half-effective concentration of 0.5 to 1.0 nM. We conclude that the lack of Na dependence of the action potential upstroke on the second day of development results from the relatively depolarized level of the diastolic potential, and failure to activate the small available excitatory Na current. The change from Ca to Na dependence of the upstroke during the third to the seventh day of incubation results partly from the negative shift of the diastolic potential during this period, and in part from the increase in available Na conductance.     

Developmental increases in the inwardly-rectifying K+ current of embryonic chick ventricular myocytes

Whole-cell and single-channel inwardly-rectifying K+ currents (IK1) of early (3-day-old) and late (17-day-old) embryonic chick ventricular myocytes were compared to ascertain whether there are developmental changes in the properties of this conductance. The magnitude of the IK1 conductance in the early myocytes was small, but it was increased about five-fold in the older embryonic myocytes. It was found that the density of inwardly-rectifying K+ channels was greater (in the surface membrane) of the 17-day than in the 3-day embryonic myocyte. In addition, the single channel conductance for 17-day myocytes was several-fold larger than for the 3-day myocytes. These results suggest that cardiac inward rectifier channels may not only proliferate in number, but may also undergo structural alterations during development.     

8-Bromo-cyclic GMP inhibits the calcium channel current in embryonic chick ventricular myocytes

Superfusion with 8-bromo-cyclic GMP or intracellular injection of cyclic GMP inhibits calcium-dependent slow action potentials in embryonic chick or guinea pig ventricular cells, suggesting that cyclic GMP inhibits calcium currents. Recently, cyclic GMP has been shown to reduce cyclic AMP-stimulated calcium currents in voltage-clamped ventricular myocytes. Since earlier results in intact cells had suggested that cyclic GMP might inhibit basal (i.e., unstimulated by cyclic AMP) calcium currents, we directly investigated the effect of 8-bromo-cyclic GMP on basal calcium channel currents (using barium as the charge carrier) in voltage-clamped ventricular myocytes isolated from embryonic chick hearts. Superfusion with 1 mM 8-bromo-cyclic GMP (without prior cyclic AMP elevation) progressively decreased peak calcium channel currents (-68% at 15 min after the onset of drug exposure). In contrast, the currents were unchanged during 15 min superfusion with control solution, or 1 mM 8-bromo-GMP (the noncyclic inactive analog of 8-bromo-cyclic GMP). The present results in voltage-clamped embryonic chick heart cells indicate that cyclic GMP can inhibit basal calcium channel currents, apparently through a cyclic AMP-independent mechanism.     

The initial inward current in spherical clusters of chick embryonic heart cells

The rapid inward sodium current in spherical clusters of 11-d-old embryonic chick heart cells, ranging in size between 65 and 90 micron diameter, was studied using the two-microelectrode voltage-clamp technique. Using these preparations, it was possible to resolve the activation phase of the rapid inward current for potentials negative to -25 mV at 37 degrees C. The rapid inward current exhibited a voltage and time dependence similar to that observed in other excitable tissues. It was initiated at potential steps more positive than -45 mV. The magnitude of the current reached its maximum value at a potential of approximately -20 mV. The measured reversal potential was that predicted by the Nernst equation for sodium ions. The falling phase of the current followed a single exponential time-course with a time constant of inactivation, tau h, ranging between 2.14 ms at -40 mV and 0.18 ms at -5 mV. The time constant of inactivation, tau h, determined by a single voltage-step protocol was compared to the constant, tau c, determined by a double voltage-step protocol and no significant different between the two constants of inactivation was found. Furthermore, the time constants of inactivation and reactivation at the same potential in the same preparation were similar. The results of this study demonstrate that the sodium current of heart cells recorded at 37 degrees C can be described by Hodgkin-Huxley kinetics with speeds approximately four times faster than the squid giant axon at 15 degrees C.     

Voltage-dependent potassium channels in the molluscan neuron somatic membrane

Under voltage clamp conditions proof of the presence of two populations of potassium current channels was obtained on the molluscan neuron somatic membrane: inactivated and uninactivated. They differ from each other in their physicochemical characteristics, the property of their gating mechanisms, and the molecular structure of their current-conducting part. The inactivated potassium current is largely and selectively inhibited by cooling. Channels of the fast potassium current also are highly sensitive to temperature changes. By using parameters of gating mechanisms of the "fast" potassium channels included in the Hodgkin-Huxley model, the physicochemical properties of channels of this type were described. The density of fixd negative surface charges on the somatic membrane in the region of localization of fast potassium channels was estimated with the aid of the Gouy-Chapman theory. It is 0.3 electron charge/nm². Data on the character of interaction of potassium channels with intracellular sodium ions revealed differences in the structure of the current-conducting part of different types of potassium channels. Experiments on intracellularly perfused molluscan neurons demonstrated the particular features of interaction between intracellular calcium ions and calcium-activated channels under conditions of strictly controlled changes in the intracellular calcium concentration.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 296–307, May–June, 1984.     

去甲肾上腺素对大鼠肝细胞延迟外向钾电流的影响

目前为止国内外尚未见到有关大鼠肝细胞外向钾电流方面的报道。本文用全细胞膜片宿制技术观察了大鼠肝细胞延迟外向钾电流（Ik）及去甲肾上腺素等对人的影响。实验结果表明,在保持电位-50mV、指令电位＋140mV时大鼠肝细胞Ik为2．85±1.21nA。去甲肾上腺素明显降低IK,异丙肾上腺素和乙酰胆碱对IK无影响。     

A whole-cell and single-channel study of the voltage-dependent outward potassium current in avian hepatocytes

Voltage-dependent membrane currents were studied in dissociated hepatocytes from chick, using the patch-clamp technique. All cells had voltage-dependent outward K+ currents; in 10% of the cells, a fast, transient, tetrodotoxin-sensitive Na+ current was identified. None of the cells had voltage-dependent inward Ca2+ currents. The K+ current activated at a membrane potential of about -10 mV, had a sigmoidal time course, and did not inactivate in 500 ms. The maximum outward conductance was 6.6 +/- 2.4 nS in 18 cells. The reversal potential, estimated from tail current measurements, shifted by 50 mV per 10-fold increase in the external K+ concentration. The current traces were fitted by n2 kinetics with voltage-dependent time constants. Omitting Ca2+ from the external bath or buffering the internal Ca2+ with EGTA did not alter the outward current, which shows that Ca2+-activated K+ currents were not present. 1-5 mM 4-aminopyridine, 0.5-2 mM BaCl2, and 0.1-1 mM CdCl2 reversibly inhibited the current. The block caused by Ba was voltage dependent. Single-channel currents were recorded in cell-attached and outside-out patches. The mean unitary conductance was 7 pS, and the channels displayed bursting kinetics. Thus, avian hepatocytes have a single type of K+ channel belonging to the delayed rectifier class of K+ channels.     

Voltage-dependent potassium channels: minK and Shaker families

In the last 4 years, the molecular identity of several types of voltage-dependent potassium channels has been discovered. These include channels that terminate action potentials and control repetitive neuronal firing, as well as channels whose biological role is not yet understood. The majority of these are encoded by genes related to the Drosophila Shaker gene. The large number of genes comprising the Shaker gene family, coupled with the existence of different channels that result from alternatively spliced messages from the same gene, provide both vertebrates and invertebrates with a wide selection of channels whose voltage-dependence and kinetics can be tailored to the needs of a specific cell. Mutagenesis experiments on such channels are providing new information on those regions of the protein that govern essential aspects of channel activity, such as gating by voltage and ion permeation. Another gene, unrelated to the Shaker family, encodes a voltage-dependent potassium channel that activates much more slowly than the Shaker channels. This has been termed the MinK channel.     

Voltage-dependent removal of sodium inactivation by N-bromoacetamide and pronase.

When perfused internally through crayfish giant axons, pronase removed sodium inactivation more than three times as fast at -100 mV as compared with -30 mV. N-bromoacetamide, applied internally, removed sodium inactivation twice as fast at -100 mV as at -30 mV, and the relative rate of removal declined with membrane depolarization in proportion to steady-state sodium inactivation. We conclude that in the closed conformation the sodium inactivation gate is partially protected from destruction by N-bromoacetamide and pronase.     

Voltage-dependent inactivation gating at the selectivity filter of the MthK K+ channel

Voltage-dependent K(+) channels can undergo a gating process known as C-type inactivation, which involves entry into a nonconducting state through conformational changes near the channel's selectivity filter. C-type inactivation may involve movements of transmembrane voltage sensor domains, although the mechanisms underlying this form of inactivation may be heterogeneous and are often unclear. Here, we report on a form of voltage-dependent inactivation gating observed in MthK, a prokaryotic K(+) channel that lacks a canonical voltage sensor and may thus provide a reduced system to inform on mechanism. In single-channel recordings, we observe that Po decreases with depolarization, with a half-maximal voltage of 96 ± 3 mV. This gating is kinetically distinct from blockade by internal Ca(2+) or Ba(2+), suggesting that it may arise from an intrinsic inactivation mechanism. Inactivation gating was shifted toward more positive voltages by increasing external [K(+)] (47 mV per 10-fold increase in [K(+)]), suggesting that K(+) binding at the extracellular side of the channel stabilizes the open-conductive state. The open-conductive state was stabilized by other external cations, and selectivity of the stabilizing site followed the sequence: K(+) ≈ Rb(+) > Cs(+) > Na(+) > Li(+) ≈ NMG(+). Selectivity of the stabilizing site is weaker than that of sites that determine permeability of these ions, suggesting that the site may lie toward the external end of the MthK selectivity filter. We could describe MthK gating over a wide range of positive voltages and external [K(+)] using kinetic schemes in which the open-conductive state is stabilized by K(+) binding to a site that is not deep within the electric field, with the voltage dependence of inactivation arising from both voltage-dependent K(+) dissociation and transitions between nonconducting (inactivated) states. These results provide a quantitative working hypothesis for voltage-dependent, K(+)-sensitive inactivation gating, a property that may be common to other K(+) channels.