Are membrane enzymes regulated by the viscosity of the membrane environment?

We have examined the idea that membrane enzymes are regulated by the viscosity of surrounding lipids using data compiled from the literature for the effect of the change in membrane viscosity ([symbol: see text]) at the gel- to liquid-crystal-phase transition on the activities of several enzymes. The analysis was not extended explicitly to the problem of viscosity-dependent regulation of membrane enzymes in liquid-crystalline lipids because of the absence of exact data for values of [symbol: see text] in liquid-crystalline phases of variable composition. For most membrane enzymes studied, energies of activation are discontinuous, while kcat is continuous, at the main-phase transition. We consider that the energy of activation contains terms related to the height of the chemical barrier to reaction and terms due to the mechanical properties of the bilayer, such as the work of expansion during the catalytic cycle and the temperature dependence of [symbol: see text]. We find that the differences in energies of activation, above and below the break points in Arrhenius plots, are orders of magnitude larger than can be accounted for by the above mechanical factors. Thus, discontinuities in energies of activation at the phase transition appear to reflect changes in the chemical barrier to reaction, which is independent of [symbol: see text]. The theorectical analysis indicates too that values of [symbol: see text] for bilayers in the liquid-crystalline phase would have to be several orders of magnitude larger than those for gel phases in order to provide a basis for viscosity-dependent regulation of membrane enzymes in liquid-crystalline phases.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis and evaluation of optically pure [3H]-(+)-pentazocine, a highly potent and selective radioligand for sigma receptors

Tritium-labeled (+)-pentazocine ([3H]-1b) of specific activity 26.6 Ci/mmol was synthesized in 3 steps starting with (+)-normetazocine (2) of defined optical purity. [3H]-1b has been characterized as a highly selective ligand for labeling of sigma receptors. Competition data revealed that [3H]-1b could be displaced from guinea pig brain membrane preparations with a number of commonly used sigma receptor ligands. [3H]-1b exhibited saturable, enantioselective binding with a Kd of 5.13 +/- 0.97 nM and a Bmax of 1146 +/- 122 fmol/mg protein. Phencyclidine (PCP) displaced [3H]-1b with low affinity while MK-801 was inactive, thus indicating insignificant activity at the PCP-binding site; apomorphine failed to displace [3H]-1b indicating lack of dopamine receptor cross-reactivity. Since the affinity of [3H]-1b is about 6 times that of the two commonly employed sigma ligands ((+)-3-[3H]PPP and [3H]DTG) and since it is more selective for sigma receptors than the benzomorphan [3H]SKF-10,047, it represents the first example of a highly selective benzomorphan based sigma receptor ligand. [3H]-1b should prove useful for further study of the structure and function of sigma receptors.     

A New Pressure Probe Method to Determine the Average Volumetric Elastic Modulus of Cells in Plant Tissue

A new in vivo method was used to determine an average volumetric elastic modulus ([epsilon]ave) for nongrowing cells in plant tissue. This method requires that both the relative transpiration rate, T, of the tissue and the average turgor pressure decay rate, (dP/dt)ave, of the cells are measured after the water source is removed from the plant tissue. Then [epsilon]ave is calculated from the equation [epsilon]ave = (-dP/dt)ave/T. This method was used to determine [epsilon]ave for cortical cells in stems of pea seedlings (Pisum sativum L.). The results demonstrate that [epsilon]ave increases from virtually zero at low P (approximately 0.01MPa) to approximately 10 MPa at high P (approximately 0.5 MPa). Analyses of the results indicate that the relationship between [epsilon]ave and P can be approximated by a linear function and more accurately approximated by a saturating exponential function: [epsilon]ave = [epsilon][infinity symbol][1 - exp {-k(P - Po)}], where Po is a plateau pressure (approximately 0.01 MPa), k is a rate constant (approximately 7 per MPa), and [epsilon][infinity symbol] (approximately 10 MPa) is the hypothetical maximum value of [epsilon]ave as P -> [infinity symbol]. Solutions for the turgor pressure decay (due to transpiration) as functions of time and symplasmic water mass (after the water source is removed) are derived.     

Effects of pH and free Mg2+ on the Keq of the creatine kinase reaction and other phosphate hydrolyses and phosphate transfer reactions.

The observed equilibrium constants (Kobs) of the creatine kinase (EC 2.7.3.2), myokinase (EC 2.7.4.3), glucose-6-phosphatase (EC 3.1.3.9), and fructose-1,6-diphosphatase (EC 3.1.3.11) reactions have been determined at 38 degrees C, pH 7.0, ionic strength 0.25, and varying free magnesium concentrations. The equilibrium constant (KCK) for the creatine kinase reaction defined as: KCK = [sigma ATP] [sigma creatine] divided by ([sigma ADP] [sigma creatine-P] [H+]) was measured at 0.25 ionic strength and 38 degrees C and was shown to vary with free [Mg2+]. The value was found to be 3.78 x 10(8) M-1 at free [Mg2+] = 0 and 1.66 x 10(9) M-1 at free [Mg2+] = 10(-3) M. Therefore, at pH 7.0, the value of Kobs, defined as Kobs = KCK[H+] = [sigma ATP] [sigma creatine] divided by ([sigma ADP] [sigma creatine-P] was 37.8 at free [Mg2+] = 0 and 166 at free [Mg2+] = 10(-3) M. The Kobs value for the myokinase reaction, 2 sigma ADP equilibrium sigma AMP + sigma ATP, was found to vary with free [Mg2+], being 0.391 at free [Mg2+] = 0 and 1.05 at free [Mg2+] = 10(-3) M. Taking the standard state of water to have activity equal to 1, the Kobs of glucose-6-P hydrolysis, sigma glucose-6-P + H2O equilibrium sigma glucose + sigma Pi, was found not to vary with free [Mg2+], being 110 M at both free [Mg2+] = 0 and free [Mg2+] = 10(-3) M. The Kobs of fructose-1,6-P2 hydrolysis, sigma fructose-1,6-P2 equilibrium sigma fructose-6-P + sigma Pi, was found to vary with free [Mg2+], being 272 M at free [Mg2+] = 0 and 174 M at free [Mg2+] = 0.89 x 10(-3) M.     

Effect of a sudden and a slow concentration increase in plasma urea on its concentration in some tissues of the dog and rat

The urea concentration in the renal cortex ([RC]) of pentobarbital-anaesthetized dogs was 5.71 times higher than the plasma concentration ([P]); the liver ([L]) and the skeletal muscle ([SM]) concentrations were the same as ([P]). Rapid infusion of 20% urea (1 g/kg b.w. within 1 min) was followed by a sudden increase in [P]; [RC] and [L] rose to values nonsignificantly different from [P] and remained non-significantly different for the whole 4 hours of the experiment; at the end, [P] was still about 10 times higher than before infusion. Surprisingly, [SM] 2 and 6 min after infusion was significantly lower than [P]; later they were the same. The experiment thus does not testify to the existence of active transport of urea in the RC. The permeability of the skeletal muscle membrane for urea is lower than that of the RC and liver. Chronic uraemia was induced in rats by transplanting the trigonum vesicae into the peritoneal cavity. In addition to the chemical determination of urea, 14C-urea (marked*) was also measured. [RC]/[P] and [RC*]/[P*] fell as [P] rose; [L], [L*] [SM] and [SM*] never differed from [P] or [P*]. Fluid [PF] collected in the peritoneal cavity had the same chemically determined and radioactive urea concentration as P, but it was hypoosmolar and had a lower [Na+] than P. These experiments likewise did not testify to active urea transport in the RC. It is not clear what caused the osmolarity and sodium gradient between the PF and P, but the peritoneal wall definitely did not act as a simple dialyzing membrane.     

Roles of CO2, O2, and acid in arteriovenous [H+] difference during muscle contractions

To determine the origins of the arteriovenous [H+] difference of muscle during contractions, arterial and muscle venous blood sample pairs were taken before and after 0.5, 5.0, and 30.0 min of 4/s isometric twitches of the gastrocnemius-plantaris muscle group of anesthetized dogs. These samples were analyzed for PO2, PCO2, and pH, the concentrations of O2, CO2, K+, Na+, La-, and Cl- in whole blood, and La-, K+, Na+, and Cl- in plasma. Whole blood was hemolyzed and analyzed for PO2, PCO2, and pH. Net O2 uptake, CO2 output, L, K+, Na+, and Cl- were calculated in addition to net output of non-CO2 acid (HA) and strong ion difference ([SID]) and common ion [SID] ([K+] + [Na+] - [Cl-] - [La-]). From these data we partitioned the origins of the arteriovenous [H+] difference via the common PCO2-pH diagram and via a [H+]-PCO2 diagram and determined whether true plasma arteriovenous [H+] differences reflect plasma and cell arteriovenous [H+] differences. The arteriovenous [H+] differences of plasma and hemolyzed blood were the same, showing that true plasma does reflect plasma and cells. K+ showed a small significant but transient output. Na+ was not significant, whereas Cl- showed a significant transient uptake. Lactate output and HA, calculated for dog blood acid-base, showed transient outputs and were the same. At 5.0 min when the arteriovenous difference was largest, CO2 alone would have increased [H+] 15.9 nmol/l whereas desaturation of Hb would have decreased [H+] 4.2 nmol/l and lactate could have raised [H+] 1.0 nmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)     

Permeability properties and intracellular ion concentrations of epithelial cells in rat duodenum.

Effects of the K+ concentration in the bathing fluid ([K+]l) on the intracellular K+, Na+ and Cl- concentrations ([K+]i [Na+]i and [Cl-]i) as well as on the electrical potential were studied in rat duodenum. Changes in the mucosal K+ concentration ([K+]m), bringing the sum of Na+ and K+ concentrations to 147.2 mM constant, had little effect on the transmural potential difference (PDt), but did induce marked changes in the mucosal membrane potential (Vm). As [K+]m increased, Vm was depolarized gradually and obeyed the Nernst equation for a potassium electrode in the range of [K+]m greater than approx. 60 mM. Experiments of ion analyses were carried out on strips of duodenum to determine the effect of changing the external K+ concentrations on [K+] i, [Na+]i and [Cl-]i. An increase in [K+]o resulted in increases in [K+]i and [Cl-]i and a decrease in [Na+]i, [K+]i approaching its maximum at [K+]o greater than 70 mM. Such changes in [K+]i and [Na+]i seem to correlate quantitatively with the changes in [K+]o and [Na+]o. The values of the ratio of permeability coefficients, Pna+/PK+ were estimated using the Vm values and intracellular ion concentrations measured in these experiments. The results suggested that there appeared a rather abrupt increase in the PNa+/PK+ ratio from 0 to approx. 0.1, as [K+]m decreased.     

Fractionation of xyloglucan fragments and their interaction with cellulose.

Tamarind seed xyloglucan was partially degraded with a purified endoglucanase (endoV) from Trichoderma viride. Analysis by high-performance anion-exchange chromatography showed that this digest was composed of fragments consisting of 1 to 10 repeating oligosaccharide units ([xg]1-[xg]10). To study the adsorption of xyloglucan fragments to cellulose in detail, this digest was fractionated on BioGel P-6. Fragments were separated satisfactorily up to 5 repeating oligosaccharide units ([xg]5). The galactose substitution of the fragments increased with increasing molecular weight. The BioGel P-6 pools, as well as polymeric xyloglucan ([xg] infinity), were tested for their ability to interact with Avicel crystalline cellulose. Quantitative binding to cellulose occurred for sequences consisting of (at least) 4 repeating units. The adsorption of [xg]4 to Avicel was very high relative to that of [xg] infinity. The dimensions of these fragments were such that they could also penetrate the smaller pores of cellulose. Apparently, the effective surface area for the polymers is much smaller. Adsorption isotherms of [xg] infinity and [xg]4 showed a pattern that is typical for polydisperse systems. However, the mechanisms underlying these patterns were different. At high xyloglucan concentrations, this polydispersity resulted in preferential adsorption of the larger molecules in the case of [xg] infinity and a more extensive colonization of the smaller pores of cellulose in the case of [xg]4. The pH influenced the interaction between xyloglucan (fragments) and cellulose to only a small extent.     

Effect of the number of nucleic acid oligomer charges on the salt dependence of stability (DeltaG 37degrees) and melting temperature (Tm): NLPB analysis of experimental data

For nucleic acid oligomers with variable chain lengths, the salt concentration ([salt]) dependences of the denaturation temperature (T(m)) and of the free energy of helix formation at 37 degrees C (Delta) are predicted using nonlinear Poisson-Boltzmann (NLPB) calculations. Analysis of experimental data reveals that the ratio of the [salt] derivative of melting temperature (ST(m) = dT(m)/d log[salt]) to the value for a polymer with the same base composition (ST(m)/ST(m, infinity)) is independent of base composition but strongly dependent on the number of DNA charges (/Z/) below approximately 8 bp for two-strand helices (formed from association of two complementary strands) and below approximately 18 bp for hairpin helices (formed from folding of one self-complementary strand). We interpret these ST(m)/ST(m, infinity) ratios in terms of the ratio of thermodynamic ion release from the oligomer (Deltan(u), per charge) to that from the same oligomer embedded in polymeric DNA (Deltan(u, infinity), per charge). Experimental values of ST(m)/ST(m, infinity) and its dependence on /Z/ are in good agreement with NLPB predictions for a preaveraged (essential structural) model of DNA. In particular, the NLPB calculations describe the stronger /Z/ dependence of ST(m) observed for melting of oligomeric hairpin helices than for melting of two-strand helices. These calculations predict an experimentally detectable (>or=10%) difference between ST(m) and ST(m, infinity) which increases strongly with decreasing length for two-strand helix lengths of <15 bp and for hairpin helix lengths of <30 bp. From NLPB values of Deltan(u)/Deltan(u, infinity), we predict Delta as a function of [salt] and /Z/. Predictions of thermodynamic and thermal stabilities of oligomeric helices as functions of length and [salt] are consistent with and represent a significant refinement of the average oligomer salt effect currently in use in nearest neighbor stability predictions.     

Role of lungs and inactive muscle in acid-base control after maximal exercise

The pulmonary responses and changes in plasma acid-base status occurring across the inactive forearm muscle were examined after 30 s of intense exercise in six male subjects exercising on an isokinetic cycle ergometer. Arterial and deep forearm venous blood were sampled at rest and during 10 min after exercise; ventilation and pulmonary gas exchange variables were measured breath by breath during exercise and recovery. Immediately after exercise, ventilation and CO2 output increased to 124 +/- 17 1/min and 3.24 +/- 0.195 l/min, respectively. The subsequent decrease in CO2 output was slower than the decrease in O2 intake (half time of 105 +/- 15 and 47 +/- 4 s, respectively); the respiratory exchange ratio was greater than 1.0 throughout the 10 min of recovery. Arterial plasma concentrations of Na+, K+, and Ca2+ increased transiently after exercise. Arterial lactate ion concentration ([La-]) increased to 14-15 meq/l within 1.5 min and remained at this level for the rest of the study. Throughout recovery there was a positive arteriovenous [La-] difference of 4-5 meq/l, associated with an increase in the arteriovenous strong ion difference ([SID]) and by a large increase in the venous Pco2 and [HCO3-]. These findings were interpreted as indicating uptake of La- by the inactive muscle, leading to a fall in the muscle [SID] and increase in plasma [SID], associated with an increase in muscle PCO2. The venoarterial CO2 content difference was 38% greater than could be accounted for by metabolism of La- alone, suggesting liberation of CO2 stored in muscle, possibly as carbamate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fracture Faces in the Cell Envelope of Escherichia coli

Freeze-fracturing of Escherichia coli cells in the presence of 30% (v/v) glycerol resulted in a double cleavage of the cell envelope exposing two convex and two concave fracture faces ([Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text]) with characteristic patterns. Complementary replicas revealed the relationship of the fracture faces to their corresponding fracture planes. The inner fracture plane splits the plasma membrane at one particular level. Apparently the outer fracture plane was located in the outer part of the wall, as it was separated by a layer ([Formula: see text]) from the fractured profile (CW1) presumably corresponding to the murein layer. The outer fracture plane did alternate toward the cell periphery, exposing complementary smooth areas ([Formula: see text] and [Formula: see text]). When cells were freeze-fractured in the absence of glycerol, the outer cell surface appeared as an etching face rather than a fracture face. A schematic representation of the relative location of the different fracture faces in the E. coli cell envelope is given.     

Identification of a Transport Mechanism for NH4+ in the Symbiosome Membrane of Pea Root Nodules

Symbiosome membrane vesicles, facing bacteroid-side-out, were purified from pea (Pisum sativum L.) root nodules and used to study NH4+ transport across the membrane by recording vesicle uptake of the NH4+ analog [14C]methylamine (MA). Membrane potentials ([delta][psi]) were imposed on the vesicles using K+ concentration gradients and valinomycin, and the size of the imposed [delta][psi] was determined by measuring vesicle uptake of [14C]tetraphenylphosphonium. Vesicle uptake of MA was driven by a negative [delta][psi] and was stimulated by a low extravesicular pH. Protonophore-induced collapse of the pH gradient indicated that uptake of MA was not related to the presence of a pH gradient. The MA-uptake mechanism appeared to have a large capacity for transport, and saturation was not observed at MA concentrations in the range of 25 [mu]M to 150 mM. MA uptake could be inhibited by NH4+, which indicates that NH4+ and MA compete for the same uptake mechanism. The observed fluxes suggest that voltage-driven channels are operating in the symbiosome membrane and that these are capable of transporting NH4+ at high rates from the bacteroid side of the membrane to the plant cytosol. The pH of the symbiosome space is likely to be involved in regulation of the flux.     

Rates of DNA sequence evolution are not sex-biased in Drosophila melanogaster and D. simulans

To determine whether male- or female-biased mutation rates have affected the molecular evolution of Drosophila melanogaster and D. simulans, we calculated the male-to-female ratio of germline cell divisions ([symbol: see text]) from germline generation data and the male-to-female ratio of mutation rate ([symbol: see text]) by comparing chromosomal levels of nucleotide divergence. We found that the ratio of germline cell divisions changes from indicating a weak female bias to indicating a weak male bias as the age of reproduction increases. The range of [symbol: see text] values that we observed, however, does not lead us to expect much, if any, difference in mutation rate between the sexes. Silent and intron nucleotide divergence were compared between nine loci on the X chromosome and nine loci on the second and third chromosomes. The average levels of nucleotide divergence were not significantly different across the chromosomes, although both silent and intron sites show a trend toward slightly more divergence on the X. These results indicate a lack of sex- or chromosome-biased molecular evolution in D. melanogaster and D. simulans.      

Measurement of the Membrane Potential of Isolated Nerve Terminals by the Lipophilic Cation [3H]Triphenylmethylphosphonium Bromide

Abstract: The lipophilic cation [³H]triphenylrnethylphosphonium bromide ([³H]TPMP⁺) was investigated as a measure of the membrane potential of synaptosomes. Conditions under which [³H]TPMP⁺ achieved an equilibrium distribution were tested. The toxicity of TPMP has been studied relative to its inhibitory effects on [³H]y-aminobutyric acid ([³H]GABA) transport. In some experiments the distribution of ⁸⁶RbZ⁺ and [³H]TPMP⁺ was changed upon incubation in the presence of elevated levels of K⁺, ouabain, or KCN, or at 0°C in a way that would be expected from the membrane potential. In normal incubation conditions a membrane potential of ∼−60 mv was calculated.     

Subcellular distribution of leukotriene C4 binding units in cultured bovine aortic endothelial cells

The subcellular distribution of specific binding sites for [3H]leukotriene C4 ([3H]LTC4) was analyzed after sedimentation of organelles from disrupted bovine aortic endothelial cells on sucrose density gradients and was shown to be in membrane fractions I (20% sucrose) and IV (35% sucrose). Saturation binding studies of [3H]LTC4 on endothelial cell monolayers at 4 degrees C demonstrated high-affinity binding sites with a dissociation constant (Kd) of 6.8 +/- 2.2 nM (mean +/- SD) and a density of 0.12 +/- 0.02 pmol/10(6) cells. At 4 degrees C, the specific binding of [3H]LTC4 by each of the subcellular fractions reached equilibrium at 30 min and remained stable for an additional 60 min. After 30 min of incubation with [3H]LTC4, the addition of excess unlabeled LTC4 to each subcellular fraction reversed more than 70% of [3H]LTC4 binding in 10 min. The [3H]LTC4 binding activities of subcellular fractions were enhanced approximately twofold to fourfold in the presence of Ca2+, Mg2+, and Mn2+, whereas Na+, K+, and Li+ were without effect. As measured by saturation experiments, the Kd and density of LTC4 binding sites in fraction I were 4.8 +/- 1.6 nM and 16.5 +/- 1.9 pmol/mg of protein, respectively, and in fraction IV were 4.7 +/- 1.5 nM and 81.4 +/- 19 pmol/mg of protein, respectively. Inhibition of [3H]LTC4 binding in membrane-enriched subcellular fractions I and IV by LTC4 occurred with molar inhibition constant (Ki) values of 4.5 +/- 0.1 nM and 4.7 +/- 1.2 nM, respectively, whereas Ki values for LTD4 were 570 +/- 330 nM and 62.5 +/- 32.8 nM, respectively, and for LTE4 were greater than 1000 nM for each fraction; LTB4 and reduced glutathione were even less active. FPL55712, a putative antagonist of the sulfidopeptide LT components of slow reacting substance of anaphylaxis, had Ki values of 1520 +/- 800 nM and 1180 +/- 720 nM for [3H]LTC4 binding sites on membrane-enriched subcellular fractions I and IV, respectively. Thus as defined by Kd, Ki, and specificity, the LTC4 binding units that are distributed to the plasma membrane and the binding units in the subcellular fraction of greater density were similar to each other. Pretreatment of the isolated subcellular membrane fractions with trypsin abolished [3H]LTC4 binding by fraction I, enriched for the plasma membrane marker 5' nucleotidase, and that by fraction IV, enriched for the mitochondrial membrane marker succinate-cytochrome C reductase.(ABSTRACT TRUNCATED AT 400 WORDS)     

Reconstruction of ionic currents in a molluscan photoreceptor.

Two-microelectrode voltage-clamp measurements were made to determine the kinetics and voltage dependence of ionic currents across the soma membrane of the Hermissenda type B photoreceptor. The voltage-dependent outward potassium currents, IA and ICa(2+)-K+, the inward voltage-dependent calcium current, ICa2+ and the light-induced current, IIgt, were then described with Hodgkin-Huxley-type equations. The fast-activating and inactivating potassium current, IA, was described by the equation; IA(t) = gA(max)(ma infinity[1-exp(-t/tau ma)])3 x (ha infinity [1-exp(-t/tau ha)] + exp(-t/tau ha)) (Vm-EK), where the parameters ma infinity, ha infinity, tau ma, and tau ha are functions of membrane potential, Vm, and ma infinity and ha infinity are steady-state activation and inactivation parameters. Similarly, the calcium-dependent outward potassium current, ICa(2+)-K+, was described by the equation, ICa(2+)-K+ (t) = gc(max)(mc infinity(VC)(1-exp[-t/tau mc (VC)]))pc (hc infinity(VC) [1-exp(-t/tau hc)] + exp(-t/tau hc(VC)])pc(VC-EK). In high external potassium, ICa(2+)-K+ could be measured in approximate isolation from other currents as a voltage-dependent inward tail current following a depolarizing command pulse from a holding potential of -60 mV. A voltage-dependent inward calcium current across the type B soma membrane, ICa2+, activated rapidly, showed little inactivation, and was described by the equation: ICa2+ = gCa(max) [1 + exp](-Vm-5)/7]-1 (Vm-ECa), where gCa(max) was 0.5 microS. The light-induced current with both fast and slow phases was described by: IIgt(t) = IIgt1 + IIgt2 + IIgt3, IIgti = gIgti [1-exp(- ton/tau mi)] exp(-ton/tau hi)(Vm-EIgti) (i = 1, 2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Origins of the transient anterior-posterior asymmetry in the frog lens fiber potential

The origin of the transient asymmetry of intracellular resting potentials between the anterior and posterior lens fibers was investigated in the isolated American bullfrog lens by a conventional microelectrode technique. In high K+, Rb+, Cs+, or NH+4 test solution applied only to the lens anterior or posterior side, anterior fibers depolarized at a slower rate than posterior ones. After a long exposure, however, the transient potential difference disappeared. The magnitude of the depolarizations of the lens fibers was in the order of K+ greater than Rb+ greater than Cs+ greater than NH+4. The resting potentials plotted as a function of external K+ concentrations ([K]0) were in agreement with Nernst equation predictions with a slope of 58 mV/decade ion concentration change. A small Na+ permeability is unmasked at a [K]0 less than 10 mM. It was concluded that the transient difference measured in potentials of anterior and posterior lens fibers on increasing external K+, Rb+, Cs+ or NH+4 depends on the anterior epithelial cell layer, which is a diffusional barrier for ions penetrating into the lens interior.