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PubMed1994

Membrane properties of two types of basal cells in Necturus taste buds.

Necturus taste buds contain two types of basal cells: presumptive stem cells and Merkel-like basal cells. Both types of basal cells are small round cells located at the base of the taste bud, indistinguishable from each other with light microscopy. However, with electron microscopy, autoradiography, or immunocytochemistry, these two types of basal cells can be easily distinguished. We isolated basal cells from taste buds, characterized their voltage-dependent currents using gigaseal whole-cell recordings, and processed the cells for electron microscopy or immunocytochemistry. We were able to distinguish two cell types electrophysiologically and to correlate cell type with membrane properties. Isolated Merkel-like basal cells had several voltage-activated currents: transient, TTX-sensitive, inward Na+ current; sustained, saturating outward K+ current; and slowly inactivating inward Ca2+ current. These currents are similar to those observed in taste receptor cells. In contrast, presumptive stem cells from Necturus taste buds only had outward K+ currents.

Animals↗

PubMed1975

Some aspects of proximal tubular sodium chloride reabsorption in Necturus kidney.

Some aspects of proximal tubular sodium chloride reabsorption in Necturus kidney. Renal tubular reabsorption of fluid and sodium was measured by clearance methods in the doubly perfused Necturus kidney in which the bicarbonate concentration was varied between 0 and 60 mEq/liter. The effects of Damox (2.2 times 10-3M), ocubain (10-5M) and ethacrynic acid (10-4M) and of acidosis were also investigated. In addition to clearance experiments, stationary microperfusion experiments were carried out on promimal tubules to measure volume flow and steady-state sodium and chloride concentration differences across the tubular epithelium. In some experiments, the transepithelial electrical potential difference was also measured using an axial electrode system. The following results were obtained: 1) Bicarbonate is not essential to the operation of renal tubular fluid and sodium transport. 2) Total renal and proximal tubular fluid and sodium transport are partially inhibited by Diamox, ouabian and ethacrynic acid. 3) The proximal tubule maintains a significant transepithelial sodium and chloride concentration difference and a significant electrical potential difference (lumen-negative) in the presence of a poorly permeant nonelectrolyte. The direction and magnitude of the electrical polarization fully accounts for the observed chloride concentration difference. The data support the thesis that sodium chloride transport accross the proximal tubular epithelium takes place by active sodium transport and electically coupled passive chloride reabsorption. Important species differences with respect to mammalian transport mechanisms are discussed.

Action Potentials↗

PubMed1977

Kinetics of Na+ transport in Necturus proximal tubule.

The dependence of proximal tubular sodium and fluid readsorption on the Na(+) concentration of the luminal and peritubular fluid was studied in the perfused necturus kidney. Fluid droplets, separated by oil from the tubular contents and identical in composition to the vascular perfusate, were introduced into proximal tubules, reaspirated, and analyzed for Na(+) and [(14)C]mannitol. In addition, fluid transport was measured in short-circuited fluid samples by observing the rate of change in length of the split droplets in the tubular lumen. Both reabsorptive fluid and calculated Na fluxes were simple, storable functions of the perfusate Na(+) concentration (K(m) = 35-39 mM/liter, V(max) = 1.37 control value). Intracellular Na(+), determined by tissue analysis, and open-circuit transepithelial electrical potential differences were also saturable functions of extracellular Na(+). In contrast, net reabsorptive fluid and Na(+) fluxes were linearly dependent on intracellular Na(+) and showed no saturation, even at sharply elevated cellular sodium concentrations. These concentrations were achieved by addition of amphotericin B to the luminal perfusate, a maneuver which increased the rate of Na(+) entry into the tubule cells and caused a proportionate rise in net Na(+) flux. It is concluded that active peritubular sodium transport in proximal tubule cells of necturus is normally unsaturated and remains so even after amphotericin-induced enhancement of luminal Na(+) entry. Transepithelial movement of NaCl may be described by a model with a saturable luminal entry step of Na(+) or NaCl into the cell and a second, unsaturated active transport step of Na(+) across the peritubular cell boundary.

Animals↗

PubMed1980

Conductive properties of the proximal tubule in Necturus kidney.

The electrical properties of the proximal tubule of the in vivo Necturus kidney were investigated by injecting current (as rectangular waves) into the lumen or into the epithelium of single tubules and by studying the resulting changes of transepithelial (VL) and/or cell membrane potential (VC) at various distances from the source. In some experiments paired measurements of VL and VC were performed at two abscissas x and x'. The luminal length constant of about 1,030 micrometer was shown to provide a good estimate of the transepithelial resistance, specific resistance (RTE = 420 omega.cm2) and/or per unit length (rTE = 1.3 x 10(4) omega.cm). The apparent intraepithelial length constant was subject to distortions arising from concomitant current spread in the lumen. The resistances of luminal membrane (rL), basolateral membrane (rB), and shunt pathway (rS) were estimated by two independent methods at 3.5 x 10(4), 1.2 x 10(4), and 1.7 x 10(4) omega.cm, respectively. The corresponding specific resistances were close to 1,200, 600, and 600 omega.cm2. There are two main conclusions of this study. (a) The resistances of cell membranes and shunt pathway are of the same order of magnitude. The figure of the shunt resistance is at variance with the notion that the proximal tubule of Necturus is a leaky epithelium. (b) A rigorous assessment of the conductive properties of concentric cylindrical double cables (such as renal tubules) requires that electrical interactions arising from one cable to another be taken into account. Appropriate equations were developed to deal with this problem.

Animals↗

PubMed1977

The permeability of the skin of a neotenous urodele amphibian, the mudpuppy Necturus maculosus.

1. The permeability of the isolated skin of a neotenous urodele amphibian, the mudpuppy Necturus maculosus, to Na, Cl, urea and water was measured. 2. Unidirectional transcutaneous flux measurements and the action of ouabain and amiloride, showed that there was normally no active Cl or Na transport, nor a Cl/Cl exchange diffusion process. 3. Amphotericin B initiated a transcutaneous potential difference and short-circuit current, which could be inhibited by ouabain. 4. The short-circuit current was nearly equivalent to the net Na Transport and this was also inhibited by ouabain. 5. A transcutaneous active Na transport mechanism thus appears to be incipient in the mudpuppy but is limited by a low permeability of the outer barrier of the cells. 6. Vasotocin increased the skin's diffusion permeability for water but had no effect on the influx of Na or urea. 7. The function of Necturus skin is in several respects unique compared to that of other amphibians.

Amiloride↗

PubMed1978

Further studies on ion permeation in proximal tubule of necturus kidney.

In the perfused Necturus kidney, the ratio of transepithelial sodium-to-chloride permeabilities (PNa/PCl) was estimated from dilution potentials at 1:7. The ratio of transepithelial permeabilities PHCO3/cl was estimated from bi-ionic potentials at 1:4 and that of Plact/PCl at 1:12. The permeability sequence, therefore, is PCl greater than PHCO3 greater than than PNa or PnaCl greater than PNaHCO3. The latter is similar to the situation in the rat. However, because of the low Na permeability, passive NaCl absorption by solvent drag is probably less important in Necturus than in the rat. The measurement of transepithelial input conductance during replacement of extracellular chloride by either bicarbonate or lactate shows that the former reduces the transmural conductance to 31% of control values as compared with 40% for the latter. Such discrepancies between permeability and conductance sequences are consistent either with an interference of the test anion with chloride permeation or with the presence of positive fixed changes within the shunt pathway.

Animals↗

PubMed1978

Transcellular and paracellular tracer chloride fluxes in Necturus proximal tubule.

Necturus proximal tubule lumen was filled with solutions of Na36Cl or [36Cl]tetramethylammonium ([36Cl]TMA) and the tracer disappearance was measured. With these tracers it was possible to differentiate between chloride fluxes across the cellular and the extracellular shunt pathways. Since it was previously shown that chloride does not enter tubule cells from the lumen unless Na is also present in the lumen, the [36Cl]TMA disappearance rate gave the shunt flux of chloride while the Na36Cl disappearance rate gave the sum of the transcellular and the shunt fluxes. The transcellular tracer chloride flux was unaffected by changes in the transepithelial potential difference, and the rate constant for the chloride flux from lumen to cell was identical to that previously reported for luminal sodium entry. These observations support the conclusion that a coupled transport of NaCl, in an electrically silent form, occurs across the luminal membrane of the Necturus proximal tubule cell. Shunt chloride flux was directly proportional to the electrical driving force, indicating diffusional chloride movement out of the lumen into the shunt pathway.

Animals↗

PubMed1975

Micropuncture study of the renal responses of the urodele amphibian Necturus maculosus to injections of arginine vasotocin and an anti-aldosterone compound.

1. Necturus maculosus kidney function has been examined using standard clearance techniques and renal tubular micropuncture methodology. 2. Throughout, cyanocobalamin (vitamin B12) has been used to monitor glomerular filtration rate (GFR) and tubular water movements. It was established that this substance was handled by the Necturus kidney in a similar manner to inulin. It can be readily analysed, together with renal electrolytes, by electron microprobe techniques. 3. Profiles of transtubular gradients (TF:P ratios) along the nephron were established for osmolarity, sodium, potassium, calcium and cobalt (of cyanocobalamin). 4. Ureteral urine is always hyposmotic with respect to plasma and the site of dilution of the plasma ultrafiltrate is within the distal segment. 5. Up to 30% of the filtrate is isosmotically reabsorbed along the proximal tubule; the tubular fluid:plasma ratio for osmolarity and sodium is around 1, and the TF:P for cobalt of cyanocobalamin is about 1.4 by the end of this segment. 6. The renal effects of the neurohypophysial hormone arginine vasotocin (AVT) and an aldosterone antagonist (SC14266; Soldactone) have been examined. 7. AVT was consistently antidiuretic causing both a decreased GFR and an enhanced distal tubular reabsorption of water. 8. SC14266 also increased distal tubular reabsorption of water. Such an effect differs from that found in higher vertebrates, and may indicate a "glucocorticoid-type" of renal action for aldosterone in amphibians.

Aldosterone↗

PubMed1984

Seasonal variations in the fine structure of the Necturus maculosus urinary bladder epithelium: low transporters and high transporters.

Although the urinary bladder of Necturus maculosus provides an important model system for studying the mechanisms of active Na absorption, little critical attention has been paid to the fine structure of its epithelium. Moreover, two distinct groups of urinary bladders, low and high Na transporters, have been described based on short-circuit current or transepithelial potential difference. In the present study, over an 11-month period, stable electrical parameters (short-circuit current, transepithelial potential difference, and resistance) were recorded from 63 chamber-mounted bladders. Analysis of these parameters revealed a highly significant difference between two groups (low transporters and high transporters) occurring at different times of the year. Consistent with these data, in urine collected from the bladders, the Na concentration in low transporters was significantly higher than that in high transporters. A subpopulation of these bladders was subsequently fixed and examined at the light and/or electron microscopic level. Low-transporting bladders were characterized unequivocally by a thin, stratified squamous epithelium only 6-15 micron thick. High-transporting bladders were composed predominantly of columnar-shaped granular cells up to 70 micron in height, with ciliated, mitochondria-rich, and basal cells present in small numbers. There is thus a correlation between transport activity, as measured by electrophysiological techniques and urine sodium analysis, and the structure of the tissue. Moreover, these parameters exhibit significant seasonal variation, the underlying mechanisms of which remain obscure.

Animals↗

PubMed1987

Ultrastructure of apical specializations of taste cells in the mudpuppy, Necturus maculosus.

The first interaction of taste stimuli with lingual chemoreceptors occurs on the apical membrane of taste cells, since only that portion is exposed to the oral cavity. To gain better insight into this interaction, we examined the pore region of taste buds in Necturus maculosus with scanning electron microscopy (SEM), transmission electron microscopy, and high-voltage electron microscopy. SEM of the pore reveals a patchwork distribution of three morphologically distinct types of apical specializations: long and branched (LB) microvilli, short and unbranched (SU) microvilli, and bundles of stereocilia. As demonstrated in thin and thick sections, LB microvilli are specializations of dark cells, SU microvilli are the apical specializations of light cells, and stereocilia arise from a cell that has the cytoplasmic markers characteristic of light cells. When left in place, the pore mucus completely covers the SU microvilli and partially covers the LB microvilli. However, stereocilia project above the surface and thus are highly exposed to taste stimuli in the oral cavity. These three morphologically distinct types of apical specializations may reveal functional differences among taste cells. The initial interaction between chemical stimulus and taste cell, and possibly chemoreceptor specificity itself, may be influenced by the morphology of the apical ending.

Animals↗

PubMed1984

Electron microscopic study of the innervation of the renal tubules and urinary bladder epithelium in Rana catesbeiana and Necturus maculosus.

The fine structure of the kidney and the bladder of the bullfrog (Rana catesbeiana), the bullfrog tadpole, and the mudpuppy (Necturus maculosus) were studied with special attention to the innervation of renal tubule cells and bladder epithelial cells. In the bullfrog kidney, nerve terminals and varicosities were frequently associated with the tubule cells, apparently in an increasing order from the proximal tubule to the connecting tubule. Although these terminals and varicosities did not directly contact the tubular cell membrane, an aggregation of synaptic vesicles on the side facing the tubule was considered as morphological evidence that neurotransmitter can be released here and can affect the transport activity of the tubule cells. The association of nerve varicosities with canaliculi cells in the connecting tubule was also demonstrated. In the bullfrog tadpoles, renal tubule cells were occasionally innervated. In the mudpuppy, renal tubule cells were only poorly innervated. The epithelium of the bullfrog bladder was commonly innervated. Nerve terminals with synaptic vesicles were located very near basal cells and even contacted them directly on rare occasions. In the mudpuppy, the innervation of the bladder epithelium was observed infrequently. The bullfrog tadpoles did not possess an apparent bladder. In all materials studied, renal arterioles and bladder smooth muscle cells were innervated.

Animals↗

PubMed1989

Electron-cytochemical localization of alkaline phosphatase to G cells of Necturus maculosus antrum.

Electron-cytochemical localization of alkaline phosphatase activity was performed on G cells of Necturus maculosus antral mucosa. Alkaline phosphatase activity was localized to the nuclear membrane, the Golgi/endoplasmic reticulum, and the limiting membranes of G cell peptide-secretion vesicles. There was no specific localization of alkaline phosphatase activity to the plasma membrane. Treatment of the tissues with levamisole (an alkaline phosphatase inhibitor) did not markedly reduce the specific alkaline phosphatase activity. Specific lead deposition was reduced by removal of the substrate from the reaction mixture. The results from this study on N. maculosus G cells demonstrate that alkaline phosphatase activity can be found in a non-mammalian gastric endocrine cell and that specific activity was localized primarily to those intracellular structures involved with protein biosynthesis.

Alkaline Phosphatase↗

PubMed1988

Ca2+-activated K+ currents in Necturus choroid plexus.

The tight-seal whole-cell recording method has been used to study Necturus choroid plexus epithelium. A cell potential of -59 +/- 2 mV and a whole cell resistance of 56 +/- 6 M omega were measured using this technique. Application of depolarizing step potentials activated voltage-dependent outward currents that developed with time. For example, when the cell was bathed in 110 mM NaCl Ringer solution and the interior of the cell contained a solution of 110 mM KCl and 5 nM Ca2+, stepping the membrane potential from a holding value of -50 to -10 mV evoked outward currents which, after a delay of greater than 50 msec, increased to a steady state in 500 msec. The voltage dependence of the delayed currents suggests that they may be currents through Ca2+-activated K+ channels. Based on the voltage dependence of the activation of Ca2+-activated K+ channels, we have devised a general method to isolate the delayed currents. The delayed currents were highly selective for K+ as their reversal potential at different K+ concentration gradients followed the Nernst potential for K+. These currents were reduced by the addition of TEA+ to the bath solution and were eliminated when Cs+ or Na+ replaced intracellular K+. Increasing the membrane potential to more positive values decreased both the delay and the half-times (t1/2) to the steady value. Increasing the pipette Ca2+ also decreased the delay and decreased t1/2. For instance, when pipette Ca2+ was increased from 5 to 500 nM, the delay and t1/2 decreased from values greater than 50 and 150 msec to values less than 10 and 50 msec. We conclude that the delayed currents are K+ currents through Ca2+-activated K+ channels. At the resting membrane potential of -60 mV, Ca2+-activated K+ channels contribute between 13 to 25% of the total conductance of the cell. The contribution of these channels to cell conductance nearly doubles with membrane depolarization of 20-30 mV. Such depolarizations have been observed when cerebrospinal fluid (CSF) secretion is stimulated by cAMP and with intracellular Ca2+. Thus the Ca2+-activated K+ channels may play a specific role in maintaining intracellular K+ concentrations during CSF secretion.

Animals↗

PubMed1988

Kinetics of voltage- and Ca2+ activation and Ba2+ blockade of a large-conductance K+ channel from Necturus enterocytes.

Potassium channels in membranes of isolated Necturus enterocytes were studied using the patch-clamp technique. The most frequent channel observed had a conductance of 170 pS and reversal potential of 0 mV in symmetrical potassium-rich solutions. Channels were highly K- selective. Channel activity was modulated by membrane potential and cytosolic Ca2+ concentration. Channel openings occurred in characteristic bursts separated by long closures. During bursts openings were interrupted by brief closures. Two gating modes controlled channel opening. The primary gate's sensitivity to intracellular Ca2+ concentration and membrane potential crucially determined long duration closures and bursting. In comparison, the second gate determining brief closures was largely insensitive to voltage and intracellular Ca2+ concentration. The channel was reversibly blocked by cytosolic barium exposure in a voltage-sensitive manner. Blockade reduced open-state probability without altering single-channel conductance and could be described, at relatively high Ca2+ concentration, by a three-state model where Ba2+ interacted with the open channel with a dissociation constant of about 10(-4) M at 0 mV.

Animals↗

PubMed1984

Electrical effects of potassium and bicarbonate on proximal tubule cells of Necturus.

The effects of stepwise concentration changes of K+ and HCO3- in the basolateral solution on the basolateral membrane potential (Vbl) of proximal tubule cells of the doubly-perfused Necturus kidney were examined using conventional microelectrodes. Apparent transference numbers were calculated from changes in Vb1 after alterations in external K+ concentration from 1.0 to 2.5 mM (tK, 1.0-2.5), 2.5 to 10, and in external HCO3- concentration (at constant pH) from 5 to 10 mM (tHCO3, 5-10), 10 to 20, or 10 to 50. tK, 2.5-10 was 0.38 +/- 0.02 under control conditions but was sharply reduced to 0.08 +/- 0.03 (P greater than 0.001) by 4 mM Ba++. This concentration of Ba++ reduced Vb1 by 9 +/- 1 mV (at 2.5 external K+). Perfusion with SITS (5 X 10(-4)M) for 1 hr hyperpolarized Vb1 by 10 +/- 3 mV and increased tK, 2.5-10 significantly to 0.52 +/- 0.01 (P less than 0.001). Ba++ application in the presence of SITS depolarized Vb1 by 22 +/- 3 mV. In control conditions tHCO3, 10-50 was 0.63 +/- 0.05 and was increased to 0.89 +/- 0.07 (P less than 0.01) by Ba++ but was decreased to 0.14 +/- 0.02 (P less than 0.001) by SITS. In the absence of apical and basolateral chloride, the response of Vb1 to bicarbonate was diminished but still present (tHO3, 10-20 was 0.35 +/- 0.03). Intracellular pH, measured with liquid ion-exchange microelectrodes, increased from 7.42 +/- 0.19 to 7.57 +/- 0.17 (P less than 0.02) when basolateral bicarbonate was increased from 10 to 20 mM at constant pH. These data show that the effects of bicarbonate on Vb1 are largely independent of effects on the K+ conductance and that there is a significant current-carrying bicarbonate pathway in the basolateral membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗

PubMed1989

Osmotic water permeability of Necturus gallbladder epithelium.

An electrophysiological technique that is sensitive to small changes in cell water content and has good temporal resolution was used to determine the hydraulic permeability (Lp) of Necturus gallbladder epithelium. The epithelial cells were loaded with the impermeant cation tetramethylammonium (TMA+) by transient exposure to the pore-forming ionophore nystatin in the presence of bathing solution TMA+. Upon removal of the nystatin a small amount of TMA+ is trapped within the cell. Changes in cell water content result in changes in intracellular TMA+ activity which are measured with intracellular ion-sensitive microelectrodes. We describe a method that allows us to determine the time course for the increase or decrease in the concentration of osmotic solute at the membrane surface, which allows for continuous monitoring of the difference in osmolality across the apical membrane. We also describe a new method for the determination of transepithelial hydraulic permeability (Ltp). Apical and basolateral membrane Lp's were assessed from the initial rates of change in cell water volume in response to anisosmotic mucosal or serosal bathing solutions, respectively. The corresponding values for apical and basolateral membrane Lp's were 0.66 x 10(-3) and 0.38 x 10(-3) cm/s.osmol/kg, respectively. This method underestimates the true Lp values because the nominal osmotic differences (delta II) cannot be imposed instantaneously, and because it is not possible to measure the true initial rate of volume change. A model was developed that allows for the simultaneous determination of both apical and basal membrane Lp's from a unilateral exposure to an anisosmotic bathing solution (mucosal). The estimates of apical and basal Lp with this method were 1.16 x 10(-3) and 0.84 x 10(-3) cm/s.osmol/kg, respectively. The values of Lp for the apical and basal cell membranes are sufficiently large that only a small (less than 3 mosmol/kg) transepithelial difference in osmolality is required to drive the observed rate of spontaneous fluid absorption by the gallbladder. Furthermore, comparison of membrane and transepithelial Lp's suggests that a large fraction of the transepithelial water flow is across the cells rather than across the tight junctions.

Animals↗

PubMed1982

Coupled NaCl entry into Necturus gallbladder epithelial cells.

NaCl entry into Necturus maculosus gallbladder epithelial cells was studied by determination of the rate of fluid movement into the cell when the Na+-K+-ATPase was inhibited by 10(-4) M ouabain in the serosal bathing solution. The cell swelling was due to continuing entrance of NaCl into the cell across the apical membrane, which increased the solute content of the cell; the resultant rise in cell osmolality induced water flow and cell swelling. The rate of swelling was 4.3% of the cell volume per minute, equivalent to a volume flow across the apical membrane of 1.44 x 10(-6) cm/s, similar in magnitude to the normal rate of fluid absorption by the gallbladder. We determined the mechanism of NaCl entry by varying the ionic composition of the mucosal bath; when most of the mucosal Na+ or Cl- was replaced, cell volume did not increase during pump inhibition. The rate of NaCl entry was a saturable function of Na+ or Cl- in the mucosal bathing solution with K1/2 values of 26.6 mM for Na+ and 19.5 mM for Cl-. The mode of NaCl entry was probably not the parallel operation of Na+-H+ and Cl(-)-HCO-3 exchangers because of the lack of effect of bicarbonate removal or of the inhibitors amiloride and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. NaCl entry was reversibly inhibited by bumetanide in the mucosal bathing solution. Transepithelial NaCl and water absorption is the result of the coupled, carrier-mediated movement of NaCl into the cell across the apical membrane and the active extrusion of Na+ by the Na+-K+-ATPase in the basolateral membrane.

Animals↗

PubMed1982

Volume regulation by Necturus gallbladder: apical Na+-H+ and Cl(-)-HCO-3 exchange.

Necturus gallbladder epithelial cells exhibited volume regulatory swelling when exposed to a hypertonic mucosal bathing solution. The initial, osmotically induced shrinkage was followed by a rapid increase in cell volume back to the control value despite continuing hypertonicity of the mucosal perfusate. This volume regulatory increase occurred by osmotic water flow accompanying the transient cellular uptake of NaCl from the mucosal bathing solution. Volume regulatory increase required Na+ and Cl- in the mucosal bath; it was inhibited by amiloride or 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid but not by bumetanide or ouabain. The K1/2 for Na+ was 2.8 mM, the K1/2 for Cl- was 1.9 mM, and maximum velocity of fluid flow into the cell for both ions was greater than 10 x 10(-6) cm/s. Both volume regulatory increase and transepithelial fluid absorption involve NaCl flux across the apical membrane into the cells, but the nature of the NaCl fluxes differ in the two processes. During volume regulatory increase NaCl enters the cells by parallel Na+-H+ and Cl(-)-HCO-3 exchanges, whereas during transepithelial fluid absorption NaCl enters the cell by the coupled flux of NaCl.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗