Membrane properties and cell ultrastructure of taste receptor cells in Necturus lingual slices

1. Whole cell patch-clamp recordings and electron micrographs were obtained from cells in Necturus taste buds in lingual slices to study their membrane properties and to correlate these properties with cell ultrastructure. 2. Two different populations of taste receptor cells could be identified: one type possessed voltage-gated Na+ and K+ (noninactivating) currents (group 1 cells); the other type possessed only K+ (inactivating) currents (group 2 cells). 3. The zero-current ("resting") potential (Vo) and whole cell resistance (Ro) of these two types of taste cells differed significantly. For group 1 cells, on average, Vo = -75 mV and Ro = 24.6 G omega, and for group 2 cells, Vo = -49 mV and Ro = 48.9 G omega. The difference in Ro was not explained completely by differences in cell sizes, suggesting that intrinsic membrane properties differed between the populations. 4. Cells injected with biocytin were the electron microscope after tissues were reacted with majority (14 of 16) of cells with voltage-gated Na+ and K+ currents (group 1 cells) were characterized by abundant rough endoplasmic reticulum and dense granular packets in the apical process. These are features of dark cells. All the cells that only possessed K+ currents (group 2 cells) were characterize by well-developed smooth endoplasmic reticulum and an absence granular packets. These features characterize light cells. 5. These findings indicate that there is a good, although not exact, correlation between electrophysiological properties and cell morphotype in Necturus taste bud cells. All dark cells possessed Na+ and K+ currents and thus would be expected to be capable of generating action potentials. Most light cells only possessed outward K+ currents and thus would be incapable of generating action potentials.

Removal of extracellular chloride suppresses transmitter release from photoreceptor terminals in the mudpuppy retina

Removal of extracellular Cl- has been shown to suppress light-evoked voltage responses of ON bipolar and horizontal cells, but not photoreceptors or OFF bipolar cells, in the amphibian retina. A substantial amount of experimental evidence has demonstrated that the photoreceptor transmitter, L-glutamate, activates cation, not Cl-, channels in these cells. The mechanism for Cl-free effects was therefore reexamined in a superfused retinal slice preparation from the mudpuppy (Necturus maculosus) using whole-cell voltage and current clamp techniques. In a Cl-free medium, light-evoked currents were maintained in rod and cone photoreceptors but suppressed in horizontal, ON bipolar, and OFF bipolar cells. Changes in input resistance and dark current in bipolar and horizontal cells were consistent with the hypothesis that removal of Cl- suppresses tonic glutamate release from photoreceptors. The persistence of light-evoked voltage responses in OFF bipolar cells, despite the suppression of light-evoked currents, is due to a compensatory increase in input resistance. Focal application of hyperosmotic sucrose to photoreceptor terminals produced currents in bipolar and horizontal cells arising from two sources: (a) evoked glutamate release and (b) direct actions of the hyperosmotic solution on postsynaptic neurons. The inward currents resulting from osmotically evoked release of glutamate in OFF bipolar and horizontal cells were suppressed in a Cl-free medium. For ON bipolar cells, both the direct and evoked components of the hyperosmotic response resulted in outward currents and were thus difficult to separate. However, in some cells, removal of extracellular Cl- suppressed the outward current consistent with a suppression of presynaptic glutamate release. The results of this study suggest that removal of extracellular Cl- suppresses glutamate release from photoreceptor terminals. Thus, it is possible that control of [Cl-] in and around photoreceptors may regulate glutamate release from these cells.

Computer simulations of voltage clamping retinal ganglion cells through whole-cell electrodes in the soma

1. Computer simulations of voltage-clamp experiments in retinal ganglion cells were implemented to better understand the insights that can be obtained with this physiological approach. 2. Simulation studies of voltage clamping were based on the contemporary approach of using whole-cell recordings with low resistance electrodes attached to the soma. Realistic ganglion cell morphologies were provided by cell staining experiments in the mudpuppy retina; selected cells included small-, medium-, and large-field neurons whose morphologies were entered into a computer through a neuron tracing program. 3. Values for the specific membrane resistance (Rm) varied from 5,000 to 100,000 omega/cm2 to conform to the range of Rm values obtained with intracellular sharp electrodes and whole-cell recordings. 4. Synaptic input currents were simulated by injecting current with and without an underlying conductance change into different regions of the dendritic tree. The time-variant waveform of the current included a combined transient and sustained component similar to the waveform of ON-bipolar activation. 5. Simulations were base on 1) intact structures, which included the soma and the entire dendritic tree, and 2) a more limited cell geometry that included representation of the soma, but only part of the dendritic tree, to represent the restricted morphology that might be rendered after cutting the retina into 150-microns cross sections for retinal slice experiments. 6. The results of this study indicate that voltage clamping from the soma, with optimal, low resistance electrodes and series resistance compensation, provides an error-free voltage clamp for slow signals that are generated within a small electrotonic distance from the soma (approximately 0.1 lambda). 7. The ideal voltage-clamp conditions are optimized when synaptic conductances are small and nonlinear membrane elements are minimally activated: small-field neurons best approximate these conditions, but clamping errors are evident in these cells when more distal branches are activated. The degree of error in voltage clamping was much greater when medium-and large-field neurons were evaluated. 8. It was not possible to clamp action potentials (nonpropagating) even when they were generated near the soma in any of the three model cells examined. 9. Experimental paradigms were developed to demonstrate that inadequate voltage clamping can lead to errors in the interpretation of experimental data when relevant variables are not taken into consideration. Suggestions are made for determining and optimizing favorable clamp conditions.

Ketoconazole activates chloride and fluid secretion by Necturus gallbladder at low pH

Necturus gallbladder epithelium, normally a reabsorptive epithelium, was stimulated to secrete chloride and fluid by the combined effects of ketoconazole and a reduction in perfusate pH to 7.0. The reversal in the direction of net fluid transport was accompanied by inhibition of the conductance of the apical cell membrane to sodium, potassium, and a striking stimulation of the conductance to chloride. The results are consistent with a previously unidentified mechanism for regulation of the apical cell membrane transport properties of reabsorptive epithelia.

Milk and egg phospholipids act as protective surfactants against luminal acid in Necturus gastric mucosa

BACKGROUND

Our previous studies indicate that milk phospholipids have anti-ulcer properties in rats and humans, possibly by forming a hydrophobic surfactant layer at the epithelial surface. In the present study we measured intracellular pH and parameters of membrane resistances in gastric epithelium exposed to luminal acid using a microelectrode technique.

METHODS

Chambered isolated Necturus maculosus antral mucosa was exposed to pH 2.3, with or without 20-25 min pre-treatment with milk or egg phospholipids. The pH in surface epithelial cells was measured with double-barrelled liquid sensor pH/PD-microelectrodes.

RESULTS

Pre-treatment with phospholipids (2500-5000 micrograms P/mL) significantly (P < 0.01, n = 14) opposed intracellular acidification. Phospholipids significantly (P < 0.05, n = 14) increased the ratio of apical and basal membrane resistances, suggesting that they primarily affect the apical cell membrane. In contrast, there was no significant change in transmucosal resistance suggesting lack of effect on paracellular shunts in the 'leaky' epithelium.

CONCLUSIONS

Exogenous phospholipids of dietary origin may be used to form a protective layer in the gastric mucosa against irritants.

Arachidonic-acid-activated membrane conductances in dissociated cardiac parasympathetic neurons from Necturus

1. Characteristics of the membrane currents activated by arachidonic acid (AA) in dissociated mudpuppy parasympathetic neurons have been determined using the perforated-patch whole cell recording technique. 2. In a sodium-containing physiological solution with 12.5 mM potassium, AA (10-50 microM) increased total membrane current produced by either depolarizing or hyperpolarizing voltage steps delivered from a holding potential of -40 mV. Decreasing the external potassium concentration from 12.5 to 2.5 mM shifted the reversal potential of the AA-induced current by 10 mV rather than the approximately 42 mV predicted for a highly potassium-selective channel. 3. In cells kept in sodium solution plus 12.5 mM potassium and treated with 20 microM nordihydroguaiaretic acid (NDGA), an inhibitor of the lipoxygenase pathway of AA metabolism, AA activated only inward currents following hyperpolarizing voltage steps. In this condition, the shift in reversal potential of the AA-induced current was 40 mV when extracellular potassium concentration was changed fivefold. Consequently, in cells treated with NDGA, AA appeared to activate only an inwardly rectifying potassium current. 4. Decreasing the extracellular chloride concentration by approximately 90% did not alter the reversal potential of the AA-activated current when the extracellular sodium concentration was kept constant and the external potassium concentration was 2.5 mM. In the low-chloride solution, AA potentiated both inward and outward current amplitudes. These results suggested that AA did not activate a chloride current in these cells. 5. In a sodium-deficient, N-methyl-D-glucamine (NMG)-containing solution, AA only activated currents for voltage steps to potentials more negative than the holding potential. In the NMG-substituted solution, changing the extracellular potassium concentration fivefold shifted the reversal potential of the AA-induced current by 40 mV. Therefore, in the NMG solution, AA primarily activated an inwardly rectifying potassium current. 6. Exchanging the control solution containing AA to an external solution containing AA and barium (barium blocks the inwardly rectifying potassium current) shifted the current-voltage relationship to more positive voltages such that the extrapolated reversal potential was approximately 0 mV. In other experiments, using the barium-containing solution, the reversal potential for the AA-induced current was -3.3 +/- 2.4 (SE) mV. 7. In conclusion, the results of the present study indicate that at least two membrane currents are activated in the presence of AA: an inwardly rectifying potassium current and an NDGA-sensitive, sodium-dependent current that has a reversal potential more positive than the potassium equilibrium potential. We suggest the second current component is due to the activation of a nonselective cationic conductance.

Estimation of the junctional resistance between electrically coupled receptor cells in Necturus taste buds

Junctional resistance between coupled receptor cells in Necturus taste buds was estimated by modeling the results from single patch pipette voltage clamp studies on lingual slices. The membrane capacitance and input resistance of coupled taste receptor cells were measured to monitor electrical coupling and the results compared with those calculated by a simple model of electrically coupled taste cells. Coupled receptor cells were modeled by two identical receptor cells connected via a junctional resistance. On average, the junctional resistance was approximately 200-300 M omega. This was consistent with the electrophysiological recordings. A junctional resistance of 200-300 M omega is close to the threshold for Lucifer yellow dye-coupling detection (approximately 500 M omega). Therefore, the true extent of coupling in taste buds might be somewhat greater than that predicted from Lucifer yellow dye coupling. Due to the high input resistance of single taste receptor cells (> 1 G omega), a junctional resistance of 200-300 M omega assures a substantial electrical communication between coupled taste cells, suggesting that the electrical activity of coupled cells might be synchronized.

Regulation of intracellular pH in isolated Necturus gastric mucosa during short-term exposure to luminal acid

BACKGROUND/AIMS

Continuous exposure to gastric acid implies efficient control mechanisms of intracellular pH (pHi) in the gastric epithelium. This study assessed the roles of Na+, H+, and HCO3- transport mechanisms in controlling pHi during short-term exposure of the gastric epithelium to luminal acid.

METHODS

pHi and Na+ activity (aiNa) were measured with liquid sensor microelectrodes in isolated Necturus antral mucosa, modulating ion transport mechanisms by ion removal and pharmacological inhibition.

RESULTS

Short-term exposure to luminal acid (pH 2.3) acidified pHi by 0.3 pH units, whereafter pHi stabilized. This was associated with transient increase in aiNa. Blocking of Na+/H+ exchange (in the presence of HCO3-/CO2) by removal of Na+ or addition of amiloride eliminated the increase in aiNa and resulted in uncontrolled acidification of pHi. Similarly, blocking of HCO3- transport (in the presence of Na+) by removal of HCO3-/CO2 or addition of 4-acetamido-4-isothiocyanatostilbene-2,2-disulfonic acid resulted in uncontrolled acidification of pHi despite increase in aiNa. Blocking of Na+/K+ exchange with ouabain eliminated the recovery of aiNa and also resulted in uncontrolled acidification of pHi.

CONCLUSIONS

The data indicate that during short-term exposure of the gastric mucosa to luminal acid, both Na+/H+ antiport and HCO3- transport are needed to control pHi and maintain it within physiological ranges.

Neuropeptide galanin inhibits omega-conotoxin GVIA-sensitive calcium channels in parasympathetic neurons

1. We determined the effect of the neuropeptide galanin on barium currents (IBa) flowing through voltage-gated calcium channels. We voltage clamped parasympathetic neurons dissociated from mudpuppy cardiac ganglia using both the standard whole cell and the perforated-patch variations of the patch-clamp technique. 2. Galanin produced a concentration-dependent inhibition of IBa. The maximal inhibition was 50-60% and the concentration that produced half-maximal inhibition (IC50) was 0.42 nM. In mud-puppy parasympathetic neurons, omega-conotoxin-GVIA (CTX)-sensitive channels are the predominant type of calcium channels, and only a small portion of IBa is contributed by dihydropyridine-sensitive channels. Galanin preferentially inhibited a portion of the CTX-sensitive current. 3. In currents recorded with the standard whole cell technique, activation of IBa was slowed in the presence of galanin. In contrast, in the majority of neurons studied with the perforated-patch technique, galanin decreased IBa without altering the kinetics of current activation. With both recording methods, the decrease in IBa was greatest with voltage steps to 0 mV and persisted with steps to +50 mV. For control currents, large depolarizing voltage steps (+70 to +120 mV) did not markedly facilitate IBa when either recording technique was used. However, the degree of facilitation in galanin was significantly greater with the standard whole cell recording technique. 4. IBa exhibited inactivation under the conditions of these experiments. Inactivation of IBa recorded during a 900-ms depolarizing voltage step was fitted to a double exponential. Galanin decreased the amplitude of IBa but did not alter the time constants of inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium is not involved in the cAMP-mediated stimulation of Cl- conductance in the apical membrane of Necturus gallbladder epithelium

The permeability properties of the forskolin-stimulated Cl- conductance in the apical membrane of Necturus gallbladder epithelium and the possible participation of intracellular Ca2+ in its stimulation have been investigated. The anion selectivity sequence as derived from biionic potential measurements (SCN- > I- approximately NO3- > Br- > Cl- >> ISE-) differed from the sequence derived from measurements of apical membrane resistance (NO3- approximately Br- approximately Cl- > SCN- > I- approximately ISE-). Accordingly, the conductance was inhibited by SCN- and I- which, from the potential measurements, appeared to be more permeable than Cl-. This finding agrees with observations of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel reported recently. However, none of the commonly used Cl- channel blockers, such as 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), anthracene-9-carboxylic acid (9-AC) and glibenclamide reduced this conductance in Necturus gallbladder. In contrast to the situation in most other epithelia, elevation of intracellular Ca2+ concentration ([Ca2+]i) by ionomycin stimulated only K+ conductance and not that of Cl- in the apical cell membrane. Chelation of intracellular Ca2+ did not prevent the stimulation of Cl- conductance by forskolin. This indicates that [Ca2+]i does not have even a permissive role in the cyclic adenosine monophosphate-(cAMP)-mediated stimulation process, as would have been expected if exocytosis was involved. Further evidence against the involvement of exocytosis in the stimulation process came from the observation that the stimulation was not associated with an increase in apical membrane capacitance and was not suppressed by disruption of the cytoskeleton by preincubation of the tissue with cytochalasin D. The data indicate that Necturus gallbladder epithelium contains homologues of the CFTR Cl- channel which reside permanently in the apical cell membrane and which can be stimulated by a cAMP-dependent phosphorylation process without involvement of cell Ca2+ or exocytosis.

The significance of apical K+ channels in mudpuppy feeding behavior

Behavioral experiments were conducted to determine the effects of known potassium channel blockers, minnow extract (a natural food substance), and other taste stimuli on feeding behavior in the mudpuppy (Necturus maculosus). Groups of 10-15 mudpuppies were presented with one or more of the following chemicals dissolved in gelatin cubes: H2O (control), NaCl (1.0 M; 0.2 M), crude minnow extract (0.1 g/ml), CaCl2 (1.0 M), citric acid (0.1 M; pH 2, 0.01 M; pH 3), quinine HCl (0.1 M), tetraethylammonium chloride (TEA; 0.1 M), KCl (1.0 M) and an amino acid mixture containing L-phenylalanine, L-arginine, L-histidine, L-valine, L-glutamic acid and L-lysine at concentrations of 0.02 M and 0.04 M each. Crude minnow extract was centrifuged and separated into components consisting of the following fractions: > 500 D, > 1000 D and > 14,000 D, each of which was presented to mudpuppies in cubes. Results were expressed as the percentage of animals rejecting the cube within 5 min of taking the cube into the oral cavity. Statistical analysis using the Fisher exact test indicated that cubes containing chemicals known to block (CaCl2, TEA, citric acid and quinine) or permeate (KCl) apical K+ channels in mudpuppy taste cells were more aversive than control cubes, and aversion was concentration-dependent. In contrast, cubes containing minnow extract and components of minnow extract < 14,000 D were significantly preferred over control cubes. Cubes containing a mixture of minnow extract and an aversive chemical were significantly less aversive than cubes containing only an aversive chemical. These findings suggest that activation of the apical K+ conductance in mudpuppy taste cells triggers an aversive response, but the aversion can be ameliorated by low molecular weight compounds present in minnow extract.

Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus

1. The interaction between K+, Cl- and H2O fluxes was studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. 2. Three experimental strategies were adopted: the osmolarity of the ventricular solution was increased abruptly by addition of (i) mannitol or (ii) KCl; (iii) Na+ in the ventricular solution was replaced isosmotically by K+. 3. The mannitol experiments showed that H2O had two pathways across the ventricular membrane. One was purely passive, with a water permeability, L'p, of 0.64 x 10(-4) cm s-1 (osmol l-1)-1. This operated in parallel with an ion-dependent pathway of similar magnitude which was abolished in Cl(-)-free solutions. 4. When KCl was added there was a flow of H2O into the cell. Surprisingly, this took place despite the osmotic gradient which favoured an efflux of H2O. The effect was blocked by frusemide (furosemide), in which case KCl had the same effects as applications of NaCl or mannitol. 5. Replacement of Na+ with K+ caused an influx of H2O. This flux could proceed against osmotic gradients implemented by mannitol. 6. The present data and those of earlier publications show that the interdependence of the fluxes of K+, Cl- and H2O in the exit membrane can be described as cotransport. The fluxes have a fixed stoichiometry of 1:1:500, the flux of one species is able to energize the flux of the two others, and the transport exhibits saturation and is specific for K+ and Cl-. 7. A molecular model based upon a mobile barrier in a membrane spanning protein gives an accurate quantitative description of the data.

Triflocin, a novel inhibitor for the Na-HCO3 symport in the proximal tubule

1. Triflocin, applied at millimolar concentration hyperpolarizes the basolateral membrane of Necturus proximal convoluted tubular cells, in vivo. 2. Barium, 2.5 x 10(-3) M, ouabain, 10(-3) M, or amiloride 10(-4) M, fail to prevent this hyperpolarization. 3. Triflocin has no effect on the intracellular chloride activity. 4. In physiological acid base conditions, Triflocin increases intracellular pH. 5. Upon an acute isohydric hypercapnia, Triflocin depolarizes the basolateral membrane potential. 6. It is concluded that, Triflocin inhibits the basolateral electrogenic Na-(HCO3)n > 1 cotransport in proximal tubules.

Bidirectional synaptic transmission in Necturus taste buds

Pairs of taste cells were impaled with intracellular recording microelectrodes in intact taste buds in slices of Necturus lingual epithelium. Applying short pulses of 140 mM KCl or 200 mM CaCl2 solutions to the apical pore elicited receptor potentials in taste receptor cells. Chemostimulation of receptor cells elicited postsynaptic responses in basal cells in the taste bud. Postsynaptic responses in basal cells had a threshold for activation and did not saturate with increasing doses of chemical stimulus applied to the receptor cells. We directly depolarized individual receptor cells and tested whether this would evoke postsynaptic responses in basal cells. Depolarizing receptor cells to approximately 0 mV evoked small depolarizing responses in basal cells in 16% of the experiments. The properties of these responses were consistent with their being mediated by a chemical synapse. A comparison of the responses in basal cells evoked by depolarizing single receptor cells, with responses evoked by stimulating the entire receptor cell population with KCl suggests that there is extensive synaptic convergence from receptor cells onto each basal cell. We also tested whether electrical excitation of basal cells would elicit (retrograde) synaptic responses in receptor cells. Single depolarizing pulses (up to 1 sec duration) applied to basal cells through the intracellular recording microelectrode never evoked synaptic responses in receptor cells. However, when repetitive electrical stimuli were applied to basal cells (four to six 1 sec depolarizations to approximately 0 mV every 12 sec) we observed prolonged effects on receptor cells in 11 of 23 experiments. These effects included an increase in the amplitude of receptor potentials elicited by KCI (mean +/- SD = +19 +/- 5%), an increase in membrane input resistance of receptor cells (+27 +/- 11%), and a hyperpolarization of receptor cells (3-10 mV). In control experiments, repetitive stimulation of one receptor cell never elicited such effects in another receptor cell. We investigated the possibility that serotonin (5-HT), released from basal cells, mediated the above modulatory effects on receptor cells. Bath-applied 5-HT (100 microM) mimicked the effects produced by repetitive basal cell stimulation (KCI responses increased by 23 +/- 12%; input resistance increased by 24 +/- 11%; hyperpolarization of 5-15 mV; N = 14). We conclude that basal cells release 5-HT onto adjacent taste receptor cells and that this enhances the electrotonic propagation of receptor potentials from the apical (chemosensitive) tip to the basal (synaptic) processes of receptor cells. The net effect is that activation of basal cells effectively increases the chemosensitivity of taste receptor cells.

Regulation of NaCl entry into Necturus gallbladder epithelium by protein kinase C

The role of protein kinase C in the regulation of the mode of NaCl entry into Necturus gallbladder epithelial cells was determined from the rate and magnitude of ouabain-induced cell swelling in the presence of inhibitors. Stimulation of protein kinase C by phorbol ester increased the rate of cell swelling from the control value of 2.9% to 4.7%/min and caused the predominant apical membrane transport mechanism for NaCl to switch from bumetanide-sensitive Na-Cl cotransport to amiloride-sensitive parallel exchange. Na-Cl cotransport could be restored as the predominant mode of NaCl entry by treatment of stimulated tissues with the kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) and calphostin C. Therefore the mechanism of NaCl transport across the apical membrane can be controlled by the activity of protein kinase C.

Tight-junction tightness of Necturus gall bladder epithelium is not regulated by cAMP or intracellular Ca2+. II. Impedance measurements

In the preceding publication we have reported that, contrary to the prevailing opinion in the literature, the tight-junction tightness of Necturus gall bladder epithelium is not up-regulated by cAMP-mediated or by Ca(2+)-mediated stimulation. This conclusion was based on our observation that the stimulant-induced increase in transepithelial resistance (Rt) occurred only when the lateral intercellular spaces were allowed to collapse, which suggested that the increase reflected primarily or exclusively the increasing resistance of the lateral spaces (Rlis) rather than the postulated increase in tight-junction resistance (Rj). An alternative explanation could have been that the constancy of Rt after space dilatation reflected an increase Rj that was masked by a concomitant fall in apical and basolateral cell membrane resistances Ra and Rbl. To decide between those possibilities we have performed impedance measurements with transepithelial and intracellular microelectrodes on Necturus gall bladder epithelium. Applying previously developed analysis procedures, the measurements readily showed that elevation of intracellular Ca2+ concentration increased Rlis, but left Rj as well as Ra and Rbl quasi constant. Experiments with forskolin, theophylline or isobutylxanthine, on the other hand, were less clear. These stimulants activated an apical Cl- conductance, which drastically reduced Ra and apparently caused low-frequency polarization effects that could not be accounted for by the classical epithelial equivalent circuit. After elimination of the polarization phenomena by uni- or bilateral substitution of Cl- by isethionate or sulphate, however, we were able to demonstrate that Rj remains constant under cAMP-mediated stimulation irrespective of whether the lateral spaces are kept open or are allowed to collapse. We conclude that the tight-junction resistance of Necturus gall bladder epithelium is not controlled by intracellular Ca2+ or by cAMP-mediated stimulation.

Tight-junction tightness of Necturus gall bladder epithelium is not regulated by cAMP or intracellular Ca2+. I. Microscopic and general electrophysiological observations

Following the publications by Duffey et al. [Nature 294:451 (1981)] and Palant et al. [Am J Physiol 245: C203 (1983)] it is generally accepted that tight-junction tightness of Necturus gall bladder epithelium is up-regulated by cAMP-mediated and Ca(2+)-mediated stimulation. This conclusion was mainly based on observed increases in transepithelial resistance (Rt). However, since in leaky epithelia Rt cannot be simply equated with the tight junction resistance (Rj), but may include large contributions from the lateral space resistance (Rlis), we asked whether the observed increases in Rt resulted indeed from Rj or whether Rlis also increased. The experiments were performed on Necturus gall bladders using forskolin or the Ca2+ ionophore A23187 as stimulants. Forskolin (2 mumol/l) had a biphasic effect. In the first 5 min Rt decreased from 128 +/- 13 to 119 +/- 14 omega cm2 (P < 0.05, n = 10) which probably reflects stimulation of an apical cell membrane Cl- conductance (see accompanying paper). Subsequently Rt increased in approximately 30 min to 184 +/- 20 omega cm2 and then remained fairly constant. Simultaneously the lateral spaces collapsed. If the spaces were now transiently opened by passing mucosa-positive direct current across the epithelium, Rt fell transiently to 111 +/- 7 omega cm2, but returned gradually to its elevated level when the spaces collapsed again. When the spaces were constantly dilated by a serosa-positive hydrostatic pressure of 1 cm H2O, forskolin neither affected the space width nor increased Rt, and current passage was virtually ineffective, although the cells depolarized in response to forskolin as usual.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane currents evoked by excitatory amino acid agonists in ON bipolar cells of the mudpuppy retina

1. Whole-cell patch-clamp recordings were obtained from ON bipolar cells in a retinal slice preparation of the mudpuppy, Necturus maculosus. The effects of excitatory amino acid (EAA) agonists applied in the presence of cobalt (2-5 mM) were examined. 2. At the holding potential of -50 mV, L-2-amino-4-phosphonobutanoic acid (L-AP4, 5-10 microM) evoked an outward current accompanied by a conductance decrease. The zero current potential of the L-AP4-evoked current was near 0 mV independent of whether the intracellular Ringer solution contained CsCl or CsCH3SO4. The currents evoked by light were also accompanied by a conductance decrease and reversed near 0 mV. Replacing external sodium with choline or N-methyl-D-glucamine generated an outward current and suppressed the response to L-AP4. The response to L-AP4 was enhanced by removing extracellular calcium and suppressed by increasing extracellular calcium. These results indicate that L-AP4 closes nonspecific cation channels that are blocked by extracellular calcium. 3. In 2 mM cobalt, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 50-100 microM) evoked membrane currents that were accompanied by a conductance increase. AMPA-evoked currents exhibited a significant chloride dependence and were suppressed by gamma-aminobutyric acid-A (GABAA) antagonists bicuculline and picrotoxin; a GABA uptake blocker, nipecotic acid; and a glycine antagonist, strychnine. AMPA-induced currents were virtually absent in the presence of 5 mM cobalt and nominally 0 mM extracellular calcium. These results indicate that the conductance increase induced by AMPA in the presence of 2 mM cobalt is largely the result of calcium-dependent synaptic inputs onto GABAA and glycine receptors of ON bipolar cells. 4. N-methyl-D-aspartic acid (250 microM) was ineffective when applied in the presence of 100 microM cadmium or 2 mM cobalt. 5. 1S,3R/1R,3S-1-aminocyclopentane-1,3-dicarboxylic acid (100-200 microM) evoked an outward current accompanied by a conductance decrease and appears to be an agonist at the L-AP4 receptor. 6. The findings of this study suggest that the only type of EAA receptor in mudpuppy ON bipolar cells is the L-AP4 receptor and that L-AP4 receptor activation results in the closing of nonspecific cation channels that are blocked by extracellular calcium.

Independently gated multiple substates of an epithelial chloride-channel protein

We have purified a protein from Necturus maculosus gallbladder cells that forms chloride channels in an artificial membrane. The same protein apparently can form channels that are highly selective for chloride but can have conductances varying from 9 to about 150 pS. The high-conductance channels are blocked by the monoclonal antibody used to purify the protein, but this antibody has no effect on the 9-pS channels. The observation that gating of the low- and high-conductance states is independent and that the antibody affects only the latter has implications regarding the control of chloride conductance in cell membranes and the different types of channels described in those cells.

Whole-cell currents in isolated resting Necturus gastric oxynticopeptic cells

1. Necturus gastric mucosa secretes Cl- actively across the gastric glands which are composed almost entirely of acid- and enzyme-secreting oxynticopeptic cells. Single channel studies on Necturus oxynticopeptic cells have shown that the basolateral membrane possesses multiple K(+)-selective channels but no observable Cl- channels while the apical membrane has Cl- channels but no observable K+ channels. To relate these channel properties to the conductance of the whole cell we have investigated the macroscopic membrane currents with conventional whole-cell patch-clamp techniques. 2. When bathed in amphibian Ringer solution, gastric oxynticopeptic cells had a membrane resistance of 47.8 +/- 2.8 M omega and a membrane capacitance of 75.5 +/- 2.7 pF (n = 82). This gave a specific membrane resistance of 3260 +/- 160 omega cm2 (n = 82). Reversal potentials of the oxynticopeptic cells were -13.8 +/- 1.2 mV (n = 45) for an intracellular Cl- concentration ([Cl-]i) of 42 mM and were significantly more negative -24.4 +/- 3.1 mV (n = 31, P < 0.001) for [Cl-]i = 22 mM. 3. In the absence of ATP in the pipette solution, there was an 80% reduction of the whole-cell current with a typical half-time (t1/2) of 5 min. The run-down was not observed when the pipette solution contained 4 mM ATP. 4. A slow and voltage-independent inhibition of 80% of the whole-cell currents occurred after addition of NPPB (35 microM). Ba2+ (10 mM) produced a reversible inhibition of 20% of the total current. Together, 35 microM NPPB and 10 mM Ba2+ eliminated 95% of the whole-cell currents. These data suggest that in the resting oxynticopeptic cells Cl- carried the major fraction of the current while K+ ions carried only a small fraction. 5. Total replacement of Cl- in the pipette and bath solution by gluconate- increased the membrane resistance to 751 +/- 104 M omega (n = 53) and shifted the reversal potential to -38.1 +/- 2.8 mV (n = 53). 6. Increasing the bath K+ concentration from 6 to 91 mM activated a current which had a high selectivity for K+ over choline+, Li+, Na+, Rb+ and Cs+ and was independent of Cl-. The activation of this K+ current (IK*) by high external K+ was not seen with ATP-free pipette solution. 7. Ba2+ or Cs+ had a voltage-dependent blocking effect of this inward K+ current. Ouabain (1 mM) or SCH 28080 (200 microM), specific inhibitors of the Na+,K(+)-ATPase and H+,K(+)-ATPase, had no effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Preepithelial mucus-HCO-3 layer protects against intracellular acidosis in acid-exposed gastric mucosa

The role of the preepithelial mucus-HCO-3 layer in protection against intracellular acidosis was investigated in isolated Necturus gastric antral mucosa exposed to luminal acid by simultaneous measurement of intracellular pH (pH(i)) and extracellular surface pH (pHs) in surface epithelium with microelectrode technique. Acidification of the luminal perfusate to pH 2.5 acidified pH(i) in surface epithelial cells from 7.33 +/- 0.02 to 7.20 +/- 0.04, whereas pHs fell from 6.75 +/- 0.21 to 5.20 +/- 0.25 (P < 0.01; n = 9), followed by a steady state for at least 2 h. Inhibition of epithelial HCO-3 secretion and transport by removal of serosal HCO-3 and CO2 (HEPES and O2 substitution) during acid exposure provoked a progressive acidification of pHs from 5.60 +/- 0.41 to 2.74 +/- 0.14 in 30 min (P < 0.01; n = 9), which was accompanied, after a 5- to 10-min delay, by acidification of pH(i) from 7.21 +/- 0.03 to 5.68 +/- 0.26 (P < 0.01). Digestion of the surface mucus gel by pepsin (5% wt/vol) at pH 2.5 caused a slow acidification of pHs from 5.22 +/- 0.59 to 3.60 +/- 0.46 within 2 h. This was followed by a more rapid acidification to 2.53 +/- 0.38 (P < 0.01; n = 7), with concomitant acidification of pH(i) from 7.19 +/- 0.05 to 6.03 +/- 0.33 (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Aminergic and peptidergic elements and actions in a cardiac parasympathetic ganglion

Correlated histochemical, immunocytochemical, and electrophysiological experiments have been undertaken to identify putative neurotransmitter-neuromodulator substances in cells and fibers in the parasympathetic cardiac ganglion of the mudpuppy, Necturus maculosus, and to determine the action of these agents on the properties of the parasympathetic postganglionic neurons. The mudpuppy cardiac ganglion contains two neuron types: large parasympathetic postganglionic neurons and smaller intrinsic neurons initially identified as small intensely fluorescent cells. We have shown that the postganglionic neurons contain both acetylcholine and a galanin-like neuropeptide. Also, we have demonstrated that the intrinsic neurons contain a number of different biogenic amines such as dopamine and serotonin, as well as neuropeptides including a substance P-like peptide and a galanin-like peptide. The results of these studies indicate that the anatomical and histochemical organization of the mudpuppy cardiac ganglion is more complex than that seen in other amphibians and is very similar to that found in most mammalian species. Previously, we showed that galanin has actions that make it of interest as a potential inhibitory neurotransmitter in the mudpuppy cardiac ganglion. Galanin hyperpolarizes and decreases membrane excitability in most parasympathetic neurons. Here we show that galanin initiates membrane hyperpolarization by activating a voltage- and time-dependent potassium conductance. We also present the initial results of ongoing studies which indicate that calcitonin gene-related peptide can depolarize some of the parasympathetic neurons as well as evidence that serotonin initiates depolarization in many parasympathetic neurons. This serotonin-induced depolarization consists of an initial transient depolarization followed by a longer, more slowly developing depolarization. Action potential activity is stimulated during the initial period of depolarization, but depressed during the later, slow depolarization. The results of these electrophysiological experiments suggest that many of the bioactive substances that have been identified in the different cells and nerve fibers within the cardiac ganglion affect the excitability of the postganglionic neurons. In conclusion, we suggest that the results of the studies summarized in this review demonstrate that the cardiac ganglion in the mudpuppy is not simply a relay station. Rather, the cardiac ganglion has a complex organization and exhibits a diversity of physiological responses, indicating that it very likely is another site of integration for control of cardiac function.

Intercellular signaling in Necturus taste buds: chemical excitation of receptor cells elicits responses in basal cells

1. Taste cells in intact taste buds in slices of Necturus lingual epithelium were impaled with microelectrodes for intracellular recording. Two types of cells were investigated: taste receptor cells and basal cells. 2. Impaling cells in the apical end of taste buds resulted in intracellular records from taste receptor cells. Applying short pulses (100- to 200-ms duration) of 140 mM KCl solution to the apical pore elicited receptor potentials in the taste receptor cells. 3. Impaling cells in the base of the taste bud resulted in intracellular records from taste receptor cells and basal cells. KCl applied to the taste pore elicited responses in the basal region that varied greatly in both magnitude and time of onset. The latency of these responses (time of onset compared with the onset of the receptor potential) ranged from 0 to hundreds of milliseconds. 4. Impaled cells were identified by injecting Lucifer yellow after recording KCl responses for 21 cells. KCl responses recorded from identified basal cells all had latencies of greater than 75 ms. KCl responses from identified receptor cells all had latencies of less than 75 ms. 5. One explanation for the long latency of KCl responses recorded in basal cells is that the responses represent postsynaptic potentials. In agreement with this interpretation, long-latency responses, but not short-latency responses, were reversibly reduced by the Ca antagonist Cd (1 mM, 10- to 20-min bath exposure). 6. Long-latency responses also differed from short-latency responses in their voltage dependence. Short-latency responses had the same voltage dependence as apically recorded receptor potentials, increasing with hyperpolarization from resting potential with an extrapolated reversal potential near 0 mV. Long-latency responses were much less dependent on voltage in this range. 7. We measured the spread of exogenously applied KCl with potassium-sensitive electrodes. Long-latency responses were not generated by diffusion of applied KCl to the basal region of the taste bud. A small transient increase in extracellular potassium occurred at the base of the taste bud after chemostimulation at the apical pore. This increase was due to depolarization-evoked release of potassium from taste cells and did not cause the long-latency responses in basal cells. 8. We conclude that short-latency (less than 75 ms) responses recorded from cells situated in the bases of taste buds are electrotonically conducted receptor potentials generated at the apical region. Long-latency (greater than 75 ms) responses are consistent with recording postsynaptic responses in basal cells.

Stretch- and volume-activated channels in isolated proximal tubule cells

Apical and basolateral channels were studied in isolated proximal tubule cells of Necturus kidney. Many of these isolated cells maintained their polarity, with clearly delineated apical and basolateral regions. A 20-pS stretch-activated (SA) cation-selective channel was identified at the apical side of these cells. This channel was permeable to Ca, K, and Na but was not significantly gated by either membrane potential or cytosolic Ca. Negative pipette pressure (15 cmH2O) increased the open probability (Po) of this channel from 0.04 +/- 0.02 to 0.26 +/- 0.08 (n = 6). Two types of Ca-independent, mechanosensitive, K-selective (SAK) channels were identified at the basolateral surface of polarized proximal tubule cells, i.e., a 30-pS long-open time (50 +/- 7 ms) channel (n = 9), and a 46-pS short-open time (1.3 +/- 0.7 ms) channel (n = 10). Pipette suction (-12 cmH2O) increased the Po of the short-open time channels from 0.008 to 0.015 and increased the Po of the long-open time channel from 0.03 to 0.19. The effect of swelling was studied with isolated cells suspended at the tip of patch pipettes. A 50% dilution of the bath doubled cell volume, hyperpolarized the membrane potential by 11 +/- 0.7 mV, and increased the Po of the basolateral SAK channels. This was followed by a spontaneous regulatory volume decrease (RVD), repolarization of the membrane potential, and a decrease in Po. In contrast, isosmotic (bath side) replacement of an impermeant anion (methanesulfonate) with a permeant anion (Cl) doubled cell volume in 5 min but without a subsequent RVD. This sustained swelling hyperpolarized the cell potential by 5.5 +/- 0.7 mV (n = 16) and increased the Po of short-open time channel by a factor of 2.3 from 0.03 +/- 0.01 to 0.07 +/- 0.02 (n = 6). The increase in Po was primarily produced by a reduction in the interburst closed time, which decreased from 142 +/- 43 ms in K methanesulfonate to 36 +/- 11 ms in KCl solutions. These results are consistent with the hypothesis that cell swelling activates Ca-independent K channels at the basolateral membrane of renal proximal tubule. Efflux of K through these channels may partially mediate renal cell volume regulation.

Water permeability of ventricular cell membrane in choroid plexus epithelium from Necturus maculosus

1. The osmotic water permeability Lp and the relations between the flows of H2O, K+ and Cl- were studied in the ventricular membrane of the epithelium from the choroid plexus of Necturus maculosus. 2. The flows were induced by abrupt changes in external osmolarity of the ventricular solution. Solution changes were convective and no effects of unstirred layers could be detected on measured parameters. 3. The initial rate of change in intracellular concentrations of K+ and Cl- was monitored by double-barrelled ion-selective microelectrodes. 4. The initial rate of flux of H2O could be monitored as the changes in the concentration of intracellular choline ions (Ch+i). When 0.5 mmol l-1 of choline chloride was added to the external solutions, Ch+i attained values of 1-5 mmol l-1. The dilution or concentration of Ch+i could be recorded by K+ electrodes since the sensitivity of these to Ch+ is more than 50 times greater than to K+. 5. The Lp of the ventricular membrane of the epithelium was 1.4-2.1 x 10(-4) cm s-1 (osmol l-1)-1 and independent of the direction of the induced water flux. Lp was unchanged in tissues adapted to osmolarities of half the physiological value. 6. The efflux of H2O induced by mannitol was associated with an instantaneous efflux of K+ which was inhibited by furosemide. The fluxes had a ratio of 40 mmol l-1. The influx of H2O induced by the removal of NaCl from the ventricular solution was associated with an instantaneous influx of K+. The H2O influx had a ratio to the flux of K+ of 70 mmol l-1. 7. The efflux of H2O induced by mannitol was associated with an efflux of Cl- which was inhibited by furosemide. The ratio of the two fluxes was in the range 15-44 mmol l-1. 8. The conclusion is that the Ch+ method gives a reliable measure of the movement of H2O across the ventricular membrane. The magnitude of the Lp and its relevance to transepithelial transport are discussed. The osmotically induced H2O movement is accompanied by furosemide-sensitive fluxes of K+ and Cl- of the same magnitude. This suggests that co-transport between H2O and KCl can take place in the membrane.