Chloride-sensitive nature of the histamine-induced Ca2+ entry in cultured human aortic endothelial cells

1. Whole-cell currents and intracellular Ca2+ concentration ([Ca2+]i) were recorded in cultured human aortic endothelial cells (HAECs) to study the mechanisms underlying Cl--sensitive Ca2+ entry. 2. In the absence of histamine the membrane potential ranged between -90 and +5 mV and showed bimodal distribution with peaks at -17.8 and -67.5 mV. 3. Histamine (1-100 microM) activated an outward current, followed by a sustained inward current at -50 mV. The reversal potential (Vrev) was more negative than -60 mV for the initial outward current, and approximately -30 mV for the sustained inward current with normal Tyrode solution and internal solution containing 30 mM Cl-. 4. Vrev of the sustained inward current was hardly affected by varying the external concentrations of K+, Na+ and Ca2+, but was greatly changed by varying the external Cl- concentration ([Cl-]o). The relationship between Vrev and log[Cl-]o showed a slope of -44.8 mV per tenfold increase of [Cl-]o. 5. The Cl- channel blockers 9-anthracene carboxylic acid (1 mM), N-phenylanthranilic acid (0.1 mM) and niflumic acid (0.1 mM) all depressed the histamine-induced inward current. The non-selective cation channel blocker Gd3+ (10 microM) was without effect on the current. 6. In the absence of histamine, [Ca2+]i was not affected by varying the membrane potential. During the continuous presence of histamine, however, hyperpolarization increased and depolarization decreased [Ca2+]i, indicating that Ca2+ entry through the plasma membrane was activated by histamine. 7. Vrev of the histamine-induced Cl- current, measured by the gramicidin-perforated patch clamp method, was -28.4 +/- 6.6 mV (n = 8), which gave an intracellular Cl- concentration of approximately 34 mM. Under the current clamp condition, the membrane potential varied from cell to cell in the control, but application of histamine induced either depolarization or hyperpolarization, depending on the membrane potential before histamine application, and the membrane potential became stable near the equilibrium potential for Cl-. 8. We conclude that the histamine-induced inward current is carried mainly by Cl-. Although Ca2+ entry was also activated, we consider that its amplitude was too small to be resolved by the patch clamp method. The Cl- current may play a functional role in the sustained [Ca2+]i elevation by providing a constant driving force for Ca2+ entry in the presence of histamine.

The effect of gramicidin, a topical contraceptive and antimicrobial agent with anti-HIV activity, against herpes simplex viruses type 1 and 2 in vitro

The effect of an anti-HIV compound, gramicidin, previously used as a topical antibiotic and vaginal contraceptive, on the replication of herpes simplex viruses (HSV) type 1 and 2 has been examined. Human WI-38 fibroblasts were inoculated with either HSV type in the presence of serial dilutions of gramicidin and reduction in viral yield was measured by ELISA. The 50% inhibitory dose (IC50) of gramicidin against 3 HSV-1 and 4 HSV-2 isolates was equal to 0.3 microgram/ml and was comparable to the efficacy of the anti-HSV agent acyclovir (ACV). The IC50 of gramicidin required to protect WI-38 from cytolytic effect of HSV was 10 micrograms/ml at day 5 postinfection, indicating that at this time point the activity of gramicidin was inferior than that of ACV. Nevertheless, gramicidin suppressed the replication of ACV-resistant thymidine kinase and DNA polymerase HSV mutants at doses effective against ACV-sensitive strains. The results suggest that the antimicrobial and spermostatic agent, gramicidin, has potential against sexually transmitted diseases (STDs) and for prophylaxis of sex-borne HIV and HSV infections.

Effects of NaCl removal on osmolyte fluxes and regulatory volume decrease in cultured astrocytes

Cultured cerebellar astrocytes exhibit regulatory volume decrease (RVD) after anisosmotic swelling by extrusion of potassium (K), chloride (Cl), and organic osmolytes. The volume decay curve after swelling in 50% hyposmotic solutions has a first-order rate constant k min(-1) of 0.18 and 70% cell volume recovery is attained after 15 min. Replacing NaCl in the hyposmotic medium by sugars and polyalcohols markedly accelerated RVD increasing k to 0.37-0.39 min(-1) and full recovery in 3-5 min. NaCl removal increased (80%) K efflux (measured as 86Rb), delayed the inactivation of Cl efflux (measured as 125I) and enhanced taurine and D-aspartate release by 25%. These effects appear due to NaCl removal rather than to specific actions of the organic molecules. Replacing Na by other cations or Cl by gluconate increased k to 0.22 min(-1) and 0.26 min(-1), respectively, and 86Rb efflux by 4-23% and 39, respectively. It is concluded that the electrochemical gradient after NaCl removal increasing the K driving force, delaying the equilibrium of Cl and increasing amino acid efflux, accounts for the observed RVD acceleration. Other consequences of NaCl removal possibly modulating the K efflux pathway include changes in the surface charge screening, decreased ion strength, a rise in [Ca]i and a decrease in intracellular Na.

Spectroscopic studies of the interactions of the pyrethroid insecticide fenvalerate with gramicidin

Fenvalerate is a pyrethroid insecticide which interacts with ionic channels. Using circular dichroism technique we have studied the interaction of fenvalerate with gramicidin, a model channel peptide which transports ions. In most organic solvents, gramicidin exists as a double helix except in trifluoroethanol where it exists as a channel forming single stranded beta6.3 helical monomer. In model lipid membranes, under certain experimental conditions, gramicidin exists as a channel forming single stranded beta6.3 helical dimer. Our results show that fenvalerate interacts more with the single stranded beta6.3 helical monomer or dimer than with the double helical form of gramicidin. This was further confirmed by an increase in the rate of gramicidin mediated proton transport in liposomes by fenvalerate, using the pH sensitive fluorophore, pyranine.

Swelling-activated potassium currents of Ehrlich ascites tumour cells

The K+ and Cl- currents activated by Ca2+-ionophore treatment or by hypotonic cell swelling have been studied in Ehrlich ascites tumour cells by the patch-clamp technique. A charybdotoxin-inhibitable K+ current was activated by increasing intracellular Ca2+ concentration. In contrast, the K+ current activated by cell swelling was insensitive to charybdotoxin as well as to apamin, suggesting that channels different from those sensitive to Ca2+ are responsible for regulatory volume adjustments in these cells. The magnitude of the K+ and Cl- currents activated by hypotonic challenge was markedly temperature-dependent, possibly reflecting the temperature-dependence of enzymes involved in the intracellular signalling of cell volume regulation.

Characterization of a nucleoside/proton symporter in procyclic Trypanosoma brucei brucei

Adenosine transport at 22 degrees C in procyclic forms of Trypanosoma brucei brucei was investigated using an oil-inhibitor stop procedure for determining initial rates of adenosine uptake in suspended cells. Adenosine influx was mediated by a single high affinity transporter (Km 0.26 +/- 0.02 microM, Vmax 0.63 +/- 0.18 pmol/10(7) cells s-1). Purine nucleosides, with the exception of tubercidin (7-deazaadenosine), and dipyridamole inhibited adenosine influx (Ki 0.18-5.2 microM). Purine nucleobases and pyrimidine nucleosides and nucleobases had no effect on adenosine transport. This specificity of the transporter appears to be similar to the previously described P1 adenosine transporter in bloodstream forms of trypanosomes. Uptake of adenosine was Na+-independent, but ionophores reducing the membrane potential and/or the transmembrane proton gradient (monitored with the fluorescent probes bis-(1,3-diethylthiobarbituric acid)-trimethine oxonol and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein acetoxymethyl ester, respectively) inhibited adenosine transport. Similarly, an increase in extracellular pH from 7.3 to 8.0 reduced adenosine influx by 30%. A linear correlation was demonstrated between the rate of adenosine transport and the protonmotive force. Adenosine uptake was accompanied by a proton influx in base-loaded cells and was also shown to be electrogenic. These combined results indicate that transport of adenosine in T. brucei brucei procyclics is protonmotive force-driven and strongly suggest that the adenosine transporter functions as an H+ symporter.

Energy-transducing thylakoid membranes remain highly impermeable to ions during protein translocation

We investigated the operation of a posttranslational protein translocation pathway to determine whether ions are excluded from the translocase during protein transport. The membrane capacitance during protein translocation across chloroplast thylakoid membranes was monitored via electric-field-indicating carotenoid electrochromic bandshift measurements. Evidence is presented that shows that the membrane ion conductance is not increased during the complete cycle of binding, transport, and substrate release by the DeltapH-dependent translocase; i.e., the membrane remains ion-tight during protein translocation. We further demonstrate that a synthetic targeting peptide that directs proteins across this membrane does not gate translocation pores. We conclude that protein transport across the thylakoid membrane does not compromise its ability to maintain ion gradients and is, thus, unlikely to affect its functions in energy transduction.

Gramicidin-perforated patch revealed depolarizing effect of GABA in cultured frog melanotrophs

1. In frog pituitary melanotrophs, GABA induces a transient stimulation followed by prolonged inhibition of hormone secretion. This biphasic effect is inconsistent with the elevation of cytosolic calcium and the inhibition of electrical activity also provoked by GABA in single melanotrophs. In the present study, standard patch-clamp configurations and gramicidin-perforated patches were used to investigate the physiological GABAA receptor-mediated response and intracellular chloride concentration ([Cl-]i) in cultured frog melanotrophs. 2. In the gramicidin-perforated patch configuration, 1 microM GABA caused a depolarization associated with an action potential discharge and a slight fall of membrane resistance. In contrast, at a higher concentration (10 microM) GABA elicited a depolarization accompanied by a transient volley of action potentials, followed by a sustained inhibitory plateau and a marked fall of membrane resistance. Isoguvacine mimicked the GABA-evoked responses, indicating a mediation by GABAA receptors. 3. In gramicidin-perforated cells, the depolarizing excitatory effect of 1 microM GABA was converted into a depolarizing inhibitory action when 0.4 microM allopregnanolone was added to the bath solution. 4. After gaining the whole-cell configuration, the amplitude and/or direction of the GABA-evoked current (IGABA) rapidly changed before stabilizing. After stabilization, the reversal potential of IGABA followed the values predicted by the Nernst equation for chloride ions when [Cl-]i was varied. 5. In gramicidin-perforated cells, the steady-state I-V relationships of 10 microM GABA- or isoguvacine-evoked currents yielded reversal potentials of -37.5 +/- 1.6 (n = 17) and -38.6 +/- 2.0 mV (n = 8), respectively. These values were close to those obtained by using a voltage-ramp protocol in the presence of Na+, K+ and Ca2+ channel blockers. The current evoked by 1 microM GABA also reversed at these potentials. 6. We conclude that, in frog pituitary melanotrophs, chloride is the exclusive charge carrier of IGABA. In intact cells, the reversal potential of IGABA is positive to the resting potential because of a relatively high [Cl-]i (26.5 mM). Under these conditions, GABA induces a chloride efflux responsible for a depolarization triggering action potentials. However, GABA at a high concentration or in the presence of the potentiating steroid allopregnanolone exerts a concomitant shunting effect leading to a rapid inhibition of the spontaneous firing.

[Interaction of actinomycin D with suspended mycelium of streptomyces]

Binding of exogenous actinomycin D (AMD) by washed mycelium of streptomycetes i.e. variants of Streptomyces chrysomallus producing and not producing actinomycins and Streptomyces lividans not synthesizing the antibiotics was studied. Dependence of the bound quantity of AMD on its concentration, incubation time and temperature, energy source availability, influence of respiration inhibitors and the membranotropic antibiotic gramicidin S was shown. The intracellularly localized portion of the bound AMD likely penetrated to the cells by diffusion and was strongly bound presumably to DNA in the AMD sensitive S.lividans and to the specific intracellular actinomycin-binding proteins in the AMD resistant variants of S.chrysomallus. The ratio of AMD strongly bound by the mycelium and AMD easily washed with physiological solution and probably localized on the surface was determined. The ratio depended on sensitivity of the culture to AMD and for the variants of S.chrysomallus on the age of the culture and its ability to synthesize actinomycins.

Characterisation of Ca2+ membrane permeability of renal A6 cells upon different osmotic conditions

The nature of the calcium-transporting mechanisms involved in A6 cell calcium homeostasis under iso- and hypo-osmotic conditions was investigated using fura-2 (AM) as a cell calcium indicator. Under steady-state conditions, intracellular calcium (Ca2+i) was increased by Bay K8644 or by gramicidin, an ionophore which depolarises A6 cell membranes. The Ca2+i increase following calcium addition (to calcium-depleted cells) or membrane depolarisation was blocked by nifedipine but not by verapamil or omega-conotoxin, indicating that the membrane calcium permeability may be mediated by voltage-dependent and dihydropyridine-sensitive calcium channels. Ca2+i could also be increased by a hypo-osmotic shock having a linear relationship with the osmolarity change. This osmotically induced Ca2+i increase had an extracellular origin since it was absent when cells were suspended in a calcium-free medium and it was not affected by thapsigargin or TMB-8 application. In addition, it was inhibited by the calcium channel inhibitor, nifedipine. Furthermore, under hypo-osmotic conditions, an additional Ca2+i increase, sensitive to nifedipine, was measured when cells were depolarised by gramicidin or K-gluconate addition. It is proposed that the hypo-osmotically induced cell calcium increase implies the activation of voltage-dependent and nifedipine-sensitive calcium channels, presenting the same pharmacological characteristics as those involved in cell calcium homeostasis under iso-osmotic conditions. The initial Ca2+i increase was transient and stabilised to a value nevertheless higher than the iso-osmotic level; this secondary and incomplete regulatory phase did not occur in the presence of thapsigargin or TMB-8, thus providing evidence of intracellular calcium storage.

Toxicity of sea nettle toxin to human hepatocytes and the protective effects of phosphorylating and alkylating agents

The sea nettle jellyfish toxin (SNTX), which contains several polypeptides, was highly toxic to human hepatocytes. The Cytosensor microphysiometer was used continuously to monitor cell media acidification rate as an index of cellular metabolic activity. Cells exposed to > 1 microg SNTX protein/ml media exhibited a transient increase in metabolic activity, followed by a sharp decrease and cell death within minutes. The kinetics of the transient increase and subsequent decline increased with higher concentrations of SNTX. The biphasic and time-dependent response of hepatocytes to SNTX suggests that more than one mechanism may be involved in the toxicity of its different polypeptides. SNTX-induced cytotoxicity of hepatocytes was reduced by the presence of high titer antibodies against a heterologous jellyfish. Phenobarbital-induced cells became more vulnerable to SNTX, suggesting that some toxin component(s) require(s) bioactivation. Short-term exposure (1-2 h) to 10 microg/ml of the calcium ionophore calcimycin, or the non-selective monovalent cation ionophore gramicidin, had no effect on metabolic activity. However, 165 microg/ml gramicidin or 53 microg/ml calcimycin produced slight transient activation followed by steady decline in metabolic activity, while 20 h exposure to either ionophore produced total cell death. Exposure to even a 10-fold lower concentration of either ionophore killed 88% and 75%, respectively. This contrasts with the toxicity of SNTX which is detectable in minutes with as little as 3 microg/ml. Since pre-exposure to the organophosphate anticholinesterases VX and paraoxon, or the chemotherapeutic alkylating agents cyclophosphamide and mechlorethamine reduced the cytotoxic effects of SNTX, it suggests that phosphorylation or alkylation of cell protein(s) interferes with SNTX toxicity.

cis-FFA do not alter membrane depolarization but block Ca2+ influx and GH secretion in KCl-stimulated somatotroph cells. Suggestion for a direct cis-FFA perturbation of the Ca2+ channel opening

It has been reported that cis-unsaturated free fatty acids (cis-FFA) block intracellular Ca2+ rise in EGFR T17 and GH3 cells by perturbing the generation of Ins(1,4,5)P3. In the present work, it was found that cis-FFA did not alter potassium-induced cell depolarization in GH3 cells, while blocking Ca2+ rise and GH secretion. Interestingly enough, saturated or trans-unsaturated FFA exert the opposite actions, i.e., they block cell depolarization without altering Ca2+ rise and hormone secretion. As depolarization activates GH3 cells via direct opening of Ca2+ channels with no generation of intracellular mediators, these results suggest that cis-FFA act by a direct perturbation of the Ca2+ channel opening.

Adenosine stimulates Cl- channels of nonpigmented ciliary epithelial cells

Ciliary epithelial cells possess multiple purinergic receptors, and occupancy of A1 and A2 adenosine receptors is associated with opposing effects on intraocular pressure. Aqueous adenosine produced increases in short-circuit current across rabbit ciliary epithelium, blocked by removing Cl- and enhanced by aqueous Ba2+. Adenosine's actions were further studied with nonpigmented ciliary epithelial (NPE) cells from continuous human HCE and ODM lines and freshly dissected bovine cells. With gramicidin present, adenosine (> or = 3 microM) triggered isosmotic shrinkage of the human NPE cells, which was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB) and niflumic acid. At 10 microM, the nonmetabolizable analog 2-chloroadenosine and AMP also produced shrinkage, but not inosine, UTP, or ATP. 2-Chloroadenosine (> or = 1 microM) triggered increases of whole cell currents in HCE cells, which were partially reversible, Cl- dependent, and reversibly inhibited by NPPB. Adenosine (> or = 10 microM) also stimulated whole cell currents in bovine NPE cells. We conclude that occupancy of adenosine receptors stimulates Cl- secretion in mammalian NPE cells.

Heterogeneity of astrocyte resting membrane potentials and intercellular coupling revealed by whole-cell and gramicidin-perforated patch recordings from cultured neocortical and hippocampal slice astrocytes

Astrocytes are thought to regulate the extracellular potassium concentration by mechanisms involving both voltage-dependent and transport-mediated ion fluxes combined with intercellular communication via gap junctions. Mechanisms regulating resting membrane potential (RMP) play a fundamental role in determining glial contribution to buffering of extracellular potassium and uptake of potentially toxic neurotransmitters. We have investigated the passive electrophysiological properties of cultured neocortical astrocytes and astrocytes recorded in hippocampal slices from 18-25 d postnatal rats. These experiments revealed a wide range of astrocyte RMPs that were independent of developmental factors, length of culturing, cellular morphology, the electrophysiological techniques used (whole-cell vs perforated recording), cell-specific expression of Na+/2HCO3- co-transporters, or voltage-dependent Na+ channels. Exposure of cultured astrocytes to differentiation-inducing factors (such as cAMP) or inhibition of proliferation (by serum deprivation) did not significantly influence RMP. Expression of ATP-sensitive potassium channels was absent in these glia; thus, K(ATP)-related mechanisms did not contribute to cell resting potential. In both cultured and slice astrocytes, spontaneous electrophysiological changes were commonly observed. These reversible events, which resulted in differential sensitivity to potassium channel blockers (cesium and barium) and sudden current-voltage profile changes, were attributable to dynamic changes in cell-to-cell coupling, as confirmed by recordings from isolated pairs of cells. We conclude that the heterogeneity of astrocytic RMP and intercellular coupling both in culture and in situ are intrinsic properties of glia that may contribute to transcellular transport of potassium. We propose a model in which spatial buffering may be facilitated by heterogeneous mechanisms controlling glial RMP in combination with dynamic changes in intercellular coupling.

Characterization of gramicidin S synthetase aggregation substance: control of gramicidin S synthesis by its product, gramicidin S

An aggregation substance of gramicidin S synthetases was found and purified by DEAE-cellulose chromatography and CM-chromatography from cell debris of Bacillus brevis Nagano. It specifically aggregated and inactivated gramicidin S synthetases 1 (GS1) and 2 (GS2). On the basis of amino acid composition analysis, reversed-phase HPLC, FAB mass spectrometry, amino acid sequence analysis, and antibacterial activity, this substance (GrS-aggregation substance) was identified as gramicidin S. A gramicidin S derivative bearing a lysine residue in place of one ornithine residue was also detected as a minor component of GrS-aggregation substance. The extent of the aggregation was dependent on the concentration and relative amount of gramicidin S. The inhibition of the enzyme activities was irreversible and the inhibition was proportional to the amount of gramicidin S, like the aggregation of the enzymes. The degree of GS2 inhibition in the amino acid-dependent ATP-PPi exchange reaction varied with the amino acids of gramicidin S and increased in order of the amino acid sequence of gramicidin S. The degree of inhibition of the overall synthesis of gramicidin S was the same as that in the leucine-dependent exchange reaction.

Assessment of the effects of gramicidin, formaldehyde, and surfactants on Escherichia coli by flow cytometry using nucleic acid and membrane potential dyes

Two membrane potential sensitive dyes (Rhodamine 123 and bis-oxonol) and three nucleic acid dyes (propidium iodide, SYTO-13, and SYTO-17) were used to assess the effect of surfactants on Escherichia coli. The ability of E. coli to be stained by these probes was validated at different physiological states. Propidium iodide was used to assess the integrity of cell envelopes. Two double staining methods based on propidium iodide with SYTO-13 and bis-oxonol with SYTO-17 were used to improve the discrimination between bacteria and micelles or aggregated particles generated by the presence of surfactants. A rapid (1 h contact time between cells and surfactants, and less than 5 min for staining and obtaining data) Rhodamine 123 flow cytometric assay was developed to assess the bactericidal effect of surfactants.

Competitive, non-competitive and cooperative interactions between substrates of P-glycoprotein as measured by its ATPase activity

We have studied the interaction between verapamil and other modulators of the P-glycoprotein ATPase from membranes of CR1R12 Chinese hamster ovary cells. Four major categories of interaction were identified. (i) Non-competitive inhibition of verapamil's stimulation of enzyme activity was found with vanadate. (ii) Competitive inhibition of the ATPase was found for the pair verapamil and cyclosporin A. (iii) Allosteric inhibition with an increase in the Hill number for verapamil was found in the cases of daunorubicin, epirubicin, gramicidin S and D, vinblastine, amiodarone, and colchicine. (iv) Cooperative stimulation of verapamil-induced ATPase activity was found with progesterone, diltiazem, amitriptyline, and propranolol. At high levels, progesterone and verapamil mutually enhanced each other's inhibitory action on the ATPase. Our data show that the substrate binding behavior of P-glycoprotein is complex with more than one binding site being present. This information could form the basis for the development of improved modulators of P-glycoprotein.

Water transport by the Na+/glucose cotransporter under isotonic conditions

Solute cotransport in the Na+/glucose cotransporter is directly coupled to significant water fluxes. The water fluxes are energized by the downhill fluxes of the other substrates by a mechanism within the protein itself. In the present paper we investigate the Na+/glucose cotransporter expressed in Xenopus oocytes. We present a method which allows short-term exposures to sugar under voltage clamp conditions. We demonstrate that water is cotransported with the solutes despite no osmotic differences between the external and intracellular solutions. There is a fixed ratio of 195:1 between the number of water molecules and the number of Na+ ions transported, equivalent to 390 water molecules per glucose molecule. Unstirred layer effects are ruled out on the basis of experiments on native oocytes incubated with the ionophores gramicidin D or nystatin.

Cellular respiration during hypoxia. Role of cytochrome oxidase as the oxygen sensor in hepatocytes

We previously reported that hepatocytes exhibit a reversible suppression of respiration during prolonged hypoxia (PO2 = 20 torr for 3-5 h). Also, isolated bovine heart cytochrome c oxidase undergoes a reversible decrease in apparent Vmax when incubated under similar conditions. This study sought to link the hypoxia-induced changes in cytochrome oxidase to the inhibition of respiration seen in intact cells. Hepatocytes incubated at PO2 = 20 torr exhibited decreases in respiration and increases in [NAD(P)H] after 2-3 h that were reversed upon reoxygenation (PO2 = 100 torr). Respiration during hypoxia was also inhibited when N,N,N',N'-tetramethyl-p-phenylenediamine (0.5 mM) and ascorbate (5 mM) were used to reduce cytochrome c, suggesting that cytochrome oxidase was partially inhibited. Similarly, liver submitochondrial particles revealed a 44% decrease in the apparent Vmax of cytochrome oxidase after hypoxic incubation. In hepatocytes loaded with tetramethylrhodamine ethyl ester (10 nM) to quantify mitochondrial membrane potential, acute hypoxia (<30 min) produced no change in fluorescence, consistent with the absence of an acute change in respiration. However, fluorescence increased during acute reoxygenation after prolonged hypoxia, suggesting an increase in potential. The control exhibited by NADH over mitochondrial respiration was not altered during hypoxia. Thus, changes in the Vmax of cytochrome oxidase during prolonged hypoxia correlate with the changes in respiration and mitochondrial potential. This suggests that the oxidase functions as an oxygen sensor in the intact hepatocyte.

Measurement of intracellular Na+ concentration by a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate, in porcine adrenal chromaffin cells--usage of palytoxin as a Na+ ionophore

Palytoxin was found to equilibrate sodium ions (Na+) across the cell membrane much faster than dose gramicidin, which has been frequently used to calibrate the intracellular Na+ concentration ([Na+]in), in cells treated with a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate (SBFI). Palytoxin was capable of equilibrating Na+ in cells treated with SBFI-acetoxymethyl ester (SBFI-AM) and in voltage-clamped cells loaded with SBFI through a patch pipette. Nicotine caused a dose-dependent increase in ([Na+]in) in porcine adrenal chromaffin cells treated with SBFI-AM and caused a simultaneous increase in [Na+]in and inward current in the voltage-clamped cells loaded with SBFI. Palytoxin has an advantage of calibrating ([Na+]in) in a shorter time than dose gramicidin because of its powerful ionophoretic activity.

Hyperpolarization of isolated capillaries from guinea-pig heart induced by K+ channel openers and glucose deprivation

1. The present study was designed to test if microvascular coronary endothelial cells express ATP-sensitive K+ channels (KATP channels). We performed microfluorometric measurements of the membrane potential of freshly isolated guinea-pig coronary capillaries equilibrated with the voltage-sensitive dye bis-oxonol (bis-[1,3-dibutylbarbituric acid] trimethineoxonol, [DiBAC4(3)]). 2. The resting membrane potential of capillaries in physiological salt solution was -46 +/- 4.2 mV (n = 8) at room temperature (22 degrees C) as determined after calibration of the fluorescence using the Na(+)-K+ ionophore gramicidin in the presence of different K+ concentrations. Spontaneous membrane potential fluctuations of 10-20 mV amplitude were often observed. 3. A reversible, sustained hyperpolarization to a new membrane potential close to the K+ equilibrium potential (EK) could be induced by application of the K+ channel openers HOE 234 (100 nM to 1 microM), diazoxide (10 PM to 100 nM) or pinacidil (100 nM). Subsequent addition of glibenclamide (200 nM to 2 microM) reversed this hyperpolarization. 4. A glibenclamide-sensitive hyperpolarization of coronary capillaries to values near EK was also observed upon omission of D-glucose (10 mM) from the superfusing solution or by substituting L-glucose for D-glucose. Maximum hyperpolarization was reached in less than 10 min. 5. Our results suggest that microvascular coronary endothelial cells express KATP channels which may be activated during hypoglycaemia.

A flow cytometric study of antibiotic-induced damage and evaluation as a rapid antibiotic susceptibility test for methicillin-resistant Staphylococcus aureus

Flow cytometry using the anionic membrane potential-sensitive fluorescent probe, bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)), enabled assessment of antibiotic-induced membrane perturbation in five clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and two antibiotic-sensitive reference strains, NCTC 6571 and 8325-4, after establishment of steady-state growth in liquid cultures inoculated from single colonies. Flow cytometric indications of the enhanced DiBAC4(3) uptake after treatment with vancomycin at 0.1, 1, 4 and 10 x MIC showed excellent comparison with viability losses quantified as cfu on solid agar in MRSA isolate QC. The antibiotic susceptibility patterns to benzylpenicillin, methicillin and vancomycin for all isolates used in this study could be determined in 2-4 h from an overnight plate culture. This technique thus provides a rapid and reproducible antibiotic sensitivity test which may be applicable in routine clinical practice.

Volume regulation in NIH/3T3 cells not expressing P-glycoprotein. I. Regulatory volume decrease

Exposure of NIH/3T3 fibroblasts not expressing P-glycoprotein to 50, 30, 20, and 10% hyposmotic solutions led to cell volume increases of 70, 32, 21, and 12%, respectively. After swelling, NIH/3T3 cells exhibited regulatory volume decrease (RVD), attaining complete volume recovery after 30 min except in 50% hyposmotic solution, in which volume recovery was 76%. RVD was accelerated by gramicidin and inhibited by the Cl channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoic acid, 1,9-dideoxyforskolin, dipyridamole, and niflumic acid and by the K channel, blocker quinidine. RVD was reduced 15% by removal of extracellular Ca. The pathway opened by hypotonicity was highly permeable to K and Rb and only partly permeable to other cations. Most anions were able to permeate, with a permeability ranking of nitrate > benzoate = iodide > thiocyanate > chloride > > gluconate. The pathway was permeable to neutral amino acids, with a permeability ranking of glycine > alanine > glutamate > taurine > gamma-aminobutyric acid > glutamine. The pathway was not permeable to basic amino acids. These results show that, despite the absence of P-glycoprotein, NIH/3T3 cells exhibit RVD with properties similar to those expressed in most cell types.

Cell volume regulation by the mouse zygote: mechanism of recovery from a volume increase

Mouse zygotes regulate their volumes after cell swelling. This regulatory volume decrease (RVD) is rapid and complete. RVD in zygotes was inhibited by K+ or Cl- channel blockers, indicating the participation of such channels in volume recovery. The channels are separate entities, as indicated by the ability of the cation ionophore gramicidin to restore RVD when K+ channels are blocked but not when Cl- channels are blocked. Intracellular Ca2+ concentration increased with cell swelling. Nevertheless, RVD occurred normally in zygotes loaded with the Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, which prevented Ca2+ from increasing above its normal resting concentration. Thus an increase in intracellular Ca2+ is not necessary for zygote RVD; consistent with this, inhibitors of Ca(2+)-activated K+ channels had little or no effect on RVD. RVD in zygotes was also completely inhibited by millimolar amounts of extracellular ATP. ATP has been shown to inhibit current passed by the volume-sensitive organic osmolyte-Cl- channel in other cells, and thus zygotes may have such a channel participating in RVD.

Mitochondrial membrane potential measurement in rat cerebellar neurons by flow cytometry

Mitochondrial membrane potential (MMP) in dissociated rat cerebellar neurons was measured using rhodamine 123 (Rh 123) as fluorescent dye, and flow cytometry. Dye distribution was studied by confocal scanning microscopy. Propidium iodide (PI)-marked cells (dead cells) were not stained by Rh 123, while the green fluorescence of living cells was restricted to mitochondria. Incubation of cells with different ionophores resulted in a maximal inhibition of Rh 123 fluorescence of 27.0 +/- 5.9% (valinomycin), 55.6 +/- 7.2% (ionomycin), and 37.3 +/- 5.1% (gramicidin). Ionophores decreased cell viability at high concentrations, measured as the number of propidium iodide-marked cells. Exposure of cell suspensions to the mitochondrial specific uncoupling agent CCCP caused a decrease in Rh 123 fluorescence (40 +/- 6.1%). Conversely, oxidative stress induced by H2O2 did not affect Rh 123 fluorescence. Impairment of glucose bioavailability reduced Rh 123 fluorescence. 2-Deoxy-D-glucose decreased the MMP with a maximal inhibition of 24.0 +/- 4.4%. Lack of glucose in the incubation medium also resulted in a decrease in MMP. Moreover, application of L-glutamate and N-methyl-D-aspartate (NMDA) (the excitatory amino acids) decreased Rh 123 uptake in a dose-dependent manner, which suggests that the measurement of MMP in dissociated cerebellar neurons by flow cytometry is a suitable method to detect the activity of drugs acting on glutamate receptors.

Measurement of changes in cell volume based on fluorescence quenching

The intracellular fluorescence of 6-methoxy-N-(3-sulfopropyl)-quinolinium (SPQ), a Cl(-) -sensitive fluorescent dye, is quenched by intracellular organic anions and proteins of unknown identity. The concentration of these intracellular quenchers (ICQs), however, is dependent on cell volume. In the absence of Cl-, changes in the observed SPQ fluorescence may therefore reflect changes in cell volume. This concept has been applied to determine relative changes in cell volume of cultured corneal endothelium in response to anisosmotic shocks, using NO3- as the Cl- substituent. SPQ fluorescence increased with decreasing osmolarity and vice versa. A 20 mosM hypertonic shock was needed to detect a change in SPQ fluorescence with a signal-to-noise ratio of >25. Assuming dynamic quenching by ICQs, we applied an extension of the Stern-Volmer equation to develop a simple relationship between the measured SPQ fluorescence and relative changes in cell volume. For large hyposmotic shocks, regulatory volume decrease (RVD) was observed. The rate of RVD could be enhanced by exposure to 0.5 microM gramicidin in low-Na+ Ringer solution (i.e., K+-NO3- efflux), indicating that K+ conductance is rate limiting for RVD. These results demonstrate the principle of using fluorescence quenching to measure changes in cell volume in real time. Because SPQ is sensitive to Cl-, its usefulness as a quenching probe is limited. However, a structure-activity study can be expected to yield useful Cl(-)-insensitive analogs.

A calcium-dependent chloride current in insulin-secreting beta TC-3 cells

Ca(2+)-dependent conductances have been hypothesized to play a role in the bursting pattern of electrical activity of insulin-secreting beta cells in response to high plasma glucose. A Maxi K+ channel has received the most attention, while a low-conductance Ca(2+)-activated K+ current has also been identified. We used an increasingly popular beta cell model system, the beta TC-3 cell line, and the perforated-patch technique to describe the properties of a novel Ca(2+)-dependent Cl- current [ICl(Ca)] in insulin-secreting pancreatic beta cells. The reported ICl(Ca) could be activated under physiological Ca2+ concentrations and is the first of its kind to be described in pancreatic insulin-secreting cells. We found that long depolarizing steps above -20 mV elicited an outward current which showed slow inward relaxation upon repolarization to negative membrane potentials. Both the outward currents and the inward tails showed dependence on Ca2+ influx: their current/voltage (I/V) relations followed that of the "L-like" Ca2+ current (ICa) present in these cells; they were blocked completely by the removal of external Ca2+ or application of Cd2+ at concentrations sufficient for complete block of ICa; and their magnitude increased with the depolarizing step duration. Moreover, the inward tail decayed monoexponentially with a time constant which at voltages negative to activation of ICa showed a weak linear voltage dependence, while at voltages positive to activation of ICa it followed the voltage dependence of ICa. This Ca(2+)-dependent current reversed at -21.5 mV and when the external Cl- concentration was reduced from 159 mM to 62 mM the reversal potential shifted by approximately +20 mV as predicted by the Nernst relation for a Cl(-)-selective current. Cl- channel blockers such as DIDS (100 microM) and niflumic acid (100 microM) blocked this current. We concluded that this current was a Ca(2+)-dependent Cl- current [ICl(Ca)]. From substitution of the external Cl- with various monovalent anions and from the reversal potentials we obtained the following permeability sequence for ICl(Ca): I- > NO3- > Br- > Cl- > Acetate.

Ca(2+)-dependent changes of acetylcholine release and IP3 mass in Torpedo cholinergic synaptosomes

The aim of the present study was to investigate possible changes of inositol 1,4,5-trisphosphate (IP3) mass in Torpedo cholinergic synaptosomes in conditions promoting stimulated acetylcholine (ACh) release. For this purpose, we used a radioreceptor IP3 mass assay and a chemiluminescent method for ACh detection. Torpedo cholinergic synaptosomes have consistent IP3 mass levels under resting conditions. The IP3 mass was neither modified by changes in external Ca2+ nor by a Ca(2+)-free medium containing EGTA. IP3 mass and ACh release, measured in the same conditions and in parallel, were increased by depolarization with high K+ and by the ionophores A-23187 and gramicidin-D in a manner dependent on external Ca2+ emphasizing that Ca2+ entry, independently of the influx mechanism involved, leads to an IP3 increase. The phospholipase C beta inhibitors U-73122 and U-73343 reduced K(+)-stimulated IP3 levels while K(+)-evoked ACh release was almost completely blocked suggesting an additional effect of these drugs on depolarization-neurotransmitter secretion coupling. The effect reported showing an increase of IP3 by agents that stimulate ACh release may suggest a possible link between IP3 metabolism and the neurotransmitter release mechanism. However, such a link is probably not a direct one as implied by the results obtained with the inhibitors of phospholipase C.

Electrodiffusion, barrier, and gating analysis of DIDS-insensitive chloride conductance in human red blood cells treated with valinomycin or gramicidin

Current-voltage curves for DIDS-insensitive Cl- conductance have been determined in human red blood cells from five donors. Currents were estimated from the rate of cell shrinkage using flow cytometry and differential laser light scattering. Membrane potentials were estimated from the extracellular pH of unbuffered suspensions using the proton ionophore FCCP. The width of the Gaussian distribution of cell volumes remained invariant during cell shrinkage, indicating a homogeneous C1- conductance among the cells. After pretreatment for 30 min with DIDS, net effluxes of K+ and Cl- were induced by valinomycin and were measured in the continued presence of DIDS; inhibition was maximal at approximately 65% above 1 microM DIDS at both 25 degrees C and 37 degrees C. The nonlinear current-voltage curves for DIDS-insensitive net Cl- effluxes, induced by valinomycin or gramicidin at varied [K+] o, were compared with predictions based on (1) the theory of electrodiffusion, (2) a single barrier model, (3) single occupancy, multiple barrier models, and (4) a voltage-gated mechanism. Electrodiffusion precisely describes the relationship between the measured transmembrane voltage and [K+]o. Under our experimental conditions (pH 7.5, 23 degrees C, 1-3 microM valinomycin or 60 ng/ml gramicidin, 1.2% hematocrit), the constant field permeability ratio PK/PCl is 74 +/- 9 with 10 microM DIDS, corresponding to 73% inhibition of PCl. Fitting the constant field current-voltage equation to the measured Cl- currents yields PCl = 0.13 h-1 with DIDS, compared to 0.49 h-1 without DIDS, in good agreement with most previous studies. The inward rectifying DIDS-insensitive Cl- current, however, is inconsistent with electrodiffusion and with certain single-occupancy multiple barrier models. The data are well described either by a single barrier located near the center of the transmembrane electric field, or, alternatively, by a voltage-gated channel mechanism according to which the maximal conductance is 0.055 +/- 0.005 S/g Hb, half the channels are open at -27 +/- 2 mV, and the equivalent gating charge is -1.2 +/- 0.3.

Tryptophan-N-formylated gramicidin causes growth inhibition of Plasmodium falciparum by inducing potassium efflux from infected erythrocytes

In a study of the supposed selective action of tryptophan-N-formylated gramicidin (NFG) on infected erythrocytes as well as the relationship between the ability of NFG to inhibit parasite growth and its capacity to induce potassium leakage from infected cells, a series of experiments was performed in which in vitro cultures of Plasmodium falciparum were incubated with NFG or gramicidin. Those cultures were subsequently assayed for intracellular sodium and potassium contents, cell lysis, and/or parasite viability. It is shown and discussed that although NFG can attack both infected and uninfected erythrocytes, resulting in potassium efflux from and sodium influx into these cells, the effects are much greater on infected erythrocytes than on uninfected ones. Furthermore, the results strongly suggest that NFG-mediated potassium efflux is the direct cause of parasite death.

Development of a tonic form of synaptic inhibition in rat cerebellar granule cells resulting from persistent activation of GABAA receptors

1. To investigate the origin and functional significance of a recently described tonic GABAA receptor-mediated conductance in cerebellar granule cells we have made recordings from cells in cerebellar slices from rats of different ages (postnatal days P4 to P28). 2. During development there was a dramatic change in the properties of GABA-mediated synaptic transmission. The contribution to GABAA receptor-mediated charge transfer from the tonic conductance (GGABA), relative to that resulting from discrete spontaneous postsynaptic currents (sPSCs), was increased from 5% at P7 to 99% at P21. GGABA was reduced by bicuculline, tetrodotoxin and by lowering extracellular Ca2+, and was initially present only in those cells which exhibited sPSCs. 3. At P7 sPSCs were depolarizing, occasionally triggering a single action potential. By P18 the GABA reversal potential was shifted close to the resting potential and GGABA produced a shunting inhibition. Removal of GGABA by bicuculline increased granule cell excitability in response to current injection. 4. This novel tonic inhibition is present despite the low number of Golgi cell synapses on individual granule cells and appears to result from 'overspill' of synaptically released GABA leading to activation of synaptic and extrasynaptic GABAA receptors.

Synergistic effect of gramicidin and EDTA in inhibiting sperm motility and cervical mucus penetration in vitro

Gramicidin, a linear polypeptide with antiviral and antimicrobial properties, was compared in vitro with a commonly used spermicidal detergent-nonoxynol-9 (N9). The inhibition of sperm functions was evaluated by computer-assisted semen analysis (CASA) for sperm motility, in cervical mucus penetration assay, and by colorimetric tetrazolium salt and lactate dehydrogenase release assays routinely employed for testing the toxicity of drugs. The effective 100% inhibitory concentration (IC100) of gramicidin in a 2-min sperm immobilization assay by CASA was equal to 4 micrograms/ml, whereas IC100 of N9 was equal to 200 micrograms/ml. The presence of 0.1% of chelating agent, EDTA, reduced IC100 of gramicidin to 10 ng/ml, while less than a twofold enhancement in N9 activity was observed upon combination with EDTA. Likewise, the gramicidin/EDTA combination was 100,000 times more potent than N9/EDTA in the sperm penetration assay. Quantitative toxicity tests confirmed that gramicidin is a potent spermostatic rather than spermicidal agent. Further development of a gramicidin/EDTA formulation is warranted as a nontoxic topical contraceptive with activity against viral and microbial sexually transmitted diseases (STDs).

Na+-dependent release of Mg2+ from an intracellular pool in rat sublingual mucous acini

Muscarinic stimulation induces release of Mg2+ from an intracellular pool in rat sublingual mucous acini (Zhang, G. H., and Melvin, J. E. (1992) J. Biol. Chem. 267, 20721-20727). In the present study we examined the interdependence of Mg2+ mobilization on intracellular Na+ and Ca2+ by monitoring the intracellular free concentrations of Na+ ([Na+]i), Mg2+ ([Mg2+]i), and Ca2+ ([Ca2+]i) using ion-sensitive fluorescent indicators. Gramicidin increased the intracellular concentrations of all three ions. Comparable to agonist-stimulated mobilization of Mg2+, the gramicidin-induced [Mg2+]i increase was independent of extracellular Mg2+ indicating release of Mg2+ from an intracellular pool. Clamping the [Ca2+]i near 30 nM with the Ca2+-selective chelator BAPTA failed to alter the [Na+]i or [Mg2+]i increases generated by gramicidin. In contrast, depletion of intracellular Na+ markedly suppressed the muscarinic-stimulated [Mg2+]i increase, whereas the [Ca2+]i increase was similar to that seen in physiological extracellular Na+. These results revealed that intracellular Mg2+ mobilization did not directly relate to the [Ca2+]i, but required an increase in [Na+]i. Consistent with this hypothesis, increasing [Na+]i by activating Na+ influx via the Na+/H+ exchanger also increased the [Mg2+]i. The Na+/Mg2+ exchange inhibitor quinidine suppressed both the gramicidin- and muscarinic-induced discharge of internal Mg2+. These results suggest that release of Mg2+ from an intracellular pool is mediated by a Na+-dependent Mg2+ transport mechanism in salivary acinar cells.

Excitatory GABA responses in embryonic and neonatal cortical slices demonstrated by gramicidin perforated-patch recordings and calcium imaging

Gramicidin perforated-patch-clamp recordings in brain slices were used to obtain an accurate assessment of the developmental change in the GABAA receptor reversal potential (EGABAA) in embryonic and early postnatal rat neocortical cells including neuroepithelial precursor cells, cortical plate neurons, and postnatal neocortical neurons. Our results demonstrate that there is a progressive negative shift in EGABAA with the most positive values found in the youngest cortical precursor cells. At the early stages of neocortical development, EGABAA is determined by the chloride (Cl-) gradient, and the internal chloride concentration ([Cl-]i) decreases with development. EGABAA is positive to the resting potential, indicating that GABA serves to depolarize developing neocortical cells. Consistent with this conclusion, GABAA receptor activation with muscimol was found-to increase the internal calcium concentration ([Ca2+]i) in both embryonic and early postnatal neocortical cells through the activation of voltage-gated calcium channels (VGCCs). Postnatal cells exhibit spontaneous postsynaptic synaptic currents, which are eliminated by bicuculline methiodide (BMI) but not glutamate receptor antagonists and reverse at the Cl- equilibrium potential. Likewise, brief spontaneous increases in [Ca2+]i, sensitive to BMI and TTX, are observed at the same ages, suggesting that endogenous synaptic GABAA receptor activation can depolarize cells and activate VGCCs. These results suggest that GABAA receptor-mediated depolarization may influence early neocortical developmental events, including neurogenesis and synaptogenesis, through the activation of Ca(2+)-dependent signal transduction pathways.

Modulation of structure and antibacterial and hemolytic activity by ring size in cyclic gramicidin S analogs

We have evaluated the effect of ring size of gramicidin S analogs on secondary structure, lipid binding, lipid disruption, antibacterial and hemolytic activity. Cyclic analogs with ring sizes ranging from 4 to 14 residues were designed to maintain the amphipathic character as found in gramicidin S and synthesized by solid phase peptide synthesis. The secondary structure of these peptides showed a definite periodicity in beta-sheet content, with rings containing 6, 10, and 14 residues exhibiting beta-sheet structure, and rings containing 8 or 12 residues being largely disordered. Peptides containing 4 or 6 residues did not bind lipopolysaccharide, whereas longer peptides showed a trend of increasing binding affinity for lipopolysaccharide with increasing length. Destabilization of Escherichia coli outer membranes was only observed in peptides containing 10 or more residues. Peptides containing fewer than 10 residues were completely inactive and exhibited no hemolytic activity. The 10-residue peptide showed an activity profile similar to that of gramicidin S itself, with activity against Gram-positive and Gram-negative microorganisms as well as yeast, but also showed high hemolytic activity. Differential activities were obtained by increasing the size of the ring to either 12 or 14 residues. The 14-residue peptide showed no antibiotic activity but exhibited increased hemolytic activity. The 12-residue peptide lost activity against Gram-positive bacteria, retained activity against Gram-negative microorganisms and yeast, but displayed decreased hemolytic activity. Biological activities in the 12-residue peptide were optimized by a series of substitutions in residues comprising both hydrophobic and basic sites resulting in a peptide that exhibited activities comparable with gramicidin S against Gram-negative microorganisms and yeast but with substantially lower hemolytic activity. Compared with gramicidin S, the best analog showed a 10-fold improvement in antibiotic specificity for Gram-negative microorganisms and a 7-fold improvement in specificity for yeast over human erythrocytes as determined by a therapeutic index. These results indicate that it is possible to modulate structure and activities of cyclic gramicidin S analogs by varying ring sizes and further show the potential for developing clinically useful antibiotics based on gramicidin S.

Gramicidin channels--a solvable membrane "protein" folding problem

The linear gramicidins are peptide antibiotics that form cation-selective channels in lipid bilayers. Gramicidin channels have very well-defined functional characteristics, and the structure of membrane-spanning gramicidin A channels is known at atomic resolution. These features make the gramicidins well suited to study how the amino acid sequence encodes the structure and function of a membrane-spanning channel. We show how one can use electrophysiological measurements to obtain structural information about conducting channels and to quantify the conformational preferences of sequence-substituted gramicidin mutants.

Effects of gramicidin and tryptophan-N-formylated gramicidin on the sodium and potassium content of human erythrocytes

In order to get a better understanding in the mechanism by which tryptophan-N-formylated gramicidin (NFG) and gramicidin kill the malaria parasite Plasmodium falciparum in vitro, we studied the capacity of these peptides to change the potassium, as well as the sodium, composition of normal human erythrocytes, and their ability to cause cell lysis. It is shown that both peptides are able to induce potassium leakage from, and sodium flux into, erythrocytes in such a manner that it is most likely that they are able to form cation channels in the membrane of these cells. For both peptides, potassium efflux proceeds at a faster rate than sodium influx, but this difference is greater for NFG than for gramicidin. This explains the observation that gramicidin is more lytic than NFG is, even when comparing concentrations that show the same antimalarial activity. The finding that gramicidin is approximately 10 times more active than NFG in causing potassium efflux from normal erythrocytes, as well as in killing the malaria parasite, supports the hypothesis that peptide-induced parasite death is related to their capacity to induce potassium leakage from infected erythrocytes. Finally, the observation that erythrocytes are able to restore their normal ion contents after losing more than 50% of their potassium content by incubation with NFG or gramicidin, suggests that, in vivo, and upon treatment with drug concentrations that cause full inhibition of parasite growth, these cells would not be irreversibly damaged by action of the drugs.

Perforated patch-clamp recording in cardiac myocytes using cation-selective ionophore gramicidin

Whole cell currents were recorded in single myocytes dissociated from guinea pig ventricles by the patch-clamp technique. The addition of 0.1 mg/ml gramicidin D, a cation-selective ionophore, into the pipette solution induced a gradual spontaneous perforation of the patch membrane under a conventional cell-attached configuration. The access resistance, measured at approximately 12 min after formation of a gigaohm seal, was 9.2 +/- 1.5 M omega (n = 12). The perforated patch membrane exhibited ionic selectivity for various monovalent cations, with a relative order of Cs+ (1.11) > K+ (1.0) > Na+ (0.65) >> tris(hydroxymethyl)aminomethane+ (approximately 0) but was not permeable for Cl-. Under the gramicidin-perforated patch recording configuration, the cells showed the typical electrophysiological properties for ventricular cells reported previously. The intracellular Cl- concentration, estimated from the reversal potential of the catecholamine-induced Cl- current, was 36.3 +/- 2.9 mM (n = 17). We thus conclude that the gramicidin-perforated patch recording mode provides a useful tool for recording the ionic currents while maintaining the intracellular Cl- concentration.

Interactions between gramicidin S and its producer, Bacillus brevis

Gramicidin S (GS) inhibition of germination outgrowth of Bacillus brevis spores was reversed completely by a short pretreatment with sodium dodecyl sulfate, moderately by ethanol or by incubation at pH 10 but not by incubation at pH 4. Of five metal ions tested (Na+, Mg2+, Fe2+, Cu2+, Ca2+), only Ca2+ reversed GS inhibition. When Ca2+ (but not the other four metal ions) was added to the growth medium, there was a considerable portion of the biosynthesized GS found in the extracellular fluid. These findings are interpreted in terms of the binding of GS to the external layers of the B. brevis spore.

Highly supralinear feedback inhibition of Ca2+ uptake by the Ca2+ load of intracellular stores

Net Ca2+ uptake into intracellular Ca2+ stores of homogenized cells is transient, even when the extravesicular Ca2+ concentration is kept constant. To study the mechanism underlying the phenomenon, we have investigated 45Ca2+ uptake by HL-60 cell homogenates. The initial rate of Ca2+ uptake as well as the final amount of stored Ca2+ were a function of the extravesicular Ca2+ concentration. However, Ca2+ uptake stopped independently of the extravesicular Ca2+ concentration after approximately 10 min. Studies using Ca2+-ATPase inhibitors demonstrated that the transient nature of the net uptake was not due to Ca2+ efflux. Monovalent cation ionophores did not influence the Ca2+ uptake curves, excluding a relevant involvement of pH and membrane potential. Together with the observation of a continued Ca2+ uptake in the presence of the intralumenal Ca2+ chelator oxalate, these results strongly suggest a feedback inhibition of Ca2+ uptake by the Ca2+ load of intracellular stores. The concentration-inhibition relationship between the Ca2+ load and the rate of Ca2+ uptake was highly supralinear (slope factor >/= 4). IC50 and maximum of the dose-inhibition curve, but not the slope factor were a function of the extravesicular free Ca2+ concentration. A series of three logistic equations derived from our data allowed an appropriate description of the behavior of Ca2+ uptake. Our results suggest, in addition to its well known activation by cytosolic Ca2+ concentration, a highly supralinear feedback inhibition of Ca2+ uptake by the Ca2+ load of intracellular stores. The steepness of the feedback inhibition might have a profound effect on spatial and temporal behavior of the Ca2+ signal.

Gramicidin S is active against both gram-positive and gram-negative bacteria

Four linear and four cyclic analogs of gramicidin S (GS) in which D-Phe was replaced with either D-His, D-Ser, D-Tyr or D-Asn have been prepared by solid-phase peptide synthesis and characterized with respect to antibacterial, antifungal and hemolytic activity. Unlike previous reports, GS and a number of cyclic analogs were found to be active against gram-positive as well as gram-negative bacteria. GS showed MICs ranging from 3 to 12.5 micrograms/mL for gram-negative bacteria, compared to MICs of 3 micrograms/mL for gram-positive bacteria. Furthermore, these analogs were also found to exhibit antifungal activity. Unlike the cyclic analogs, all linear analogs were found to be inactive against a wide range of microorganisms tested, and showed low levels of hemolytic activity. The antibacterial activity was found to be highly dependent on the type of assay used, with solution-based assays showing greater activity against gram-negative bacteria than agar-based assays. The GS cyclic analogs were all less toxic than GS itself, with the analog containing the D-Phe to D-Tyr substitution showing the greatest activity of the synthetic analogs. Hemolytic activity in solution against human and sheep red blood cells paralleled antibiotic activity, with those peptides exhibiting greater antibiotic activity generally showing greater hemolytic activity. Membrane destabilization as monitored using the hydrophobic probe N-phenyl-1-naphthylamine was also found to parallel antibacterial and hemolytic activity of cyclic and linear analogs. These results indicate that GS and certain related analogs may have applications as broad-spectrum antibiotics and should be reevaluated for such purposes.

Subunit structure of vacuolar proton-pyrophosphatase as determined by radiation inactivation

Vacuolar proton-pyrophosphatase (H(+)-PPase) of mung bean seedlings contains a single kind of polypeptide with a molecular mass of approx. 73 kDa. However, in this study, a molecular mass of approx. 140 kDa was obtained for the purified vacuolar H(+)-PPase by size-exclusion gel-filtration chromatography, suggesting that the solubilized form of this enzyme is a dimer. Radiation inactivation analysis of tonoplast vesicles yielded functional masses of 141.5 +/- 10.8 and 158.4 +/- 19.5 kDa for PP1 hydrolysis activity and its supported proton translocation respectively. These results confirmed the in situ dimeric structure of the membrane-bound H(+)-PPase of plant vacuoles. Further target-size analysis showed that the functional unit of purified vacuolar H(+)-PPase was 71.1 +/- 6.7 kDa, indicating that only one subunit of the purified dimeric complex would sufficiently display its enzymic reaction. Moreover, in the presence of valinomycin and KCl, the functional size of membrane-bound H(+)-PPase was decreased to approx. 63.4 +/- 6.3 kDa. A working model was proposed to elucidate the structure of native H(+)-PPase on vacuolar membrane as a functional dimer. Factors that would disturb the membrane, e.g. membrane solubilization and the addition of valinomycin and KCl, may induce an alteration in its enzyme structure, subsequently resulting in a different functional size.

Ammonium ion minisensors form self-assembled bilayer lipid membranes using gramicidin as an ionophore. Modulation of ammonium selectivity by platelet-activating factor

The present paper reports the electrochemical investigation of ion transport through self-assembled bilayer lipid membranes on a metal support and represents a technology suitable for development of an ammonium ion minisensor. Gramicidin D was used as a channel-forming ionophore for selective conduction of ammonium ion through lipid bilayers composed of egg phosphatidylycholine. The ammonium ion sensor exhibited good mechanical stability and longevity (routinely over 48 h) and constant sensitivity and response to a given concentration of ammonium ion in solution. The use of stabilized metal-supported BLMs has allowed the electrochemical investigation of the reversibility of response to ammonium ions and of ionophore binding to lipid membranes. The effects of pH and some possible interferents were examined. Semisynthetic platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, AGEPC) was found to enhance transport of ammonium ions by gramicidin and to reduce potassium interference, whereas the non-acetylated derivative of AGEPC did not exert any alterations in transport ammonium ions. The present microfabricated ammonium sensor based on thin lipid film technology provides advantages of extremely fast response times (to millisecond speeds) to alterations of ammonium ion concentration (0.02-5 mmol L-1), high selectivity to ammonium ions in the presence of volatile amines, and the capability of analyzing small volumes of samples. A detection limit of ammonium ions of approximately 1 x 10(-6) M was attained using BLMs modified with PAF. Furthermore, a device can now simply and reliably be fabricated at low cost and therefore can be used as a disposable sensor.

Mechanisms of glutamate-stimulated Mg2+ influx and subsequent Mg2+ efflux in rat forebrain neurones in culture

1. Mag-fura-2 fluorescence microscopy and whole-cell patch-clamp recordings were used to measure glutamate-induced changes in the intracellular free Mg2+ concentration ([Mg2+]i) and Mg2+ currents, respectively, in cultured forebrain neurones from fetal rats in the absence of extracellular Na+ (Nao+) and Ca2+ (Cao2+). 2. Increasing the extracellular Mg2+ concentration ([Mg2+]o) from 9 to 70 mM significantly enhanced the maximum [Mg2+]i induced by a 5 min 100 microM glutamate plus 1 microM glycine stimulation ([Mg2+]i,5 min) from 2.04 +/- 0.07 to 2.98 +/- 0.20 mM. Increasing [Mg2+]o from 9 to 70 mM also significantly enhanced the initial rate of rise in [Mg2+]i upon glutamate stimulation from 0.41 +/- 0.02 to 0.81 +/- 0.08 mM min-1. 3. The glutamate-stimulated increase in [Mg2+]i was not altered by prior depletion of intracellular free Na+ (Nai+). For paired stimulations in single neurones, the mean [Mg2+]i,5 min was 1.95 +/- 0.17 mM under Na(+)-depleted conditions and 1.94 +/- 0.16 mM under control conditions. 4. The glutamate-stimulated increase in [Mg2+]i was significantly reduced when NMDA channel-permeant Cs+ or K+ ions were used as the Na+ substitute instead of the presumably NMDA channel-impermeant ions N-methyl-D-glucamine (NMDG), Tris or sucrose. The mean [Mg2+]i,5 min was 0.56 +/- 0.06 and 0.74 +/- 0.08 mM in the presence of Cs+ or K+, respectively, compared with 2.13 +/- 0.10, 1.93 +/- 0.11 and 2.07 +/- 0.22 mM in the presence of NMDG, Tris or sucrose, respectively. 5. In whole-cell recordings performed with Cs+ as the primary intracellular cation, application of 100 microM NMDA plus 10 microM glycine induced inward currents that reversed around -55 mV in an extracellular solution containing 70 mM Mg2+ and 31 mM NMDG as the only cations. The currents were reversibly inhibited by DL-2-amino-5-phosphonovaleric acid (APV). In an extracellular solution containing 2 mM Mg2+ and 140 mM NMDG, NMDA plus glycine activated outward currents at potentials more depolarized than -90 mV. 6. In whole-cell recordings made with NMDG as the principal cation in the patch pipette, application of NMDA plus glycine in the 70 mM Mg2+ extracellular solution induced inward currents at voltages more negative than +15 mV. The ratio of the current measured under these conditions to the current measured in an extracellular solution containing Na+ as the principal cation (0.073:1) was nearly constant from cell to cell. 7. Following a 5 min glutamate stimulation in the presence of 9 mM Mg2+, [Mg2+]i returned to basal levels at a mean rate of 58.1 +/- 2.1 microM min-1. Complete removal of Nao+ significantly inhibited the rate of recovery to 31% of control. Raising [Mg2+]o to 30 mM in combination with removal of Nao+ did not inhibit recovery significantly more than either manipulation alone (28% of control). 8. These results suggest that glutamate-stimulated increases in [Mg2+]i that occur in the absence of Nao+ and Cao2+ result from Mg2+ entry through NMDA-activated ion channels. Furthermore, recovery from a glutamate-induced Mg2+ load appears to be primarily due to Mg2+ efflux via a mechanism whose characteristics are consistent with Na(+)-Mg2+ exchange.

[Absorption of various actinomycins by Staphylococcus aureus cells]

The absorption of actinomycin D by the cell suspension of Staphylococcus aureus via diffusion linearly depended on the antibiotic concentration in the suspension within the ranges of 2 to 15 micrograms/ml. The absorption of active actinomycins C2, C3 and Au6 was the same as that of actinomycin D. The Staphylococcus intact membranes limited the inlet of the actinomycins to the cells since the membranotropic substances such as gramicidin S and its derivatives and thyrocidin increased their absorption by 30-70 per cent. The absorption of a low active actinomycin D0 and inactive actinomycinic acid even after the exposure to the membranotropic substances was not detectable. These compounds did not form any complexes with DNA. The level of the absorption of the actinomycins by the cells was likely defined by their ability to complex with DNA.

Filled pore approximation: a theoretical framework for solute-solvent coupling in narrow water channels

A phenomenological model is presented of water and solute transport that is applicable to water pores with radii less than approximately 2 A. This includes such examples as gramicidin A, the proximal tubule basolateral membrane, and the aquaporin 1 (CHIP28) water channel. The model differs from the conventional single-file model by allowing for a variation of unoccupied volume within the pores. It is shown that the accessible or free portion of the unoccupied volume can be related to the mechanical frictional coefficients and thereby to the filtration and diffusive permeabilities by the filled pore approximation. In general, the smallness of the unoccupied volume represents the compactness of the molecules within the pore and is indicative of the single-file character of the motion of water and solute moving together. When that volume is equal to a single water volume, the results are identical to the conventional single-file model. An important result is that, despite very low diffusive permeabilities, the reflection coefficient of a solute can remain at approximately 0.5 if its frictional interaction with the channel walls is comparable with its frictional interaction with neighboring water molecules. This is consistent with values previously reported for NaCl in cell membranes of proximal tubule. The model predicts a minimum effective pore radius for a water channel of 1.78 A and corresponds to a maximum filtration-to-diffusion permeability ratio that is proportional to the length of the effective pore or channel. This limiting condition corresponds to a water channel completely filled by water and may be applicable to the aquaporin 1 water channel.

Sodium-bicarbonate cotransport in guinea pig ileal crypt cells

Prior studies show that ileal HCO3- secretion is of crypt origin, possibly involving Na+-HCO3- cotransport. To test for the latter, we isolated crypt cells from guinea pig ileum and determined effects of medium HCO3-, Na+, K+, disulfonic stilbenes, and gramicidin on intracellular pH [pHi;2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein fluorescence], cell volume (electronic sizing), and Na+ efflux from 22Na+ -preloaded cells. Ileal crypt cells alkalinized when placed in sodium gluconate-HCO3- medium containing N-5-methyl-5-isobutyl amiloride (1 microM), bumetanide (10 microM) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (250 microM which blocks Cl-/HCO3- exchange but not Na+ dependent HCO3- uptake). Depolarization with either gramicidin (50 microM) or 50 mM K+ caused a further 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS)-inhibitable increase in pHi. Gramicidin also caused SITS-inhibitable cell swelling. Both gramicidin effects were Na+ dependent: at 0 mM Na+, gramicidin acidified and did not alter cell volume; at 25 mM, gramicidin also acidified; at 90 and 140 mM, gramicidin alkalinized and induced cell swelling. HCO3- -dependent SITS-inhibitable Na+ efflux from 22Na+ -preloaded cells was also seen. We conclude that ileal crypt cells engage in electrogenic Na+ -HCO3- symport.

Effect of membrane potential on phosphatidylserine synthesis and calcium movements in control and CD3-activated Jurkat T cells

CD3 mAb induced calcium movements are unaffected by hyperpolarization of the membrane in Jurkat T cells treated with valinomycin. By contrast, the CD3 induced Ca2+ influx was impaired by depolarization of the membrane with either gramicidin or by equimolar substitution of KCl for NaCl in the medium. In depolarized cells, the synthesis of phosphatidylserine was strongly diminished as a result of impaired transport of the [3H]serine substrate. In depolarized cells, the CD3-induced release of Ca2+ from intracellular stores (endoplasmic reticulum) was unaffected. Emptying of the Ca2+ stores by CD3 was shown by the lack of effect of additional treatment of the cells with the Ca2+ ionophore, ionomycin. The empty status of the calcium stores was also confirmed by measurements of phosphatyidylserine synthesis through the Ca2+ -dependent base exchange enzyme system that was found to be significantly decreased despite the low amount synthesized in the presence of a defective [3H]serine transport in depolarized cells.

Competition of hydrophobic peptides, cytotoxic drugs, and chemosensitizers on a common P-glycoprotein pharmacophore as revealed by its ATPase activity

The aim of the present study was to demonstrate that the modulation of P-glycoprotein (Pgp) ATPase activity by peptides, drugs, and chemosensitizers takes place on a common drug pharmacophore. To this end, a highly emetine-resistant Chinese hamster ovary cell line was established, in which Pgp constituted 18% of plasma membrane protein. Reconstituted proteoliposomes, the Pgp content of which was up to 40%, displayed a basal activity of 2.6 +/- 0.45 micromol of Pi/min/mg of protein, suggesting the presence of an endogenous Pgp substrate. This basal ATPase activity was stimulated (up to 5.2 micromol of Pi/min/mg of protein) by valinomycin and various Pgp substrates, whereas, to our surprise, gramicidin D, an established Pgp substrate, was inhibitory. Taking advantage of this novel inhibition of Pgp ATPase activity by gramicidin D, a drug competition assay was devised in which gramicidin D-inhibited Pgp ATPase was coincubated with increasing concentrations of various substrates that stimulate its ATPase activity. Gramicidin D inhibition of Pgp ATPase was reversed by Pgp substrates, including various cytotoxic agents and chemosensitizers. The inhibition of the basal ATPase activity and the reversal of gramicidin D inhibition of Pgp ATPase by its various substrates conformed to classical Michaelis-Menten competition. This competition involved an endogenous substrate, the inhibitory drug gramicidin D, and a stimulatory substrate. We conclude that the various MDR type substrates and chemosensitizers compete on a common drug binding site present in Pgp.

Endothelial cell oxidant generation during K(+)-induced membrane depolarization

We tested the hypothesis that membrane depolarization may initiate oxidant generation in the endothelial cell. Depolarization was produced in bovine pulmonary arterial endothelial cells (BPAEC) in monolayer culture with varying external K+, or with glyburide (10 microM), tetraethylammonium (TEA, 10 mM), gramicidin (1 microM), or nigericin (2 microM). Evaluation of bisoxonol fluorescence of BPAEC indicated concentration-dependent depolarization by high K+ (2% change in fluorescence/mV change in membrane potential in the 5.9-48 mM range of K+) and essentially complete depolarization with glyburide. Generation of oxidants was assessed with o-phenylenediamine dihydrochloride (o-PD) oxidation in the presence of horseradish peroxidase (HRP). There was a time-dependent increase in o-PD oxidation with 24 mM K+, nigericin, and gramicidin over 2 hours compared with control. In 1 hour o-PD oxidation increased 2.8-fold for 24 mM and 3.7-fold for 48 mM K+ compared with control. Catalase reduced 24 mM K(+)-induced o-PD oxidation by 50%, while Cu/Zn-superoxide dismutase (SOD) abolished the increase. Oxidation of o-PD was reduced by 57% in the absence of HRP in the system. With K+ channel blockade, o-PD oxidation increased 3.8-fold with glyburide and 4.6-fold with TEA compared with control. These data indicate formation of H2O2 and possibly other oxidants with depolarization and suggest involvement of K(+)-channels in this process.