Conduction and selectivity in potassium channels

Pinacidil opens K+-selective channels causing hyperpolarization and relaxation of noradrenaline contractions in rat mesenteric resistance vessels

1. The effects of pinacidil on noradrenaline-induced tone, smooth muscle membrane potential and 42K- and 86Rb-efflux from isolated mesenteric resistance vessels (internal diameter 200 microns) of the rat have been studied. 2. Pinacidil (0.3-10 microM) produced concentration-dependent suppression of noradrenaline-induced tone. 3. Pinacidil (0.3-10 microM) caused concentration-dependent hyperpolarization of the smooth muscle. 4. In rat resistance vessels loaded with 42K, pinacidil (1-10 microM) significantly increased the 42K-efflux rate constant. 5. With the use of 86Rb as a marker for K+, 1 microM pinacidil did not affect the 86Rb-efflux rate constant, while 10 microM pinacidil transiently increased the 86Rb rate constant. 6. The results indicate that the relaxant action of pinacidil in these vessels is due to the opening of K+-channels and consequent hyperpolarization. The K+-channels opened are selective for 42K over 86Rb.

Calcium-activated non-selective cation channel in ventricular cells isolated from adult guinea-pig hearts

1. A class of Ca2+-activated non-selective cation channel was identified in ventricular cells, which were dissociated from adult guinea-pig hearts using collagenase. 2. Under cell-attached conditions the patch electrode filled with a Na+-rich solution recorded no obvious single-channel current at the resting membrane potential. Subsequent superfusion of the ventricular cell with a Na+-free Tyrode solution induced an inward-going single-channel current as well as contracture of the cell. Kinetics of this channel were not affected by varying the membrane potential. 3. Single-channel currents showing a conductance similar to those observed in the cell-attached patches were recorded in isolated inside-out membrane patches when the inner side of the membrane was exposed to a free Ca2+ concentration ([Ca2+]i) higher than 0.3 microM. The slope conductance of the channel was 14.8 +/- 2.9 pS (mean +/- S.D., n = 17) at 20-25 degrees C. 4. The reversal potential examined in the inside-out patch was about 0 mV irrespective of the Na+-rich, K+-rich, Li+-rich or Cs+-rich solutions on either side of the membrane, thereby indicating that the channel was almost equally permeable to these cations. 5. The open probability of the channel was increased by raising [Ca2+]i with the maximum value of 0.93 +/- 0.17 (n = 4) at about 10 microM [Ca2+]i. The dose-response relation was fitted to the saturation kinetics with a Hill coefficient of 3.0 and a half-maximum concentration of 1.2 microM [Ca2+]i. 6. The gating kinetics were complex; both the open and closed time histograms showed at least two exponential components with time constants of 3.8 +/- 1.3 ms and 140 +/- 110 ms for open time and 1.8 +/- 1.1 ms and 14.9 +/- 5.3 ms for closed time (n = 4) at 10 microM [Ca2+]i. Reduction of [Ca2+]i resulted in both a decrease of the time constant of the slow component in the open time histogram and an increase of the two time constants of the closed time histogram. 7. Contribution of the channel to the whole-cell current was discussed based on an estimation of the channel density, presumably about 0.04 approximately 0.4/microns 2. Maximum activation of the channel would produce 7.2 approximately 72 nS of membrane conductance, which would explain the reported magnitude of the Ca2+-mediated background conductance of the single myocyte. The channel may also contribute, at least in part, to the transient inward current which develops in Ca2+-overloaded cardiac cells.

Structural aspects of the sarcoplasmic reticulum K+ channel revealed by gallamine block

We have studied single-channel conductance fluctuations of K+ channels present in the sarcoplasmic reticulum (SR) membrane systems of rabbit cardiac and skeletal muscle. K+ conductance through the channels is reversibly blocked by gallamine. Conductance block occurs only from the trans side of the channel and is resolved as a smooth reduction in the open state conductance. At a fixed K+ concentration, conduction decreases with increasing gallamine concentration and the data can be fitted to a single-site inhibition scheme. The degree of block seen at a constant gallamine concentration decreases as K+ concentration is increased, indicating competition between gallamine and K+. Gallamine block is voltage dependent, the degree of block increasing with increasing negative holding potential. Quantitative analysis of block yields a zero voltage dissociation constant of 55.3 +/- 16 microM and an effective valence of block of 0.93 +/- 0.12. We conclude that gallamine blocks by interacting with a site or sites located at an electrical distance 30-35% into the voltage drop from the trans side of the channel. This site must have a cross-sectional area of at least 1.2 nm2. The results of this study have been used to modify and extend our view of the structure of the channel's conduction pathway.

K+ channels activated by L-alanine transport in isolated Necturus enterocytes

Using the patch-clamp technique, we demonstrate here the opening of K+ channels evoked by the actively transported amino acid L-alanine in isolated Necturus enterocytes. These channels had a conductance of about 30 pS and their activation was dependent on transmembrane electrical potential and cytosolic Ca2+.

Multiple products of the Drosophila Shaker gene may contribute to potassium channel diversity

K+ channels are known through electrophysiology and pharmacology to be an exceptionally diverse group of channels. Molecular studies of the Shaker (Sh) locus in Drosophila have provided the first glimpse of K+ channel structure. The sequences of several Sh cDNA clones have been reported; none are identical. We have isolated and examined 18 additional Sh cDNAs in an attempt to understand the origin, extent, and significance of the variability. The diversity is extensive: we have already identified cDNAs representing at least nine distinct types, and Sh could potentially encode 24 or more products. This diversity, however, fits a simple pattern in which variable 3' and 5' ends are spliced onto a central constant region to yield different cDNA types. These different Sh cDNAs encode proteins with distinct structural features.

Multiple types of voltage-dependent Ca2+-activated K+ channels of large conductance in rat brain synaptosomal membranes

K+-selective ion channels from a mammalian brain synaptosomal membrane preparation were inserted into planar phospholipid bilayers on the tips of patch-clamp pipettes, and single-channel currents were measured. Multiple distinct classes of K+ channels were observed. We have characterized and described the properties of several types of voltage-dependent, Ca2+-activated K+ channels of large single-channel conductance (greater than 50 pS in symmetrical KCl solutions). One class of channels (Type I) has a 200-250-pS single-channel conductance. It is activated by internal calcium concentrations greater than 10(-7) M, and its probability of opening is increased by membrane depolarization. This channel is blocked by 1-3 mM internal concentrations of tetraethylammonium (TEA). These channels are similar to the BK channel described in a variety of tissues. A second novel group of voltage-dependent, Ca2+-activated K+ channels was also studied. These channels were more sensitive to internal calcium, but less sensitive to voltage than the large (Type I) channel. These channels were minimally affected by internal TEA concentrations of 10 mM, but were blocked by a 50 mM concentration. In this class of channels we found a wide range of relatively large unitary channel conductances (65-140 pS). Within this group we have characterized two types (75-80 pS and 120-125 pS) that also differ in gating kinetics. The various types of voltage-dependent, Ca2+-activated K+ channels described here were blocked by charybdotoxin added to the external side of the channel. The activity of these channels was increased by exposure to nanomolar concentrations of the catalytic subunit of cAMP-dependent protein kinase. These results indicate that voltage-dependent, charybdotoxin-sensitive Ca2+-activated K+ channels comprise a class of related, but distinguishable channel types. Although the Ca2+-activated (Type I and II) K+ channels can be distinguished by their single-channel properties, both could contribute to the voltage-dependent Ca2+-activated macroscopic K+ current (IC) that has been observed in several neuronal somata preparations, as well as in other cells. Some of the properties reported here may serve to distinguish which type contributes in each case. A third class of smaller (40-50 pS) channels was also studied. These channels were independent of calcium over the concentration range examined (10(-7)-10(-3) M), and were also independent of voltage over the range of pipette potentials of -60 to +60 mV. Type III channels were unaffected by internal TEA concentrations <50 mM. Our results also indicate that the study of K+ channels in lipid bilayers may allow the identification and characterization of novel K+ channels from brain regions otherwise inaccessible to conventional recording techniques.

Kinetic analysis of voltage- and ion-dependent conductances in saccular hair cells of the bull-frog, Rana catesbeiana

1. By the use of whole-cell and excised-patch tight-seal recording techniques, we studied ionic conductances in voltage-clamped solitary hair cells isolated from the bull-frog's sacculus. As a basis for assessing their contributions to hair cell electrical resonance, we developed kinetic models describing voltage-dependent Ca2+ and Ca2+-dependent K+ conductances. 2. A transient K+ current (IA) was activated by steps to potentials positive to -50 mV from holding potentials more negative than -70 mV. In the steady state, the current was fully inactivated at the normal resting potential. Possibly due to the dissipation of a Donnan potential between the pipette's interior and the cell, the voltage dependence of IA inactivation slowly shifted in the negative direction during whole-cell recording. 3. The voltage-gated Ca2+ current (ICa) was isolated by blocking IA with 4-aminopyridine (4-AP) and Ca2+-activated K+ current with tetraethylammonium (TEA). The ICa was activated at potentials more positive than -60 to -50 mV and was maximal at about -10 mV. Its magnitude was highly variable among cells, with an average value of -240 pA at -30 mV. Its activation could be fitted well by a third-order (m3) gating scheme. 4. A Ca2+-activated K+ current (IK(Ca)) was isolated as the component of membrane current blocked by TEA. This current was activated at potentials more positive than -60 to -50 mV and had an average value of 1.5 nA at -30 mV. The Ca2+-activated K+ conductance (gK(Ca)) showed a high apparent voltage dependence, increasing e-fold every 3 mV at potentials between -50 and -40 mV. 5. The Ca2+-activated K+ current displayed rapid activation and deactivation kinetics. The current reached half-maximal activation in 2-4 ms at voltages between -50 and -30 mV, and the tail current decayed exponentially with a time constant of 1.0 ms at -70 mV. The activation rate and magnitude of IK(Ca) were reduced by lowering the extracellular Ca2+ concentration. 6. The open probability of Ca2+-activated K+ channels was estimated by ensemble-fluctuation analysis of whole-cell currents evoked by voltage steps to -30 mV. The average open probability was estimated to be 0.8 at this potential. 7. K+-selective channels with a high conductance (140-200 pS) were examined in excised, inside-out membrane patches. The activity of these channels depended on intracellular Ca2+ and membrane potential. These properties suggest that the channels underlie the whole-cell Ca2+-activated K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

Failure of Leiurus quinquestriatus venom to affect potassium movements in pancreatic islets

The venom from the Israeli scorpion Leiurus quinquestriatus failed to affect 86Rb and 45Ca outflow from rat pancreatic islets perifused in the presence of tetrodotoxin and stimulated by the Ca2+-ionophore A23187 or the hypoglycaemic sulfonylurea tolbutamide. In non-stimulated islets, the venom components whose effects are insensitive to tetrodotoxin did not affect 45Ca and 86Rb outflow. Last, the venom did not alter 86Rb inflow. These findings suggest that 86Rb, 45Ca fluxes and more specifically the Ca2+-activated K+ permeability in the pancreatic B-cell are insensitive to the venom.

Role of calcium-activated potassium channels in transmitter release at the squid giant synapse

1. Several compounds known to block Ca2+-activated K+ channels were microinjected into squid 'giant' presynaptic terminals to test the hypothesis that these channels mediate Ca2+-dependent neurotransmitter release. 2. Injection of tetrapentylammonium, nonyl-triethylammonium and decamethonium all reversibly blocked transmission evoked by presynaptic action potentials. 3. All three of these compounds blocked presynaptic Ca2+ channels. The actions of tetrapentylammonium on presynaptic Ca2+ influx were examined in detail and found to be quantitatively sufficient to account for the ability of this compound to inhibit transmitter release. 4. Injection of Ba2+, another agent known to block Ca2+-activated K+ channels, also reversibly blocked evoked transmitter release. Ba2+ simultaneously enhanced basal (asynchronous) transmitter release and thus may be decreasing evoked release by depleting transmitter quanta available for release. 5. None of these results provide any support for the hypothesis that Ca2+-activated K+ channels mediate Ca2+-dependent release of transmitter at the squid synapse. However, our results have identified a new class of compounds that block Ca2+ channels from their cytoplasmic surface.

Cell volume regulation by the thin descending limb of Henle's loop

Thin descending limb cells from Henle's loop (from the inner strip of the outer medulla of long loops) were studied with optical and video techniques to identify the mechanisms of ion transport and cell volume regulation. Increasing the K+ concentration in the basolateral solution from 5 to 90 mM caused the cells to swell. This K+-induced swelling was inhibited by exposure of the basolateral membrane to 9 mM Ba2+ and was abolished by removing Cl- from the perfusion solutions. Decreasing the perfusion osmolality caused an increase in cell volume followed by a return to the preexposure volume. The latter regulatory decrease in hypoosmolality was slowed by basolateral Ba2+ and the removal of HCO-3 from the solutions. Further slowing occurred when both HCO-3 and Cl- were removed. Exposure of cells to ouabain abolished volume regulation. These data suggest that the basolateral cell membrane of the thin descending limb has a Cl- -dependent K+ permeability, which is important in cell volume regulation. The cells also possess Cl- and HCO-3 transport pathways that participate in volume regulation. Finally, volume regulation is dependent upon the operation of the Na/K pump.

Open-state substructure of inwardly rectifying potassium channels revealed by magnesium block in guinea-pig heart cells

1. Outward single-channel currents through inwardly rectifying K+ channels of cardiac myocytes were studied in the open cell-attached configuration to clarify the mechanism of the rectification. The outward currents, which were not recorded in the cell-attached configuration, appeared after the inner surface of the patch was exposed to low-Mg2+ solution by rupturing a part of the cell membrane. 2. The single-channel current-voltage (I-V) relation was linear in the absence of Mg2+ and crossed the voltage axis near the equilibrium potential for K+ (EK). The channel conductance was 22 and 16 pS (15-16 degrees C) at external K+ concentrations of 150 and 40 mM, respectively. 3. The channel rapidly closed on stepping the membrane potential of the patch to values more positive than EK. Decay of the average current during depolarization was fitted with a single-exponential function. The time constant appeared voltage dependent, but also tended to increase slowly with time after opening the cell to the bath solution. 4. Mg2+ on the cytoplasmic side blocked the outward currents without affecting the inward currents. The half-saturation concentration of the Mg2+ block was 1.7 microM as examined by measuring the mean patch current at +70 mV. 5. In the presence of internal Mg2+ at a micromolar level (2-10 microM), the outward single-channel current fluctuated between four levels including two intermediate levels (sublevels) in addition to the fully open channel current and the zero-current levels. The I-V relations of each sublevel were equally spaced with an interval of about 7 pS. Corresponding sublevels were found spontaneously in the inward direction. 6. Occupancy at each level was estimated from reconstructed traces at various Mg2+ concentrations and voltages, and compared with the value predicted from the binomial theorem. At different probabilities for the blocked state, the distribution of the current levels showed reasonable agreement with the binomial theorem. These findings suggest that the inwardly rectifying K+ channel of cardiac cells is composed of three identical conducting subunits and each subunit is blocked by Mg2+ independently. 7. Dwell times in each substate were distributed exponentially. On the assumption of the above model, the blocking (mu) and unblocking (lambda) rates were calculated. The value of mu increased with higher Mg2+ concentrations or larger depolarizations, while lambda ranged between 50 and 90 s-1 and seemed independent of Mg2+. 8. Owing to the voltage-dependent block by Mg2+, the average current decayed exponentially on depolarization beyond EK.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of Cs+, Li+ and Na+ on the potassium conductance and gating kinetics in the frog node of Ranvier

The effects of monovalent internal cations Cs+, Li+ and Na+ on potassium channel conductance in the frog node of Ranvier were studied by means of the voltage clamp. As previously reported, when 10-80% of the internal K+ was replaced by one of the above cations, the steady-state current-voltage relationship was significantly modified. The main effect was a voltage-dependent attenuation of the currents. We demonstrate that the current attenuation is associated with a change in the channel gating kinetics. For small depolarizations the kinetics can be described by the usual potassium conductance activation time constant, tau n. However, under certain experimental conditions (e.g. substitution of the intracellular K+ with 10% Cs+), during larger depolarizations, stepping the membrane potential to values above 40-60 mV, the conductance develops with two time constants: tau n and a new, slower time constant that, in contrast to tau n, grows with membrane potential. These results can be explained by assuming that the cations may occupy two different sites in the channel; when the first site is occupied the channel is blocked, while occupation of the second site results in slowing of the gating kinetics in the affected channels.

K channels of human alveolar macrophages

The activation of macrophages has been reported to be associated with Ca-activated K permeability change. In order to study this permeability change in human alveolar macrophages, we examined alveolar macrophages electrophysiologically at a single channel level. We observed two types of Ca-activated K channel currents having conductances of 218 +/- 2 and 32 +/- 0.6 picosiemens in symmetrical 154 mmol/L KCl solutions. The characteristics, such as voltage dependency and Ca sensitivity, as well as channel conductance, were different between these two types of channel currents. Quinine (a blocker of Ca-activated K conductance), 0.5 mmol/L, reduced these channel currents by 45 +/- 8% and 31 +/- 8%. Quinine, 0.5 mmol/L, also inhibited chemiluminescence and leukotriene B4 release by 82 +/- 6 to 88 +/- 3% and 88 +/- 2%, respectively. These results suggest the presence of two types of Ca-activated K channels, which may be related to the release of inflammatory mediators from human alveolar macrophages.

Single-channel recordings from cultured human retinal pigment epithelial cells

We have applied patch-clamp techniques to on-cell and excised-membrane patches from human retinal pigment epithelial cells in tissue culture. Single-channel currents from at least four ion channel types were observed: three or more potassium-selective channels with single-channel slope conductances near 100, 45, and 25 pS as measured in on-cell patches with physiological saline in the pipette, and a relatively nonselective channel with subconductance states, which has a main-state conductance of approximately 300 pS at physiological ion concentrations. The permeability ratios, PK/PNa, measured in excised patches were 21 for the 100-pS channels, 3 for the 25-pS channels, and 0.8 for the 300-pS nonselective channel. The 45-pS channels appeared to be of at least two types, with PK/PNa's of approximately 41 for one type and 3 for the other. The potassium-selective channels were spontaneously active at all potentials examined. The average open time for these channels ranged from a few milliseconds to many tens of milliseconds. No consistent trend relating potassium-selective channel kinetics to membrane potential was apparent, which suggests that channel activity was not regulated by the membrane potential. In contrast to the potassium-selective channels, the activity of the nonselective channel was voltage dependent: the open probability of this channel declined to low values at large positive or negative membrane potentials and was maximal near zero. Single-channel conductances observed at several symmetrical KCl concentrations have been fitted with Michaelis-Menten curves in order to estimate maximum channel conductances and ion-binding constants for the different channel types. The channels we have recorded are probably responsible for the previously observed potassium permeability of the retinal pigment epithelium apical membrane.

Activation by odorants of a multistate cation channel from olfactory cilia

Single-channel records were obtained after fusion of ciliary membranes from the olfactory epithelium of Rana catesbeiana to planar lipid bilayers, and odorant-activated cation-selective channels were identified. In addition, a 190-pS potassium-selective channel and a 40-pS cation-selective channel were found in a 0.2 M salt-containing buffer. Odorant-sensitive channels were directly and reversibly activated by nanomolar concentrations of the bell pepper odorant 3-isobutyl-2-methoxypyrazine and the citrus odorant 3,7,-dimethyl-2,6-octadienenitrile. These channels display burst kinetics, multiple conductance levels between 35 and 420 pS, and open times in the millisecond range. With increasing concentrations of odorant, the probability of populating the higher conductance levels increases. These results show that direct activation of channels by odorants may mediate excitation of the olfactory receptor cell.

Potassium channels in human and avian fibroblasts

The cell-attached and excised inside-out patch-clamp techniques were used to study single-channel characteristics of potassium channels in cultured human and avian fibroblasts. Six different potassium channels were distinguished with conductances of 235 +/- 25, 190 +/- 57, 114 +/- 27, 77 +/- 14, 40 +/- 6 and 21 +/- 4 pS in symmetric 140 mM potassium solutions. The channels were separable by their conductances, ion-selectivities, voltage-sensitivities and kinetic properties. All six channels were found in both fully differentiated human skin fibroblasts and primary cultures of 72 h chick sclerotome. The largest channel (235 pS) had a steep bimodal voltage dependence, being open only around the resting membrane potential. It was imperfectly selective for potassium, having a relative sodium:potassium permeability of 0.3. The 190 pS channel was very potassium-selective, had an S-shaped voltage sensitivity and was calcium-dependent. The two intermediate-size channels (114 and 77 pS) had open probabilities of less than 0.5 under all of the conditions we used. They were not completely selective for potassium and were not voltage-sensitive. The two smallest channels (40 and 21 pS) were not well characterized. They both had open probabilities of less than 0.2 and showed no evidence of voltage-sensitivity. The 40 pS channel seemed highly potassium-selective. A suction stimulus was used to test all observed channels for mechanosensitivity but none of the six potassium channels was mechanosensitive. Another small channel, with very clear mechanical sensitivity, was seen on a few occasions; this channel has not yet been characterized.

Osmotically induced basolateral K+ conductance in turtle colon: lidocaine-induced K+ channel noise

The basolateral membrane of amphotericin-treated turtle colon can exhibit two distinct types of K+ conductance, one of which is associated with cell swelling and is blocked by quinidine or lidocaine. Fluctuations in basolateral K+ currents were analyzed under swelling (mucosal KCl) and nonswelling (mucosal K gluconate) conditions. Under nonswelling conditions, it was not possible to detect a spontaneous Lorentzian component in the power density spectrum (PDS) and the addition of lidocaine neither inhibited the macroscopic current nor induced a Lorentzian component in the PDS. Under swelling conditions, however, lidocaine induced a Lorentzian component in the PDS and the corner frequency increased linearly with blocker concentration as expected for reversible blockade of the channel. The gating and conductance properties of osmotically induced channels estimated from a two-state model were similar to those determined recently in single-channel recordings from isolated colonic cells.

Three kinetically distinct potassium channels in mouse neuroblastoma cells

1. Mouse neuroblastoma cells were utilized to examine the electrical properties of single K+ channels which might underlie multiple components of outward current in vertebrate neurones. The conductance, kinetics of activation, inactivation, and pharmacology of three types of channels were compared. 2. Two types of voltage-dependent channels, primarily permeable to K+, were identified which did not require the presence of internal Ca2+. The first had gating kinetics best classified as a delayed rectifier. The conductance of the open channel was 35 pS (22 degrees C) in solutions having symmetrical 125 mM-K+ concentrations. 3. The second type of channel had a conductance of 14 pS under identical conditions. The gating kinetics of this type of channel were distinct from those of the delayed rectifier. The mean first latency, and lifetime of the open state at any voltage, were longer. The maximum probability of an open channel was smaller, so that this parameter appeared less sensitive to the membrane potential. The rate of inactivation of the channel was slower. Further, at the more negative membrane potentials tested, the level of steady-state inactivation was less for this type of channel. 4. The delayed rectifier channel was more sensitive to the blocking action of 4-aminopyridine than the channel with low conductance. 5. A Ca2+ -activated, voltage-dependent K+ channel, having a conductance of 140 pS, was also identified. The maximum probability of an open channel increased, and the voltage for half-maximal activation shifted to a more negative potential as the internal Ca2+ was increased. 6. The time course of inactivation of K+ currents recorded from the whole cell declined in two phases, probably due to the presence of the two types of voltage-dependent K+ channels.

Mechanism of sugar transport through the sugar-specific LamB channel of Escherichia coli outer membrane

Lipid bilayer experiments were performed with the sugar-specific LamB (maltoporin) channel of Escherichia coli outer membrane. Single-channel analysis of the conductance steps caused by LamB showed that there was a linear relationship between the salt concentration in the aqueous phase and the channel conductance, indicating only small or no binding between the ions and the channel interior. The total or the partial blockage of the ion movement through the LamB channel was not dependent on the ion concentration in the aqueous phase. Both results allowed the investigation of the sugar binding in more detail, and the stability constants of the binding of a large variety of sugars to the binding site inside the channel were calculated from titration experiments of the membrane conductance with the sugars. The channel was highly cation selective, both in the presence and absence of sugars, which may be explained by the existence of carbonyl groups inside the channel. These carbonyl groups may also be involved in the sugar binding via hydrogen bonds. The kinetics of the sugar transport through the LamB channel were estimated relative to maltose by assuming a simple one-site, two-barrier model from the relative rates of permeation taken from M. Luckey and H. Nikaido (Proc. Natl. Acad. Sci. USA 77:165-171 (1980a)) and the stability constants for the sugar binding given in this study.

Inhibition by quinine of endothelium-dependent relaxation of rabbit aortic strips

1 The effects of quinine sulphate, tetramethylammonium chloride (TMA) and tetraethylammonium chloride (TEA) (all blockers of the Ca2+-activated K+ channels) on the relaxations induced by acetylcholine (ACh), calcium ionophore A23187 and sodium nitrite were studied in helical strips of rabbit aorta. 2 The strips were contracted to a moderate stable tone with phenylephrine (10(-7) M). ACh (4 X 10(-9) to 10(-6) M) as well as A23187 (10(-8) to 3 X 10(-7) M) reduced this tone in a concentration- and endothelium-dependent manner. 3 Pretreatment of the tissues with quinine (2.5 X 10(-5) to 10(-4) M) for 60 min produced a concentration-dependent inhibition of the relaxation induced by ACh. Also 90 min incubation of the strips with TMA (3 X 10(-3) to 6.5 X 10(-2) M) or TEA (10(-3) to 3 X 10(-2) M) inhibited the ACh-evoked relaxation in a manner similar to quinine. 4 Quinine (10(-4) M, 60 min), TMA (6.5 X 10(-2) M, 90 min) or TEA (3 X 10(-2) M, 90 min) produced 5 to 10 fold reductions in the relaxant EC50 values of A23187 and ACh and depressed (by 40 to 95%) the maximal relaxations to the ionophore and ACh. 5. On a molar basis, quinine was more effective than the two tetraalkylammonium ions in reducing the endothelium-dependent relaxations of the aortic strips induced by ACh or A23187. The inhibitory actions were reversible after 60 to 90 min washout. 6. Exposure of the strips to either quinine (10-4M, 60 min), TMA (6.5 x 10-2 M, 90 min) or TEA (3 X 10-2 M, 90 min), however, did not influence significantly the relaxations evoked by sodium nitrite, a direct smooth muscle relaxant. 7. These results suggest that stimulation of the Ca2+-activated K' channels could be, at least partially, responsible for the endothelium-dependent relaxations induced by ACh or A23 187. Their activation might not be required for the endothelium-independent relaxant effects of sodium nitrite.

Mechanisms of Cs+ blockade in a Ca2+-activated K+ channel from smooth muscle

Large unitary conductance Ca2+-activated K+ channels from smooth muscle membrane were incorporated into phospholipid planar bilayers, and the blockade induced by internally and externally applied Cs+ was characterized. Internal Cs+ blockade is voltage dependent and can be explained on the basis of a Cs+ binding to a site that senses 54% of the applied voltage, with an apparent dissociation constant, Kd(0), of 70 mM. On the other hand, external Cs+ blocks the channel in micromolar amounts, and the voltage dependence of blockade is a function of Cs+ concentration. The fractional electrical distance can be as large as 1.4 at 10 mM Cs+. This last result suggests that the channel behaves as a multi-ion pore. At large negative voltages the I-V relationships in the presence of external Cs+ show an upturn, indicating relief of Cs+ block. External Cs+ blockade is relieved by increasing the internal K+ concentration, but can be enhanced by increasing the external K+. All the characteristics of external Cs+ block can be explained by a model that incorporates a "knock-on" of Cs+ by K+.

Effect of Ba2+ and furosemide on K+ and Rb+ secretion and absorption in isolated frog skin

The aim of the present investigation was to explore whether in isolated frog skin there should be different pathways for K+ absorption and secretion. Therefore, the unidirectional fluxes of 42K+ and the K+-like isotope 86Rb+ were measured. By using various transport inhibitors, separate pathways for active K+ absorption and secretion were detected. The data obtained indicate that the transepithelial K+ and Rb+ transport across isolated frog skin is made up of four different components: one passive and three active. One of the active components is directed from the apical to the basolateral solution, whereas the other two are in the opposite direction. The direction of the net K+ transport depends on the activities of these three active transport components. The active uptake mechanism, which is present in the epithelial cells, discriminates between K+ and Rb+. The ratio between the K+ and Rb+ influxes, K/Rb, is about 3. The presence of Ba2+, furosemide or ouabain in the apical solution had no effect on the K+ influx. The active secretion of K+ takes place via two different pathways, namely the skin glands and the epithelial cells. The K+ secretion via the glands is inhibited by furosemide (basolateral), but is unaffected by Ba2+ (apical) and does not discriminate between K+ and Rb+. The active K+ secretion via the epithelial cells is blocked by apical Ba2+, and it discriminates between K+ and Rb+. The ratio between the K+ and Rb+ effluxes is about 2. The data presented show that the K+ channels in the apical and the basolateral membrane of the epithelial cells discriminate between K+ and Rb+.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane properties of rabbit basilar arteries and their responses to transmural stimulation

Changes in membrane potential of rabbit basilar arteries were recorded in response to transmural stimuli applied by means of a suction electrode. Responses to current pulses of long duration and low intensity showed that the passive electrical properties of basilar arteries were similar to those of other vascular smooth muscles. In contrast to peripheral arteries, action potentials were readily evoked by depolarizing currents. Action potentials were graded in amplitude from 17-60 mV according to stimulus strength. Amplitudes and rates of rise of the directly evoked action potentials increased with increasing external calcium and were abolished by cobalt, manganese and magnesium. Brief electrical stimuli which might have been expected to activate perivascular nerves produced slow depolarizing responses whose amplitude and duration increased with increasing stimulus intensity. These responses were not blocked by tetrodotoxin, lowered external calcium, or sympathetic denervation. They do not appear to be due to the release of a conventional neurotransmitter.

Exact continuum solution for a channel that can be occupied by two ions

The classical Nernst-Planck continuum equation is extended to the case where the channel can be occupied simultaneously by two ions. A two-dimensional partial differential equation is derived to describe the steady-state channel. This differential equation is of the form of the generalized Laplace equation, but it has the novel feature that the boundary conditions are periodic. The finite difference solution takes approximately 8 s on a large computer. The equations are solved for the special case of a cylindrical channel with a fixed charge in the center. It is assumed that the forces on the ions result entirely from the sum of the Born image potential, the fixed charge potential, the interaction potential between the two ions, and the applied voltage. Approximate simple analytical expressions are derived for these potential terms, based on the assumption that the electric field perpendicular to the channel wall is zero. The potentials include the contribution from a diffuse charge (Debye-Huckel) reaction field in the bulk solution for the monovalent cation flux was obtained for channels with a radius of 4 A and lengths of 16 and 32 A and a fixed charge valence of -1 and -1.5. For these channels, a significant fraction (up to 90%) of the total resistance is contributed by the bulk solution and results were obtained for the case where the "channel" included 8 A of bulk solution at each channel end. These results for the two-ion channel were compared with the analytical solution for a one-ion channel. The one-ion channel is a fair approximation to the two-ion channel for a fixed charge of -1, underestimating the flux at high concentrations by approximately 30%. However, for a fixed charge of -1.5, the one-ion model is a poor approximation, with the two-ion flux about seven times that of the one-ion model at high concentrations. The absolute conductance and concentration dependence of these channels (with a fixed charge of -1) mimic the behavior of the large conductance K+ channel and the acetylcholine receptor channel.

Temperature dependence of Ca2+-activated K+ currents in the membrane of human erythrocytes

The currents through single Ca2+-activated K+ channels were studied in excised inside-out membrane patches of human erythrocytes. The effects of temperature on single-channel conductance, on channel gating and on activation by Ca2+ were investigated in the temperature range from 0 up to 47 degrees C. The single-channel conductance shows a continuous increase with increasing temperature; an Arrhenius plot of the conductance gives the activation energy of 29.6 +/- 0.4 kJ/mol. Reducing the temperature alters channel-gating kinetics which results in a significant increase of the probability of the channel being open (Po). The calcium dependence of Po is affected by temperature in different ways; the threshold concentration for activation by Ca2+ is not changed, the Ca2+ concentration of half-maximal channel activation is reduced from 2.1 mumol/l at 20 degrees C to 0.3 mumol/l at 0 degrees C, the saturation level of the dependence is reduced for temperatures higher then about 30 degrees C. The relevance of the obtained data for the interpretation of the results known from flux experiments on cells in suspensions is discussed.

Single-channel activity in sea urchin sperm revealed by the patch-clamp technique

Ionic fluxes are deeply involved in the response of spermatozoa to the egg. Using the patch-clamp technique, we show for the first time single ion channel activity in sea urchin spermatozoa and spermatozoa heads. Due to their small size gigaseals were obtained in suspended cells by applying suction through the pipette. The rate of gigaseal formation was very low and improved to 6% (n = 1145) when flagella were detached from sperm. Current-voltage curves created from single-channel events showed conductances of approx. 65 and 170 pS, suggesting the presence of two types of channels. At least one appears to be a K+ channel.

Single calcium-activated potassium channels recorded from cultured rat sympathetic neurones

1. The properties of single Ca2+-activated K+ channels in cultured rat superior cervical ganglionic neurones were studied in cell-attached and excised patches using the patch-clamp technique. 2. In cell-attached patches using an external K+ concentration ([K+]o) of 150 mM, approximately equal to the internal [K+], the channel slope conductance was approximately 200 pS and independent of membrane voltage between -50 and +50 mV. Using [K+]o of 4.7 mM (providing a near physiological K+ gradient), the I-V relationship was non-linear with a slope conductance of approximately 120 pS at 0 mV. 3. The channel was selective for K+ over Cs+ and Na+ which were impermeant from either side of the membrane. Both Na+ and Cs+ also blocked the movement of K+ through the channel. Cs+ was active on either side of the membrane, whereas Na+ apparently blocked the channel only when applied to the cytoplasmic side. 4. The channel was activated by increasing the Ca2+ concentration on the inside of the membrane ([Ca2+]i). The channel was virtually inactive when [Ca2+]i = 0.01 microM. Depolarizing the patch at a constant [Ca2+]i usually further increased the opening probability. 5. The gating properties of the channel were studied using cell-attached patches. At potentials more negative than the resting membrane potential, the open-time distribution was described by a single exponential. On depolarization, two exponentials were required. The closed-time distribution was fitted by three exponentials. 6. Depolarization of the patch caused the long mean open lifetime to increase whilst the short mean open and closed lifetimes were unaffected. Both the intermediate and long mean closed lifetimes decreased with depolarization from -60 to +60 mV. 7. In cell-attached patches, the long mean open lifetimes were usually smaller than those observed in excised patches at depolarized potentials (greater than 0 mV). 8. A fourth closed state, possibly representing an inactivated form of the channel, was infrequently observed. A 50% substate of the full single-channel current was also observed occasionally. This substate was always associated with openings to the full current state. 9. The channel was blockable by external tetraethylammonium (25 microM-1 mM), Ba2+ (1-10 mM), and quinine (10-200 microM). External d-tubocurarine (25-100 microM) also blocked this IC channel. However it was insensitive to apamin (100-300 nM), muscarine (10 microM) and 4-aminopyridine (1-3 mM). The channel was also blocked by internal tetraethylammonium (5-10 mM) or Ba2+ (0.3-1 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular characterization of Shaker, a Drosophila gene that encodes a potassium channel

The Drosophila Shaker (Sh) gene appears to encode a type of voltage-sensitive potassium (K+) channel called the A channel. We have isolated Sh as part of a 350 kb chromosomal walk. The region around Sh contains four identified transcription units. We find that Sh corresponds to a very large transcription unit encompassing a total of about 95 kb of genomic DNA and split by a major 85 kb intron. Sh has multiple hydrophobic domains that have a high probability of being membrane-spanning, consistent with the proposal that it encodes an ion channel.

Selectivity and patch measurements of A-current channels in Helix aspersa neurones

1. The ionic selectivity of A-current K+ channels has been measured in single Helix aspersa neurones by recording the reversal potential shift in test solutions containing various monovalent cations. 2. The A-current channel is permeable to Tl+, K+, Rb+, NH4+ and Cs+. The channels may also be sparingly permeable to Na+ and Li+. Organic cations have an apparent small permeability as judged from their reversal potentials, but this may be an artifact of K+ accumulation. 3. A large patch electrode (3 microns tip) isolated a region that appeared to contain only A-current channels. This may indicate that A-current channels are found in the membrane as rafts of at least 3 microns in diameter. 4. The single-channel conductance calculated from single-channel current steps was 14 pS.

Trapping single ions inside single ion channels

Single Ca++-activated K+ channels from rat muscle plasma membranes are inhibited by Ba++. A single Ba++ entering the channel's conduction pore induces a long-lived blocked state. This study employs Ba++ as a probe of the channel's conduction pathway to show that the channel can be forced to close with a single Ba++ ion inside the pore. A Ba++ ion inside the closed channel is trapped and cannot escape until the channel opens. The results demonstrate that in the channel's closed state, the cytoplasmic side of the conduction pore is obstructed to the passage of ions.

Potassium conductance in straight proximal tubule cells of the mouse. Effect of barium, verapamil and quinidine

The present study has been performed to test for the influence of verapamil and quinidine on the potential difference across the basolateral cell membrane (PDbl) and on the basolateral potassium conductance of isolated perfused segments of the mouse proximal tubule. PDbl was recorded continuously with conventional microelectrodes during rapid alterations of bath or luminal perfusate composition. The contribution of the basolateral potassium conductance to the conductance of both cell membranes (tk) was estimated from the effects of altered bath potassium concentration on PDbl. Under control conditions tk approaches 0.8, i.e. the basolateral cell membrane is mainly conductive to potassium. Neither quinidine nor verapamil affect PDbl at concentrations below 10 mumol/l. At higher concentrations both substances depolarize the basolateral cell membrane mimicking the effect of 1 mmol/l barium. In the presence of 0.1 mmol/l verapamil tk is virtually abolished at 5 to 10 mmol/l bath potassium concentration but is almost unaffected at bath potassium concentrations between 20 and 40 mmol/l. 1 mumol/l ionophore A-23187 does not change the depolarizing effect of 0.1 mmol/l verapamil on cell membrane potential. In the presence of 0.1 mmol/l quinidine, tk is reduced to some 50%, irrespective of the bath potassium concentration. It is concluded that the potassium conductance in straight proximal tubules is inhibited not only by barium but as well by high concentrations of verapamil and quinidine. The effect is probably direct and not related to alterations in the intracellular calcium activity.

Reconstitution in phospholipid vesicles of calcium-activated potassium channel from outer renal medulla

A barium-sensitive Ca-activated K+ channel in the luminal membrane of the tubule cells in thick ascending limb of Henle's loop is required for maintenance of the lumen positive transepithelial potential and may be important for regulation of NaCl reabsorption. In this paper we examine if the K+ channel can be solubilized and reconstituted into phospholipid vesicles with preservation of its native properties. The K+ channel in luminal plasma membrane vesicles can be quantitatively solubilized in CHAPS at a detergent/protein ratio of 3. For reconstitution, detergent is removed by passage over a column of Sephadex G 50 (coarse). K+-channel activity is assayed by measurement of 86Rb+ uptake against a large opposing K+ gradient. The reconstituted K+ channel is activated by Ca2+ in the physiological range of concentration (K1/2 approximately 2 X 10(-7) M at pH 7.2) as found for the K+ channel in native plasma membrane vesicles and shows the same sensitivity to inhibitors (Ba2+, trifluoperazine, calmidazolium, quinidine) and to protons. Reconstitution of the K+ channel into phospholipid vesicles with full preservation of its native properties is an essential step towards isolation and purification of the K+-channel protein. Titration with Ca2+ shows that most of the active K+ channels in reconstituted vesicles have their cytoplasmic aspect facing outward in contrast to the orientation in plasma membrane vesicles, which requires also addition of Ca2+ ionophore in order to observe Ca2+ stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

A mutant Paramecium with a defective calcium-dependent potassium conductance has an altered calmodulin: a nonlethal selective alteration in calmodulin regulation

The Paramecium mutant, pantophobiac A, has a defect that results in an in vivo loss of calcium-dependent potassium efflux channel activity. This defect is corrected fully by the microinjection of wild-type Paramecium calmodulin into pantophobiac A cells and is partially restored by calmodulins from other organisms, but it cannot be restored by microinjection of pantophobiac calmodulin. Overall, these results suggested that wild-type Paramecium calmodulin has unique features that allow it to restore fully a normal phenotype and that the defect in pantophobiac A might be an altered calmodulin molecule. Previous studies established the amino acid sequence of wild-type calmodulin and showed that Paramecium calmodulin has several differences from other calmodulins, including the presence of dimethyllysine at residue 13. To test directly the possibility that calmodulin from the pantophobiac mutant might be altered, we purified the mutant calmodulin and compared its properties to those of wild-type Paramecium calmodulin. We found one amino acid sequence difference between the two Paramecium calmodulins: a phenylalanine in the mutant protein, instead of a serine, at residue 101. This change is at a calcium-liganding residue in the third calcium-binding loop. These and previous studies demonstrate that comparatively subtle changes in the structure of calmodulin can result in quantitative alterations in in vivo activity, provide insight into the in vivo roles of calmodulin and the regulation of ion channels, and demonstrate that functional alterations of calmodulin are not necessarily lethal.

Ba2+-inhibitable 86Rb+ fluxes across membranes of vesicles from toad urinary bladder

86Rb+ fluxes have been measured in suspensions of vesicles prepared from the epithelium of toad urinary bladder. A readily measurable barium-sensitive, ouabain-insensitive component has been identified; the concentration of external Ba2+ required for half-maximal inhibition was 0.6 mM. The effects of externally added cations on 86Rb+ influx and efflux have established that this pathway is conductive, with a selectivity for K+, Rb+ and Cs+ over Na+ and Li+. The Rb+ uptake is inversely dependent on external pH, but not significantly affected by internal Ca2+ or external amiloride, quinine, quinidine or lidocaine. It is likely, albeit not yet certain, that the conductive Rb+ pathway is incorporated in basolateral vesicles oriented right-side-out. It is also not yet clear whether this pathway comprises the principle basolateral K+ channel in vivo, and that its properties have been unchanged during the preparative procedures. Subject to these caveats, the data suggest that the inhibition by quinidine of Na+ transport across toad bladder does not arise primarily from membrane depolarization produced by a direct blockage of the basolateral channels. It now seems more likely that the quinidine-induced elevation of intracellular Ca2+ activity directly blocks apical Na+ entry.

A calcium-dependent potassium current is increased by a single-gene mutation in Paramecium

The membrane currents of wild type Paramecium tetraurelia and the behavioral mutant teaA were analyzed under voltage clamp. The teaA mutant was shown to have a greatly increased outward current which was blocked completely by the combined use of internally delivered Cs+ and external TEA+. This, along with previous work (Satow, Y., Kung, C., 1976, J. Exp. Biol. 65:51-63) identified this as a K+ current. It was further found to be a calcium-activated K+ current since this increased outward K+ current cannot be elicited when the internal calcium is buffered with injected EGTA. The mutation pwB, which blocks the inward calcium current, also blocks this increased outward K+ current in teaA. This shows that this mutant current is activated by calcium through the normal depolarization-sensitive calcium channel. While tail current decay kinetic analysis showed that the apparent inactivation rates for this calcium-dependent K+ current are the same for mutant and wild type, the teaA current activates extremely rapidly. It is fully activated within 2 msec. This early activation of such a large outward current causes a characteristic reduction in the amplitude of the action potential of the teaA mutant. The teaA mutation had no effect on any of the other electrophysiological parameters examined. The phenotype of the teaA mutant is therefore a general decrease in responsiveness to depolarizing stimuli because of a rapidly activating calcium-dependent K+ current which prematurely repolarizes the action potential.

Voltage dependence of the Ca2+-activated K+ conductance of human red cell membranes is strongly dependent on the extracellular K+ concentration

The conductance of the Ca2+-activated K+ channel (gK(Ca)) of the human red cell membrane was studied as a function of membrane potential (Vm) and extracellular K+ concentration ([K+]ex). ATP-depleted cells, with fixed values of cellular K+ (145 mM) and pH (approximately 7.1), and preloaded with approximately 27 microM ionized Ca were transferred, with open K+ channels, to buffer-free salt solutions with given K+ concentrations. Outward-current conductances were calculated from initial net effluxes of K+, corresponding Vm, monitored by CCCP-mediated electrochemical equilibration of protons between a buffer-free extracellular and the heavily buffered cellular phases, and Nernst equilibrium potentials of K ions (EK) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of 42K at (Vm-EK) approximately equal to 0, using a single-file flux ratio exponent of 2.7. Within a [K+]ex range of 5.5 to 60 mM and at (Vm-EK) greater than or equal to 20 mV a basic conductance, which was independent of [K+]ex, was found. It had a small voltage dependence, varying linearly from 45 to 70 microS/cm2 between 0 and -100 mV. As (Vm-EK) decreased from 20 towards zero mV gK(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 microS/cm2. The zero-current conductance was not significantly dependent on [K+]ex (30 to 156 mM) corresponding to Vm (-50 mV to 0). A further increase in gK(Ca) symmetrically around EK is suggested as (Vm-EK) becomes positive. Increasing the extracellular K+ concentration from zero and up to approximately 3 mM resulted in an increase in gK(Ca) from approximately 50 to approximately 70 microS/cm2. Since the driving force (Vm-EK) was larger than 20 mV within this range of [K+]ex this was probably a specific K+ activation of gK(Ca).

IN CONCLUSION

The Ca2+-activated K+ channel of the human red cell membrane is an inward rectifier showing the characteristic voltage dependence of this type of channel.

Interactions of sodium transport, cell volume, and calcium in frog urinary bladder

The volume of individual cells in intact frog urinary bladders was determined by quantitative microscopy and changes in volume were used to monitor the movement of solute across the basolateral membrane. When exposed to a serosal hyposmotic solution, the cells swell as expected for an osmometer, but then regulate their volume back to near control in a process that involves the loss of KCl. We show here that volume regulation is abolished by Ba++, which suggests that KCl movements are mediated by conductive channels for both ions. Volume regulation is also inhibited by removing Ca++ from the serosal perfusate, which suggests that the channels are activated by this cation. Previously, amiloride was observed to inhibit volume regulation: in this study, amiloride-inhibited, hyposmotically swollen cells lost volume when the Ca++ ionophore A23187 was added to Ca++-replete media. We attempted to effect volume changes under isosmotic conditions by suddenly inhibiting Na+ entry across the apical membrane with amiloride, or Na+ exit across the basolateral membrane with ouabain. Neither of these Na+ transport inhibitors produced the expected results. Amiloride, instead of causing a decrease in cell volume, had no effect, and ouabain, instead of causing cell swelling, caused cell shrinkage. However, increasing cell Ca++ with A23187, in both the absence and presence of amiloride, caused cells to lose volume, and Ca++-free Ringer's solution (serosal perfusate only) caused ouabain-blocked cells to swell. Finally, again under isosmotic conditions, removal of Na+ from the serosal perfusate caused a loss of volume from cells exposed to amiloride. These results strongly suggest that intracellular Ca++ mediates cell volume regulation by exerting a negative control on apical membrane Na+ permeability and a positive control on basolateral membrane K+ permeability. They also are compatible with the existence of a basolateral Na+/Ca++ exchanger.

Epinephrine activates outward rectifying K channel in Madin-Darby canine kidney cells

Patch-clamp recordings were used to study the epinephrine dependent activation of ion channels in the cell membrane of cultured subconfluent renal epithelial (MDCK) cells. The patch-current was dominated by two populations of K channels. The spontaneously active population of K channels shows an inward rectifying behavior. Addition of epinephrine to the cell exterior, after the patch-pipette had been sealed to the cell membrane, increased the open probability of the inward rectifying K channel and shifted the membrane potential in the hyperpolarizing direction. The epinephrine induced hyperpolarization occurs in the range of seconds and is caused by activation of outward-rectifying K channels. The outward-rectifying K channel could not be observed under control conditions. Epinephrine activated channels always appeared in clusters of four to nine channels. Both populations of K channels are modulated in their open probability by cytoplasmic free calcium and voltage.

Recent advances in the characterization of epithelial ionic channels

Physiologists have long recognized the importance of channels in the functioning of neurons and excitable membranes. This brief review has been an attempt to illustrate how channel properties are also essential to an understanding of epithelial transport physiology. Among their more important functions, channels influence membrane potentials and serve as conduits for ion movements. As the need to understand the molecular basis for ion transport continues to develop, it is crucial to be able to distinguish between different channel properties. For example, apparent voltage-dependent properties can arise because of a voltage-dependent gating process, or alternatively, because of a rectification of channel conductance. Voltage-dependent effects can also be only indirect, mediated by changes in cell volume, intracellular ion levels, the levels of secondary intracellular messengers such as Ca2+ (perhaps through voltage-dependent membrane Ca2+ channels), or possibly even by morphological changes. An important area for future research is to differentiate mechanisms which modulate the activity of open channels. For example, a decrease in channel number, a reduction in open-channel conductance or a decline in the probability of channel opening can all underlie changes in macroscopic permeability. The factors which mediate hormonal activation of epithelial channels particularly need to be understood. Specifically, the mechanisms of aldosterone and anti-diuretic hormone activation of apical membrane Na+ channels need to be identified. In conclusion, we are witnessing a new era in epithelial electrophysiology which promises to resolve many issues concerning the cellular regulation of ion transport and open new, unanticipated avenues of inquiry.

Characterization of the basolateral membrane conductance of Necturus urinary bladder

Necturus urinary bladders stripped of serosal muscle and connective tissue were impaled through their basolateral membranes with microelectrodes in experiments that permitted rapid changes in the ion composition of the serosal solution. The transepithelial electrical properties exhibited a marked seasonal variation that could be attributed to variations in the conductance of the shunt pathway, apical membrane selectivity, and basolateral Na+ transport. In contrast, the passive electrical properties of the basolateral membrane remained constant throughout the year. The apparent transference numbers (Ti) of the basolateral membrane for K+ and Cl- were determined from the effect on the basolateral membrane equivalent electromotive force of a sudden increase in the serosal K+ concentration from 2.5 to 50 mM/liter or a decrease in the Cl- concentration from 101 to 10 mM/liter. TK and TCl were 0.71 +/- 0.05 and 0.04 +/- 0.01, respectively. The basolateral K+ conductance could be blocked by Ba2+ (0.5 mM), Cs+ (10 mM), or Rb+ (10 mM), but was unaffected by 3,4-diaminopyridine (100 microM), decamethonium (100 microM), or tetraethylammonium (10 mM). We conclude that a highly selective K+ conductance dominates the electrical properties of the basolateral membrane and that this conductance is different from those found in nerve and muscle membranes.

Acetylcholine dose-response relation and the effect of cesium ions in the rat adrenal chromaffin cell under voltage clamp

Acetylcholine(ACh)-induced inward current was examined in the rat adrenal chromaffin cell in culture. Cells were voltage clamped with one electrode using the giga-seal technique. The ACh dose-response relationship was determined by applying various concentrations of ACh with the "puffer" method. Although there were several technical difficulties, we found tentative figures as 135 to 480 microM for the dissociation constant and 1.2 to 1.4 for the Hill coefficient. External Cs ions had a depressing effect on the ACh response. Cs ions may slowly block ACh receptor channels from outside at the resting state. The ACh response in normal saline recorded with the Cs-filled electrode reversed the polarity at -7.1 mV. When the amplitude of ACh-induced current was plotted against holding membrane potential the curve showed strong rectification at the region more negative than the reversal potential. Between +10 and +30 mV there was a region of a negative slope. We discussed possible mechanisms for this characteristic I-V curve.

High-conductance K+ channel in apical membranes of principal cells cultured from rabbit renal cortical collecting duct anlagen

Using the patch clamp technique, one type of K+ channel was identified in the apical cell membrane of cultured principal cells of rabbit renal collecting ducts in the cell-attached or excised-patch configuration. The channel was highly selective for K+ over Na+ (typically 30-70-fold) and had a conductance of 180, SD +/- 39 pS (n = 6), referred to a situation of 140 mmolar K+-Ringer solution present on either surface of the patch membrane. Channel activity was completely blocked by Ba2+ (5 mmol/l) and partially inhibited by Na+. The latter was evidenced by a deviation from Goldman rectification at high cytoplasm-positive membrane potentials, which was observed when Na+ competed with K+ for channel entrance from the cytoplasmic surface. Channel open probability depended strongly on membrane voltage and cytoplasmic Ca2+ concentration. Open-close kinetics exhibited double exponential behaviour, with a strong voltage dependence of the slow open time constant. Infrequently also a substate conductance level was identified. The voltage and calcium dependence suggest that the channel plays a role in adjusting K+ secretion to Na+ absorption in the fine regulation of cation excretion in renal collecting ducts.