Effects of external cesium and rubidium on outward potassium currents in squid axons

We have studied the effects of external cesium and rubidium on potassium conductance of voltage clamped squid axons over a broad range of concentrations of these ions relative to the external potassium concentration. Our primary novel finding concerning cesium is that relatively large concentrations of this ion are able to block a small, but statistically significant fraction of outward potassium current for potentials less than approximately 50 mV positive to reversal potential. This effect is relieved at more positive potentials. We have also found that external rubidium blocks outward current with a qualitatively similar voltage dependence. This effect is more readily apparent than the cesium blockade, occurring even for concentrations less than that of external potassium. Rubidium also has a blocking effect on inward current, which is relieved for potentials more than 20-40 mV negative to reversal, thereby allowing both potassium and rubidium ions to cross the membrane. We have described these results with a single-file diffusion model of ion permeation through potassium channels. The model analysis suggests that both rubidium and cesium ions exert their blocking effects at the innermost site of a two-site channel, and that rubidium competes with potassium ions for entry into the channel more effectively than does cesium under comparable conditions.     

Gating and flickery block differentially affected by rubidium in homomeric KCNQ1 and heteromeric KCNQ1/KCNE1 potassium channels

The voltage-gated potassium channel KCNQ1 associates with the small KCNE1 subunit to form the cardiac IKs delayed rectifier potassium current and mutations in both genes can lead to the long QT syndrome. KCNQ1 can form functional homotetrameric channels, however with drastically different biophysical properties compared to heteromeric KCNQ1/KCNE1 channels. We analyzed gating and conductance of these channels expressed in Xenopus oocytes using the two-electrode voltage-clamp and the patch-clamp technique and high extracellular potassium (K) and rubidium (Rb) solutions. Inward tail currents of homomeric KCNQ1 channels are increased about threefold upon substitution of 100 mM potassium with 100 mM rubidium despite a smaller rubidium permeability, suggesting an effect of rubidium on gating. However, the kinetics of tail currents and the steady-state activation curve are only slightly changed in rubidium. Single-channel amplitude at negative voltages was estimated by nonstationary noise analysis, and it was found that rubidium has only a small effect on homomeric channels (1.2-fold increase) when measured at a 5-kHz bandwidth. The apparent single-channel conductance was decreased after filtering the data at lower cutoff frequencies indicative of a relatively fast "flickery/block" process. The relative conductance in rubidium compared to potassium increased at lower cutoff frequencies (about twofold at 10 Hz), suggesting that the main effect of rubidium is to decrease the probability of channel blockage leading to an increase of inward currents without large changes in gating properties. Macroscopic inward tail currents of heteromeric KCNQ1/KCNE1 channels in rubidium are reduced by about twofold and show a pronounced sigmoidal time course that develops with a delay similar to the inactivation process of homomeric KCNQ1, and is indicative of the presence of several open states. The single channel amplitude of heteromers is about twofold smaller in rubidium than in potassium at a bandwidth of 5 kHz. Filtering at lower cutoff frequencies reduces the apparent single-channel conductance, the ratio of the conductance in rubidium versus potassium is, however, independent of the cutoff frequency. Our results suggest the presence of a relatively rapid process (flicker) that can occur almost independently of the gating state. Occupancy by rubidium at negative voltages favors the flicker-open state and slows the flickering rate in homomeric channels, whereas rubidium does not affect the flickering in heteromeric channels. The effects of KCNE1 on the conduction properties are consistent with an interaction of KCNE1 in the outer vestibule of the channel.     

The localization of transport properties in the frog lens.

The selectivity of fiber-cell membranes and surface-cell membranes in the frog lens is examined using a combination of ion substitutions and impedance studies. We replace bath sodium and chloride, one at a time, with less permeant substitute ions and we increase bath potassium at the expense of sodium. We then record the time course and steady-state value of the intracellular potential. Once a new steady state has been reached, we perform a small signal-frequency-domain impedance study. The impedance study allows us to separately determine the values of inner fiber-cell membrane conductance and surface-cell membrane conductance. If a membrane is permeable to a particular ion, we presume that the conductance of that membrane will change with the concentration of the permeant ion. Thus, the impedance studies allow us to localize the site of permeability to inner or surface membranes. Similarly, the time course of the change in intracellular potential will be rapid if surface membranes are the site of permeation whereas it will be slow if the new solution has to diffuse into the intercellular space to cause voltage changes. Lastly, the value of steady-state voltage change provides an estimate of the lens' permeability, at least for chloride and potassium. The results for sodium are complex and not well understood. From the above studies we conclude: (a) surface membranes are dominated by potassium permeability; (b) inner fiber-cell membranes are permeable to sodium and chloride, in approximately equal amounts; and (c) inner fiber-cell membranes have a rather small permeability to potassium.     

Partial purification of a potassium channel with low permeability for sodium from tonoplast membranes of Hordeum vulgare cv. Gerbel

Summary A potassium-specific tonoplast channel was identified by reconstitution of tonoplast polypeptides into planar lipid bilayer membranes. Highly purified tonoplast membranes were solubilized in Triton X-100-containing buffer and fractionated by size-exclusion chromatography. The protein fractions were assayed for ion channel activity in a planar bilayer system, and the potassium channel was routinely recovered in specific fractions corresponding to an apparent molecular mass of 80 kDa. In symmetrical electrolyte solutions of 100 mM potassium chloride, the potassium channel had a single-channel conductance of 72 pS. Substates of the channel with conductances of 17, 33 and 52 pS were frequently observed. After identification of the channel in low or high KCl, addition of sodium acetate or sodium chloride caused only insignificant conductance changes. This result suggested that the channel was not or little permeable for sodium or chloride, whereas it had similar single-channel conductance for rubidium and caesium ions as compared with potassium ions. The channel is presumably responsible for the equilibration of potassium between the vacuole and the cytosol. The role of the channel in the physiology of the barley cell under salt stress is discussed.The authors would like to thank U. Heber for many helpful discussions. This work was supported by grants of the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 176, projects B3 and B7) and by the Fonds der Chemischen Industrie.     

Anomalous permeabilities of the egg cell membrane of a starfish in K(+)-Tl(+) mixtures

The electrical properties of “inward” rectifying egg cell membranes of the starfish mediastera aequalis have been studied in the presence of K(+)-Tl(+) mixtures. When the ratio of the external concentrations of these ions is changed while their sum is kept constant, both the conductance and the zero-current membrane potential go through a minimum, showing clear discrepancies from theoretical results based on conventional electrodiffusion models (E.g., Goldman’s equation). By contrast, when the ration of the two concentrations is fixed and their sum varied, the potential follows an ideal Nernst slope, consistent with Goldman’s equation. The membrane conductance which, according to previous studies on similar membranes, is to be viewed as a function of the displacement of the membrane potential from its resting value δV, shows marked differences between the cases in which K(+) or Tl(+) are the predominant ions: when K(+) is the predominant permeant ion in solution, the addition of small amounts of Tl(+) inhibits the current, while corresponding blocking effects of K(+) on the current are not observed when Tl(+) is the predominant permeant ion. Also, the time course of the conductance during voltage clamp is different in the two cases, being much faster in Tl(+) than in K(+) solution for comparable values of δV. Most of the above features are accounted for by a model in which it is assumed that the ionic channels have external binding sites for cations and that their permeability properties depend on the species of the cation bound (K(+)or Tl(+) in the present experiments).     

Rubidium reduces potassium permeability and fluid secretion in Malpighian tubules of Locusta migratoria, L

The basal membrane potential (V(b)) of Locusta Malpighian tubule cells in control saline results from its relatively high permeability to potassium. In the presence of 1 mM barium added to the control saline V(b) hyperpolarized from a mean resting potential of -72.1 mV to -90.1 mV. On substituting rubidium for potassium in the control saline, V(b) also hyperpolarized to a value of -91.4 mV. Rubidium was also similarly effective in hyperpolarizing the basal membrane even in the presence of control concentrations of potassium in the bathing medium. Substitution of rubidium for potassium also effected a approximately 50% reduction in the rate of fluid secretion. The action of inhibitors on V(b) in the presence of rubidium showed that V(b) under these conditions probably originated from the bafilomycin-sensitive electrogenic potential generated across the apical membrane by a V-type ATPase. The responses of V(b) to potassium, barium and rubidium and their inhibition of fluid secretion suggest the presence of a substantial rubidium-blockable potassium conductance located on the basal membrane of Locusta Malpighian tubule cells.     

Cation permeability ratios of sodium channels in normal and grayanotoxin-treated squid axon membranes

Summary Permeabilities of squid axon membranes to various cations at rest and during activity have been measured by voltage clamp before and during internal perfusion of 4×10^–5 m grayanotoxin I. The resting sodium and potassium permeabilities were estimated to be 6.85×10^–8 cm/sec and 2.84×10^–6 cm/sec, respectively. Grayanotoxin I increased the resting sodium permeability to 7.38×10^–7 cm/sec representing an 11-fold increase. The potassium permeability was increased only by a factor of 1.24. The resting permeability ratios as estimated by the voltage clamp method before application of grayanotoxin I were Na (1): Li (0.83): formamidine (1.34): guanidine (1.49): Cs (0.87): methylguanidine (0.86): methylamine (0.78). Grayanotoxin I did not drastically change the resting permeability ratios with a result of Na (1): Li (0.95): formamidine (1.27): guanidine (1.16): Cs (0.47): methylguanidine (0.72): methylamine (0.46). The membrane potential method gave essentially the same resting permeability ratios before and during application of grayanotoxin I if corrections were made for permeability to choline as the cation substitute and for changes in potassium permeability caused by test cations. The permeability ratio choline/Na was estimated to be 0.72 by the voltage clamp method and 0.65 by the membrane potential method. Grayanotoxin I decreased the ratio to 0.43. The permeability ratios during peak transient current were estimated to be Na (1): Li (1.12): formamidine (0.20): guanidine (0.20): Cs (0.085): methylguanidine (0.061): methylamine (0.036). Thus the sodium channels for the peak current are much more selective to cations than the resting sodium channels. It appears that the resting sodium channels in normal and grayanotoxin I-treated axons are operationally different from the sodium channels that undergo a conductance increase upon stimulation.     

The Effects of GPX-1 Knockout on Membrane Transport and Intracellular Homeostasis in the Lens

Glutathione peroxidase-1 (GPX-1) is an enzyme that protects the lens against H₂O₂-mediated oxidative damage. The purpose of the present study was to determine the effects of GPX-1 knockout (KO) on lens transport and intracellular homeostasis. To investigate these lenses we used (1) whole lens impedance studies to measure membrane conductance, resting voltage and fiber cell gap junction coupling conductance; (2) osmotic swelling of fiber cell membrane vesicles to determine water permeability; and (3) injection of Fura2 and Na⁺-binding benzofuran isophthalate (SBFI) into fiber cells to measure [Ca²⁺] _i and [Na⁺] _i , respectively, in intact lenses. These approaches were used to compare wild-type (WT) and GPX-1 KO lenses from mice around 2 months of age. There were no significant differences in clarity, size, resting voltage, membrane conductance or fiber cell membrane water permeability between WT and GPX-1 KO lenses. However, in GPX-1 KO lenses, coupling conductance was 72% of normal in the outer shell of differentiating fibers and 45% of normal in the inner core of mature fibers. Quantitative Western blots showed that GPX-1 KO lenses had about 50% as much labeled Cx46 and Cx50 protein as WT, whereas they had equivalent labeled AQP0 protein as WT. Both Ca²⁺ and Na⁺ accumulated significantly in the core of GPX-1 KO lenses. In summary, the major effect on lens transport of GPX-1 KO was a reduction in gap junction coupling conductance. This reduction affected the lens normal circulation by causing [Na⁺] _i and [Ca²⁺] _i to increase, which could increase cataract susceptibility in GPX-1 KO lenses.     

Relative roles of Ca2+, Sr2+, Ba2+ and Mg2+ in controlling lens permeability characteristics

The effects of barium, strontium and magnesium upon lens permeability characteristics were studied in the presence and absence of 2 mM calcium in the bathing medium. Permeability characteristics were determined by measuring lens potential, resistance and ⁴²K efflux rates. Barium and strontium at equimolar concentrations to calcium were able to substitute for calcium in controlling lens sodium permeability. Magnesium was ineffective in this respect.Small changes in resistance and ⁴²K efflux rates occurred when calcium was eliminated from bathing solution containing either 2 mM barium or strontium. These changes were interpreted to be the result of an increase in lens permeability to potassium. When 2 mM strontium was added to calcium-containing solution, there was no significant change in the electrical or flux parameters of the lens. However, the addition of 2 mM barium to calcium-containing solution resulted in a 54% increase in lens resistance and a 13 mV depolarization. These observations indicated a barium-induced decrease in lens permeability to potassium, and this was confirmed by an observed decrease in ⁴²K efflux rate constant under similar experimental conditions.The rapid time course of all the observed changes implies that they are the result of changes in the permeability characteristics of membranes lying close to the surface of the lens.     

137Cs penetration by contact exchange through isolated plant cuticles: cuticles as asymmetric transport membranes

The penetration of ¹³⁷Cs by contact exchange through cuticular membranes from the adaxial surface of leaves of Pyrus communis and Prunus cerasus has been investigated. The resistance of the cuticles to the caesium penetration was dependent on the counter-ions associated with the fixed negative ion exchange sites in the membrane. The mobility of hydrated potassium ions and their tenuous connection to -COOH^?-groups in the membrane encouraged caesium permeation in contrast to cuticular membranes with predominantly protonized ion exchange sites. Divalent calcium ions caused a strong reduction (4–20 times) of the caesium permeability which is decisive for the calculation of the caesium uptake by the intact leaf. Under these conditions, a penetration rate of the deposited caesium of 0·11±0·05% h^?1 for pear cuticles and of 0·036±0·025% h^?1 for cherry cuticles was measured after the adjustment to steady state conditions. Approximately 12–24% and 4·5–7·5% of initially retained caesium could be absorbed by the leaves of pear and cherry, respectively, in a rain-free period of 7 d in the area of Munich after wet deposition of fallout from the Chernobyl reactor accident. Furthermore, the caesium penetration from the physiological inside to the outside of the membrane was found to be smaller by a factor of 100–150 compared with that of the opposite direction.     

Ouabain-insensitive salt and water movements in duck red cells. II. Norepinephrine stimulation of sodium plus potassium cotransport

Catecholamines induce net salt and water movements in duck red cells incubated in isotonic solutions. The rate of this response is approximately three times greater than a comparable effect observed in 400 mosmol hypertonic solutions in the absence of hormone (W.F. Schmidt and T. J. McManus. 1977 a.J. Gen. Physiol. 70:59-79. Otherwise, these two systems share a great many similarities. In both cases, net water and salt movements have a marked dependence on external cation concentrations, are sensitive to furosemide and insensitive to ouabain, and allow the substitution of rubidium for external potassium. In the presence of ouabain, but the absence of external potassium (or rubidium), a furosemide-sensitive net extrusion of sodium against a large electrochemical gradient can be demonstrated. When norepinephrine-treated cells are incubated with ouabain and sufficient external sodium, the furosemide-sensitive, unidirectional influxes of both sodium and rubidium are half- maximally saturated at similar rubidium concentrations; with saturating external rubidium, the same fluxes are half-maximal at comparable levels of external sodium. In the absence of sodium, a catecholamine-stimulated, furosemide-sensitive influx of rubidium persists. In the absence of rubidium, a similar but smaller component of sodium influx can be seen. We interpret these results in terms of a cotransport model for sodium plus potassium which is activated by hypertonicity or norepinephrine. When either ion is absent from the incubation medium, the system promotes an exchange-diffusion type of movement of the co-ion into the cells. In the absence of external potassium, net movement of potassium out of the cell leads to a coupled extrusion of sodium against its electrochemical gradient.     

Molecule-based magnets with TC above room temperature: Improved synthesis of vanadium–chromium Prussian blue analogues with inserted alkali cations

We report the synthesis of several new vanadium–chromium Prussian blue analogues with a Curie temperature above room temperature. A catalytic amount of vanadium(III) during the synthesis allows to get improved magnetic properties, stable Curie temperatures (T_C) and a magnetisation at saturation in satisfying agreement with the stoichiometry. The insertion of alkali cations (potassium, rubidium, caesium) allows to reach a V:Cr stoichiometry close to 6:5, hence to enhance the number of magnetic neighbours compared to the usual V:Cr 3:2 stoichiometry and to increase accordingly the Curie temperature. For the same stoichiometry, the ferrimagnetic ordering temperature varies slightly with the nature of the inserted alkali cation (T_C/K = 360, 346 and 340 K for the potassium, rubidium and caesium derivatives, respectively). X-ray absorption spectroscopy allows to characterise the local surroundings of the chromium and vanadium ions and to propose an explanation of the magnetic properties.     

Electrophysiological properties of cellular and paracellular conductive pathways of the rabbit cortical collecting duct

Summary Microelectrode techniques were applied to the rabbit isolated perfused cortical collecting duct to provide an initial quantitation and characterization of the cell membrane and tight junction conductances. Initial studies demonstrated that the fractional resistance (ratio of the resistance of the apical cell membrane to the sum of the resistances of the apical and basolateral membranes) was usually independent of the point along the tubule of microelectrode impalement—implicating little cell-to-cell coupling—supporting the application of quantitative techniques to the cortical collecting duct. It was demonstrated that in the presence of amiloride, either reduction in the luminal pH or the addition of barium to the perfusate selectively reduced the apical membrane potassium conductance. From the changes inG ^te and fractional resistance upon reducing the luminal pH or addition of barium to the perfusate, the transepithelial, apical membrane, basolateral membrane and tight junction conductances were estimated to be 9.3, 6.7, 8.1 and 6.0 mS cm^–2, respectively. Ninety to ninety-five percent of the apical membrane conductance reflected the barium-sensitive potassium conductance in the presence of amiloride with an estimated potassium permeability of 1.1×10^–4 cm sec^–1. Reduction in the perfusate pH to 4.0 caused a 70% decrease in the apical membrane potassium conductance, implying a blocking site with an acidic group having a pK _a near 4.4. It is concluded that both the transcellular and paracellular pathways of the cortical collecting tubule have high ionic conductances, and that the apical membrane conductance primarily reffects a high potassium conductance. Furthermore, both reduction in the perfusate pH and addition of barium to the perfusate selectively block the apical potassium channels, although the site of inhibition likely differs since the two ions display markedly different voltage-dependent blocks of the channel.     

Rubidium as a probe for function and transport of potassium in the yeast Candida utilis NCYC 321, grown in chemostat culture

Attempts to grow the yeast Candida utilis in continuous culture, using media in which all the potassium had been replaced by other monovalent cations, revealed that neither lithium, sodium, caesium nor ammonium ions could functionally substitute for potassium. However, potassium could be effectively replaced by rubidium which gave (on a molar basis, and under conditions where cation availability limited growth) the same yield of cells as did potassium.Comparison of potassium- and rubidium-limited cultures showed them to be virtually identical in all the measured parameters, with the single exception of the maximum growth rate value which was considerably decreased in the rubidium-containing culture (0.35 h^-1 as compared with 0.55 h^-1).When, with variously-limited chemostat cultures, both potassium and rubidium were supplied in equimolar amounts, these ions were taken up by the cells in a ratio that varied with both the growth rate and the nature of the growth limitation. With glucose-, phosphate- or magnesium-limited cultures, the molar ratio K⁺:Rb⁺ was 1:0.6 (at D=0.1 h^-1) and 1:0.17 (at D=0.5 h^-1). In contrast, ammonia-limited cultures took up increased amounts of rubidium when growing at a low rate such that the ratio was 1:1.2, at D=0.1 h^-1, though still 1:0.17 at the higher growth rate value (D=0.5 h^-1).From a comparison of glucose- and ammonialimited cultures growing first with an equimolar mixture of potassium and rubidium, and then with rubidium alone, it was noted that the yield on oxygen was significantly decreased when potassium was absent.These results are discussed in relation to the transport and possible functions of monovalent cations in micro-organisms. It was concluded that, on the basis of these experiments, some objections could be raised against estimation of potassium transport rates by means of the tracer ⁸⁶Rb.     

Ionic permeability of K, Na, and Cl in potassium-depolarized nerve. Dependency on pH, cooperative effects, and action of tetrodotoxin.

The passive ionic membrane conductances (gj) and permeabilities (Pj) of K, Na, and Cl of crayfish (Procambarus clarkii) medial giant axons were determined in the potassium-depolarized axon and compared with that of the resting axon. Passive ionic conductances and permeabilities were found to be potassium dependent with a major conductance transition occurring around an external K concentration of 12-15 mM (Vm = -60 to -65 mV). The results showed that K, Na, and Cl conductances increased by 6.2, 6.9, and 27-fold, respectively, when external K was elevated from 5.4 to 40 mM. Permeability measurements indicated that K changed minimally with K depolarization while Na and Cl underwent an order increase in permeability. In the resting axon (K0 = 5.4 mM, pH = 7.0) PK = 1.33 X 10(-5), PCl = 1.99 X 10(-6), PNa = 1.92 X 10(-8) while in elevated potassium (K0 = 40 mM, pH 7.0), PK = 1.9 X 10(-5), PCl = 1.2 X 10(-5), and PNa = 2.7 X 10(-7) cm/s. When membrane potential is reduced to 40 mV by changes in internal ions, the conductance changes are initially small. This suggests that resting channel conductances depend also on ion environments seen by each membrane surface in addition to membrane potential. In elevated potassium, K, Na, and Cl conductances and permeabilities were measured from pH 3.8 to 11 in 0.2 pH increments. Here a cooperative transition in membrane conductance or permeability occurs when pH is altered through the imidazole pK (approximately pH 6.3) region. This cooperative conductance transition involves changes in Na and Cl but not K permeabilities. A Hill coefficient n of near 4 was found for the cooperative conductance transition of both the Na and Cl ionic channel which could be interpreted as resulting from 4 protein molecules forming each of the Na and Cl ionic channels. Tetrodotoxin reduces the Hill coefficient n to near 2 for the Na channel but does not affect the Cl channel. In the resting or depolarized axon, crosslinking membrane amino groups with DIDS reduces Cl and Na permeability. Following potassium depolarization, buried amino groups appear to be uncovered. The data here suggest that potassium depolarization produces a membrane conformation change in these ionic permeability regulatory components. A model is proposed where membrane protein, which forms the membrane ionic channels, is oriented with an accessible amino terminal group on the axon exterior. In this model the ionizable groups on protein and phospholipid have varied associations with the different ionic channel access sites for K, Na, and Cl, and these groups exert considerable control over ion permeation through their surface potentials.     

Potassium currents in the giant axon of the crabCarcinus maenas

Summary Measurements were made of the kinetic and steadystate characteristics of the potassium conductance in the giant axon of the crabCarcinus maenas. These measurements were made in the presence of tetrodotoxin, using the feedback amplifier concept introduced by Dodge and Frankenhaeuser (J. Physiol. (London) 143:76–90). The conductance increase during depolarizing voltage-clamp pulses was analyzed assuming that two separate potassium channels exist in these axons. The first potassium channel exhibited activation and fast inactivation gating which could be fitted using them ³ h, Hodgkin-Huxley formalism. The second potassium channel exhibited the standardn ⁴ Hodgkin-Huxley kinetics. These two postulated channels are blocked by internal application of caesium, tetraethylammonium and sodium ions. External application of 4 amino-pyridine also blocks these channels.     

Effets du rubidium et du césium sur la croissance et la nutrition minerale des characees

Résumé Le rubidium et le césium introduits à l'état de chlorure dans le milieu de culture ont à faible dose un effet stimulant sur la croissance de Chara fragilis et de Chara vulgaris. La résistance de ces végétaux à l'action toxique des deux ions est accrue par l'addition de potassium au milieu.Les analyses chimiques confirment que le rubidium et le césium sont antagonistes vis-à-vis du potassium et du sodium alors qu'ils ne modifient pas de manière significative le taux de calcium.

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Chara fragilis and Chara vulgaris were cultivated in a natural medium containing rubidium and caesium as chloride.The growth of Characeae was increased after culture in the solutions containing Rb and Cs in small amount. The resistance to the toxic effects of these two ions is enhanced if potassium chloride is added to the medium.Quantitative analyses indicate that Rb and Cs decrease the rate of Na and K but have no significative influence on the rate of Ca.

Université de Dijon, Laboratoire de Nutrition minérale des Végétaux     

Differential blockage of two types of potassium channels in the crab giant axon

Summary Measurements were made of the kinetic and steady-state characteristics of the potassium conductance in the giant axon of the crabsCarcinus maenas andCancer pagirus. The conductance increase during depolarizing voltage-clamp pulses was analyzed assuming that two separate types of potassium channels exist in these axons (M. E. Quinta-Ferreira, E. Rojas and N. Arispe,J. Membrane Biol. 66:171–181, 1982). It is shown here that, with small concentrations of conventional K⁺-channel blockers, it is possible to differentially inhibit these channels. The potassium channels with activation and fast inactivation gating (m³h, Hodgkin-Huxley kinetics) were blocked by external application of 4 amino-pyridine (4-AP). The potassium channels with standard gating (n⁴, Hodgkin-Huxley kinetics) were preferentially inhibited by externally applied tetraethylammonium (TEA). The differential blockage of the two types of potassium conductance changes suggests that they represent two different populations of potassium channels.It is further shown here that blocking the early transient conductance increase leads to the inhibition of the repetitive electrical activity induced by constant depolarizing current injection in fibers fromCardisoma guanhumi.     

Diurnal modulation of photoreceptor potassium conductance in the locust

Single electrode clamp techniques demonstrated diurnal changes in photoreceptor membrane conductance, recorded intracellularly in the intact, dark-adapted retina of the locust Schistocerca gregaria. In the day, locust photoreceptors exhibited the membrane properties of fast cells, as previously defined in rapidly moving diurnal Diptera. Depolarization activated a powerful potassium conductance with two kinetic components, one rapidly activating close to resting potential and the other activating more slowly when further depolarized, giving a pronounced delayed rectification. There was little inactivation. At night, locust photoreceptors resembled slow cells, as defined in weakly flying crepuscular and nocturnal Diptera. Depolarization rapidly activated an outward current which then inactivated over 100 ms to reduce rectification. The change from day to night state was mimicked by applying 10 mM serotonin extracellularly to the retina. We conclude that the potassium conductances of locust photoreceptor membranes are modulated according to a diurnal rhythm, possibly by serotonin. This neuromodulation is used to match photoreceptor membrane properties to photic habitat. Our findings suggest a definite and potentially widespread function for serotonin as a mediator of diurnal changes in the insect visual system.