Routes of nonelectrolyte permeability in gallbladder. Effects of 2,4,6- triaminopyrimidinium (TAP)

The organic cation 2,4,6-triaminopyrimidinium (TAP), which blocks the tight junction channels for cation permeability across gallbladder, also inhibits gallbladder permeability (P) to urea and glycerol without significantly affecting P to Cl(-), sucrose, 1,7-heptanediol, or water (osmotic or diffusional permeabilities). These effects together with the comparisons of P's in frog with P's in rabbit gallbladder suggest that sucrose migrates exclusively through the leakage pathway (through where Cl(-) permeates), and that urea and glycerol permeate in addition through both, the tight junction channels for cations and a polar pathway at the cell membranes. Water and 1,7-heptanediol probably permeate mainly through the epithelial cell membranes.     

On the cross-reactivity of amiloride and 2,4,6 Triaminopyrimidine (TAP) for the cellular entry and tight junctional cation permeation pathways in epithelia

Summary 2,4,6 Triaminopyrimidine (TAP) has been previously shown to inhibit the passive tight junctional cation permeation pathway in various leaky epithelia. Amiloride has been shown to be an effective inhibitor of the cation cellular entry pathway in tight epithelia. In this paper we demonstrate that TAP and amiloride at appropriate concentrations are able to block either of these epithelial cation permeation pathways. TAP was found to block the Na entry pathway in frog skin with the following characteristics: it (1) inhibits from the external solution only, (2) is completely reversible, (3) increases the transepithelial resistance, (4) is active in the monoprotonated form, (5) is noncompetitive with Na, (6) displays saturation kinetics which obey a simple kinetic model (K _I=1×10^–3 m), (7) is independent of external calcium, (8) is dependent on external buffering capacity, and (9) is competitive with amiloride. Amiloride inhibition of the junctional permeation in gallbladder had the following characteristics: it (1) increases the transepithelial resistance, (2) decreases cation conductance without affecting the anion conductance, (3) displays saturation kinetics which obey a simple kinetic model (K _I=1×10^–3 m), and (4) possesses inhibitory activity in both its protonated and unprotonated form. These results not only indicate that a similar inhibitory site may exist in both of these cation permeation pathways, but also provide information on the chemical nature and possible location of these inhibitory sites.     

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.     

Effect of oxytocin on the electrical potential and ion permeability of the apical membrane of frog gall bladder epithelial cells

The influence of oxytocin on the intracellular Na+ and K+ concentrations, the level of transmembrane potential differences, and on the relative ionic permeability (PNa/PK) of the apical zones of the superficial epithelium membrane was studied in experiments on the isolated frog gallbladder (GB). Oxytocine introduced into the outer incubation solution in a dose of 20 mulliunits/ml caused a reduction of transmembrane potential difference, and an increase of PNa/pk coefficient and an insignificant shift of the Na+ and K+ concentrations in the intracellular medium. Thirty minutes after the oxytocine action of the organ the membrane potential (MP) of the cells decreased from 52.7 mV to 38.7 mV (the cell is negatively charged inside), and PNa/PK increased from 0,083 (control) to 0,175 (test) with a simultaneous increase in the intracellular Na+ concentration by 18.3 milliequiv./kg of (H2O)i. Such a shift in the intracellular Na+ and K+ concentrations may cause a decrease of the MP by only--0.7 mV, but actually the membrane potential decreased by--14.0 mV. Thus, the reduction of the transmembrane potential difference results from increase of PNa/PK under the influence of oxytocine. No electrogenic ionic transport through the apical membrane of frog gallbladder epithelial cells was revealed.     

Arg352 is a major determinant of charge selectivity in the cystic fibrosis transmembrane conductance regulator chloride channel

The cystic fibrosis transmembrane conductance regulator forms an anion-selective channel. We previously showed that charge selectivity, the ability to discriminate between anions and cations, occurs near the cytoplasmic end of the channel. The molecular determinants of charge selectivity, however, are unknown. We investigated the role of Arg352, a residue flanking the predicted cytoplasmic end of the M6 segment, in the mechanism of charge selectivity. We determined the Cl- to Na+ permeability ratio (PCl/PNa) from the reversal potential measured in a 10-fold NaCl gradient. For the wild type, PCl/PNa was 36 (range of 28-51). For the R352H mutant, PCl/PNa was dependent on cytoplasmic pH. At pH 5.4, the PCl/PNa was 33 (range of 27-41), similar to that of the wild type, but at pH 7.2, where the histidine should be largely uncharged, PCl/PNa was 3 (range of 2.9-3.1). For the R352C and R352Q mutants, PCl/PNa was 7 (range of 6-8) and 4 (range of 3.5-4.4), respectively. Furthermore, Na+ which does not carry a significant fraction of the current through the wild type is measurably conducted through R352Q. Thus, the charge of the side chain at position 352 is a strong determinant of charge selectivity. In the wild type, the positive charge on Arg352 contributes to an electrostatic potential in the channel that forms a barrier to cation permeation. Mutation of Arg352 did not alter the halide selectivity sequence. Selectivity among halides must involve other residues.     

Occlusion of Na+ by the Na,K-ATPase in the presence of oligomycin

Oligomycin occludes Na+ in an E1-form of the Na,K-ATPase. The rate constants for the release of Na+ from the E1-form and for the transition to the E2-form are about 0.5 s-1. The effect of oligomycin is not seen using other cations which also have a Na+-like effect on the enzyme conformation. The inhibitory effect of oligomycin on the ADP-ATP dependent Na:Na exchange but not on the accompanying ADP-ATP exchange can be explained from a decrease in the rate of release of Na+ from an E1 approximately phosphoform with Na+ occluded, E'1 approximately P (Na3), i.e. with Na+ in the membrane phase, to an E"1 approximately PNa3 form with Na+ not occluded. E"1 approximately PNa3 is at a step before formation of E2-P, and disappears at a high rate when ADP reacts with E"1 approximately P (Na3).     

Monovalent and divalent cation permeability and block of neuronal nicotinic receptor channels in rat parasympathetic ganglia

Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh- evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5- 6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.     

Ionic blockage of the light-regulated sodium channels in isolated rod outer segments.

We have investigated, with osmotic techniques, the light-regulated Na+ channels in rod outer segments (ROS) and ROS fragments freshly isolated from the frog retina. Values of Na+ permeability (PNa) similar to those observed electrophysiologically in the retina were observed using the osmotic technique (continuous flow) described by Korenbrot and Cone. In the other osmotic techniques that we explored, PNa was greatly diminished, if not completely suppressed; however, we found with these techniques that antioxidant conditions (N2 atmosphere or EDTA) significantly increased PNa, suggesting that the Na+ channels are highly sensitivive to membrane oxidation. Using the continuous flow technique, we investigated the H+ and Ca++ dependence of the Na+ channels and found that both of these ions, at micromolar activities, can block the channels. Raising the external H+ activity decreases PNa (reversibly) in a single "sigmoidal" response with an apparent pKa of 5.8. Similarly, in the presence of the ionophores X537A or A23187 which allow equilibration of Ca++ across membranes, the Na+ channels are blocked when the external Ca++ activity is increased from 10(-7) to 10(-5) M. This high sensitivity to both H+ and Ca++ ions suggests that high field strength anionic sites may exist in or near the Na+ channels and that the channels are blocked when these sites bind H+ or Ca++ ions.     

Anion-cation interactions in the pore of neuronal background chloride channels

Background Cl channels in neurons and skeletal muscle are significantly permeable for alkali cations when tested with asymmetrical concentrations of the same salt. Both anion and cation permeation were proposed to require binding of an alkali cation with the pore (Franciolini, F., and W. Nonner. 1987. Journal of General Physiology. 90:453-478). We tested this hypothesis by bilaterally substituting large alkali cations for Na and found no significant changes of unitary conductance at 300 mM symmetrical concentrations. In addition, all organic cations examined were permeant in a salt gradient test (1,000 mM internal@300 mM external), including triethanolamine, benzyltrimethylamine, and bis-tris-propane (BTP, which is divalent at the tested pH of 6.2). Inward currents were detected following substitution of internal NaCl by the Na salts of the divalent anions of phosphoric, fumaric, and malic acid. Zero-current potentials in gradients of the Na and BTP salts of varied anions (propionate, F, Br, nitrate) that have different permeabilities under bi-ionic conditions, were approximately constant, as if the permeation of either cation were coupled to the permeation of the anion. These results rule out our earlier hypothesis of anion permeation dependent on a bound alkali cation, but they are consistent with the idea that the tested anions and cations form mixed complexes while traversing the Cl channel.     

Stretch-activated currents in cardiomyocytes isolated from rabbit pulmonary veins

Evidence is growing of a relationship between atrial dilation and atrial fibrillation (AF), the most prevalent type of arrhythmia. Pulmonary veins, which are important ectopic foci for provoking AF, are of increasing interest in relation to the early development of AF. Here, using single cardiomyocytes isolated from rabbit pulmonary veins, we characterised the stretch-activated currents induced by swelling and axial mechanical stretching. Swelling induced both a stretch-activated nonselective cationic current (NSC) and a Cl(-) current. The swelling-induced Cl(-) current (I Cl,swell) was inhibited by DIDS, whereas the swelling-induced NSC (I NSC,swell) was inhibited by Gd3+. The cationic selectivity of the I NSC,swell was K+ >Cs+ >Na+ >Li+, whilst the PK/PNa, PCs/PNa, and PLi/PNa permeability ratios were 2.84, 1.86, and 0.85, respectively. Activation of the I NSC,swell was faster than that of the I Cl,swell. Given a high K+ concentration in the bath solution, the I NSC,swell showed limited amplitude (<-70 mV). Mechanical stretching induced an immediate Gd3+- and streptomycin-sensitive NSC (I NSC,stretch) that was permeable to Na+, K+, Cs+ and NMDG. Persistent stretching activated a DIDS-sensitive current (I Cl,stretch). The I NSC,stretch, but not the I NSC,swell, was completely blocked by 400 microM streptomycin; therefore, the two currents may not be associated with the same channel. In addition, the type of current induced may depend on the type of stretching. Thus, stretch-induced anionic and cationic currents are functionally present in the cardiomyocytes of the main pulmonary veins of rabbits, and they may have pathophysiological roles in the development of AF under stretched conditions.     

Electrical properties of the cellular transepithelial pathway in Necturus gallbladder. II. Ionic permeability of the apical cell membrane.

Microelectrode techniques were employed to study the ionic permeability of the apical cell membrane of Necturus gallbladder epithelium. Results obtained from continuous records in single cells, and from several cellular impalements shortly after a change in solution, were similar and indicate that both the apical membrane equivalent electromotive force (Va) and electrical resistance (Ra) strongly depend on external [K]. Cl substitutions produced smaller effects, while the effects of Na substitutions with N-methyl-D-glucamine on both Va and Ra were minimal. These results indicate that the permeability sequence of the apical membrane is PKgreater thanPClgreater than PNa. From the calculated absolute value of PNa it is possible to estimate the diffusional Na flux from the mucosal solution into the cells (from the cell potential and an assumed intracellular Na concentration). The calculated flux is roughly three orders of magnitude smaller than the measured net transepithelial flux in this tissue and in gallbladders of other species. Thus, only a minimal portion of Na entry can be attributed to independent diffusion. From estimations of the electrochemical potential gradient across the apical membrane, Cl transport at that site must be active. At the serosal cell membrane, Na transport takes place against both chemical and electrical potentials, while a significant portion of the Cl flux can be passive, if this membrane has a significant Cl conductance. The changes in shunt electromotive force and in transepithelial potential after mucosal substitutions were very similar, indicating that transepithelial bi-ionic potentials yield appropriate results on the properties of shunt pathway.     

Facilitated transport of di- and trinitrophenolate ions across lipid membranes by valinomycin and nonactin.

The conductance of black lipid membranes in the presence of 2,4,6-trinitrophenol (or 2,4-dinitrophenol) is considerably enhanced, if the cation carriers valinomycin, enniatin B or nonactin are added. The effect is, however, largely independent of the cation concentration and is identical for the cations Li+, Na+ and Ba2+. This finding, as well as the sign and magnitude of the diffusion potential in the presence of a gradient of picrate are consistent with the assumption that the transport of picrate anions is facilitated by the above-mentioned macrocyclic compounds, but that cations are not directly involved. A model is suggested which, based on the generation of mobile defect structures by the incorporation of large molecules, allows one to explain facilitated transport without the assumption of stable chemical bonds between a carrier and its transported substrate. If K+ is present in the aqueous phase, the conductance is largely determined by the permeation of the cation complexes of valinomycin and nonactin. The conductance is, however, increases by adsorption of picrate anions to the membrane surface. The negative surface potential generated by the adsorption layer seems to be responsible for the saturation of the conductance at high picrate concentrations in the absence of valinomycin and nonactin.     

Permeation of Na+ through a delayed rectifier K+ channel in chick dorsal root ganglion neurons

In whole-cell patch clamp recordings from chick dorsal root ganglion neurons, removal of intracellular K+ resulted in the appearance of a large, voltage-dependent inward tail current (Icat). Icat was not Ca2+ dependent and was not blocked by Cd2+, but was blocked by Ba2+. The reversal potential for Icat shifted with the Nernst potential for [Na+]. The channel responsible for Icat had a cation permeability sequence of Na+ >> Li+ >> TMA+ > NMG+ (PX/PNa = 1:0.33:0.1:0) and was impermeable to Cl-. Addition of high intracellular concentrations of K+, Cs+, or Rb+ prevented the occurrence of Icat. Inhibition of Icat by intracellular K+ was voltage dependent, with an IC50 that ranged from 3.0-8.9 mM at membrane potentials between -50 and -110 mV. This voltage- dependent shift in IC50 (e-fold per 52 mV) is consistent with a single cation binding site approximately 50% of the distance into the membrane field. Icat displayed anomolous mole fraction behavior with respect to Na+ and K+; Icat was inhibited by 5 mM extracellular K+ in the presence of 160 mM Na+ and potentiated by equimolar substitution of 80 mM K+ for Na+. The percent inhibition produced by both extracellular and intracellular K+ at 5 mM was identical. Reversal potential measurements revealed that K+ was 65-105 times more permeant than Na+ through the Icat channel. Icat exhibited the same voltage and time dependence of inactivation, the same voltage dependence of activation, and the same macroscopic conductance as the delayed rectifier K+ current in these neurons. We conclude that Icat is a Na+ current that passes through a delayed rectifier K+ channel when intracellular K+ is reduced to below 30 mM. At intracellular K+ concentrations between 1 and 30 mM, PK/PNa remained constant while the conductance at -50 mV varied from 80 to 0% of maximum. These data suggest that the high selectivity of these channels for K+ over Na+ is due to the inability of Na+ to compete with K+ for an intracellular binding site, rather than a barrier that excludes Na+ from entry into the channel or a barrier such as a selectivity filter that prevents Na+ ions from passing through the channel.     

Cation selective channel in fetal alveolar type II epithelium.

A cation selective channel was identified in the apical membrane of fetal rat (Wistar) alveolar type II epithelium using the patch clamp technique. The single channel conductance was 23 +/- 1.2 pS (n = 16) with symmetrical NaCl (140 mM) solution in the bath and pipette. The channel was highly permeable to Na+ and K+ (PNa/PK = 0.9) but essentially impermeant to chloride and gluconate. Membrane potential did not influence open state probability when measured in a high Ca2+ (1.5 mM) bath. The channel reversibly inactivated when the bath was exchanged with a Ca(2+)-free (less than 10(-9) M) solution. The Na+ channel blocker amiloride (10(-6) M) applied to the extracellular side of the membrane reduced P(open) relative to control patches; P(control) = 0.57 +/- 0.11 (n = 5), P(amiloride) = 0.09 +/- 0.07 (n = 4, p less than 0.01), however, amiloride did not significantly influence channel conductance (g); g(control) 19 +/- 0.9 pS (n = 5), 18 +/- 3.0 pS (n = 4). More than one current level was observed in 42% (16/38) of active patches; multiple current levels (ranging from 2 to 6) were of equal amplitude suggesting the presence of multiple channels or subconductance states. Channel activity was also evident in cell attached patches. Since monolayers of these cells absorb Na+ via an amiloride sensitive transport mechanism we speculate that this amiloride sensitive cation selective channel is a potential apical pathway for electrogenic Na+ transport in the alveolar region of the lung.     

Mutations in M2 alter the selectivity of the mouse nicotinic acetylcholine receptor for organic and alkali metal cations

We measured the permeability ratios (PX/PNa) of 3 wild-type, 1 hybrid, 2 subunit-deficient, and 22 mutant nicotinic receptors expressed in Xenopus oocytes for alkali metal and organic cations using shifts in the bi-ionic reversal potential of the macroscopic current. Mutations at three positions (2', 6', 10') in M2 affected ion selectivity. Mutations at position 2' (alpha Thr244, beta Gly255, gamma Thr253, delta Ser258) near the intracellular end of M2 changed the organic cation permeability ratios as much as twofold and reduced PCs/PNa and PK/PNa by 16-18%. Mutations at positions 6' and 10' increased the glycine ethyl ester/Na+ and glycine methyl ester/Na+ permeability ratios. Two subunit alterations also affected selectivity: omission of the delta subunit reduced PCs/PNa by 16%, and substitution of Xenopus delta for mouse delta increased Pguanidinium/PNa more than twofold and reduced PCs/PNa by 34% and PLi/PNa by 20%. The wild-type mouse receptor displayed a surprising interaction with the primary ammonium cations; relative permeability peaked at a chain length equal to four carbons. Analysis of the organic permeability ratios for the wild-type mouse receptor shows that (a) the diameter of the narrowest part of the pore is 8.4 A; (b) the mouse receptor departs significantly from size selectivity for monovalent organic cations; and (c) lowering the temperature reduces Pguanidinium/PNa by 38% and Pbutylammonium/PNa more than twofold. The results reinforce present views that positions -1' and 2' are the narrowest part of the pore and suggest that positions 6' and 10' align some permeant organic cations in the pore in an interaction similar to that with channel blocker, QX-222.     

Hepatic versus gallbladder bile composition: in vivo transport physiology of the gallbladder in rainbow trout

Ion and water transport across the teleost Oncorhynchus mykiss gallbladder were studied in vivo by comparing flow and composition of hepatic bile, collected by chronic catheter, to volume and composition of terminally collected gallbladder bile. Differences in composition were comparable with those of other vertebrates, whereas bile flow (75 microl. kg(-1). h(-1)) was below values reported for endothermic vertebrates. The gallbladder concentrates bile acids five- to sevenfold and exhibits higher net Cl(-) than Na(+) transport in vivo, in contrast to the 1:1 transport ratio from gallbladders under saline/saline conditions. Transepithelial potential (TEP) in the presence of bile, at the apical surface, was -13 mV (bile side negative) but +1.5 mV in the presence of saline. Bile acid in the apical saline reversed the TEP, presumably by a Donnan effect. We propose that ion transport across the gallbladder in vivo involves backflux of Na(+) from blood to bile resulting in higher net Cl(-) than Na(+) flux. This Na(+) backflux is driven by a bile side negative TEP and low Na(+) activity in bile due to the complexing effects of bile acids.     

Study of claudin function by RNA interference

Claudins are tight junction proteins that play a key selectivity role in the paracellular conductance of ions. Numerous studies of claudin function have been carried out using the overexpression strategy to add new claudin channels to an existing paracellular protein background. Here, we report the systematic knockdown of endogenous claudin gene expression in Madin-Darby canine kidney (MDCK) cells and in LLC-PK1 cells using small interfering RNA against claudins 1-4 and 7. In MDCK cells (showing cation selectivity), claudins 2, 4, and 7 are powerful effectors of paracellular Na+ permeation. Removal of claudin-2 depressed the permeation of Na+ and resulted in the loss of cation selectivity. Loss of claudin-4 or -7 expression elevated the permeation of Na+ and enhanced the proclivity of the tight junction for cations. On the other hand, LLC-PK1 cells express little endogenous claudin-2 and show anion selectivity. In LLC-PK1 cells, claudin-4 and -7 are powerful effectors of paracellular Cl- permeation. Knockdown of claudin-4 or -7 expression depressed the permeation of Cl- and caused the tight junction to lose the anion selectivity. In conclusion, claudin-2 functions as a paracellular channel to Na+ to increase the cation selectivity of the tight junction; claudin-4 and -7 function either as paracellular barriers to Na+ or as paracellular channels to Cl-, depending upon the cellular background, to decrease the cation selectivity of the tight junction.     

南湖霉素选择地运载Li^＋,Na^＋通过磷脂双层

南湖霉素具有抑制枯草杆菌生长和抗鸡球虫病效应,是个新的多醚类抗生素,先前利用神经肌肉标本进行的研究提示,它对生物膜的作用都可用“Ｎａ＾＋载体”来解释。本文观察了南湖霉素对人工脂双层离子通透性的影响,获得的主要结果为：南湖霉素引起脂双层依剂量提高而增大的膜电导升高;通过测定在不同溶液系统中的平衡电位,确定膜电导的变化来源于脂双层对阳离子,特别是对Ｌｉ＾＋,Ｎａ＾＋通透性的升高,ＰＬｉ：ＰＮａ：ＰＫ：     

The electrical potential profile of gallbladder epithelium.

In this study the relative ionic permeabilities of the cell membranes of Necturus gallbladder epithelium have been determined by means of simultaneous measurement of transmural and transmucosal membrane potential differences (PD) and by ionic substitution experiments with sodium, potassium and chloride ions. It is shown that the mucosal membrane is permeable to sodium and to potassium ions. The baso-lateral membrane PD is only sensitive to potassium ions. In both membranes chloride conductance is negligible or absent. The ratio of the resistances of the mucosal and baso-lateral membranes, RM/RS, increases upon reducing the sodium concentration in the mucosal solution. The same ratio decreases when sodium is replaced by potassium which implies a greater potassium than sodium conductance in the mucosal membrane. The relative permeability of the shunt for potassium, sodium and chloride ions is: PK/PNa/PCl=1.81:1.00:0.32. From the results obtained in this study a value for the PK/PNa ratio of the mucosal membrane could be evaluated. This ratio is 2.7. From the same data the magnitude of the electromotive forces generated across the cell membranes could be calculated. The EMF's are -15mV across the mucosal membrane and -81mV across the baso-lateral one. Due to the presence of the low resistance shunt the transmucosal membrane PD is -53.2mV (cell inside negative) and the transmural PD is +2.6mV (serosal side positive). The change in potential profile brought about by the low resistance shunt favors passive entry of Na ions into the cell across the mucosal membrane. Calculations show that this passive Na influx is maximally 64% of the net Na flux estimated from fluid transport measurements. The C-1 conductive of the baso-lateral membrane is too small to allow electrogenic coupling of C1 with Na transport across this membrane. Experiments with rabbit gallbladder epithelium indicate that the membrane properties in this tissue are qualitatively similar to those of Necturus gallbladder epithelium.