The inward rectifier K+ current underlies oscillatory membrane potential behaviour in bovine pigmented ciliary epithelial cells

Hyperpolarization-activated inward potassium and calcium-sensitive chloride currents in beating pacemaker insect neurosecretory cells (dorsal unpaired median neurons)

Hyperpolarization-activated inward currents were studied in single adult cockroach Periplaneta americana pacemaker neurosecretory cells, identified as dorsal unpaired median neurons using the whole-cell patch-clamp technique. Under current clamp, injection of negative current produced a hyperpolarization of the cell membrane with a sag in the membrane potential toward the resting value. Under voltage clamp, the whole-cell current-voltage relationship exhibited an unexpected biphasic aspect. The global hyperpolarization-activated inward current could be dissociated by means of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and tetraethylammonium chloride sensitivity, ionic selectivity, voltage dependence and activation threshold as inward potassium and calcium-sensitive chloride currents. The inward potassium current was activated around -80 mV. The reversal potential followed the potassium equilibrium potential when the extracellular potassium concentration was raised. This current was not dependent on the external sodium concentration and was sensitive to 10 mM tetraethylammonium chloride or 5 mM barium chloride. The hyperpolarization-activated inward calcium-sensitive chloride current was activated in a range of potential 20 mV more positive than the potassium current. The estimated reversal potential (-71 mV) was very close to the equilibrium potential for chloride ions ( 73 mV). Intracellularly applied 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and external application of 1 mM zinc chloride, calcium-free saline or high concentrations of intracellular 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate blocked the inward chloride current. Current-clamp experiments indicated that the inward potassium current accounted for inward rectification of dorsal unpaired median neurons. Our findings report, for the first time in pacemaker neurosecretory cells, the co-existence of two distinct hyperpolarization-activated inward currents which have specialized function in pacemaker activity.

Effect of coupling on volume-regulatory response of ciliary epithelial cells suggests mechanism for secretion

The ciliary epithelium of the eye secretes the aqueous humor. It is a double epithelium arranged so that the apical surfaces of the nonpigmented ciliary epithelial (NPCE) and pigmented ciliary epithelial (PCE) cells face each other and the basolateral membranes face the inside of the eye and the blood, respectively. We have investigated the volume responses of both single cells and coupled pairs from this tissue to osmotic challenge. Both NPCE and PCE cells undergo regulatory volume increase (RVI) and decrease (RVD) when exposed to hyper- and hyposmotic solution, respectively. In hyposmotic solution single cells swell and return to their original volumes within approximately 3 min. In nonpigmented cells RVD could be inhibited by blockers of volume-activated Cl- channels [tamoxifen (100%) > quinidine (87%) > DIDS (84%) > 5-nitro-2-(3-phenylpropylamino)benzoic acid (80%) > SITS (58%)] and K+ channels [Ba2+ (31%)]. However, in PCE cells these inhibitors and additionally tetraethylammonium and Gd3+ were without effect. Only bumetanide, an inhibitor of Na+-K+-2Cl- cotransport, was found to have any effect on RVD in PCE cells. NPCE-PCE cell coupled pairs also underwent RVD, but with altered kinetics. The onset of RVD of the PCE cell in a pair occurred approximately 80 s before that of the NPCE cell, and the peak swell was reduced. This is consistent with fluid movement from the PCE to the NPCE cell. The effect of the volume-activated Cl- channel inhibitor tamoxifen was to eliminate this difference in the times of onset of RVD in coupled cell pairs and to inhibit RVD in both the NPCE and PCE cells partially. On the basis of these observations we suggest that fluid is transferred from the PCE to the NPCE cell in coupled pairs during cell swelling and the subsequent RVD. Furthermore, we speculate that reciprocal RVI-RVD could underlie aqueous humor secretion.

Modification of membrane currents in mouse neuroblastoma cells following infection with rabies virus

1. The effect on membrane currents of infection of mouse neuroblastoma NA cells with rabies virus was studied by using the whole-cell patch clamp technique. 2. Three types of membrane currents, namely voltage-dependent Na+ current (I(Na)), delayed rectifier K+ current (I(K-DR)) and inward rectifier K+ current (I(K-IR)) were elicited in uninfected cells. 3. In cells 3 days after infection with the virus, no detectable change was observed in morphology and membrane capacitance, but I(Na) and I(K-IR) were significantly decreased in amplitude without any appreciable difference in the time course of current activation and inactivation. The voltage-dependence of I(Na) activation was significantly shifted in the positive direction along the voltage axis with a decreased slope. I(K-DR) remained almost unaltered after the viral infection. 4. The resting membrane potential, measured with a physiological K+ gradient across the cell membrane, was decreased (depolarized) after the viral infection. The depolarization was associated with the decreased amplitude of I(K-IR). 5. These results suggest that infection of mouse neuroblastoma NA cells with rabies virus causes reduction of functional expression of ion channels responsible for I(Na) and I(K-IR), and provide evidence for possible involvement of the change in membrane properties in the pathogenesis of rabies disease.

Adrenergic regulation of calcium-activated potassium current in cultured rabbit pigmented ciliary epithelial cells

1. The effects of adrenergic agonists on K+ currents were studied in cultured rabbit pigmented ciliary epithelial (PCE) cells. 2. Outward K+ current (IK) was reduced by tetraethylammonium chloride, the Ca2+-activated K+ (K(Ca)) channel blocker iberiotoxin (IbTX), or Ca2+-free external Ringer solution. The calcium ionophore ionomycin increased an IbTX-sensitive IK in PCE cells. 3. The adrenergic agonists adrenaline and phenylephrine increased IK in PCE cells. The induced current was blocked by IbTX and the alpha1-antagonist prazosin, suggesting that adrenergic agonists activate IK(Ca) via alpha1-adrenoreceptors. 4. Internal dialysis of D-myo-inositol 1,4, 5-trisphosphate (IP3) increased IK, whilst pre-incubation of PCE cells with thapsigargin or the phospholipase C (PLC) inhibitor U-73122 reduced phenylephrine-induced increases in IK(Ca). Adrenergic increases in IK(Ca) were mediated by a pertussis toxin-insensitive G protein. 5. These results demonstrate that IK(Ca) channels in rabbit PCE cells are coupled to alpha1-adrenergic receptors and a PLC/IP3 signalling pathway. Activation of these channels may modulate fluid secretion by the ciliary epithelium.

Inwardly rectifying currents in hair cells and supporting cells in the goldfish sacculus

1. Inwardly rectifying ionic currents were studied using patch-clamp recording methods in oscillatory-type and spike-type hair cells and supporting cells dissociated from the goldfish sacculus. These cells had different types of inwardly rectifying currents. The biophysical properties of these currents were investigated. 2. A unique potassium current (Isc) was the sole ionic current recognized in supporting cells. Isc was active throughout the membrane potential range between +30 and -170 mV, but showed weak inward rectification and no inactivation. 3. In spike-type hair cells, inwardly rectifying current (Ik1) was selectively permeable to K+ (K+:Na+ permeability ratio, 1:0.0021). Ik1 could underlie the high negative resting potential of these hair cells because it is partially active at this potential. The strong inward rectification of Ik1 contributed to the low negative plateau potential seen in spike-type hair cells. 4. In oscillatory-type hair cells, hyperpolarization-activated potassium-sodium current (Ih), which had properties similar to that in photoreceptor and other neurons, was present instead of inwardly rectifying K+ current. 5. In the cell-attached and inside-out modes with 125 microM external K+ ([K+]o), IK1 channel had a unitary conductance of 27 pS and showed inactivation with increasing hyperpolarization. Putative Ih and Iso single channels had unitary conductances of 7 and 61 pS, respectively, in the cell-attached mode with 125 microM Ko+.

Transient activation of K+ channels by carbachol in bovine pigmented ciliary body epithelial cells

The action of carbachol (CCh) on isolated pigmented ciliary epithelial cells was examined using whole cell patch-clamp recording. Application of 100 microM CCh caused transient, occasionally oscillatory, increases in the inward and outward currents, followed by a long-term decrease in both currents. Caffeine produced transient responses similar to those of CCh. The responses to CCh were blocked by the muscarinic receptor antagonist atropine and the inositol 1,4,5-trisphosphate receptor blocker heparin (200 micrograms/ml in patch pipette). Manipulation of the internal ionic concentrations indicated that only K+ conductances were affected by CCh. Changing intracellular Ca2+ concentration ([Ca2+]i) with the calcium ionophore ionomycin demonstrated that both the inward rectifier K+ current and the outward current exhibited Ca2+ dependence. There was no Cl- current stimulated either directly by CCh or indirectly by modulators of [Ca2+]i, and any Cl- currents present arose from osmotic effects. In the short term, muscarinic stimulation will activate K+ channels by causing a transient increase in [Ca2+]i. This effect only lasts for 1-5 min, however, and, in the long term, the conductance decreases below its original level. The effect of such a transient increase in [Ca2+]i on secretion would be complex, involving effects on gap junction communication between the pigmented and nonpigmented cell layers and the activation state of Cl- channels in the nonpigmented cells. This complexity probably accounts for the variable reports of the effects of muscarinic stimulation of the ciliary body in vivo.

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)

A tetraethylammonium-insensitive inward rectifier K+ channel in Müller cells of the turtle (Pseudemys scripta elegans) retina

Ion channels present in isolated glial (Müller) cells from the retina of the turtle (Pseudemys scripta elegans) were studied with the patch clamp technique. The predominant conductance in these cells was due to an inward rectifying potassium current. The whole-cell conductance of the inward rectifier was 20.2 +/- 1.9 nS (n = 7 cells) in a standard extracellular saline solution (3 mM extracellular potassium). This conductance was dependent on the extracellular potassium concentration, with a 2.88-fold change in conductance per tenfold shift in concentration. The relative permeability sequence to potassium of the inward rectifier was found to be: potassium (1.0) > rubidium (0.7) > ammonium (0.2) > lithium (0.1) = sodium (0.1), which corresponded to the Eisenman sequence IV or V for a strong-field-strength potassium binding site on the channel. The single channel conductance measured in cell-attached patches with potassium chloride (150 mM) in the pipette was 68.5 +/- 6.0 pS (n = 3 patches). The inward rectifier current was not blocked by extracellular tetraethylammonium (TEA+, 20 mM), but was blocked by extracellular barium (5 mM) or cesium (5 mM). The TEA+ insensitivity of the inward rectifier potassium channel in Müller cells is unusual, given that this type of channel in most excitable cells is sensitive to micromolar concentrations of this compound, and may be a characteristic of inward rectifier potassium channels that are primarily involved with extracellular potassium regulation.

Subthreshold rectification in neostriatal spiny projection neurons

Intracellular recordings from slice preparations were used to assess the subthreshold electrophysiological behavior of rat neostriatal projection neurons. Both current steps and ramp currents were used to estimate the current-voltage relationship (I-V plot). Inward rectification in the subthreshold range was a characteristic of most neurons. The amount of rectification varied greatly, and it was complex: membrane voltage trajectories in response to ramps were made up by almost piece-wise changes in the rate of voltage rise, suggesting that multiple conductances contribute to the subthreshold range. Inward current blockers such as tetrodotoxin (TTX) or Cd2+ decreased inward rectification, whereas outward current blockers such as tetraethylammonium (TEA) or 4-aminopyridine (4-AP) increased inward rectification. However, most inward rectification was due to TEA- and Cs(+)-sensitive conductances and not to TTX- or Cd(2+)-sensitive conductances. Cs(+)-sensitive conductances predominated at more negative membrane potentials, whereas 4-AP-sensitive conductances predominated at just +/- 10 mV below the firing threshold. In spite of a very slow activation, there was evidence for transient outward currents modulating the response, i.e., 4-AP-sensitivity, and voltage-sensitivity for firing frequency and threshold. TEA-sensitive conductances also contributed toward fixing the firing threshold. These results imply the contribution of various ion conductances on the shaping of the characteristic physiological firing recorded in vivo. Modulation of these responses by transmitters or peptides may help to understand neural processing in the neostriatum.

Membrane potential oscillation from a novel combination of ion channels

Single pigmented epithelial cells from the ciliary body of the eye were studied using the whole cell voltage and current clamp, permeabilized patch recording, and patch-clamp recording. These cells can produce two types of oscillation. Both are slow, with a period in the range of 1-2 min; one has a low amplitude and oscillates between -60 and -80 mV, and the second is larger, with biphasic hyperpolarizing and depolarizing phases. The latter was seen when the membrane potential was driven negative by a constant current and results from the interplay between the inward rectifier K+ channel and a hyperpolarizing-activated cation channel. The hyperpolarization is caused by the constant current acting on a decreasing conductance as the inward rectifier inactivates, and the depolarization drive results from the activation of cation channels. It is suggested that the constant current would be provided by the Na+ pump in vivo, and such an interplay of channels and pumps could drive the uptake of cations in absorbing epithelia or provide an increased driving force for chloride exit in secretory epithelia.