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

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

The anion transport inhibitor DIDS activates a Ba2+-sensitive K+ flux associated with hepatic exocrine secretion

4,4'-Diisothiocyanatostilbene-2,2'-disulfonate (DIDS), an anion transport inhibitor and choleretic organic anion, was used to study the relationship between putative DIDS-sensitive K channels and exocrine secretion in the isolated and bile duct cannulated perfused rat liver. Bile flow, DIDS excretion, and effluent perfusate K+ content were measured. DIDS (125 µM) caused a doubling in bile generation concomitant with its appearance in bile, confirming earlier reports. Furthermore, DIDS induced a transient increase in perfusate K+ concentration that peaked prior to the biliary parameters and, after 10 min, reversed to net uptake that fully compensated for the initial release. The K channel blocker Ba2+ (1 mM) strongly inhibited the release phase along with the accompanying choleresis and DIDS excretion. Ouabain (13.5 µM) alone was choleretic and hyperkalemic and, when applied in combination with DIDS, depressed DIDS excretion, choleresis, and DIDS-sensitive K+ uptake. To obtain further evidence for the presence of DIDS-sensitive K channels K+ flux was measured under the influence of different gradients of the cation. Perfusate K+ at 26 and 80 mM changed the DIDS-activated K+ flux from a transient outward to a sustained inward flux, and both DIDS excretion and bile flow decreased. Mean net K+ flux over 20 min DIDS perfusion changed from -1.3 ± 1.1 µmol/g with 5.9 mM K+ to -1304 ± 55 µmol/g with 80 mM K+ in the perfusate. K+ efflux was fully and reversibly blocked by Ba2+ and influx was ouabain-insensitive, suggesting that the DIDS-activated K+ flux was channel mediated. The results show that a significant fraction of DIDS-induced bile generation is associated with K+ release that may be mediated by Ba2+-sensitive K channels, possibly of the inward rectifying type.Key words: hepatocyte, inward rectifier, 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS), K+ channel, bile formation.

Evidence for norepinephrine-activated Ca2+ permeable channels in guinea-pig hepatocytes using a patch clamp technique

To determine whether the hepatocyte plasma membrane possesses a Ca2+ channel. we applied a patch clamp technique to isolated guinea-pig hepatocytes. In a cell-attached configuration, using an internal pipette solution of 110 mM BaCl2 or CaCl2, we observed sporadic inward single channel currents (Po = 0.004 +/- 0.002, n = 6) at various membrane potentials. The unit amplitude was 0.60 +/- 0.15 pA (n = 6) at resting membrane potential. The single channel conductance was 20.4 +/- 4.6 pS (n = 6) and this channel showed no rectification and no voltage dependence. Bay K 8644, a dihydropyridine Ca2+ channel activator, did not affect this channel activity. Although norepinephrine in the pipette solution did not activate this channel, its external application increased channel activity. These observations suggest that guinea-pig hepatocytes possess Ca2+ permeable channels that differ from the voltage-operated Ca2+ channels found in excitable cells and that such channels are responsible for the agonist-stimulated Ca2+ entry in hepatocytes.

Novel variants of voltage-operated calcium channel alpha 1-subunit transcripts in a rat liver-derived cell line: deletion in the IVS4 voltage sensing region

Using reverse transcriptase-PCR and Northern analysis, we have shown that the H4IIE cell line, derived from the Reuber H35 rat hepatoma, contains significant amounts of transcripts for the CaCh3 (neuroendocrine) and CaCh1 (skeletal muscle) L-type voltage-operated calcium channel alpha 1-subunits. Two of the CaCh3 transcripts have a 45 bp deletion in the IVS4 membrane-spanning region which is the result of a mutation in genomic DNA. The deduced amino acid sequences of the PCR-derived clones of CaCh3 indicate that the mutation causes the loss of 15 amino acids from the IVS4 region, including three of the six positively charged residues, which are thought to be part of the voltage-sensing mechanism of voltage-operated Ca2+ channels. Quantitative-PCR and Northern analysis indicate that one of the novel CaCh3 transcripts is present in sufficient amounts to imply it could play a functional role in Ca2+ inflow. RT-PCR analysis of hepatocytes isolated from rat liver detected transcripts of CaCh3 (without the IVS4 mutation) and CaCh2, but at considerably lower levels than observed for the isoforms in the H4IIE cell line. Transcripts of CaCh1 and CaCh2 were also detected at low levels in Jurkat T lymphocytes. Fluorimetric studies with the Ca(2+)-sensitive probe, Fluo-3, have shown that H4IIE cells exhibit receptor-activated and store-activated (thapsigarin-induced), but not depolarisation (extracellular KCl)-induced Ca2+ inflow. The mutant transcripts are unlikely to produce Ca2+ channels that are opened by membrane depolarisation. The idea that they may be opened by other mechanisms is briefly discussed.

Existence, properties, and functional expression of "Maxi-K"-type, Ca2+-activated K+ channels in short-term cultured hepatocytes

A large-conductance, Ca2+-activated K+ channel was identified and characterized in embryonic chick hepatocytes using the patch-electrode voltage-clamp technique. The channel conductance was 213 pS in excised patches bathed in symmetrical 145 mM KCI and 1 mM Ca2+. Current-voltage relationships were linear with high K+ on both sides of the membrane but showed constant field rectification as the K+ gradient was increased. The reversal potential shifted 58 mV per 10-fold change in the ratio of external to internal K+. Channel openings occurred at potentials higher than +50 mV in cell-attached patches. The open probability X voltage relationship shifted to more negative potentials in excised, inside-out patches exposed to a solution containing high Ca2+. The voltage sensitivity of the channel was not significantly affected by changes in internal Ca2+ concentration. Conversely, channel gating, reflected in the half-activation potential, shifted 118 mV per 10-fold change in internal Ca2+ at concentrations less than approximately 2 microM, although at higher Ca2+, this parameter was Ca2+ insensitive. Channel open probability in cell-attached patches increased significantly following exposure of the cells to either the Ca2+ ionophore A-23187 or L-alanine, a cell-volume modulator. Channel density increased with time spent in culture from no observations in 10-hr cells, through 13 and 80% of patches in 24-and 48-hr cultured cells, respectively. The implications of delayed functional expression for ion channel studies in acutely dissociated cells is discussed.

Outward K+ current in epithelial cells isolated from intermediate portion of endolymphatic sac of guinea pigs

Ion currents in epithelial cells isolated from the intermediate portion of endolymphatic sac (ES) in guinea pigs were investigated with the use of the whole cell patch-clamp technique. Depolarizing voltage steps from a holding potential of -60 mV induced a time- and voltage-dependent outward current, which is comparable to that of delayed rectifying K+ currents. The average resting membrane potential in the current-clamp mode was -54.8 +/- 11 mV (n = 45), which was similar to the value of zero current potential (-55.6 +/- 0.8 mV, n = 32) obtained from current-voltage (I-V) relationships of outward currents in voltage-clamp mode. The I-V relationship of the tail current exhibited a reversal potential (Erev) of -78.1 +/- 0.9 mV (n = 19) in standard external solution. The Erev of the outward current was linearly related to the logarithm of extracellular K+ concentrations. The slope was 48 mV per 10-fold change in extracellular K+ concentrations. The time constants of K+ current activation, inactivation, and K+ tail current deactivation were voltage dependent. The steady-state activation and inactivation of K+ current exhibited a sigmoidal relationship to voltage. The 50% maximal activation voltage and slope factor were -21 and 11 mV (n = 8), respectively. The 50% maximal inactivation voltage and slope factor were -45 and 13 mV (n = 7), respectively. The K+ current was blocked by externally applied 1 mM 4-aminopyridine (4-AP), 5 mM Ba2+ and 20 mM tetraethylammonium chloride (TEA). The sensitivity of the current to 4-AP and Ba2+ was higher than that to TEA. Elimination of external Ca2+ and increase of internal Ca2+ failed to significantly change the current, suggesting that the K+ current may be Ca2+ independent. The results show that epithelial cells in the intermediate portion of the ES possess a delayed-rectifier K+ current, which may be involved in membrane stability or in the ion balance between the cytosol and the extracellular environment.

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 degrees C. In cell-attached patches, channels open upon membrane hyperpolarization and are present in about 90% of cell-attached 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 msec-voltage 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.

Properties of single potassium channels in guinea pig hepatocytes

The patch-clamp technique of cell-attached and inside-out configurations was used to study the single potassium channels in isolated guinea pig hepatocytes. The single potassium channels in isolated guinea pig hepatocytes were recorded at different K+ concentrations. A linear single-channel current-voltage relationship was obtained at the voltage range of -80 to -20 mV with slope conductance of 70 +/- 6 pS (n = 10). Under symmetrical high K+ concentration of 148 mM in the cell-attached patch membrane, the I-V curve exhibited a mild inward rectification at potentials positive to + 20 mV. The values of reversal potential was +5 +/- 2 mV (n = 10). When the external potassium concentration ([K+]o) was decreased to 74 mM and 20 mM, the slope conductance was decreased to 48 +/- 2 pS (n = 4) and 24 +/- 3 pS (n = 3), respectively. The reversal potential was changed by 58 mV for a tenfold change in [K+]o, indicating that this channel was highly selective for K+. Open probabilities (Po) of the channel were 73-93% without apparent voltage dependence. The distributions of open time of the channels were fitted to two exponentials, while those of closed time were fitted to three exponentials, exhibiting no voltage dependence. The success rate of K+ channel activity to be recorded was 28% at room temperature, and there were no increases in the success rate nor in the channel opening probabilities at a temperature of 34-36 degrees C. Po in inside-out patches was not changed by application of 1 microM Ca2+ nor 1 mM Mg2+ to the internal side of patch membranes. It is concluded that a novel type of the K+ channels in guinea pig hepatocytes had different properties of slope conductance, channel kinetics, and sensitivity to [Ca2+]i, from those in other species.

A voltage-sensitive transient potassium current in mouse pancreatic acinar cells

We describe, for the first time, a potassium current in acutely isolated mouse pancreatic acinar cells. This current is activated by depolarization and has many of the characteristics of the fast transient potassium current of neurones where roles in shaping action potential duration and frequency have been proposed. Although acinar cells do not carry action potentials, our experiments indicate that the primary regulator of the current in these cells is the membrane potential. In whole-cell patch-clamped cells we demonstrate an outward current activated by depolarization. This current was transient and inactivated over the duration of the pulse (100-500 ms). The decay of the inactivation was adequately fitted by a single exponential. The time constant of decay, tau, at a membrane potential of +20 mV was 34 +/- 0.6 ms (mean +/- SEM, n = 6) and decreased with more positive pulse potentials. The steady-state inactivation kinetics showed that depolarized holding potentials reduced the amplitude of the current observed with a half-maximal inactivation at a membrane potential of -40.6 +/- 0.33 mV (mean +/- SEM, n = 5). These activation and inactivation characteristics were not affected by low intracellular calcium (10(-10) mol.l-1) or by an increase in calcium (up to 180 nmol.l-1). In addition we found no effect on the current of dibutyryl cyclic adenosine monophosphate (db-cAMP) or the agonist acetylcholine. The current was blocked by 4-aminopyridine (Kd approximately 0.5 mmol.l-1) but not affected by 10 mmol.l-1 tetraethylammonium.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium currents in type II vestibular hair cells isolated from the guinea-pig's crista ampullaris

Type II vestibular hair cells were isolated from cristae ampullares of guinea-pig and maintained in vitro for 2-3 h. Outward membrane currents were studied under whole-cell voltage-clamp conditions. Type II hair cells had resting potentials of about -45 mV. Depolarizing voltage steps from a holding potential of -80 or -90 mV induced time- and voltage-dependent outward currents which slowly decayed to a sustained level. Tail currents reversed at about -70 mV, indicating that the outward currents were mainly carried by potassium ions. The currents had an activation threshold around -50 mV. The transient component was completely removed by a depolarizing pre-pulse positive to -10 mV. While bath application of 4-aminopyridine (5 mM) reduced both components, extracellular tetraethylammonium (10 mM) or zero calcium preferentially diminished the sustained current. We conclude that at least two potassium conductances are present, a delayed rectifier with a relatively fast inactivation and a calcium-dependent potassium current. Depolarizing current injections induced an electrical resonance in the voltage responses, with a frequency of 25-100 Hz, larger currents causing higher frequencies.

Properties of a cell volume-sensitive potassium conductance in isolated guinea-pig and rat hepatocytes

1. Whole-cell voltage clamp and intracellular recording techniques were used to study the increase in K+ conductance that accompanies swelling in isolated guinea-pig and rat hepatocytes in short-term culture at 37 degrees C. 2. Swelling was induced (i) by the application of pressure (15 cmH2O) to the shank of the patch pipette, (ii) by exposing the cells to hypotonic solutions and (iii) as a consequence of leakage of electrolyte from an intracellular microelectrode. 3. Applying pressure to the patch pipette caused a large outward current (approximately 600 pA) to develop in guinea-pig hepatocytes voltage clamped to 0 mV. This current reversed direction at -86 mV, close to the reversal potential for K+, EK (-93 mV), and is attributable to the activation of a K+ conductance. 4. Spectral analysis of current noise during this response suggested a single-channel conductance of 7 pS, though this may well be an underestimate. The power spectrum could be fitted by the sum of two Lorentzian components, with half-power frequencies of 7 and 152 Hz. Seventy per cent of the variance was associated with the lower frequency component. 5. The steady-state current-voltage relationship for guinea-pig hepatocytes, as determined by whole-cell recording, was linear over the range -70 to +40 mV both before and during the increase in K+ conductance induced by swelling. 6. Confirming earlier work, intracellular recording using microelectrodes filled with 1 M-potassium citrate sometimes resulted in a slow hyperpolarization and a large rise in input conductance. These changes are also attributable to an increase in K+ conductance as the cell swelled because of leakage from the electrode. 7. Application of hypotonic external solutions during intracellular recording caused hyperpolarization and an increase in conductance. Conversely, hypertonic solution evoked depolarization and a fall in conductance in partly swollen cells. 8. The volume-activated K+ conductance was reversibly blocked by cetiedil, which caused half-maximal inhibition at 2.3 microM. Bepridil, quinine and barium were also effective, with IC50s (concentrations giving 50% maximal inhibition) of 2.7, 12 and 67 microM respectively. 9. Much greater concentrations of cetiedil and bepridil (IC50 approximately 1 mM and 77 microM respectively) were required to inhibit the loss of K+ which follows the application of angiotensin II (100 nM) to guinea-pig hepatocytes, and which occurs via Ca(2+)-activated K+ channels. Our evidence suggests that the activation of K+ channels by cell swelling is Ca2+ independent.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of hyperosmotic medium on hepatocyte volume, transmembrane potential and intracellular K+ activity

Hepatocyte transmembrane potential (Vm) behaves as an osmometer and varies with changes in extracellular osmotic pressure created by altering the NaCl concentration in the external medium (Howard, L.D. and Wondergem, R. (1987) J. Membr. Biol. 100, 53). We now have demonstrated similar effects on Vm by increasing external osmolality with added sucrose and not altering ionic strength. We also have demonstrated that hyperosmotic stress-induced depolarization of Vm results from changes in membrane K+ conductance, gK, rather than from changes in the K+ equilibrium potential. Vm and aKi of hepatocytes in liver slices were measured by conventional and ion-sensitive microelectrodes, respectively. Cell water vols. were estimated by differences in wet and dry weights of liver slices after 10-min incubations. Effect of hyperosmotic medium on membrane transference number for K+, tK, was measured by effects on Vm of step-changes in external [K+]. Hepatocyte Vm decreased 34, 52 and 54% when tissue was superfused with medium made hyperosmotic with added sucrose (50, 100 and 150 mM). Correspondingly, aKi increased 10, 18 and 29% with this hyperosmotic stress of added sucrose. Tissue water of 2.92 +/- 0.10 kg H2O/kg dry weight in control solution decreased to 2.60 +/- 0.05, 2.25 +/- 0.06 and 2.22 +/- 0.05 kg H2O/kg dry weight with additions to medium of 50, 100 and 150 mM sucrose, respectively. Adding 50 mM sucrose to medium decreased tK from 0.20 +/- 0.01 to 0.05 +/- 0.01. Depolarization by 50% with hyperosmotic stress (100 mM sucrose) also occurred in Cl-free medium where Cl- was substituted with gluconate. We conclude that hepatocytes shrink during hyperosmotic stress, and the aKi increases. The accompanying decrease in Vm is opposite to that expected by an increase in aKi, and at least in part results from a concomitant decrease in gK. Changes in membrane Cl- conductance most likely do not contribute to osmotic stress-induced depolarization, since equivalent decreases in Vm occurred with added sucrose in cells depleted of Cl- by superfusing tissue with Cl-free medium.

Characterization of voltage-gated and calcium-activated potassium currents in toadfish saccular hair cells

Patch clamp methods were used to study calcium activated (IKCa) and voltage-gated (IK) potassium currents in enzymatically disassociated hair cells from the saccule of the toadfish Opsanus tau. In one population of hair cells, tetraethylammonium bromide (TEA) blocked all outward current, leaving only an inward calcium current (ICa). This current blocked by TEA was also blocked by barium (5 mM) and cadmium (0.2 mM) but only partially blocked by zero external calcium. In the majority of the cells, after TEA (25 mM) was used to block IKCa, a second outward current remained. This current was resistant to block by apamin, barium (5 mM) and cadmium (0.2 mM). Its kinetics of activation and deactivation were considerably slower than those of IKCa. Because of the current/voltage characteristics, its resistance to block by the above agents and voltage-gated activation, this current was termed IK. Study of the rates of activation and deactivation of the two currents in hair cells exhibiting either fast or slow total outward current activation showed that these two kinetic parameters were linked in a cell, i.e., cells with fast IKCa kinetics exhibit faster IKCa kinetics than cells with slower IKCa kinetics. Cell attached and inside out recordings showed a high conductance channel with short open times and a lower conductance channel with longer open times active over the same voltage ranges as those seen in whole cell recordings. Since these two currents with quite different but linked kinetics are active over the same voltage range, their co-existence may be of some importance to sensory coding in the hair cells.

Ionic currents in isolated vestibular hair cells from the guinea-pig crista ampullaris

Ionic currents have been recorded under whole cell patch clamp in cells isolated from the guinea-pig vestibular system. Type I and type II cells were separately identified. Type II cells were further classified as short (less than 15 microns in length) or tall (greater than 15 microns). Under whole cell voltage clamp, cells showed an outward current which activated at potentials above about -50 mV, and tail currents which reversed near the potassium equilibrium potential. The outward current was reduced in the presence of external 10 mM tetraethylammonium or cadmium ions and when calcium was removed from the external medium. A small cadmium-sensitive transient inward current, a putative calcium current, was observed in cells loaded with caesium from the patch pipette. In 27 out of 64 cells a component of the recorded outward current inactivated. Such current components were most common in tall type II cells. This inactivating component was blocked by 4-aminopyridine and removed by depolarizing prepulses consistent with it being an A-type potassium current. Type I cells, on the other hand, showed mainly a non-inactivating outward current which slowly relaxed on repolarization to resting potentials. When membrane potentials were measured under current clamp, injections of less than 100 pA produced a single, highly damped transient followed by a plateau in Type II cells. No such transient was present in Type I cells. There is thus little evidence for an electrical resonance in these cells.

The nature and mechanism of activation of the hepatocyte receptor-activated Ca2+ inflow system

Progress in elucidation of the properties of the hepatocyte receptor-activated Ca2+ inflow system (RACIS) has been hampered by difficulties in measuring rates of Ca2+ inflow to hepatocytes. These difficulties have led, for example, to different conclusions about the relationship between the extracellular Ca2+ concentration and the movement of Ca2+ through the RACIS. The hepatocyte RACIS admits Mn2+ and a number of other divalent cations as well as Ca2+. Many of these cations also inhibit the movement of Ca2+ through this system. While the RACIS is inhibited by high concentrations of verapamil and by some other Ca2+ antagonists, it is relatively insensitive to inhibition by organic compounds which inhibit other Ca2+ channels and Ca2+ transporters. There is circumstantial evidence which suggests that the hepatocyte RACIS is an exchange system, possibly one which catalyses Ca(2+)-H+ exchange or the co-transport of Ca2+ and OH-. Other circumstantial evidence suggests that the RACIS is a channel, with some similarities to voltage-operated Ca2+ channels in excitable cells. However, experiments using the patch-clamp technique have not yet detected agonist-stimulated Ca2+ movement across the hepatocyte plasma membrane. The molecular components of the RACIS probably differ from those which facilitate the large inflow of Ca2+ to hepatocytes which occurs in the absence of an agonist. The mechanism by which agonists activate the RACIS has not been elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)

A potassium channel in cultured chondrocytes

Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.

Calcium sequestration in the liver

Hepatic parenchymal cells maintain intracellular total and cytosolic free Ca2+ levels by: entry of Ca2+ through channels, extrusion of Ca2+ by an outwardly directed Ca2+ pump, and controlled sequestration into intracellular pools. The mechanism of Ca2+ inflow is poorly characterized. The plasma membrane Ca2+ channels seem to share some of the characteristics of Ca2+ channels in excitable cells, but also differ from them. The outwardly directed plasma membrane Ca2(+)-ATPase is a calmodulin independent, P-type enzyme. Ca2+ uptake into the endoplasmic reticulum is due to the activity of a different Ca2(+)-ATPase, which is similar in molecular weight and shares antigenic determinants with the sarcoplasmic reticulum enzyme. In addition, mitochondria and nuclei also take up calcium. The exact mechanism by which Ca2+ is released from intracellular organelles is not well known. Several mechanisms for Ca2+ release from the endoplasmic reticulum were reported, including IP3 and GTP-induced. The most effective identified way of eliciting Ca2+ release from microsomal fraction is by the oxidation of critical -SH groups. This mechanism is likely to be involved in the rise of cytosolic Ca2+ observed in many situations of hepatocellular injury. In addition to being sequestered into subcellular organelles, some of the intracellular Ca2+ is bound to specific Ca2+ binding proteins. Both calmodulin and members of the annexin family were identified in the liver. Stimulation of the liver with gluconeogenic hormones results in increased Ca2+ entry into the cell, the release of Ca2+ from intracellular pools, and an oscillatory increase in free cytosolic Ca2+ levels. Extensive research is still needed for the elucidation of the exact mechanisms by which these events occur.

Voltage-dependent currents in isolated cells of the frog retinal pigment epithelium

1. Retinal pigment epithelial (RPE) cells were isolated enzymatically from bullfrog retinae. The patch-clamp technique was employed to investigate whole-cell currents under voltage-clamp conditions. 2. Isolated RPE cells were columnar or cuboidal in form, often with long processes protruding from the apical surface. Distinct apical and basal membrane domains were maintained for several hours following isolation. 3. The mean membrane capacitance was 62 pF. The resting potential averaged -30 mV, but it was as high as -75 mV in some cells. 4. Three voltage-dependent currents were observed: a time-independent and inwardly rectifying current and two time-dependent outwardly rectifying currents that had distinct kinetic properties. 5. Voltage pulses from a holding potential of -70 mV to potentials ranging from -30 to -120 mV produced membrane currents that were essentially time independent. The I-V relationship in this voltage range depended on the resting potential. It was usually inwardly rectifying in cells with resting potentials negative to about -50 mV, but tended to be linear in cells with more positive potentials. Three observations strongly suggested that the inwardly rectifying current is carried by K+. First, increasing the extracellular K+ concentration [( K+]) from 2 to 112 mM shifted the zero-current potential of the I-V relationship in the positive direction from an average value of -60 mV to 0 mV. Second, the addition of the K+ channel blockers Ba2+ (2 mM) or Cs+ (5 mM) to the extracellular solution inhibited a major component of the inwardly rectifying current. Finally, the reversal potential (Vr) of the Ba2(+)-sensitive current averaged -90 mV, near the K+ equilibrium potential (EK). 6. In approximately 50% of the cells, depolarizing voltage pulses to potentials more negative than -30 mV evoked an outward current that resembled the delayed rectifier present in other non-excitable cells. It activated with sigmoidal kinetics in less than 100 ms following a brief delay and then declined exponentially with a time constant of approximately 1 s. The peak chord conductance associated with this current was half-maximal at +14 mV. Several observations indicated that this outwardly rectifying current is carried primarily by K+: its Vr closely matched EK over a wide range of extracellular [K+]; it was inhibited 80% by exposure to the K+ channel blockers 4-aminopyridine (1 mM) and tetraethylammonium (20 mM); and it was abolished by intracellular dialysis with a K(+)-free solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Two components of potassium current activated by depolarization of single smooth muscle cells from the rabbit portal vein

1. Using the patch-clamp technique at 20-23 degrees C membrane currents were recorded from single smooth muscle cells enzymatically isolated from the rabbit portal vein. Single-channel currents were observed in outside-out patches excised from these. 2. Outward current elicited upon depolarization from -70 mV was not activated as a result of Ca2+ influx. It could be divided into two components: an inactivating, 4-aminopyridine- and phencyclidine-sensitive low-noise current (IdK), and a non-inactivating, tetraethylammonium (TEA)- and charybdotoxin-sensitive high-noise current (IcK). 3. IdK activated with a threshold around -40 mV and was carried by K+. It was substantially inhibited by 4-aminopyridine (5 mM) or phencyclidine (0.1 mM) but was insensitive to TEA+ (4 mM), charybdotoxin (0.1 microM) or apamin (0.1 microM). Upon stepping to 0 mV it reached a maximum within about 0.2 s. The time course of its activation could be described by a fourth-order single exponential; the time constants of these exponentials changed e-fold every 56 mV. It inactivated in a time- and voltage-dependent manner with a fast and slow component, and was about 50% available at -30 mV. From single-channel recordings in isolated patches single channels underlying this current have a small unitary conductance (around 5 pS). 4. IcK did not inactivate significantly over 6 s. It activated with a less negative threshold than IdK, usually near 0 mV when the pipette solution contained 0.8 mM-EGTA with no added calcium. It was blocked by TEA (4 mM) or charybdotoxin (0.1 microM), but not by 4-aminopyridine (5 mM), phencyclidine (0.1 mM) or apamin (0.1 microM). Estimates of the single-channel conductance from the noise variance of the whole-cell current IcK indicated a value at +80 mV of 115 pS, very similar to that of the large-conductance Ca2(+)-activated K+ channels studied in these cells using single-channel recording. 5. The results suggest that outward current evoked by depolarization from the resting potential can be carried by 100 pS Ca2(+)-activated K+ channels and by small-conductance delayed-rectifier K+ channels. It is likely that opening of both types of channel contributes to the repolarization phase of the action potential in this smooth muscle.

Voltage-dependent potassium channels in cultured mammalian Schwann cells

Voltage-dependent ionic currents were recorded from cultured mammalian Schwann cells (rabbits, mice, rats and humans), by a whole-cell voltage-clamp technique to clarify the properties of voltage-dependent K+ currents in Schwann cells of various species. Voltage-dependent K+ channels were classified into 3 groups according to the degree of inactivation and sensitivity to blockade by tetraethylammonium (TEA): (1) little inactivation and TEA sensitivity (rabbit); (2) rapid inactivation and TEA resistance (rat and human); and (3) intermediate degree of both inactivation and sensitivity to TEA (mouse). In rabbit Schwann cells TEA blocked K+ channels predominantly from outside of the cells, while in the other species K+ channels were blocked by TEA inside the Schwann cells. The voltage-dependent K+ channels in different species had different electrophysiological and pharmacological properties, nevertheless the K+ channels in mammalian Schwann cells were similar to those observed in human or murine T-lymphocytes which are very sensitive to blockade by quinine. Although the culture conditions of human Schwann cells were different from those of other species, K+ channels in human Schwann cells were more similar to rat K+ channels than to those of rabbits or mice.

Post-natal development of K+ currents studied in isolated rat pineal cells

1. The voltage-activated outward currents in diencephalon-derived neuroendocrine pineal cells, dissociated from rats aged 1 day to 3 weeks post-natal, were studied with the whole-cell variation of the patch-clamp technique and compared with those of adult rats (1-3 months post-natal). 2. Thirty-five per cent of the 1-week-old cells displayed a single slowly inactivating outward current that had properties which distinguished it from the classical IA and IK currents. This current, named IK(d) for developmental, activated at potentials near -35 mV. Its time to half-maximal activation (t 1/2) ranged from 16 ms at -30 mV to 4 ms at + 15 mV. No other membrane currents were apparent with depolarizing steps up to +80 mV. 3. IK(d) displayed slow inactivation at depolarized potentials. The time constant for this inactivation was on the order of several hundred milliseconds. The curve for steady-state inactivation disclosed that the current was 50% inactivated near -90 mV. This current was not found in cells dissociated from animals 4 or more weeks of age. 4. The reversal potential determined from the amplitude of the tail current at various repolarizing voltages was -76 mV. Tetraethylammonium and 4-aminopyridine reduced the amplitude of the current. The amplitude and time course of this current was not affected by the removal of external Ca2+. Similarly, removal of Cl- did not affect the current characteristics. 5. Sixty-five per cent of the 1-week-old cells displayed IA and IK. IK rose slowly with time and displayed a threshold of activation near -20 mV. No current decay was observed during a 160 ms pulse. IA activated with step potentials positive to -50 mV. This current rose faster than IK(d) and IK, and it had a significant decay over a 160 ms pulse. 6. IA and IK were observed as early as 1 day after birth. Comparison of the time course of activation of IA and IK from young and adult animals showed a small increase (2-3 ms at 0 mV) in the time to peak and half-maximal current, respectively. With a step potential to -20 mV, the time constant of decay of IA increased from 34.6 ms in 2-day-old animals to 42.9 ms in adult animals. 7. The results indicate that unlike adult pineal cells, some cells from young animals express a kinetically distinct outward current (IK(d)) which was observed in the absence of IA and IK.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of isolated rat liver cells

The electrophysiological properties of isolated rat liver cells were studied using the patch clamp method in whole-cell configuration. The membrane potential in isolated hepatocytes was -42 +/- 7 mV (n = 20). The input resistance (Rin) and the time constant (tau m) were 51 +/- 17 M (the range of 34 to 180 M omega) (n = 20) and 4.2 +/- 1.0 msec (the range of 3 to 16.5 ms) (n = 20). Assuming that the specific membrane capacitance is 1 microF/cm2, the membrane resistance and membrane capacitance were 42. +/- 9.0 K omega cm2 and 87 +/- 27 pF. These values indicate that isolated rat hepatocytes are not abnormally permeable or leaky. The current-voltage relationship was linear with no rectification. The depolarizing pulse from the resting potential did not induce fast or slow inward currents even when norepinephrine or high Ca2 (3.6 mM) were applied. This indicates that there is no voltage-sensitive Ca2+ channel in the isolated hepatocytes.

Voltage-gated potassium channels in brown fat cells

We studied the membrane currents of isolated cultured brown fat cells from neonatal rats using whole-cell and single-channel voltage-clamp recording. All brown fat cells that were recorded from had voltage-gated K currents as their predominant membrane current. No inward currents were seen in these experiments. The K currents of brown fat cells resemble the delayed rectifier currents of nerve and muscle cells. The channels were highly selective for K+, showing a 58-mV change in reversal potential for a 10-fold change in the external [K+]. Their selectivity was typical for K channels, with relative permeabilities of K+ greater than Rb+ greater than NH+4 much greater than Cs+, Na+. The K currents in brown adipocytes activated with a sigmoidal delay after depolarizations to membrane potentials positive to -50 mV. Activation was half maximal at a potential of -28 mV and did not require the presence of significant concentrations of internal calcium. Maximal voltage-activated K conductance averaged 20 nS in high external K+ solutions. The K currents inactivated slowly with sustained depolarization with time constants for the inactivation process on the order of hundreds of milliseconds to tens of seconds. The K channels had an average single-channel conductance of 9 pS and a channel density of approximately 1,000 channels/cell. The K current was blocked by tetraethylammonium or 4-aminopyridine with half maximal block occurring at concentrations of 1-2 mM for either blocker. K currents were unaffected by two blockers of Ca2+-activated K channels, charybdotoxin and apamin. Bath-applied norepinephrine did not affect the K currents or other membrane currents under our experimental conditions. These properties of the K channels indicate that they could produce an increase in the K+ permeability of the brown fat cell membrane during the depolarization that accompanies norepinephrine-stimulated thermogenesis, but that they do not contribute directly to the norepinephrine-induced depolarization.