Permeability Of Na Research Articles

PH Alterations “Reset” Ca2+ Sensitivity of Brain Na+ Channel 2, a Degenerin/Epithelial Na+ Ion Channel, in Planar Lipid Bilayers

Members of the degenerin/epithelial Na(+) channel superfamily of ion channels subserve many functions, ranging from whole body sodium handling to mechanoelectrical transduction. We studied brain Na(+) channel 2 (BNaC-2) in planar lipid bilayers to examine its single channel properties and regulation by Ca(2+). Upon incorporation of vesicles made from membranes of oocytes expressing either wild-type (WT) BNaC-2 or BNaC-2 with a gain-of-function (GF) point mutation (G433F), functional channels with different properties were obtained. WT BNaC-2 resided in a closed state with short openings, whereas GF BNaC-2 was constitutively activated; a decrease in the pH in the trans compartment of the bilayer activated WT BNaC-2 and decreased its permeability for Na(+) over K(+). Moreover, these maneuvers made the WT channel more resistant to amiloride. In contrast, GF BNaC-2 did not respond to a decrease in pH, and its amiloride sensitivity and selectivity for Na(+) over K(+) were unaffected by this pH change. Buffering the bathing solutions with EGTA to reduce the free [Ca(2+)] to <10 nm increased WT single channel open probability 10-fold, but not that of GF BNaC-2. Ca(2+) blocked both WT and GF BNaC-2 in a dose- and voltage-dependent fashion; single channel conductances were unchanged. A drop in pH reduced the ability of Ca(2+) to inhibit these channels. These results show that BNaC-2 is an amiloride-sensitive sodium channel and suggest that pH activation of these channels could be, in part, a consequence of H(+) "interference" with channel regulation by Ca(2+).

Regulation of transmembrane signaling by ganglioside GM1: interaction of anti-GM1 with Neuro2a cells.

Interaction of antibodies to ganglioside GM1 with Neuro2a cells was studied to investigate the role of GM1 in cell signaling. Binding of anti-GM1 to Neuro2a cells induced the formation of 3H-inositol phosphates (3H-IPs) and elevated the intracellular Ca2+ concentration [Ca2+]i. The rise in [Ca2+]i was due to the influx of Ca2+ from the extracellular medium and release from intracellular Ca2+ pools. The Ca2+ influx pathway did not allow the permeation of Na+ or K+. The influx was inhibited by amiloride, a specific blocker of T-type Ca2+ channels, whereas nifedipine and diltiazem, blockers of L-type Ca2+ channels, did not have any effect. Thus, anti-GM1 appears to activate a T-type Ca2+ channel in Neuro2a cells. The intracellular Ca2+ release was inhibited by pretreatment of cells with neomycin sulfate, phorbol dibutyrate, and pertussis toxin (PTx), which also inhibited the 3H-IP formation in Neuro2a cells. Addition of caffeine neither elevated the [Ca2+]i nor affected the anti-GM1-induced [Ca2+]i rise. The data reveal that the binding of anti-GM1 to Neuro2a cells activates phospholipase C via a PTx-sensitive G protein, which leads to formation of IPs and release of Ca2+ from inositol trisphosphate-sensitive pool of endoplasmic reticulum. Anti-GM1 also arrested the differentiation of Neuro2a cells in culture and significantly stimulated their proliferation. This stimulatory effect of anti-GM1 on cell proliferation was blocked by amiloride but not by PTx, suggesting that the influx of Ca2+ was essentially required for cell proliferation. Our data suggest a role for GM1 in the regulation of transmembrane signaling events and cell growth.

Bradykinin-stimulated Cl- secretion in T84 cells. Role of Ca2+-activated hSK4-like K+ channels.

Bradykinin (BK)-stimulated colonic Cl- secretion was studied in T84 colonic adenocarcinoma cells by measuring BK (50 nM)-evoked changes in cytosolic free [Ca2+] ([Ca2+]i), membrane conductance and transepithelial ion transport. In T84 cells grown on impermeable supports, BK stimulated a transient increase in [Ca2+]i as assessed by fura-2 ratio imaging. In cell-attached, patch-clamp recordings, BK transiently activated low-conductance K channels. These channels were activated/inactivated reversibly in inside-out patches by switching [Ca2+]i in the bath between 30 nM and 100 nM. Excised channels recorded with 160 mM [K+] in bath and pipette exhibited an inwardly rectifying current/voltage-relation, conductances of 10+/-1 pS and 34+/-4 pS (n=10) at positive and negative voltages, respectively, and a 15-fold lower permeability for Na+ than for K+. The mean open probability of these channels did not depend on voltage but increased with increasing [Ca2+]i with an apparent concentration for a half-maximal response (EC50) of 110 nM, resembling that of hSK4 K+ channels. Application of the reverse transcriptase-polymerase chain reaction technique showed hSK4 messenger ribonucleic acid (mRNA) to be expressed in T84 cells. Macroscopic currents in T84 cells showed a similar dependence on [Ca2+]i. Whole cell conductance (in nS/10pF) increased from 0.5+/-0. 1 (n=6) at 10 nM [Ca2+]i in the pipette solution to 1.5+/-0.2 (n=7) at 100 nM, and to 2.0+/-0.5 (n=7) at 1 microM due to activation of a K+ conductance. In Ussing-chambered T84 monolayers grown on filters, BK did not evoke a short-circuit current (Isc). When, however, the monolayers were pre-stimulated by forskolin (1 microM), BK further enhanced Cl-secretion (DeltaIsc=21+/-5 microA/cm2, n=10) transiently and biphasically. In conclusion, BK enhances cyclic adenosine monophosphate-stimulated Cl- secretion in T84 cells, probably via basolateral, Ca2+-liganded activation of low-conductance hSK4-type K+ channels.

Oncogenes and signal transduction pathways involved in the regulation of Na+ channel expression.

Recent progress in neurobiology has revealed that proteins called 'neurotrophic factors' influence development, maintenance of function, and regeneration of neurons in vertebrate nervous system. These factors include the neurotrophin family, epidermal growth factor (EGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF), which are expressed in the nervous system. Effects of the neurotrophic factors are mediated through signal transduction pathways in which several cellular protooncogenes play intrinsic roles. Furthermore, studies on mechanisms coupling membrane events to gene activation have demonstrated that transsynaptic input via action potential and neurotransmitters, and membrane depolarization play an important role in the regulation of electrical activities in neurons during and after maturation. Voltage-dependent sodium (Na+) channels mediate an increase in permeability of Na+ during the initial, rapid phase of the action potential in neurons, and are considered to be important determinants of neuronal functions. Their synthesis and expression, therefore, are crucial aspects of neural differentiation and functions. In mammals, an array of functionally distinct Na+ channels arise, at least in part, through transcriptional regulation of the multiple genes that encode distinct Na+ channel alpha subunits (Na alpha). In this review, we discuss the potential roles of the protooncogenes in the nervous system, with particular emphasis on dynamic expression of the Na+ channel gene family.

Ion transport activity of calix[n]arene (n=4, 5, 6, 7, 8) esters toward alkali-metal cations in a phospholipid bilayer membrane

Alkali-metal ion transport by a series of p-tert-butylcalix[n]arene (n=4, 5, 6, 7, 8) esters (1–5) across a soybean phospholipid bilayer membrane has been investigated by a voltage clamp method. The measurement of electric currents resulting from ion transport across the phospholipid bilayer shows that esters 1–4, show ion transport ability but that 5 does not. From an analysis of current–voltage curves, the values of ion permeability across the bilayer are determined for 1–4. The ion permeability of Na+ by 1 is >20 times that of other alkali-metal ions, suggesting that 1 acts as a selective Na+ carrier in phospholipid bilayer membranes. 2 transports K+ better than Na+, while the ion transport selectivity of 2 is poor compared to that of 1. Both 3 and 4 shows Cs+ transport selectivity. The transport selectivity (Cs+/Rb+) of 3 is better than that of 4 by a factor of 2.

Developmental changes in glucose transport, lipid composition, and fluidity of jejunal BBM.

Na(+)-dependent D-glucose uptake was studied in jejunal brush-border membrane (BBM) vesicles of chickens at 2 days and 1, 2, 5-6, and 12-14 wk of age. Both initial rates and accumulation ratios of the Na(+)-dependent D-glucose transport were significantly higher during the 1st wk than at other ages. To explain the age-related changes observed in the transport of D-glucose, the phlorizin-specific binding, Na+ permeability, lipid composition, and fluidity were studied. Transporter site density was quantified using 50 mumol/l phlorizin and found to be higher during the 1st wk. During the 2nd wk it decreased and then remained constant. Permeability of Na+, studied using 22Na+, showed that fluxes were similar during the first 6 wk, and a significant decrease was observed in the oldest group. Furthermore, membrane fluidity results showed a significant age-dependent decrease that correlated well with both the increased molar ratio of cholesterol to phospholipid and the decreased ratio of lipid to protein found during development. In conclusion, changes in the density of Na(+)-dependent D-glucose transporter as well as in lipid content and fluidity might be involved in the changes observed in D-glucose uptake during the posthatching development.

A patch clamp study of Na+ transport in maize roots

The mechanisms mediating Na(+) transpdrt in higher plant roots were investigated by applying the patch clamp technique to protoplasts isolated from the cortex and stele of maize roots. In the cortex, permeation of Na+ through a time-dependent K(+)-selective inward rectifier was negligible. Instead, Na(+) influx into maize roots probably occurs via an instantaneously-activating current. This current was partially inhibited by extracellular Ca(2+), but was insensitive to extracellular TEA(+), Cs(+) and TTX. In outside-out patches, a plasma membrane ion channel was found which mediated an inward Na(+) current which, at least in part, underlies the whole-cell instantaneously-activating current. The unitary conductance of this channel was 15 pS in 102:121 mM Na(+) (outsidexytosol). Channel gating was voltage-independent and distinct from that observed for the inwardly rectifying K(+)-selective channel in the same cell type. Increasing extracellular Ca(2+) from 0.1 to 1 mM reduced the open probability and unitary conductance of this channel. In 102 mM Na(+) : 123 mM K(+) (outside:cytosol) a P(Na):P(K) of 2.1 was calculated. It is suggested that the plasma membrane Na(+)-permeable channel identified in the cortex of maize roots represents a pathway for low affinity Na(+) uptake by intact maize roots. In the stele, permeation of Na(+) through outwardly rectifying K(+) channels was found to be negligible and the channels are thus unlikely to be involved in the transport of Na(+) from the root symplasm.

Pharmacological Studies of Physiologically Active Substances Isolated from Marine Organisms

AbstractThe present review mainly focusses on our recent pharmacological and biochemical studies on bioactive substances isolated from marine organisms. We have shown for the first time that 9-methyl-7-bromoeudistomin D, a derivative of eudistomin D isolated from a marine tunicate, acts on Ca-induced Ca release channels of sarcoplasmic reticulum at low concentrations and that maitotoxin, a dinoflagellate toxin activates voltage-independent Ca channels, thus producing a highly enhanced Ca influx. In the course of our survey of marine bioactive substances, peptide toxins (geographutoxin I and II, striatoxin, eburnetoxin and tessulatoxin) and 62-membered lactones (zooxanthellatoxin-A and-B) have been isolated from cone shells and dinoflagellates, respectively, and these marine toxins change dramatically the plasma membrane permeability of Na+ or Ca2+. From sea sponge we have isolated unique substances such as xestoquinone and purealin, which stimulate skeletal actomyosin ATPase by affecting the myosin heads ...

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.

Permeation of Na+ through open and Zn(2+)-occupied conductance states of cardiac sodium channels modified by batrachotoxin: exploring ion-ion interactions in a multi-ion channel

Mammalian heart sodium channels inserted into planar bilayers exhibit a distinctive subconductance state when single batrachotoxin-modified channels are exposed to external Zn2+. The current-voltage behavior of the open state and the Zn(2+)-induced substate was characterized in the presence of symmetrical Na+ ranging from 2 to 3000 mM. The unitary conductance of the open state follows a biphasic dependence on [Na+] that can be accounted for by a 3-barrier-2-site model of Na+ permeation that includes double occupancy and Na(+)-Na+ repulsion. The unitary conductance of the Zn2+ substate follows a monophasic dependence on [Na+] that can be explained by a similar 3-barrier-2-site model with low affinity for Na+ and single occupancy due to repulsive interaction with a Zn2+ ion bound near the external entrance to the pore. The apparent association rate of Zn2+ derived from dwell-time analysis of flickering events is strongly reduced as [Na+] is raised from 50 to 500 mM. The apparent dissociation rate of Zn2+ is also enhanced as [Na+] is increased. While not excluding surface charge effects, such behavior is consistent with two types of ion-ion interactions: 1) A competitive binding interaction between Zn2+ and Na+ due to mutual competition for high affinity sites in close proximity. 2) A noncompetitive, destabilizing interaction resulting from simultaneous occupancy by Zn2+ and Na+. The repulsive influence of Zn2+ on Na+ binding in the cardiac Na+ channel is similar to that which has been proposed to occur between Ca2+ and Na+ in structurally related calcium channels. Based on recent mutagenesis data, a schematic model of functionally important residues in the external cation binding sites of calcium channels and cardiac sodium channels is proposed. In this model, the Zn(2+)-induced subconductance state results from Zn2+ binding to a site in the external vestibule that is close to the entrance of the pore but does not occlude it.

Erythrocyte cation transport systems and membrane lipids in insulin-dependent diabetes.

The relationship between erythrocyte cation transport systems and membrane and plasma lipids was examined in normal men and patients with insulin-dependent diabetes mellitus (IDDM). Different measurements of erythrocyte transport systems were obtained in patients with IDDM and in age- and weight-matched healthy men: Na+:Li+ countertransport activity or Li(+)-stimulated Na+ efflux, Na+:K+ cotransport activity or bumetanide-sensitive Na+ efflux, Na+:K+ pump activity or ouabain-sensitive Na+ efflux, and the ouabain- and bumetanide-resistant Na+ and K+ fluxes or the ground membrane permeability for Na+ and K+ as well as the intraerythrocyte Na+, K+, and Mg2+ concentrations. Plasma cholesterol, triglycerides, phospholipids, low- and high- density lipoprotein cholesterol, and erythrocyte membrane cholesterol and phospholipid content were obtained from the fasting subjects. The patients with IDDM had an elevated (P < .05 or less) erythrocyte Na+:Li+ countertransport activity, ground membrane leak for K+, intraerythrocyte K+ concentration, and erythrocyte membrane cholesterol content, but a lower red blood cell phospholipid content. In single regression analysis, the erythrocyte Na+:Li+ countertransport, Na+:K+ cotransport, and Na+:K+ pump activity and ground membrane leak for Na+ and K+ were inversely related to the red blood cell membrane lipid content. Our data in patients with IDDM show that a decreased erythrocyte membrane lipid content was accompanied by a higher erythrocyte Na+:Li+ countertransport, Na+:K+ cotransport, and Na+:K+ pump activity.

Nonselective cation channels.

“Assuming that acetylcholine produces a large non-selective increase of ion permeability, i.e. a short-circuit,...” wrote Fatt and Katz in 1951. That was a year before Hodgkin and Huxley (1952) published their pioneering papers in which they suggested selective changes of nerve membrane permeability for Na+-ions and K+-ions as an explanation for the characteristic shape of action potentials. Selectivity for different ions is an important criterion for classification of ion channels. We can now state that acetylcholine-sensitive ion channels are indeed nonselective for monovalent cations, and that there are even many more types of ion channels characterized by different degrees of nonselectivity. With this book the authors try to demonstrate the current state of research in this field.

Flunarizine and R 56865 suppress veratridine-induced increase in oxygen consumption and uptake of 45Ca 2+ in rat cortical synaptosomes

The effect of the anti-ischemic compounds flunarizine and R 56865 on the veratridine-induced uptake of Ca 2+ and Na + was observed in cortical synaptosomes in the rat. The veratridine-induced uptake of Na + and Ca 2+ was determined by means of a measurement of synaptosomal oxygen consumption and a method for the uptake of 45Ca 2+, respectively. Veratridine (10 −5 M) was found to induce a 3-fold increase in synaptosomal oxygen consumption (uptake of Na +) and uptake of 45Ca 2+, both of which were inhibited by tetrodotoxin (10 −5 M). Nitrendipine (10 −5 M) and ω-conotoxin (5 × 10 −7 M) were ineffective on the veratridine-induced response. Nimodipine (10 −5 M) suppressed the veratridine-induced uptake of 45Ca 2+ but also diminished the unstimulated uptake of 45Ca 2+. The veratridine-induced uptake of Na + was not influenced by nimodipine. Flunarizine (3 × 10 −6–10 −5 M), as well as R 56865 (10 −6–10 −5 M), attenuated the veratridine-induced uptake of both Na + and 45Ca 2+. In conclusion, the veratridine-induced uptake of Na + and 45Ca 2+ was shown to be closely correlated to the activity of Na + channels but not to voltage-operated Ca 2+ channels. Secondly, flunarizine and R 56865 seemed to evoke their effects by interfering with the permeability of Na + channels. Since veratridine-induced uptake of Na + and Ca 2+ shares some similarities with ischaemia-induced uptake of Na + and Ca 2+, it is proposed, that flunarizine and R 56865 exert their anti-ischaemic effects by reducing ischaemia-induced Na + and Ca 2+ load, probably by inhibiting a TTX-sensitive Na + channel.

Molecular aspects of glutamate receptors and sodium-calcium exchange carriers in mammalian brain: implications for neuronal development and degeneration.

N-Methyl-D-aspartate (NMDA) and L-glutamate activate membrane receptors that produce substantial permeation of Na+, K+ and Ca2+ through the neuronal membrane. These ionic fluxes are intimately linked to processes that regulate neuronal survival, growth and differentiation. Intracellular free Ca2+ concentrations are thought to be particularly important determinants of the vulnerability of neurons to excessive excitatory stimulation produced through activation of NMDA receptors. In order to understand the molecular events involved in both NMDA receptor activation and regulation of intracellular Ca2+ levels, we have purified and reconstituted the protein complexes that form the NMDA/glutamate receptors in rat brain synaptic membranes and those that constitute the Na(+)-Ca2+ antiporters in bovine brain synaptic membrane. The molecular properties of these protein complexes are described, and information from the most recent studies of exploration of the molecular structures of these receptors and transport carriers is summarized.

Oxygen consumption stimulated by increases in permeability of Na + of mouse diaphragm muscle in vitro

1. The effects on oxygen consumption of agents that modify Na +-permeability were examined in mouse diaphragm muscle perfused with a bathing solution that contained (+)-tubocurarine in a flow-through mode. Twitch tension and levels of Na + and K + in the muscle were also measured. 2. Unstimulated preparations decreased the concentration of oxygen in the bathing solution, indicating a basal level of oxygen consumption. Electrical stimulation of the muscle further decreased the concentration of oxygen. Potassium cyanide eliminated both the basal and the stimulated consumption of oxygen. 3. Veratridine facilitated the effect of stimulation at 0.1 Hz on both the consumption of oxygen and the twitch tension. Ouabain antagonized those effects. 4. Twitch contractions were blocked in the presence of dantrolene sodium or by pretreatment with ethylene glycol. Electrical stimulation of such preparations still caused a residual but considerably decreased consumption of oxygen. Quabain and tetrodotoxin reduced the residual consumption of oxygen. 5. Ouabain significantly increased the levels of Na + in the tissue, while veratridine alone did not. The effect of ouabain was further potentiated by the simultaneous presence of veratridine. 6. These results indicate that the enhancement of the sarcolemmal permeability to Na + increases the rate oxygen consumption. this concept supports the hypothesis that Na + homeostasis depends on energy consumption.

Electrophysiological membrane characteristics of the salt tolerant Plantago maritima and the salt sensitive Plantago media

Membrane potential, electrical conductance and plasmamembrane permeability for Na+, K+ and Cl+t- of root cells of the salt tolerant Plantago maritima and the salt sensitive P. media were compared. Plants were grown with (25 mol m-3) or without NaCl. No differences were found in the membrane potentials between the species when measured in growth medium, either plus or minus NaCl. When plants were grown in 25 mol m-3 NaCl membrane potentials in both species were slightly more positive. Na and K ions both caused depolarization of the membrane potential. These were not equal for the two species, depolarization caused by K+ ions was about 3.5 times greater than that caused by Na+ ions. Membrane permeability for both ions waemed less in the salt tolerant species. Passive Na+ permeability, i.e. in the presence of the uncoupler CCCP, was about 25% of the K+ permeability in P. media, and 60% in P. maritima. From comparison of permeabilities under ‘active’ and ‘passive’ conditions and the related ion concentrations in the root cells it was concluded that in both species active, i.e. against the electrochemical gradient, Na+ efflux pumping must exist at the cortex medium boundary.

Evidence for a direct and non-receptor-mediated action of 5HT2 antagonists on transmembrane cation transport systems

Changes in transmembrane sodium fluxes have been reported in normotensive and in hypertensive subjects after ketanserin administration. In this study, the effects of the serotonergic system on transmembrane sodium transport mechanisms have been investigated in vitro. In erythrocytes drawn from ten healthy subjects, we studied the effects of serotonin (5HT) on the Na/K pump, Na/K cotransport, Na/Li countertransport, and passive permeability of Na. No significant changes were found. A direct, non-receptor-mediated action of ketanserin was then suspected, and the effects of two concentrations of ketanserin (5 x 10(-8) and 5 x 10(-7) M) were evaluated in erythrocytes from 12 normal volunteers. Both concentrations of ketanserin significantly decreased the activity of the Na/K pump and increased the activity of Na/Li countertransport. Na/K cotransport and passive permeability were not affected. Indirect evidence of the action of ketanserin on sodium transmembrane fluxes came from other experiments. In the red blood cells taken from five normal subjects and incubated for 2 hours in a plasma pool, we evaluated the changes in intracellular sodium concentration induced by the presence of 5HT or ketanserin. A significant decrease in intracellular sodium concentration occurred only with ketanserin. This study indicates that ketanserin has a direct influence on transmembrane sodium fluxes. If this action were also present in other cells, it might contribute to the actions of the drug at vascular, nervous, and renal tubular levels.

Estimation of Na + dwell time in the gramicidin A channel. Na + ions as blockers of H + currents

The permeation of Na+ through gramicidin A channels shows a simple saturation with increasing Na+ concentration that can be described by two different models. The first model assumes that one Na+ binds to the channel with high affinity (approximately 30 M-1) and that conduction occurs by a 'knock-on' mechanism requiring double occupancy of the channel; the other model assumes that Na+ binding is of low affinity (less than 1 M-1), and that double occupancy of the channel is rare. NMR measurements have shown tight Na+ binding, favoring the first model, but measurements of flux ratios and water transport support the second model. We present here a relatively model-independent measurement of the dwell time of Na+ inside the channel, in which we characterize the fluctuations in H+ current through the channel induced by 'block' from the more slowly permeating Na+ ions. The mean Na+ dwell time inside the channel is estimated to be approximately 10 ns at a membrane potential of 200 mV. This result is inconsistent with tight Na+ binding, thus favoring the second model.

The Ionic Relations of Ulva lactuca

Summary The ionic relations of the marine green alga, Ulva lactuca (L.) Grev. have been studied in seawater ([K+]o, 10.1 mol m-3; [Na+]o, 468 mol m-3; [CI-]o, 547 mol m-3). The membrane potential (Δψi,o) was -39 ∓ 1.1mV in the light and -25∓1.9mV in the dark (∓95% confidence limits), based on micro-electrode measurements. Ulva lactuca has a very low intracellular Na+ concentration ([Na+]i) of 8 mmol kg-1 and [K+]i and [CI-]i of 321 and 239mmol kg-1 respectively. 42K+ and 86Rb+ efflux experiments showed that Rb+ was a poor K+ analogue. 42K+ and 22Na+ exhibited single intracellular exchange phases and had plasmalemma fluxes of 90∓27 and 2∓1 nmol m-2 s-1 respectively. 36C1- exhibited two tracellu-lar exchange phases, the plasmalemma and tonoplast fluxes were 346∓117 and 6.9∓1.8nmol m-2 s-1 respectively. The apparent cytoplasmic [Cl-] ([Cl-]c) was about 400mmol kg-1 but it is likely that the [Cl-]c is actually about 200 mmol kg-1 because ribosomes of plants are not functional at such a high CI- concentration. Na+ is actively extruded and K+ is actively taken up; CI- appears to be actively accumulated into the vacuole in the light and dark. CI- would be near equilibrium across the plasmalemma if [CI-]c was about 200 mmol kg-1.The permeabilities of Na+, K+, and CI- (PNa + Pk + and PCl-) are about 3 pm s-1, 0.5 nm s-1 and 1 nm s-1 respectively. There is little evidence for electrogenic ion pumps in Ulva plants in seawater because the Hodgkin-Katz diffusion potential (Di,o) is similar to the observed Δψi,o and the proton motive force is approximately zero at the pH of seawater (pHo = 8.1). The power consumption of ion transport in the dark is estimated to be about 10 % of the total available from respiration but is no more than a few percent of that available in the light.

Temperature dependence of amiloride-sensitive and -insensitive components of rat taste nerve response to NaCl

The temperature dependence of the rat taste response to NaCl was examined by measuring the chorda tympani activities in response to NaCl solution of varying temperatures before and after amiloride treatment. The response to NaCl was composed of 3 components: component I which was not suppressed by amiloride and components II and III which were suppressed by amiloride. Components I, II and III show maximal responses around 30, 30 and 10 °C, respectively. The response to NaCl at 10 °C was composed mostly of amiloride-sensitive component (component III). Magnitude of the amiloride-insensitive component (I) was greatly affected by variation of anion species. The amiloride-sensitive component II also showed anion influence but the influence was different from that for component I. Another amiloride-sensitive component (III) showed practically no anion influence. Threshold concentrations of the components were 0.1 mM for component I, 3 mM for component II and 0.3 mM for component III. Equilibrium potentials of Na + at the threshold concentrations across the apical membrane of taste cells were calculated to be −120.2 mV for component I, −31.4 mV for component II and −85.4 mV for components III. Thus the equilibrium potentials of Na + at the thresholds for components I and III were more negative than the resting potentials reported, which suggested that an increase in permeabilities of Na + through receptor sites (or channels) for these components will lead to hyperpolarization, not depolarization of taste cells.