Mechanosensitive potassium channels in locust muscle membrane

Patch clamp recordings have been made from adult locust (Schistocerca gregaria) muscle membrane to study the mechanosensitivity of potassium channels (BK and IK) in cell-attached patches by transiently applying measured pressures to the contents of the patch pipettes. The aim of the investigations was to demonstrate a novel gating behaviour by pressure of the BK channel in contrast to the familiar behaviour of the IK channel. The open probability (p ₀) of the IK channel increased rapidly in response to a pressure step and monotonically during a pressure ramp. This gating was readily repeatable and rapidly reversible. The relationship between ln[p ₀/(1–p ₀)] and transmembrane pressure was linear. In comparison, p ₀ for the BK channel was also increased by pressure, but its gating was delayed, cumulative, and hysteretic. Received: 12 July 1998 / Revised version: 7 October 1998 / Accepted: 7 October 1998     

Octopaminergic modulation of locust motor neurones

The effect of octopamine on the fast extensor and the flexor tibiae motor neurones in the locust (Schistocerca gregaria) metathoracic ganglion, and also on synaptic transmission from the fast extensor to the flexor motor neurones, was examined. Bath application or ionophoresis of octopamine depolarized and increased the excitability of the flexor tibiae motor neurones. 1 mM octopamine reduced the amplitude of the fast extensor-evoked EPSP in the slow but not the fast flexor motor neurones, whereas 10 mM octopamine could reduce the EPSP amplitude in both. Octopamine broadened the fast extensor action potential and reduced the amplitude of the afterhyperpolarization, the modulation requiring feedback resulting from movement of the tibia. Octopamine also increased the frequency of synaptic inputs onto the tibial motor neurones, and could cause rhythmic activity in the flexor motor neurones, and reciprocal activity in flexor and extensor motor neurones. Octopamine also increased the frequency of spontaneous spiking in the octopaminergic dorsal unpaired median neurones. Repetitive stimulation of unidentified dorsal unpaired median neurones could mimic some of the effects of octopamine. However, no synaptic connections were found between dorsal unpaired median neurones and the tibial motor neurones. The diverse effects of octopamine support its role in mediating arousal.     

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis—a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.     

Diurnal depression of leaf hydraulic conductance in a tropical tree species

Diurnal patterns of hydraulic conductance of the leaf lamina (K_leaf) were monitored in a field‐grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance (g_s) and CO₂ uptake (A_leaf) were associated with short‐term changes in K_leaf. On days of high evaporative demand mid‐day depression of K_leaf to between 40 and 50% of pre‐dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than ?1 MPa implying a refilling mechanism, or that loss of K_leaf was not linked to cavitation. Laboratory measurement of the response of K_leaf to Ψ_leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K_leaf and Ψ_leaf predicted from measured transpiration, xylem water potential and the K_leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K_leaf, g_s and A_leaf suggests that xylem dysfunction in the leaf may lead to mid‐day depression of gas exchange in this species.     

Properties of the on-transient of the intracellular response in the barnacle photoreceptor

We have studied the on-transient of the receptor potential of the barnacle photoreceptor. Its amplitude has previously been shown to depend on light intensity and state of light-dark adaptation. We have examined its dependence on 1) the presence of a prolonged depolarizing afterpotential (PDA), 2) a background light, 3) added alcohol, or 4) decreased K⁺ concentration in the bath. We find that the relative on-transient amplitude tends to increase initially with increasing depolarization arising from 1)–4) and then to decrease again at higher depolarization. This behavior is qualitatively explainable by the cell's currentvoltage characteristics and by the adapting effect of the stimulus on the conductances arising from the PDA, the background light and the alcohol.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Tolbutamide-sensitive potassium conductance in the basolateral membrane of A6 cells

K⁺ channels sensitive to intracellular ATP (K_ATP channels) have been described in a number of cell types and are selectively inhibited by sulfonylurea drugs. To look for the presence of this type of K⁺ channel in the basolateral membrane of tight epithelia, we have used an amphibian renal cell line, the A6 cells, grown on filters. After the selective permeabilization of the apical membrane with amphotericin B, the basolateral conductance was studied under voltage-clamp conditions. Tolbutamide inhibited 65.8 ± 6.3% of the barium-sensitive current. The tolbutamide-sensitive conductance had an equilibrium potential of ?83 ± 1 mV and was inward rectifying in spite of the outwardly directed K⁺ gradient. Similar results were obtained with glibenclamide. The half-inhibition constants were 25.7 ± 3.0 μm and 0.114 ± 0.018 μm for tolbutamide and glibenclamide respectively. To study the relation between cellular ATP and the activity of this conductance, A6 cells were treated with glucose (5 mm) and insulin (250 μU/ml). This maneuver significantly increased the cellular ATP and abolished the tolbutamide-sensitive conductance. A sulfonylurea-sensitive K⁺ conductance is present and active in the basolateral membrane of A6 cells. This conductance appears to be modulated by physiological changes of intracellular ATP.     

Conditioning prepulses and kinetics of potassium conductance in the frog node

Summary The kinetics of potassium conductance were analyzed in response to voltage-clamp steps with holding potential (–75 mV) as initial condition and after a positive prepulse to-wards +45 mV of 10-msec duration. As the potassium reversal potentialE _K altered during potassium current flow, a method to obtain the conductance independent ofE _K was used. Conductance kinetics at 15°C were analyzed according to the Hodgkin-Huxley (HH) model. The time constant of potassium activation, with holding potential as initial condition, is a monotonous decreasing function of membrane potential. Its value ofca. 9 msec at –50 mV decreases to 1 msec at +30 mV. Changes inE _K did not affect the voltage dependency of this time constant. The time constant of potassium deactivation, i.e. the off-response following a 10-msec prepulse towards +45 mV, shows a completely different voltage dependency. At a membrane potential of –90 mV it is approximately 2 msec and gradually increases for more positive voltages towards a maximum value of about 6 msec, that is reached between –5 and 0 mV. At still larger values of membrane voltage this time constant starts to fall again. It is concluded that a HH-model, as applied for a single population of potassium channels, has to be rejected. Computer simulations indicate that an extension to two populations of independent potassium channels, each with HH-kinetics, is also inconsistent with the observed results.     

Basolateral membrane potassium conductance is independent of sodium pump activity and membrane voltage in canine tracheal epithelium

Summary When secretagogues stimulate Cl secretion in canine tracheal epithelium, apical membrane Cl conductance (G _a ^Cl ) increases, and then basolateral membrane K conductance (G _b ^K ) increases. Conversely, inhibition ofG _a ^Cl results in a secondary decrease inG _b ^K . The coordination of the two membrane conductances and regulation ofG _b ^K is critical for maintaining constant intracellular ion concentrations and transepithelial Cl secretion. The purpose of this study was to test two hypotheses about the regulation ofG _b ^K . First, we asked whetherG _b ^K is directly linked to the activity of the Na,K-ATPase. We found that pump activity could be dissociated from K conductance. Inhibition of the Na pump with ouabain, in nonsecreting tissues led to an increase inG _b . Elevation of the bathing solution K concentration produced a similar effect. Addition of ouabain to secreting tissues did not appear to alterG _b . These results indicate thatG _b ^K does not directly parallel Na pump activity. Second, we asked whether changes inG _b ^K are voltage dependent. We prevented secretagogue-induced depolarization of the electrical potential difference across the basolateral membrane _b by clamping _b at its resting value during stimulation of Cl secretion with epinephrine. Despite maintaining _b constant, the typical changes in transepithelial resistance and the ratio of membrane resistances persisted. This observation indicates that depolarization is not required for the secretagogue-induced increase inG _b ^K . In addition we examined the effect of depolarizing and hyperpolarizing _b by passing transepithelial current in secreting and nonsecreting epithelia. Despite depolarizing and hyperpolarizing _b within the physiologic range, we observed no significant changes in transepithelial resistance or the ratio of membrane resistance that would suggest a change inG _b ^K . This observation indicates that changes in _b are not sufficient to alterG _b ^K . Thus,G _b ^K appears to be regulated by factors other than membrane voltage, or direct coupling to the Na pump.     

G-protein activators induce a potassium conductance in murine macrophages

The whole-cell patch clamp technique was used to test whether intracellular application of G-protein activators affect ionic currents in murine macrophages. Both the J774.1 macrophage-like cell line and primary bone marrow derived macrophages were used. Cells were bathed in Na Hanks' solution and intracellularly dialyzed (via the patch pipette) with K Hanks (145 mM KCl, < 100 nM Ca) plus or minus the G-protein activators GTP gamma S (10 microM), GppNHp (10 microM), or AIF4- (200 microM AlCl3 + 5 mM KF). In the absence of G-protein activators, only two K currents, an inwardly rectifying K current (Kir) and an outward, inactivating K current (Ko) were observed. In the presence of protein activators, two effects were observed: (i) the Kir conductance, which is stable for up to 30 min under control conditions, decayed twice as fast and (ii) an outwardly rectifying, noninactivating current appeared. The induced outward current appeared < 2 min after attaining the whole-cell patch clamp configuration. The current could be distinguished from the Kir and Ko currents on the basis of its direction of rectification (outward), barium sensitivity (> 1 mM), and kinetics (no time-dependent inactivation). Intracellular application of GTP (500 microM), GDP (500 microM), cAMP (100 microM + 0.5 mM ATP), or IP3 (20 microM) did not induce the current; 100 microM ATP gamma S activated a half-maximal amount of current. Induction of outward current by 10 microM GTP gamma S could be prevented by pre-exposing cells to pertussis toxin but not cholera toxin. This current is K selective since (i) its induction was accompanied by hyperpolarization of the cell toward EK, even after Kir had "washed out", (ii) it was present after > 90% of both intracellular and extracellular Cl were replaced by isethionate, and (iii) the induced outward conductance was absent when Ki was completely replaced by Cs, and was reduced by approximately 1/3 when [K]i was reduced by 1/3. Quinidine (1 mM) and 4-aminopyridine (10 mM) inhibited the current, but apamin (1 microM) and charybdotoxin (1 microM) did not.     

Presynaptic tubular structures in photoreceptor cells

Summary After the application of fixatives including phosphotungstic acid or a mixture of osmium tetroxide and zinc iodide, complex tubular structures are evident in the presynaptic side of the synapses between photoreceptor and bipolar cells of the rat's retina. In the first case only the limiting membranes are visualized, while in the second only the content of the tubules is stained. These tubules seem to be related, on a morphological ground, with the formation of synaptic vesicles. These tubular structures are not observed when fixation is done with osmium tetroxide or glutaraldehyde-osmium tetroxide.This work has been supported by grants from the Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina, and from National Institutes of Health, U.S.A., (5 RO1 NS 06953-05 NEUA).We want to express our gratitude to Mrs. Haydée Agoff de Zimman and Mr. Alberto Saénz for their skillful technical assistance.     

Regulation of the basolateral potassium conductance of theNecturus proximal tubule

Summary Two methods, the measurement of the response of the basolateral membrane potential (V _bl) of proximal tubule cells ofNecturus to step changes in basolateral K⁺ concentration, and cellular cable analysis, were used to assess the changes in basolateral potassium conductance (G _K) caused by a variety of maneuvers. The effects of some of these maneuvers on intracellular K⁺ activity (a _K ⁱ ) were also evaluated using double-barreled ion-selective electrodes. Perfusion with 0mm K⁺ basolateral solution for 15 min followed by 45 min of 1mm K⁺ solution resulted in a fall in basolateral potassium (apparent) transference number (t _K),V _bl anda _K ⁱ . Results of cable analysis showed that total basolateral resistance,R _b , rose. The electrophysiological effects of additional manipulations, known to inhibit net sodium reabsorption across the proximal tubular epithelium ofNecturus, were also investigated. Ouabain caused a fall int _K accompanied by large decreases ina _K ⁱ andV _bl. Lowering luminal sodium caused a fall int _K and a small reduction inV _bl. Selective reduction of peritubular sodium, a maneuver that has been shown to block sodium transport from lumen to peritubular fluid, also resulted in a significant decrease int _K. These results suggest thatG _K varies directly with rate of transport of the sodium pump, irrespective of the mechanism of change in pump turnover.Part of this material has been presented at the 10th International Conference on Biological Membranes (Cohen & Giebisch, 1984).     

Modulation of small conductance calcium-activated potassium channels in C6 glioma cells

Using the patch clamp technique, we have characterized a small conductance, calcium-activated potassium (SK) channel in the C6 glioma cell line. Elevation of cytosolic Ca²⁺ concentration ([Ca²⁺] _i ) by applications of serotonin or ionomycin induced bursts of channel openings recorded in the cell-attached configuration. These channels underlie the serotonin-induced, [Ca²⁺] _i -activated whole-cell K⁺ conductance described previously. [Ca²⁺] _i directly activated SK channels in inside-out patches with a biphasic concentration dependence. Submicromolar [Ca²⁺] _i induced bursts of channel openings with a unitary conductance of about 25 pS, similar to that of the serotonin-induced channels. Supramicromolar [Ca²⁺] _i caused prolonged openings with a unitary conductance of about 35 pS, resulting in a pronounced increase of the average current in patches exposed to [Ca²⁺] _i above 100 m. The two modes of opening reflect the activity of the same SK channel. The channel conductance depended on external K⁺ concentration with K _Dof 5 m. The channel was slightly permeable to cations other than K⁺, with a permeability ratio for K⁺Ca²⁺Na⁺ of 10.0400.030, respectively. ATP was required to maintain channel activity in outside-out patches but was not essential in inside-out patches. The modulation of SK channels in C6 cells by components in their microenvironment may be related to the role of glial cells in controlling the extracellular milieu in the CNS.The authors are grateful to Dr. M. Segal for continuous support, stimulating discussions and criticism throughout the course of this work, to Dr. I. Steinberg for helpful suggestions and to Dr. H. Jarosch, for helping with the Fortran application. N.M.'s research was supported in part by BARD, the U.S.-Israel Binational Agricultural Research and Development Fund, grant no. IS-1670-89RC.     

A presumed new photoreceptor in copepod crustaceans

Summary A new photoreceptor in the Copepoda is described. The organ, previously called Gicklhorn's organ (Elofsson, 1966a), is paired and is usually situated beneath the cuticle of the front. Each member of the pair consists of two cells. From the anterolateral position, two nerves lead to the lateral part of the brain. No connexion with the nauplius eye is found. Each cell of the organ has microvilli, two nuclei, dictyosomes, and large cisternae of the endoplasmic reticulum. Except for the binucleated condition, the cells closely resemble the retinula cells of the copepod nauplius eye.It is concluded that, because of its independent position, the new photoreceptor is not a detached part of the nauplius eye. As there are no accessory structures present and no missing links so far known, it is doubtful whether it can be regarded as a vestigial compound eye. The most plausible hypothesis is that the new presumed photoreceptor is an independent structure without connexions either with crustacean compound or nauplius eyes.If the function of the nauplius eye is considered by itself the improvement contributed by the new organ is probably modest because of its low level of organization. Some experimental evidence on light reception in Copepods points to a possible function in response to directed light.This work was supported by a grant from the Swedish Natural Science Research Council 2760-3.     

Interpretation of invertebrate photoreceptor potentials in terms of a quantitative model

It is known from voltage-clamp experiments on visual cells of Limulus and Balanus that the total membrane current can be separated approximately into a dark current J _D and a light-induced current J _L such that each part has a time-and intensity-independent reversal potential. In addition J _L can be represented approximately as a product of a nonlinear, time-independent current-voltage characteristics J ⁰ _L (V) and an activation factor x _A which depends on light intensity and time. J ⁰ _L (V) can be described by a simple electrodiffusion membrane model (for J _D we use the same model). A set of kinetic equations including amplification, latency and light adaptation leads to a determination of x _A for photoisomerization of single rhodospin molecules and for arbitrary light signals. The receptor potentials calculated show many features of the experiments on Limulus, Balanus and Astacus.List of the more Important Symbols A Adaptation factor - C ratio between slopes of current-voltage curves for maximum light-induced current and dark current - d one half the thickness of the membrane - F _i effective membrane permeability, cf. Eq. (7) - g _i electrochemical potential inside membrane - g _i electrochemical potential inside and outside cell - I light intensity (quanta/sec cm²) - i (index) refers to i-th ionic species - J total current leaving cell - J _D dark current - J _L light-induced current - j _i ionic current density inside membrane in x-direction - K cf. Eq. (21) - k Boltzmann's constant - k ₀ rate constant for chain reaction (production of X) - k ₁ rate constant for decay of X ₁ - k ₂ rate constant for decay of X (closing of sites) - M total number of sites controlling light conductance - N multiplication factor in dark adapted state (maximum of Z) - n _i density of ions iaside membrane - n _i density of ions inside and outside cell - p ₁, p ₂ rate constants for adaptation kinetics - p ₃ cf. Eq. (29) - q elementary charge - q _i ionic charges - r = (x, y, z) spatial variable - T temperature - t time - u cf. Eq. (28) - V membrane potential - V _D reversal potential for the dark current - V _L reversal potential for the light current - V _i diffusion potential, cf. Eq. (8) - V potential steps at the inner and outer membrane surface - V ₀ = V ^–V - X = x _a M, number of open sites - X ₁ = x ₁M number of molecules of agent whose decay initiates chain reactions - x _A degree of activation of light sensitive membrane - Y number of sites opened by individual chain reaction - Z multiplication factor (number of sites opened by one photoisomerized rhodopsin molecule) - _i activation energy inside membrane (measured with respect to surrounding solution) - electrostatic potential - ₀ flxed-charge distribution inside membrane - cross section for photoisomerization of one rhodopsin molecule by a photon - k ₁/k ₂     

Basolateral membrane potassium conductance of A6 cells

Summary To study the properties of the basolateral membrane conductance of an amphibian epithelial cell line, we have adapted the technique of apical membrane selective permeabilization (Wills, N.K., Lewis, S.A., Eaton, D.C., 1979b, J. Membrane Biol. 45:81–108). Monolayers of A6 cells cultured on permeable supports were exposed to amphotericin B. The apical membrane was effectively permeabilized, while the high electrical resistance of the tight junctions and the ionic selectivity of the basolateral membrane were preserved. Thus the transepithelial current-voltage relation reflected mostly the properties of the basolateral membrane. Under basal conditions, the basolateral membrane conductance was inward rectifying, highly sensitive to barium but not to quinidine. After the induction of cell swelling either by adding chloride to the apical solution or by lowering the osmolarity of the basolateral solution, a large out-ward-rectifying K⁺ conductance was observed, and addition of barium or quinidine to the basolateral side inhibited, respectively, 82.4±1.9% and 90.9±1.0% of the transepithelial current at 0 mV. Barium block was voltage dependent; the half-inhibition constant (K _i) varied from 1499±97 m at 0 mV to 5.7±0.5 m at –120 mV.Cell swelling induces a large quinidine-sensitive K⁺ conductance, changing the inward-rectifying basolateral membrane conductance observed under basal conditions into a conductance with outward-rectifying properties.     

Adrenergic modulation of potassium currents in isolated human atrial myocytes

The adrenergic modulation of inwardly rectifying and depolarization-activated outward potassium currents was studied in single cardiac myocytes obtained from the human atrium. Membrane currents were recorded in enzymatically dissociated cells using the whole-cell voltage-clamp technique. It was observed that, in the presence or absence of atenolol (or 1 µM propranolol), 30 µM phenylephrine attenuated inwardly rectifying and depolarization-activated outward potassium currents including both transient and late-activated current. This suppressant effect of phenylephrine could be prevented by pretreatment with an -adrenoceptor antagonist. Isoproterenol (30 µM) increased the late outward potassium current and net transient outward current. It is concluded that, in human atrial myocytes, -adrenergic activation reduces depolarization-activated transient and late outward potassium current and inwardly rectifying background potassium current. -Adrenergic activation resulted in an increase in the depolarization-activated transient and late outward potassium current.     

Diurnal variation of phytoplankton in Loch Lomond

A study of diurnal variation over a 48 hour period was undertaken in July 1973 to ascertain the extent and timing of some major chemical, physical and biological variables in Loch Lomond. The phytoplankton population was dominated by the diatom Tabellaria fenestrata, with a maximum abundance between 04.00 and 06.00 h in surface waters on both days. A distinct diurnal variation in cell numbers was also recorded. Chlorophyll a values also showed a regular pattern of variation with a single peak between 10.00 and 14.00 h each day. Some chemical changes appeared to be a direct consequence of phytoplankton multiplication. Nitrate-nitrogen showed a decrease in concentration coinciding with the period of cell multiplication, whereas dissolved silica concentrations only fell on the completion of this process. Other common diatoms displayed less distinct patterns of variation although Fragilaria crotonensis attained its maximum abundance in surface waters. Considerable variation was recorded in the number of organisms and chlorophyll a levels at 25 cm intervals in the upper metre of the water column, with large variations in standing-crop and chemical parameters in the space of one hour. Diurnal oscillations in the position of the thermocline were recorded, with the thermal discontinuity being at its greatest depth in the early hours of the morning. The hypolimnion and thermocline regions clearly acted as a source of nutrient supply to the epilimnion. From this investigation it is apparent that for the proper understanding of diurnal variation a 24 hour study alone is insufficient and may give rise to misleading results.Department of Botany, University of GlasgowDepartment of Botany, University of Glasgow     

Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

pH modulation of large conductance potassium channel from adrenal chromaffin granules

We report here that large conductance K(+) selective channel in adrenal chromaffin granules is controlled by pH. We measured electrogenic influx of (86)Rb(+) into chromaffin granules prepared from bovine adrenal gland medulla. The (86)Rb(+) influx was inhibited by acidic pH. Purified chromaffin granule membranes were also fused with planar lipid bilayer. A potassium channel with conductance of 432+/-9 pS in symmetric 450 mM KCl was observed after reconstitution into lipid bilayer. The channel activity was unaffected by charybdotoxin, a blocker of the Ca(2+)-activated K(+) channel of large conductance. It was observed that acidification to pH 6.4 cis side of the membrane lowered the channel open probability and single channel conductance. Whereas only weak influence on the single channel current amplitude and open probability were observed upon lowering of the pH at the trans side. We conclude that a pH-sensitive large conductance potassium channel operates in the chromaffin granule membrane.     

pH modulation of large conductance potassium channel from adrenal chromaffin granules

We report here that large conductance K⁺ selective channel in adrenal chromaffin granules is controlled by pH. We measured electrogenic influx of ⁸⁶Rb⁺ into chromaffin granules prepared from bovine adrenal gland medulla. The ⁸⁶Rb⁺ influx was inhibited by acidic pH. Purified chromaffin granule membranes were also fused with planar lipid bilayer. A potassium channel with conductance of 432±9 pS in symmetric 450 mM KCl was observed after reconstitution into lipid bilayer. The channel activity was unaffected by charybdotoxin, a blocker of the Ca²⁺-activated K⁺ channel of large conductance. It was observed that acidification to pH 6.4 cis side of the membrane lowered the channel open probability and single channel conductance. Whereas only weak influence on the single channel current amplitude and open probability were observed upon lowering of the pH at the trans side. We conclude that a pH-sensitive large conductance potassium channel operates in the chromaffin granule membrane.