The relationship of K+ efflux at the inner surface of the isolated frog skin epithelium to the short circuit current.

Isolated frog skins (without chorion) were incubated with 42K+ Ringers' solution, bathing the internal surface for 2 h. All the K+ contained in the frog skin was equilibrated in specific activity with external 42K+. The kinetics of the washout of 42K+ from the internal surface of the skin exhibits one fast and one slow exponential component. Amiloride reduces the release of 42K+ corresponding to both components without affecting the K+ content of the skin. Ouabain increases the loss of 42K+ of the slow component by 200%. Since the total K+ in the skin decreases to 25% of its original value both compartments are affected. The results suggest that two distinct functional compartments exist defined by two 42K+ release ratios and that because of the large K+ contents of these compartments both are intracellular. The relation with the transepithelial Na+ transport and the morphological identification of these compartments is discussed.     

Light-induced changes of extra- and intracellular potassium concentration in photoreceptors of the leech,Hirudo medicinalis

Summary The potassium concentration was measured in the cytoplasm, perimicrovillar extracellular space (=vacuole) and intercellular space of leech photoreceptors with double-barrelled potassium-sensitive microelectrodes in darkness and upon photostimulation. The mean intracellular potassium concentration in cells with membrane potentials >50 mV was 100±34 mmol/l. Photostimulation with 90 saturating 20 ms light flashes (1/s) evoked a potassium loss of 10.6±7.6 mmol/l. In the dark, there was no potassium concentration gradient between vacuole and intercellular space (K _VAC ⁺ =4.5±0.9 mmol/l, K _ECS ⁺ =4.5±0.5 mmol/l). In both compartments the potassium concentration increased upon repetitive photostimulation. Thus, the potassium loss from the cell is due to potassium movements across both the receptive and the non-receptive membrane domains.The time courses of K⁺ accumulation and clearance differed in the two extracellular compartments: In the vacuole, potassium increased by 2.8±2.5 mmol/l to a ceiling level which was maintained during the standard train of light flashes. Potassium clearing in the dark was exponential with a half time of 60±26 s. In the intercellular space, repetitive photostimulation produced an initial rapid increase (half time <1 s) of the K⁺ concentration (mean K _max ⁺ =1.5±0.6 mmol/l). K⁺ clearing showed two superimposed components. A rapid one clears intercellular K⁺ after each light flash. The resultant K⁺ pulses ride on a slowly decreasing intercellular K+ level, and, following the last flash, K⁺ transiently undershoots the dark concentration.Ouabain or a decrease in specimen temperature affect only the slow component and abolish the poststimulation K⁺ undershoot. Thus, the rapid component is interpreted as due to passive K⁺ dispersal by diffusion through the intercellular spaces, and the slow component and the poststimulation undershoot to K⁺ clearing by active reuptake of K⁺ into the photoreceptor cells.K⁺ disappearance from the vacuole was not affected by ouabain, but a decrease in specimen temperature decreased the rate constant of K⁺ clearing, which has a Q₁₀ of 1.48. It is concluded that K⁺ clearing from the vacuole is dominated by passive processes, and that the Na⁺/K⁺-pump is possibly localized only in the non-receptive membrane domain.     

Roles of electrogenic Na+ pump and K+ conductance in the slow inhibitory postsynaptic potential of bullfrog sympathetic ganglion cells

The slow IPSP recorded from bullfrog sympathetic ganglion cells in the Ringer solution consists of two different potential components, namely ouabain-sensitive and ouabain-insensitive components. The present experimental analyses on these two potential components suggest that the former component is associated with an electrogenic Na⁺ pump while the latter with an increase of K⁺-conductance. The slow IPSP recorded in K⁺-free solution is clearly mixed with a new potential component, which is not seen in the Ringer solution and differs from the usual slow IPSP. These results do not support the concept that the slow IPSP consists of the potential component associated with a fall of Na⁺ conductance.     

Vasotocin has the potential to inhibit basolateral Na<Superscript>+</Superscript>/K<Superscript>+</Superscript>-pump current across isolated skin of tree frog in vitro<Emphasis Type="Italic">,</Emphasis> via its V<Subscript>2</Subscript>-type receptor/cAMP pathway

Adult frog skin transports Na⁺ from the apical to the basolateral side across the skin. Antidiuretic hormone (ADH) is involved in the regulation of Na⁺ transport in both mammals and amphibians. We investigated the effect of arginine vasotocin (AVT), the ADH of amphibians, on the short-circuit current (SCC) across intact skin and on the basolateral Na⁺/K⁺-pump current across apically nystatin-permeabilized skin of the tree frog, Hyla japonica, in which the V₂-type ADH receptor is expressed in vitro. In intact skin, 1 pM AVT had no effect on the SCC, but 10 nM AVT was sufficient to stimulate the SCC since 10 nM and 1 μM of AVT increased the SCC 3.2- and 3.4-fold, respectively (P > 0.9). However, in permeabilized skin, AVT (1 μM) decreased the Na⁺/K⁺-pump current to 0.79 times vehicle control. Similarly, 500 μM of 8Br-cAMP increased the SCC 3.2-fold, yet 1 mM of 8Br-cAMP decreased the Na⁺/K⁺-pump current to 0.76 times vehicle control. Arachidonic acid (10⁻⁵ M) tended to decrease the Na⁺/K⁺-pump current. To judge from these in vitro experiments, AVT has the potential to inhibit the basolateral Na⁺/K⁺-pump current via the V₂-type receptor/cAMP pathway in the skin of the tree frog.     

Continuous measurement of calcium influx in mammalian nonmyelinated nerve fibers: Effects of Na o,Ca o,and electrical activity

Summary Efflux of⁴²K⁺ was measured in frog sartorius muscles equilibrated in hyperosmotic depolarizing solutions. At the internal potentials obtained, K⁺ passes mainly through the inward rectifier potassium channels.Inhibition of K⁺ efflux by external Zn²⁺ (0.25 to 15mm) differs in three significant ways from inhibition by Ba²⁺. (1) The dose-response relation does not correspond to action at a single site. (2) The Zn²⁺-sensitivity of K⁺ efflux does not depend on [K⁺] _o at constant internal potential. (3) Zn²⁺ inhibition is reduced by hydrogen ions, while Ba²⁺ inhibition is unaffected. Further, the Ba²⁺-sensitivity of K⁺ efflux is not altered by a half-inhibiting Zn²⁺ concentration, suggesting that the two ions do not interact at a common site.The histidine-modifying reagent diethylpyrocarbonate (DEPC) reduces Zn²⁺ inhibition. After DEPC treatment Zn²⁺ inhibition is further reduced by low pH. DEPC has little effect on Ba²⁺ inhibition. Zn²⁺ inhibition is not altered by treatment with the sulfhydryl reagents 5,5-dithio-bis(2-nitrobenzoic acid) or dithiothreitol.The results can be described by either of two models in which two sites can bind Zn²⁺ and one or both of the sites may also bind H⁺. When both sites bind Zn²⁺, K⁺ efflux is inhibited, and a third site may then bind H⁺. The effects of DEPC can be accounted for by a decrease in H⁺ affinity of the first two sites by a factor of 50, and a decrease in Zn²⁺ affinity of these sites and of the H⁺ affinity of the third site by about one order of magnitude.     

Sulfatide content and (Na++K+)-ATPase activity of skin and gill during larval development of the chilean frog,Calyptocephalella caudiverbera

Summary The sulfatide content, phospholipid concentration, and (Na⁺+K⁺)-ATPase activity from skin and gills of different stages of larval development ofCalyptocephalella caudiverbera (a Chilean frog) were analyzed. Additionally, the short-circuit current in skin was studied. When skin and gills, depending on the stage of larval development, present (Na⁺+K⁺)-ATPase activity, they have a high ratio of sulfatide to amount of membrane and the phosphatidylserine concentration remains unchanged. Sulfatide content and (Na⁺+K⁺)-ATPase activity in skin are in direct relationship with the level of sodium flux present during development. The specific enzymatic hydrolysis of sulfatide with partially purified arylsulfatase of pig kidney inhibits 100% of the ouabain-sensitive (Na⁺+K⁺)-ATPase. The ouabain-insensitive ATPase remains virtually unchanged with the treatment, even with a high concentration of arylsulfatase or with ouabain present in the medium. These experiments strongly suggest a role of sulfatides in the (Na⁺+K⁺)-ATPase activity and, as a consequence, in sodium ion transport.     

Multiple fractions of sodium exchange in human lymphocytes

Human lymphocytes contain a large, saturable fraction of K⁺ that exchanges slowly with K⁺ in the external medium, and a small non-saturable fraction that exchanges rapidly. We determined whether or not Na⁺ exchanges in a similar manner with external Na⁺. Cells were pre-equilibrated to ensure absence of net ion movements. Efflux was studied by loading with ²²Na and transferring without washing to a non-labeled medium. Influx was studied by transferring to labeled medium and separating large samples of cells at 6,000g. There are fast, intermediate, and slow fractions of Na⁺ exchange, with half-times of 2, 14, and 120 minutes. At normal external K⁺, most cell Na⁺ exchanges rapidly, while at lower external K⁺ the Na⁺ that replaces cell K⁺ exchanges slowly. Parallel sources of fast and slow fractions, such as extracellular ones and subpopulations of cells, were ruled out by simultaneous ⁴²K and ²²Na fluxes and by a quantitative analysis of the combined K⁺ and Na⁺ content and flux data over a range of external K⁺ and Na⁺ levels. Five possible models of ion fluxes occurring in series were considered. Surface matrix, surface binding sites, and cytoplasmic channels with rapid nuclea exchange were eliminated as sources of the fast fractions. Therefore, the fast fractions of K⁺ and Na⁺ must reflect the permeability of the surface membrane. This left only two possible sources of the slow fractions. One, a subcellular compartment (e.g., nucleus), was eliminated by the combined content and flux data. We conclude that the slow fractions of ion flux are rate-limited by adsorption onto and desorption from cellular macromolecules. The data support the association-induction hypothesis and are understood by reference to two fundamental concepts: that of rapid solute exclusion from cell water existing in a polarized state; and that of solute accumulation limited by adsorption onto fixed anionic sites within the cell.     

Uptake of mercury by Chlorella and its effect on potassium regulation

Summary Addition of mercuric chloride at concentrations which resulted in an overall binding level of about 8 mmoles Hg/l packed cells and above caused a breakdown in the permeability of the cell membrane as indicated by a net efflux of internal K⁺. Below this level in region of 2 mmoles Hg/l packed cells the rate of K⁺ transfer across the cell surface was stimulated without affecting the internal K⁺ level. Maintainence of the stimulation was dependent both on time and dose. Enhancement of the rate of K⁺ turnover was associated with a fast component of the inorganic mercury uptake which could be removed by washing with cysteine. The mercury stimulated K⁺/K⁺ exchange was inhibited by low temperature, by the uncoupler CCCP and the energy transfer inhibitor DCCD. Overall binding concentrations of inorganic mercury below 0.5 mmoles/l packed cells had no effect on the K⁺ transport system. In contrast to mercuric chloride, methyl mercuric chloride over similar concentration ranges did not seem to induce a breakdown in the permeability barrier or directly interact with the K⁺/K⁺ exchange but more likely influenced the latter by inhibiting intracellular processes.     

Na+-dependence of 45Ca2+ uptake in adult rat isolated cardiac cells

We developed a technique that yields isolated adult rat myocytes, 70% of which are elongated and morphologically similar to intact tissue. Electrophysiological studies showed most of these cells were quiescent, Ca²⁺-tolerant and exhibited normal action potentials accompanied by contractions. We analyzed ⁴⁵Ca²⁺ uptake data in terms of instantaneous, fast and slow compartments. 69% of total exchangeable Ca²⁺ was found in the slow compartment; the rest was almost equally divided between the instantaneous and fast compartments. Replacement of extracellular Na⁺ by Li⁺ or Tris increased ⁴⁵Ca²⁺ uptake by the fast compartment; high [K⁺]_o increased this uptake further. These increases appeared to be related also to internal concentrations of Na⁺. This conclusion was supported by experiments with digitonin-treated cells. Our results indicate that the way Na⁺-dependent ⁴⁵Ca²⁺ uptake is affected by [Na⁺]_o, [Na⁺]_i and [K⁺]_o is compatible with the Na⁺-Ca²⁺ exchange mechanism. Our preparation should prove useful in studies of regulation of Ca²⁺ transport in cardiac muscles.     

Potassium transport in red blood cells of frog Rana temporaria: demonstration of a K−Cl cotransport

Pathways of K⁺ movement across the erythrocyte membrane of frog Rana temporaria were studied using ⁸⁶Rb as a tracer. The K⁺ influx was significantly blocked by 0.1 mmol·l^-1 ouabain (by 30%) and 1 mmol·l^-1 furosemide (by 56%) in the red cells incubated in saline at physiological K⁺ concentration (2.7 mmol·l^-1). Ouabain and furosemide had an additive effect on K⁺ transport in frog red cells. The ouabain-sensitive and furosemide-sensitive components of K⁺ influx saturated as f(K⁺)_e with apparent K _m values for external K _e ⁺ concentration of 0.96±0.11 and 4.6±0.5 mmol·l^-1 and V _max of 0.89±0.04 and 2.8±0.4 mmol·l cells^-1·h^-1, respectively. The residual ouabain-furosemide-resistant component was also a saturable function of K _e ⁺ medium concentration. Total K⁺ influx was significantly reduced when frog erythrocytes were incubated in NO _- ³ medium. Furosemide did not affect K⁺ transport in frog red cells in NO ₃ ^- media. At the same K _e ⁺ concentration the ouabain-furosemide-insensitive K⁺ influx in Cl^- medium was significantly greater than that in NO _- ³ medium. We found no inhibitory effect of 1 mmol·l^-1 furosemide on Na⁺ influx in frog red cells in Cl^- medium. K⁺ loss from the frog erythrocytes in a K⁺-free medium was significantly reduced (mean 58%) after replacement of Cl^- with NO _- ³ . Furosemide (0.5 mmol·l^-1) did not produce any significant reduction in the K⁺ loss in both media. The Cl^--dependent component of K⁺ loss from frog red cells was 5.7±1.2 mmol·l^-1·h^-1. These results indicate that about two-thirds of the total K⁺ influx in frog erythrocytes is mediated by a K–Cl cotransport which is only partially blocked by furosemide.Abbreviations DMSO dimethyl sulphoxide - K _e ⁺ external concentration of K⁺ - K _m apparent Michaelis constant for external - K⁺ K _e ⁺ at V _max/2 - RBC red blood cell(s) - V _max maximal velocity of the unidirectional K⁺ influx - TRIS tris(hydroxymethyl)aminomethane     

Effects of phorbol 12-myristate 13-acetate on potassium transport in the red blood cells of frog <Emphasis Type="Italic">Rana temporaria</Emphasis>

Phorbol 12-myristate 13-acetate (PMA), a stimulator of PKC, was examined for its influence on K⁺ (⁸⁶Rb) influx in the frog erythrocyte. PMA, 0.1 μM, was found to accelerate ouabain-sensitive K⁺ influx, which was suppressed by 73% with 1 mM amiloride, indicating secondary activation of the Na⁺–K⁺-pump due to stimulation of Na⁺ /H⁺ exchange. PMA-induced stimulation of the sodium pump was completely inhibited with 1 μM staurosporine and by ~50% with 20 μM chelerythrine. In contrast to Na⁺–K⁺-pump, an activity of Cl⁻-dependent K⁺ transport (K–Cl cotransport, KCC), calculated as the difference between K⁺ influxes in Cl⁻ and NO₃ ⁻-media, was substantially decreased under the influence of PMA. Staurosporine fully restored the PMA-induced inhibition of KCC, whereas chelerythrine did not exert any influence. Osmotic swelling of the frog erythrocytes was accompanied by approximately twofold stimulation of KCC. Swelling-activated KCC was inhibited by ~50 and ~83% in the presence of PMA and genistein, respectively, but not chelerythrine. Exposure of the frog erythrocytes to 5 mM fluoride (F⁻) also reduced the KCC activity in isotonic and hypotonic media, with maximal suppression of K⁺ influx in both media being observed upon simultaneous addition of PMA and F⁻. Furosemide and [(dihydronindenyl)oxy] alkanoic acid inhibited the K⁺ influx in both the media by ~50–60%. The results obtained show both the direct and indirect effects of PMA on the K⁺ transport in frog erythrocytes and a complicated picture of KCC regulation in frog erythrocytes with involvement of PKC, tyrosine kinase and protein phosphatase.     

Induction of Na+ / K+ exchange in swollen heart mitochondria

Heart mitochondria swollen passively in nitrate salts contract in a respiration-dependent reaction which can be attributed to an endogenous cation/H⁺ exchange component (or components). The rate of contraction increases with increased extent of passive swelling in both Na⁺ and K⁺ salts. Since nearly constant internal cation concentrations are maintained during osmotic swelling, this result suggests that both Na⁺/H⁺ and K⁺/H⁺ exchange is enhanced by increased matrix volume. Endogenous Mg²⁺ is also lost with increased matrix volume, and this observation, in conjunction with other evidence available in the literature, suggests that monovalent cation/H⁺ exchanges may be regulated by divalent cations. Passive exchange of Na⁺/K⁺,⁴²K⁺/K⁺, and²⁴Na⁺/Na⁺ can be readily demonstrated in mitochondria swollen in nitrate. All these exchanges are low or not detectable in unswollen control mitochondria, and it appears that they are manifestations of the activated cation/H⁺ component (or components) functioning in the absence of pH.     

Influence of exogenous ions on the events of maturation in Rana pipiens oocytes

Full-grown ovarian oocytes removed from non-hormone-treated Rana pipiens females exhibit a low level of protein synthesis, the rate of which is dependent upon the ionic environment. The highest rates of protein synthesis in these oocytes are obtained in media containing either a divalent cation (Ca⁺⁺ or Mg⁺⁺) or high levels of K⁺. The dependence of protein synthesis on ionic environment persists through about the first 18-24 hours of maturation (at 18°C). Normal maturation of oocytes in vitro also has specific ionic requirements for the first 24 hours. In this case, the process requires high ionic strength (T/2 = 1.0-1.2) and divalent cations. The kinetics of K⁺ exchange suggest that K⁺ exists in the ovarian oocyte in two compartments; one in equilibrium with the exogenous medium and freely exchangeable; the other in equilibrium with the exogenous medium and freely exchangeable; the other in equilibrium with the first internal compartment and only very slowly exchangeable. The slowly exchangeable (bound) compartment contains about 95% of all endogenous K⁺. In hormone stimulated oocytes, the kinetics of K⁺ exchange are essentially the same. Oocyte adaptation to ionic environment is discussed as a possible regulatory mechanism during maturation.     

Differential effects of glycolytic and oxidative metabolism blockers on the Na-K pump in erythrocytes of the frog, Rana temporaria

This study was undertaken to evaluate the effects of various metabolic blockers on the Na-K-pump activity and ATP content of frog erythrocytes. To eliminate K-C1 cotransport, the frog erythrocytes were incubated in nitrate media at 20 °C. Incubation of the red cells in a glucose-free medium for 2 h had no effect on cell ATP content and K⁺ influx measured as ⁸⁶Rb uptake for 60 min. The Na⁺-K⁺-pump activity was also unchanged in the frog erythrocytes incubated in a glucose-free medium containing 10 mM 2-deoxy-D-glucose or adenosine. Unexpectedly, the treatment of red cells with 1–2 mM glycolytic blocker iodoacetate produced a 2-fold increase in the ouabain-sensitive K⁺ influx. The cell ATP content declined by 9.4% after 2 h of cell incubation with iodoacetate. Incubation of the red cells for 90 min in the presence of 2 mM cyanide, 0.01 mM antimycin A or 5 mM azide resulted in a significant reduction in K⁺ influx by about 50%, 45% and 32%, respectively. The cell ATP content diminished over 60 min and 120 min of cell incubation with 2 mM cyanide by 15.6% and 31.7% of control levels, respectively. In time-course experiments, a 50% reduction in the K⁺ influx was observed when the frog erythrocytes were incubated for only 30 min in the presence of 2 mM cyanide. In contrast, 0.01–0.10 mM rotenone, a site I inhibitor, and 0.01 mM carbonyl cyanide m-chlorophenylhydrazone, an uncoupler of oxidative phosphorylation were without effect on K⁺ influx into frog erythrocytes. These results indicate that about one-half of the Na⁺ -K⁺-pump activity in frog erythrocytes is tightly functionally coupled to cytochromes via a separate “membrane-associated” ATP pool. Accepted: 12 July 1997     

Noise analysis reveals K+ channel conductance fluctuations in the apical membrane of rabbit colon

Summary In this paper we describe current fluctuations in the mammalian epithelium, rabbit descending colon. Pieces of isolated colon epithelium bathed in Na⁺ or K⁺ Ringer's solutions were studied under short-circuit conditions with the current noise spectra recorded over the range of 1–200 Hz. When the epithelium was bathed on both sides with Na⁺ Ringer's solution (the mucosal solution contained 50 m amiloride), no Lorentzian components were found in the power spectrum. After imposition of a potassium gradient across the epithelium by replacement of the mucosal solution by K⁺ Ringer's (containing 50 m amiloride), a Lorentzian component appeared with an average corner frequency,f _c=15.6±0.91 Hz and a mean plateau valueS _o=(7.04±2.94)×10^–20 A² sec/cm². The Lorentzian component was enhanced by voltage clamping the colon in a direction favorable for K⁺ entry across the apical membrane. Elimination of the K⁺ gradient by bathing the colon on both sides with K⁺ Ringer's solutions abolished the noise signal. The Lorentzian component was also depressed by mucosal addition of Cs⁺ or tetraethylammonium (TEA) and by serosal addition of Ba²⁺. The one-sided action of these K⁺ channel blockers suggests a cellular location for the fluctuating channels. Addition of nystatin to the mucosal solution abolished the Lorentzian component. Serosal nystatin did not affect the Lorentzian noise. This finding indicates an apical membrane location for the fluctuating channels. The data were similar in some respects to K⁺ channel fluctuations recorded from the apical membranes of amphibian epithelia such as the frog skin and toad gallbladder. The results are relevant to recent reports concerning transcellular potassium secretion in the colon and indicate that the colon possesses spontaneously fluctuating potassium channels in its apical membranes in parallel to the Na⁺ transport pathway.     

The H+/ATP stoichiometry of the (H+ + K+)-ATPase of dog gastric microsomes

Gastric microsomal vesicles isolated from dog fundic mucosa were shown to be relatively ion tight and have a low level of proton permeability. The H⁺ translocase, basal ATPase and K⁺-activated ATPase activities of these vesicles were measured and the H⁺/ATP stoichiometry calculated using either the total K⁺-ATPase or the K⁺-stimulatable component (total K⁺-ATPase—basal ATPase). The former estimations consistently gave stoichiometric of approximately one, whereas the use of only the K⁺-stimulatable component gave widely differing values. Measurement of the dephosphorylation of the enzyme under basal conditions revealed both a labile and a stable phosphoenzyme component. The rate of decay of the labile component completely accounted for the basal ATPase activity observed. We conclude that the basal ATPase associated with our preparations is a spontaneous dephosphorylation of the phosphoenzyme occurring in the absence of K⁺ and that the H⁺/ATP stoichiometry of the gastric ATPase is one.     

Three Types of Single Voltage-Dependent Potassium Channels in the Sarcolemma of Frog Skeletal Muscle

Patch-clamp experiments in the sarcolemma of frog skeletal muscle evidenced the presence of three types of voltage-dependent single-channel K⁺ currents. According to their unitary conductance at a membrane voltage of +40 mV, we classified them as 16-, 13-, and 7-pS K⁺ channels. The 16-pS K⁺ channels are active close to a membrane voltage of −80 mV and they do not become inactivated during voltage pulses of 100 ms. Within 10 min after beginning the recording, these channels developed rundown with an exponential time course. The 13-pS K⁺ channels are active near −60 mV; upon a 100-ms depolarization, they exhibited inactivation with an approximate exponential time course. The 7-pS K⁺ channels were recorded at voltages positive to 0 mV. In patches containing all three types of K⁺ channels, the ensemble average currents resemble the kinetic properties of the macroscopic delayed rectifier K⁺ currents recorded in skeletal muscle and other tissues. In conclusion, the biophysical properties of unitary K⁺ currents suggest that these single-channel K⁺ currents may underlie the macroscopic delayed K⁺ currents in frog skeletal muscle fibers. In addition, since the 16- and 13-pS channels were more frequently recorded, both are the main contributors to the delayed K⁺ currents.     

Colchicine effect on the permeability of the whole epithelium and of isolated cells of frog skin

The effect of 2×10^–5 M colchicine on epithelial cells isolated from frog skin was investigated. Three hours of treatment with colchicine did not change either Na⁺ and K⁺ content of isolated cells or nonelectrolyte permeability. When ADH (50 mU/ml) was added, thiourea uptake values became greater than without the hormone; the same values were found in the cells previously treated with colchicine. Na⁺ transepithelial transport, measured by means of short-circuit current, was inhibited by the antimitotic agent both under control conditions and after ADH stimulation. These results support the view that colchicine does not directly affect ADH action on membrane permeability, but influences some mechanism that controls ADH action on transepithelial transport. Intercellular junctions appear to be the location of such a mechanism.     

Effects of carbamazepine on nerve activity and transmitter release in neuroblastoma-glioma hybrid cells and the frog neuromuscular junction

Effects of the antiepileptic drug carbamazepine on nerve action potential and transmitter release in mouse neuroblastoma-glioma hybrid cells (NG108-15) and the frog neuromuscular junction were studied. Carbamazepine within a concentration range of 0.1–0.5 mmol/L reduced the peak height of the action potential of the NG108-15 cells, whereas the membrane potential and membrane resistance were unaffected. Voltage clamp revealed that the decrease in the action potential was due to the blockage of the Na⁺, delayed K⁺ and transient Ca²⁺ currents. Carbamazepine did not affect Ca²⁺-activated and A type K⁺ currents and long-lasting Ca²⁺ current. In the frog neuromuscular junction, carbamazepine decreased the mean quantal content by a parallel shift in the frequency augmentation–potentiation (FAP) relation. It is concluded that carbamazepine blocks the voltage-dependent Na⁺, delayed K⁺, and transient Ca²⁺ currents and quantal transmitter release through a decrease of nerve excitation.     

Coupled transepithelial sodium and potassium transport across isolated frog skin: Effect of ouabain,amiloride and the polyene antibiotic filipin

Summary Addition of the polyene antibiotic filipin (50 m) to the outside bathing solution (OBS) of the isolated frog skin resulted in a highly significant active outward transport of K⁺ because filipinper se increases the nonspecific Na⁺ and K⁺ permeability of the outward facing membrane. The K⁺ transport was calculated from the chemically determined changes in K⁺ concentrations in the solution bathing the two sides of the skin. The active transepithelial K⁺ transport required the presence of Na⁺ in the OBS, but not in the inside bathing solution (IBS), and it was inhibited by the Na⁺, K⁺-ATPase inhibitor ouabain. The addition of Ba⁺⁺ to the IBS in the presence of filipin in the OBS resulted in an activation of the transepithelial K⁺ transport and in an inhibition of the active Na⁺ transport. This is in agreement with the notion that Ba⁺⁺ decreases the passive K⁺ permeability of the inward facing membrane. In the presence of amiloride (which blocks the specific Na permeability of the outward facing membrane) and Ba⁺⁺ there was a good correlation between the active Na⁺ and K⁺ transport. It is concluded that the active transepithelial K⁺ transport is carried out by a coupled electrogenic Na–K pump, and it is suggested that the pump ratio (Na/K) is 1.5.