Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus

1. The interaction between Cl-, K+ and H2O fluxes were studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. The flux of H2O was measured by means of K+ electrodes as the dilution or concentration of intracellular choline ions, Ch+i. 2. In one series of experiments Cl- was readministered to the ventricular solution of tissues incubated in media with low Cl- concentrations. The resulting influx of Cl- was associated with an instantaneous influx of K+ and H2O. 3. Both the Cl- and the K+ influxes were reduced by the diuretic furosemide but were unaffected by inhibitors of Na+, K(+)-ATPase or changes in membrane potentials induced by Ba2+. Since the influx of K+ proceeds against its electrochemical gradient and is unaffected by changes in membrane potentials, the membrane exhibits secondary active, electroneutral transport of K+. 4. The influx of water, initiated simultaneously with the influx of K+ and Cl-, commenced before these ions had changed the osmolarity of the intracellular solution significantly. The influx of H2O could proceed against an osmotic gradient. The influx stopped when 100 mmol l-1 of mannitol was added to the ventricular solution at the same time as the Cl- ions. The influx of H2O was inhibited by K+ removal, furosemide or high external Ba2+ (10 mmol l-1), but not by strophanthidin, ouabain or low concentrations of Ba2+ (0.5 mmol l-1). The influx could not continue with other permeable anions, NO3-, acetate- or SCN-, replacing Cl-. 5. In another series of experiments Cl- was removed from the ventricular solution of tissues bathed in saline solutions with normal concentrations of Cl-. The resulting efflux of Cl- was associated with an instantaneous efflux of K+ and H2O. This efflux of H2O could proceed against an osmotic gradient of up to 70 mosmol l-1. This effect was inhibited by furosemide, in which case the water fluxes were entirely dependent on the osmotic gradients and the osmotic water permeability Lp of the ventricular membrane. 6. The data suggest that there is a coupling between the flux of KCl and of water in the ventricular membrane, which implies that the reflection coefficient sigma for KCl under the given circumstances is less than one. I suggest that the ability of leaky epithelia to transport against osmotic gradients depends on such a coupling, which derives from the properties of the proteins through which K+, Cl- and H2O leave the cell.

Immunocytochemistry of gamma-aminobutyric acid, glutamate, serotonin, and histamine in Necturus taste buds

Little information is currently available about which neurotransmitters are involved in signal processing in the peripheral sensory organs of taste, taste buds. Synaptic contacts between taste cells and sensory axons have long been known to exist, but what substances are active at these synapses is not known. Our objective in this study was to test for the presence of the neurotransmitter candidates, GABA, glutamate, serotonin, and histamine in taste buds of Necturus maculosus. Light microscopic immunocytochemical techniques were used to investigate the location of these substances in taste buds and surrounding epithelium. GABA and glutamate were detected in nerve fibers that innervate the taste buds, and, to a substantially lesser extent, in fine, varicose axons that penetrated the surrounding nontaste epithelium. Serotonin immunostaining was strong in basal cells in frog taste discs but was only faintly detected in Necturus taste buds. Histamine was not detected at all in taste buds. We conclude that amino acid neurotransmission may be involved in taste mechanisms and that monoamines may also play a role in chemosensory transduction in the taste bud. On the basis of our inability to detect histamine with immunocytochemical techniques, we conclude that this substance is unlikely to be a major neurotransmitter in Necturus taste buds.

Cytosolic pH regulates maxi K+ channels in Necturus gall-bladder epithelial cells

1. The patch clamp technique was used to study the effects of internal and external pH on the Ca(2+)- and voltage-activated maxi K+ channel present in the apical membrane of Necturus gall-bladder epithelial cells. 2. When the pH of the solution bathing the cytosolic side of inside-out patches (pHi) was lowered from 7.9 to 6.9, with internal free Ca2+ concentration ([Ca2+]i) buffered below saturation levels for the channel gating sites, channel open probability (Po) decreased. At saturating Ca2+ concentrations, Po was near 1.0, and unaffected by pHi. The results are consistent with a competitive interaction between Ca2+ and H+ at regulatory binding sites. Kinetic analysis assuming competitive binding yields a Hill coefficient for H+ of 1.3. 3. At sub-maximal [Ca2+]i, changing the pH of the solution bathing the extracellular surface of the patch (pHo) between 8 and 7, had no effect on maxi K+ channel Po, but lowering pHo to 6 or 5 significantly reduced Po. At saturating [Ca2+]i, Po was independent of pHo. 4. There were no effects of either pHi or pHo on single-channel conductance. 5. Inasmuch as reductions in either pHo or pHi decrease maxi K+ channel Po, changes in maxi K+ channel activity account in part for the reduction of apical membrane K+ conductance elicited by acidification of the bathing medium. The dominant effect of pH on maxi K+ channels is on the cytosolic surface of the membrane. 6. The change in Po elicited by small changes in [H+]i (delta Po/delta [H+]i) is -7.6 microM-1, compared to delta Po/delta [Ca2+]i = 2.6 microM-1, both at Vm = -30 mV and at physiological intracellular [H+] and [Ca2+]. This implies that [H+]i and [Ca2+]i have opposite effects on channel Po at physiological levels and underlines the importance of pHi in channel gating.

Potassium currents of isolated Necturus enterocytes: a whole-cell patch-clamp study

1. The whole-cell recording mode of the patch-clamp technique was used to investigate the ionic currents of enterocytes isolated from the small intestine epithelium of Necturus maculosus. 2. When enterocytes were bathed in a physiological Ringer solution and dialysed with a K(+)-rich intracellular solution containing 1.5 x 10(-7) M intracellular Ca2+, strongly outwardly rectifying currents were observed. These currents were absent from enterocytes internally dialysed with K(+)-free solutions. 3. The kinetics of activation of the outwardly rectifying current was monoexponential with rate constants decreasing with depolarization from 160 ms at 20 mV to 40 ms at 60 mV. Similar voltage dependence of the relaxations after activation were observed. 4. Strongly buffering intracellular Ca2+ with EGTA inhibited outward currents, while increasing Ca2+ increased both their magnitude and rate of activation. 5. Bath application of the K+ channel inhibitors Ba2+ and TEA greatly attenuated outwardly rectifying currents. This observation plus the fact that tail currents reverse near EK points to K+ as the charge carrier in these currents. 6. Outside-out patches showed maxi K+ channels and lower conductance K+ channels. Averaging fluctuations of the maxi K+ channels gave a kinetic behaviour similar to the whole-cell currents.

Two types of potassium currents seen in isolated Necturus enterocytes with the single-electrode voltage-clamp technique

1. The ionic permeability of Necturus maculosus small intestine epithelial cells was investigated using intracellular microelectrodes to measure membrane potential in intact tissue or by the single-electrode voltage-clamp technique in isolated cells. 2. The basolateral membrane of enterocytes appears to be K+ selective as demonstrated by the dependence of membrane potential and fractional serosal resistance measured in the intact epithelium on serosal K+ concentration. 3. Isolated cells had membrane potentials similar to those measured in the intact tissue. Voltage-clamp experiments in a physiological Ringer solution showed the presence of both large outward and inward currents. 4. Removal of Cl- from the bathing medium, linear subtraction or the use of a Cl- channel blocker revealed outwardly rectifying currents. The quasi-linear component was also revealed following K+ channel inhibition; it reversed near ECl, suggesting that the charge carrier was Cl-. 5. Outwardly rectifying currents could be kinetically resolved into two components. A fast component (tau for activation < 4 ms) accounted for 60-80% of the total current at positive potentials. A slowly activating component appeared at voltages positive to 50 mV with tau for activation of > 25 ms. 6. The slow outward current showed strong voltage dependence of both activation and relaxation, which were faster at more depolarized potentials. 7. Both fast and slow outward currents seem to be carried by K+ as they were blocked by Ba2+ and tetraethylammonium (TEA). Tail current analysis of the slow component indicated a reversal potential very similar to EK. 8. Fast outward currents were half-activated at about -40 mV whereas slow outward currents were only apparent at more positive potentials. It is proposed that the fast outward K+ current plays a role, together with Cl- currents, in determining the resting membrane potential of Necturus enterocytes.

Further studies of electrogenic Na+/HCO3- cotransport in glial cells of Necturus optic nerve: regulation of pHi

In the presence of Ba++, an increase in the bath HCO3- at constant CO2 (i.e., variable bath pH) produced a hyperpolarization. The hyperpolarizing effect of adding HCO3-/CO2 at constant bath pH was not significantly affected by the presence of 50 mumol/l strophanthidin. In the absence of Ba++, addition of HCO3-/CO2 at constant bath pH produced a Na(+)-dependent hyperpolarization. Therefore, CO2 movements, electrogenic Na+/K+ pump activity and changes in Ba++ binding do not contribute significantly to the hyperpolarization induced by HCO3-. These results along with the results of previous studies (Astion et al: J Gen Physiol 93:731, 1989) strongly suggest that the hyperpolarization induced by the addition of HCO3- is due to an electrogenic Na+/HCO3- cotransporter, which transports Na+, HCO3- (or its equivalent), and net negative charge across the glial membrane. To study the role of electrogenic Na+/HCO3- cotransport in the regulation of pHi in glial cells, we used intracellular double-barreled, pH-sensitive microelectrodes. At a bath pH of 7.5, the mean initial intracellular pH (pHi) was 7.32 (SD 0.03, n = 6) in HEPES-buffered Ringer's solution and 7.39 (SD 0.1, n = 6) in HCO3-/CO2 buffered solution. These values for pHi are more than 1.2 pH units alkaline to the pHi predicted from a passive distribution of protons; thus, these cells actively regulate pHi. Superfusion and withdrawal of 15 mmol/l NH4+ induced an acidification of 0.2 to 0.3 pH units, which recovered toward the original steady-state pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

A calcium-permeable stretch-activated cation channel in renal proximal tubule

Isolated Necturus proximal tubules were split to expose the apical membrane surface for patch clamping. When both pipette and bath solutions contained only Ca, N-methyl-D-glucamine, and methanesulfonate, inwardly directed Ca currents were observed through a stretch-activated (SA) cation channel with conductance of 18 +/- 1 pS (n = 19). The SA cation channel exhibited little discrimination among Na, K, and Ca but was at least nine times more selective for cations than anions. The channel was not significantly gated by either membrane potential or cytosolic Ca. However, application of 15 cmH2O suction to patch pipette significantly increased the mean number of open channels by a factor of 6.5, from 0.04 +/- 0.02 to 0.26 +/- 0.08 (n = 11). Ca currents through the SA cation channel were reversibly blocked by 10 microM gadolinium, which was applied to outside surface of excised patches. This is similar to gadolinium block of stretch-activated channels in Xenopus oocytes (X.-C. Yang and F. Sachs. Science Wash. DC 243: 1068-1071, 1989). A Ca-dependent, maxi-K channel (92 +/- 9 pS, n = 5) was also found at the apical membrane of the same proximal tubules. In some cases this maxi-K channel appeared to be indirectly activated by pipette suction, raising the possibility that Ca influx through the SA cation channel may regulate K efflux via the maxi-K channel. Such a process could mediate cell volume regulation and maintain electrolyte homeostasis during normal variations in Na-substrate cotransport.

Maxi K+ channels and their relationship to the apical membrane conductance in Necturus gallbladder epithelium

Using the patch-clamp technique, we have identified large-conductance (maxi) K+ channels in the apical membrane of Necturus gallbladder epithelium, and in dissociated gallbladder epithelial cells. These channels are more than tenfold selective for K+ over Na+, and exhibit unitary conductance of approximately 200 pS in symmetric 100 mM KCl. They are activated by elevation of internal Ca2+ levels and membrane depolarization. The properties of these channels could account for the previously observed voltage and Ca2+ sensitivities of the macroscopic apical membrane conductance (Ga). Ga was determined as a function of apical membrane voltage, using intracellular microelectrode techniques. Its value was 180 microS/cm2 at the control membrane voltage of -68 mV, and increased steeply with membrane depolarization, reaching 650 microS/cm2 at -25 mV. We have related maxi K+ channel properties and Ga quantitatively, relying on the premise that at any apical membrane voltage Ga comprises a leakage conductance and a conductance due to maxi K+ channels. Comparison between Ga and maxi K+ channels reveals that the latter are present at a surface density of 0.09/microns 2, are open approximately 15% of the time under control conditions, and account for 17% of control Ga. Depolarizing the apical membrane voltage leads to a steep increase in channel steady-state open probability. When correlated with patch-clamp studies examining the Ca2+ and voltage dependencies of single maxi K+ channels, results from intracellular microelectrode experiments indicate that maxi K+ channel activity in situ is higher than predicted from the measured apical membrane voltage and estimated bulk cytosolic Ca2+ activity. Mechanisms that could account for this finding are proposed.

Intracellular pH in isolated Necturus antral mucosa exposed to luminal acid

Regulation of intracellular pH in gastric epithelial surface cells exposed to luminal acid was investigated in isolated Necturus antral mucosa using microelectrode technique. Exposure of the mucosa to luminal pH 2 acidified intracellular pH from 7.21 +/- 0.01 to 6.95 +/- 0.04 (N = 50). Removal of Na+ from the perfusates or addition of amiloride (1 mM) to serosal perfusate (containing HCO3-) had no influence on intracellular pH during exposure to pH 2 (N = 6), but removal of HCO3-/CO2 from or addition of 4, acetamido-4-isothiocyanatostilbene-2,2-disulfonic acid (0.5 mM) to the serosal perfusate (containing Na+) acidified intracellular pH from 7.02 +/- 0.03 to 6.45 +/- 0.15 (p less than 0.01, N = 10) and from 6.97 +/- 0.06 to 6.58 +/- 0.26 (p less than 0.01, N = 6), respectively, in 15 min. In tissues exposed to mucosal pH 6, epithelial surface pH was about 1.3 pH units higher than pH of the mucosal bulk solution. Removal of Cl-/HCO3- from the serosal perfusate acidified epithelial surface pH by about 0.5 pH units (p less than 0.01, N = 6), suggesting that serosal HCO3- sustains intracellular pH, at least in part, by generating an alkaline buffer layer at the epithelial surface. In the absence of HCO3-/CO2, a stable intracellular pH was obtained when the tissue was exposed to mucosal pH 2.7, but in this situation intracellular pH was sensitive to Na+ removal or amiloride addition, intracellular pH decreasing from 7.00 +/- 0.07 to 6.48 +/- 0.10 (p less than 0.01, N = 6) and from 6.86 +/- 0.06 to 6.32 +/- 0.01 (p less than 0.01, N = 7), respectively, in 15 min. The data suggest that in gastric epithelium exposed to luminal acid, physiological intracellular pH is primarily maintained by the buffer action of serosal HCO3- transported to the epithelial surface to impede the entry of luminal H+ into mucosal tissue. Removal of the sheltering HCO3- unmasks a second line, Na(+)-dependent and amiloride-sensitive intracellular pH regulatory mechanism, presumably a Na+/H+ antiport.

Norepinephrine stimulation of sodium transport in Necturus urinary bladder

Norepinephrine alters the transepithelial electrical properties of an open-circuited urinary bladder from the mud puppy, Necturus maculosus. When 10(-5) M norepinephrine is superfused over the serosa of the epithelium, the transepithelial voltage (Vt) and short-circuit current (Isc) increase as the resistance (Rt) decreases. The norepinephrine-mediated changes are reversed by the addition of amiloride (5.10(-5) M) to the mucosal Ringer's solution. The serosal adrenoceptors mediating the Na+ transport are more sensitive to norepinephrine (EC50 = 1.2.10(-6) M) than to epinephrine or isoproterenol. Since the Isc is blocked selectively by the antagonist, phenoxybenzamine, stimulation of active transepithelial Na(+)-flux by catecholamines is mediated by an alpha-adrenoceptor. The apical cell membrane voltage (Va) and fractional resistance (fRa) were recorded using conventional KCl-filled microelectrodes. Untreated tissues have Va close to 0 mV while the basolateral membrane voltage (Vb) is between -85 and -95 mV. About 90% of Rt is apical cell membrane resistance (fRa). When amiloride inhibits sodium transport, Va becomes negative, Vb hyperpolarizes slightly and fRa increases to 97%. On the other hand, if the bladders are treated with norepinephrine, fRa decreases to 79% as Va becomes positive and Vb depolarizes. When Rt changes, the resistance of the paracellular pathway (Rp) is unaltered. Changes in the electrical properties of the tissue appear to be mediated primarily by alterations in Ra. Since the Necturus bladder does not respond to antidiuretic hormone, this study implies that biogenic amines regulate Na+ transport in the epithelium.

Radial density distribution of chromatin: evidence that chromatin fibers have solid centers

Fiber diameter, radial distribution of density, and radius of gyration were determined from scanning transmission electron microscopy (STEM) of unstained, frozen-dried chromatin fibers. Chromatin fibers isolated under physiological conditions (ionic strength, 124 mM) from Thyone briareus sperm (DNA linker length, n = 87 bp) and Necturus maculosus erythrocytes (n = 48 bp) were analyzed by objective image-processing techniques. The mean outer diameters were determined to be 38.0 nm (SD = 3.7 nm; SEM = 0.36 nm) and 31.2 nm (SD = 3.6 nm; SEM = 0.32 nm) for Thyone and Necturus, respectively. These data are inconsistent with the twisted-ribbon and solenoid models, which predict constant diameters of approximately 30 nm, independent of DNA linker length. Calculated radial density distributions of chromatin exhibited relatively uniform density with no central hole, although the 4-nm hole in tobacco mosaic virus (TMV) from the same micrographs was visualized clearly. The existence of density at the center of chromatin fibers is in strong disagreement with the hollow-solenoid and hollow-twisted-ribbon models, which predict central holes of 16 and 9 nm for chromatin of 38 and 31 nm diameter, respectively. The cross-sectional radii of gyration were calculated from the radial density distributions and found to be 13.6 nm for Thyone and 11.1 nm for Necturus, in good agreement with x-ray and neutron scattering. The STEM data do not support the solenoid or twisted-ribbon models for chromatin fiber structure. They do, however, support the double-helical crossed-linker models, which exhibit a strong dependence of fiber diameter upon DNA linker length and have linker DNA at the center.

Apical membrane Na+/H+ exchange in Necturus gallbladder epithelium. Its dependence on extracellular and intracellular pH and on external Na+ concentration

Intracellular microelectrode techniques and extracellular pH measurements were used to study the dependence of apical Na+/H+ exchange on mucosal and intracellular pH and on mucosal solution Na+ concentration ([Na+]o). When mucosal solution pH (pHo) was decreased in gallbladders bathed in Na(+)-containing solutions, aNai fell. The effect of pHo is consistent with titration of a single site with an apparent pK of 6.29. In Na(+)-depleted tissues, increasing [Na+]o from 0 to values ranging from 2.5 to 110 mM increased aNai; the relationship was well described by Michaelis-Menten kinetics. The apparent Km was 15 mM at pHo 7.5 and increased to 134 mM at pHo 6.5, without change in Vmax. In Na(+)-depleted gallbladders, elevating [Na+]o from 0 to 25 mM increased aNai and pHi and caused acidification of a poorly buffered mucosal solution upon stopping the superfusion; lowering pHo inhibited both apical Na+ entry and mucosal solution acidification. Both effects can be ascribed to titration of a single site; the apparent pK's were 7.2 and 7.4, respectively. Diethylpyrocarbonate (DEPC), a histidine-specific reagent, reduced mucosal acidification by 58 +/- 4 or 39 +/- 6% when exposure to the drug was at pHo 7.5 or 6.5, respectively. Amiloride (1 mM) did not protect against the DEPC inhibition, but reduced both apical Na+ entry and mucosal acidification by 63 +/- 5 and 65 +/- 9%, respectively. In the Na(+)-depleted tissues mean pHi was 6.7. Cells were alkalinized by exposure to mucosal solutions containing high concentrations of nicotine or methylamine. Estimates of apical Na+ entry at varying pHi, upon increasing [Na+]o from 0 to 25 mM, indicate that Na+/H+ exchange is active at pHi 7.4. Intracellular H+ stimulated apical Na+ entry by titration of more than one site (apparent pK 7.1, Hill coefficient 1.7). The results suggest that external Na+ and H+ interact with one site of the Na+/H+ exchanger and that cytoplasmic H+ acts on at least two sites. The external titratable group seems to be an imidazolium, which is apparently different from the amiloride-binding site. The dependence of Na+ entry on pHi supports the notion that the Na+/H+ exchanger is operational under normal transport conditions.

Electrophysiological characteristics of the Necturus proximal duodenal mucosa: effects of ion substitutions

Voltage-sensitive glass micro-electrodes were used to determine the electrical characteristics of Necturus proximal duodenal epithelium. Some comparative experiments with amiloride were performed with gastric antrum. The apical and the basolateral cell membrane potential differences in duodenum averaged -32 mV and -34 mV (cell negative) respectively. The transepithelial potential difference was -2 mV (lumen negative). The EMF across the apical cell membrane was -29 mV and that across the basolateral cell membrane -39 mV. The transepithelial resistance (Rt) of 63 omega cm2 and the paracellular pathway resistance (Rs) of 80 omega cm2 are of magnitudes similar to that previously reported for more distal amphibian small intestine. The apical and basolateral cell membrane resistances, however, were lower than those reported for distal small intestine. Ion permeabilities for Na+, K+ and Cl- across the apical cell membrane were calculated from ion substitution experiments. The permeability sequence across the apical cell membrane was PK:PCl:PNa 3.02:1.31:1.00. Luminal amiloride (10(-4)M) was without significant effect, further indicating a low duodenal membrane conductance for Na+. The low conductances for K+, Na+ and Cl- suggest that the major ion transport modes across the apical duodenal cell membrane are electroneutral in nature. In contrast, amiloride caused a marked increase in the transmembrane potentials in the antrum.

Alkaline secretion by Necturus proximal duodenal mucosa

Proximal duodenum from the amphibian Necturus was stripped of muscle layers and the mucosa was mounted as a tube for studies of alkali transport or as a flat sheet for intracellular impalement by voltage-sensitive glass micro-electrodes. The mucosa alkalinized the unbuffered luminal perfusate at a high rate (3.4 muequiv. cm-1 h-1) and developed a transepithelial electric potential difference of 5.7 mV (lumen negative). Transport was inhibited by 2,4-dinitrophenol (10(-4) M) and by furosemide (10(-3) M) and SITS (10(-3) M) on the seros but not on the mucosal side, indicating dependence on tissue metabolism and on serosal membrane Cl-/HCO3- exchange. Prostaglandin E2 (10(-7)-10(-5) M) and dibutyryl cyclic AMP (10(-6)-10(-4) M) had no effects on the secretion or transepithelial electrical potential difference. removal of serosal HCO3- decreased luminal alkalinization by 75%, indicating a contribution by passive migration of HCO3- and/or a dependence of transcellular transport on the nutrient supply of this ion. Administration of HCO3- (17.8 mM) to the luminal perfusate affected neither the transepithelial nor transmembrane electrical potential differences nor the resistance ratio. It is thus unlikely that the luminal membrane possesses any major HCO3- conductance.

Anion channels in a leaky epithelium. A patch-clamp study of choroid plexus

We have used the patch-clamp technique to characterize three anion channels in the ventricular membrane of the choroid plexus epithelium from Necturus. The most frequently occurring channel had a nonlinear IV-curve. The conductance in excised patches with 112 mM chloride at both sides was 28 pS at 0 mV, increasing towards positive membrane potentials. The selectivity ratios were PNa:PCl less than or equal to 0.1 and PNO3:PCl:PHCO3 = 1.6:1:0.43. SITS and furosemide (1 mM) on the inside reduces chloride flux to 0.15 and 0.37 times the control value. In attached patches, the most commonly observed channel had a conductance of 7.5 pS. The single-channel current for this channel reversed direction at 15 mV hyperpolarization, indicating accumulation of chloride to a factor of 1.8 above equilibrium. External stimulation of the tissue by theophylline, IBMX and dbcAMP, or by hypotonic shock did not increase the activity of this channel. In very few excised patches, we have observed a chloride channel with a conductance of 7 pS with 112 mM chloride at both sides. The 7 pS channel appears to be identical to a 2 pS channel found in attached patches. The 2 pS channel was not normally active in attached patches but was activated in 28% of the patches by external stimulation. Finally, in few excised patches we have found a 375 pS channel which inactivates within seconds when membrane potential is stepped from 0 mV to a value that differs more than 10-20 mV from zero. The channel did not conduct gluconate but PNO3:PCl = 1.08 and PNa:PCl less than or equal to 0.1. Internal SITS and furosemide (1 mM) reduced chloride flux to 0.3 and 0.5 times the control value. The channel was never seen in attached patches. The current carried through these channels can not account for the transepithelial steady state Cl- -flux measured by microelectrodes. KCl exit from the cell is suggested to be carried by KCl-cotransport or by channels that are too small to be seen in patch-clamp experiments.

Characterization of a phosphorylation-activated Cl-selective channel in isolated Necturus enterocytes

1. The cell-attached and excised inside-out configurations of the patch-clamp technique were employed to probe isolated enterocytes of Necturus maculosus for the presence of Cl(-)-selective channels. 2. Chloride-selective channels were rarely observed unless cells were previously stimulated by agonists that raise cyclic AMP. In cell-attached patches forskolin (20 microM) or dibutyryl cyclic AMP 2 mM) evoked single-channel activity that reversed, depending on the cell, between 9 and 27 mV positive to the spontaneous membrane potential. This is close to the Cl- equilibrium potential in those cells; the single-channel current-voltage relationship was linear with a unitary slope conductance between 17 and 25 pS (pipettes filled with 100 mM-NaCl). 3. Large depolarizing voltage steps also activated Cl- channels in excised inside-out membrane patches that were previously quiescent. This mode of activation produced a distinctive single-channel current-voltage relationship with strong outward rectification at depolarizing membrane potentials. Single-channel cord conductance at negative potentials was 15-18 pS and increased to 45 pS at + 100 mV. 4. Altering the Cl- concentration in the bathing solution of excised inside-out patches displaced the observed reversal potential (Erev) to values predicted for Cl- equilibrium potential. Replacement of K+ for Na+ was without effect. 5. The effect of different anions upon Erev was used to determine the channel anion selectivity in excised inside-out patches. The permeability sequence was SCN- greater than I- greater than Br- greater than Cl- greater than F- greater than HCO3- greater than gluconate which corresponds to Eisenman's sequence 1. Neither ionic size nor diffusion rates determine the permeation of ions through the channel. 6. In channels activated by depolarization the open probability (Po) was insensitive to changes in the Ca2+ concentration (less than 10(-8)-10(-3) M) bathing the cytoplasmic face of excised inside-out patches. Depolarization was also without marked effect on Po. 7. Chloride channels in excised inside-out patches were inhibited by stilbene and diphenylamine-2-carboxylate derivatives. 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS, 5 x 10(-5) M) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 1 x 10(-5) M) caused an irreversible 'flickery' blockade without altering single-channel current. 3'5-Dichlorodiphenylamine-2-carboxylic acid (DDPC, 5 x 10(-5) M) reduced the currents at every voltage without apparent effects on gating properties of the channel.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of bumetanide on tubuloglomerular feedback in Necturus maculosus

A non-invasive technique was developed to measure single-nephron glomerular blood flow (SNGBF) in Necturus maculosus. Erythrocytes labelled with rhodamine, a fluorescent dye, were injected systemically and the frequency at which labelled cells entered an arteriole was measured. Frequency was converted to flow by measuring the concentration of labelled erythrocytes in whole blood. Dependence of SNGBF on flow rate in early distal tubules was used to assess tubuloglomerular feedback (TGF). SNGBF decreased with increasing flow in the early distal tubule in a pattern typical of TGF; SNGBF decreased 25% at the highest flow rates. SNGBF increased when bumetanide was added to the perfusate, but the TGF response to flow rate persisted. IC50 (concentration that produces half-maximal inhibition) was 2.4 x 10(-10), 9.8 x 10(-10) and 1.2 x 10(-9) M bumetanide at distal perfusion rates of 5, 10 and 20 nl min-1 respectively. These results are consistent with modulation of SNGBF according to the rate of luminal entry of NaCl into early distal tubule cells. This transport rate depends on the luminal concentration of NaCl, which is tubular flow rate-dependent; NaCl and bumetanide compete.

Effects of parathyroid hormone on cyclic-AMP concentrations of in vitro Necturus maculosus gastric antrum

The effects of bovine parathyroid hormone (bPTH1-84) on the stimulation of intracellular cyclic-AMP [cAMP] were investigated in an in vitro preparation of Necturus maculosus antral mucosa. When the antrum was exposed to 1, 5, 10, or 100 nM bPTH1-84, there was an approximately 2-fold nonlinear increase in tissue [cAMP] over basal values. The pretreatment of the antral mucosa with 1 mM isobutylmethylxanthine (IBMX, a phosphodiesterase inhibitor) increased with detectability of mucosal [cAMP]. The addition of 1, 5, 10, or 100 nM bPTH1-84 to tissues pretreated with IBMX resulted in an approximately 3.5-fold linear increase in mucosal [cAMP] over basal values. The time course of the generation of mucosal cAMP to 10 nM bPTH1-84 resulted in a small but significant transient increase at 2.5 min after the addition of bPTH1-84 but no change in the medium [cAMP]. In tissues pretreated with 1 mM IBMX the response to 10 nM bPTH1-84 was a large biphasic increase of [cAMP] at 2.5 min that progressively declined to near basal values by 15 min. There was also a significant sustained increase in the [cAMP] in the bathing medium at 2.5 min of tissues pretreated with IBMX followed by 10 nM bPTH1-84. These results suggest the presence of an adenylate cyclase that can be activated by a mammalian bPTH1-84 in elevating intracellular cAMP levels in the N. maculosus antral mucosa.

Kainate receptor-mediated synaptic currents in mudpuppy inner retinal neurons reduced by D-O-phosphoserine

1. The effects of D-O-phosphoserine (DOS) were examined on proximal neurons in the superfused mudpuppy retinal-eyecup preparation by measuring their synaptically evoked whole-cell currents with the use of patch-clamp electrodes. 2. DOS reduced the light-evoked excitatory postsynaptic potentials (EPSPs) of amacrine and ganglion cells. This suppression was present even though the center responses of both ON- and OFF-bipolar cells were unaffected by DOS. 3. When recordings were done under voltage-clamp conditions. DOS diminished the magnitude of light-evoked synaptic currents associated with a reduction in synaptic conductance. 4. To determine which acidic amino acid receptor mediated the network-selective action of DOS, various glutamate agonists were tested against this excitatory amino acid receptor (EAAR) antagonist. DOS blocked the depolarizing effects of kainate (KA), but not those of N-methyl-D-aspartate (NMDA) or quisqualate (QQ). Thus DOS was a selective KA antagonist, and KA receptors appear to be the dominant EAAR subtype that mediates synaptic inputs into the inner retina of the mudpuppy.

Spatial buffering of light-evoked potassium increases by retinal Müller (glial) cells

Activity-dependent variations in extracellular potassium concentration in the central nervous system may be regulated, in part, by potassium spatial buffering currents in glial cells. The role of spatial buffering in the retina was assessed by measuring light-evoked potassium changes in amphibian eyecups. The amplitude of potassium increases in the vitreous humor was reduced to approximately 10 percent by 50 micromolar barium, while potassium increases in the inner plexiform layer were largely unchanged. The decrease in the vitreal potassium response was accurately simulated with a numerical model of potassium current flow through Müller cells, the principal glial cells of the retina. Barium also substantially increased the input resistance of Müller cells and blocked the Müller cell-generated M-wave, indicating that barium blocks the potassium channels of Müller cells. Thus, after a light-evoked potassium increase within the retina, there is a substantial transfer of potassium from the retina to the vitreous humor by potassium current flow through Müller cells.

A specific androgen-binding protein (ABP) in Necturus testis and its zonal distribution

The urodele amphibian Necturus maculosus has a zoned testis, which is advantageous for separating Leydig cells from germinal elements and for studying stage-dependent biochemical changes. Using [3H]testosterone (T) in a standard binding assay and dextran-coated charcoal (DCC) or Sephadex LH-20 to separate free and bound steroids, we identified an androgen-binding protein (ABP) in Necturus testis cytosols. This protein was of high affinity (Kd = 10(-9) M) and was saturable (Bmax = 10(-9) M) and specific for androgen (T; 5 alpha-dihydrotestosterone, DHT) but could be distinguished from the androgen receptor of Necturus testis by its relative abundance (300-550 fmol/mg protein), short half-time of dissociation (3 min at 22 degrees C), inability to adhere to DNA-cellulose, and absence from nuclear extracts. Additionally, when analyzed on sucrose gradients, the ABP of Necturus testis sedimented at 6-7 S in both low or high ionic strength buffers. In that estradiol (E2) is a poor competitor for T-binding, this protein resembles a sex steroid-binding protein previously identified in urodele serum but differs from the ABP and testosterone-estradiol-binding globulin (TEBG) of rodents, humans, goldfish, and sharks. It is differentially distributed within the testis, with the highest levels in immature lobular regions composed of Sertoli cells and germ cells in premeiotic stages and lower levels in regions composed primarily of Leydig cells. The cellular source and function of this protein in Necturus testis remain to be determined.

APB increases apparent coupling between horizontal cells in mudpuppy retina

2-Amino-4-phosphonobutyrate (APB), an agonist at a unique type of glutamate receptor on depolarizing bipolar cells, caused an apparent increase in coupling between horizontal cells as evidenced by a decrease in amplitude of responses to illumination of the receptive field center and an increase in responses to illumination of the peripheral part of the receptive field. APB also caused a hyperpolarization of horizontal cells in darkness and increased the amplitude of responses to full-field illumination, which cannot be explained by an increase in electrical coupling between horizontal cells. Possible mechanisms for these actions are discussed.

B-wave of the electroretinogram. A reflection of ON bipolar cell activity

Light-evoked intraretinal field potentials (electroretinogram, ERG) have been measured simultaneously with extracellular potassium fluxes in the amphibian retina. The application of highly selective pharmacologic agents permitted us to functionally isolate various classes of retinal neurons. It was found that: (a) application of APB (2-amino-4-phosphonobutyrate), which has previously been shown to selectively abolish the light responsiveness of ON bipolar cells, causes a concomitant loss of the ERG b-wave and ON potassium flux. (b) Conversely, PDA (cis 2,3-piperidine-dicarboxylic acid) or KYN (kynurenic acid), which have been reported to suppress the light responses of OFF bipolar, horizontal, and third-order retinal neurons, causes a loss of the ERG d-wave as well as OFF potassium fluxes. The b-wave and ON potassium fluxes, however, remain undiminished. (c) NMA (N-methyl-DL-aspartate) or GLY (glycine), which have been reported to suppress the responses of third-order neurons, do not diminish the b- or d-waves, nor the potassium fluxes at ON or OFF. This leads to the conclusion that the b-wave of the ERG is a result of the light-evoked depolarization of the ON bipolar neurons. This experimental approach has resulted in two further conclusions: (a) that the d-wave is an expression of OFF bipolar and/or horizontal cell depolarization at the termination of illumination and (b) that light-induced increases in extracellular potassium concentration in both the inner (proximal) and outer (distal) retina are the result of ON bipolar cell depolarization.

Effects of aspirin and acetic acid on intracellular pH in necturus gastric mucosa

Intracellular microelectrode techniques were employed to examine the effects of luminal aspirin and acetic acid on intracellular pH and cell membrane potential in the surface epithelial cells of Necturus antrum. Antral mucosa was mounted in a modified Ussing chamber, and intracellular pH was determined from the difference between the potentials recorded by intracellular conventional and pH-sensitive microelectrodes. Under neutral conditions (pH7), aspirin (5 mM) hyperpolarized (-7.5 +/- 1 mV, p less than 0.0001) and acetic acid (5 mM) depolarized (+4 +/- 0.08 mV, p less than 0.001) cell membrane potential. Neither agent had any significant effect on intracellular pH. Under acidic conditions (pH 4.5), aspirin (5 mM) reduced the intracellular pH from 6.99 +/- 0.03 to 6.87 +/- 0.04 (p less than 0.001) and depolarized cell membrane potential from -36.7 +/- 1.5 to -30.3 +/- 1.6 mV, p less than 0.001). Similarly, acetic acid (5 mM) acidified the cells (-0.20 +/- 0.02, p less than 0.001) and depolarized cell membrane potential (+9.6 +/- 1.9 mV, p less than 0.01). These changes suggest that, in the absence of luminal acid, small organic acids, such as aspirin and acetic acid, may have complex effects on the ionic conductances of the surface cell membranes without altering intracellular pH. In contrast, under acidic conditions, these agents increase the permeability of the apical cell membrane-to-acid back-diffusion from the gastric lumen.

Measurement of passive membrane parameters with whole-cell recording from neurons in the intact amphibian retina

1. Whole-cell recordings have been obtained from intact, photoactive retinal neurons using patch-clamp electrodes in the amphibian superfused retina eyecup preparation. 2. After removal of the vitreous humor from the surface of the retina, using a collagenase with low tryptic activity, high-resistance seals (1-10 G omega) could be formed between the patch pipette and the cell membrane by applying mild suction to the pipette. Additional suction broke the membrane patch and provided continuity between the low-resistance pipette and the interior of the neuron. 3. Measurements of input resistance and time constant were obtained from bipolar, amacrine, and ganglion cells. Assuming the membrane capacitance was 1 microF/cm2, time constant data were used to derive the specific membrane resistance. The average specific membrane resistance for the inner retinal neurons in our sample was 68,000 omega.cm2. 4. Analysis of the charging curve induced by a brief current pulse applied to the soma was used to analyze the average electrotonic length of dendrites. The charging curves of some ganglion cells were well represented by a single exponential, suggesting that they were essentially isopotential. 5. The voltage decay along an equivalent cylinder model of a ganglion cell was calculated, using the experimentally obtained values of membrane resistance to compute decay of steady-state voltages along the dendritic tree. The calculations indicate that with the high membrane resistance values implied by this study, the electrotonic length of dendritic cables were short, and there may be relatively little attenuation of the synaptic potentials irrespective of their location along the dendritic tree.