Whole-cell and single channel K+ and Cl- currents in epithelial cells of frog skin

Establishing Cell Lines from Fresh or Cryopreserved Tissue from the Great Crested Newt (Triturus cristatus):A Preliminary Protocol

This study describes a successful protocol for establishing cell lines from the threatened Triturus cristatus in terms of collection, preparing, establishing, cryopreserving, thawing and quality checking. Different parameters such as media, media change, fresh vs. cryopreserved tissue and seeding density were tested to optimize culture conditions for this species. With fresh tissue, no considerable differences in the use of two different media were found, but with cryopreserved tissue, a combination of ITS (insulin/transferrin/selenite) and 2-mercaptoethanol had a positive effect on growth. Real-time measurements on the cell lines were used, for the first time in amphibian cells, to investigate the effect of different treatments such as media change with or without washing. Media change had a positive impact on the cells, whereas the effect was negative when combined with washing. It is concluded that establishment of cell lines is possible from the great crested newt, especially when using fresh tissue, but much more challenging if the tissue has been cryopreserved. Real-time measurement during cell culture is a useful tool to visualize the sensitivity of amphibian cells during different culture treatments.

Role of basolateral membrane conductance in the regulation of transepithelial sodium transport across frog skin

Circuit analyses of the principal cell compartment of frog skin ( Rana temporaria and R. esculenta) were made using microelectrode measurements under short-circuit conditions and with the aid of the Na(+) channel blocker amiloride. Under control conditions, intracellular potential ranged between -65 and -5 mV, and the conductances of the apical and basolateral membranes were related directly to the short-circuit current and inversely to the cellular potential. Blockade of apical Na(+) uptake by amiloride hyperpolarized the cells to nearly the same value, irrespective of the potential under transporting conditions. Under these conditions, basolateral membrane conductance increased greatly, which led to paradoxical reactions of the transepithelial Na(+) transport at lower concentrations of amiloride. The half-maximal inhibitory concentration of amiloride estimated from the response of the apical membrane conductance (99+/-10 nM) was about 5 times lower than the value derived from transepithelial current or conductance in the same tissues. The results are discussed in the context of the importance of the membrane potential for acute control of membrane conductance and transepithelial transport.

Trypsin inhibits voltage-activated chloride conductance of toad skin

The effect of trypsin on the voltage-activated chloride conductance (GCl) of toad skin was investigated. Serosal application of > 0.1 mg ml-1 trypsin decreased the voltage-activated GCl without notable delay. The maximal inhibition to 38% of the control values, exerted within 15 min, was in some experiments partly or completely reversible. Chymotrypsin had much lower effect than trypsin. Mucosal application of trypsin did not have any effect. Trypsin did neither interfere with the conductive pathway opened by supramaximal concentrations of cAMP nor with the inhibitory effect of epinephrine on the voltage-activated GCl. The effect of trypsin required influx of Ca2+ from the extracellular space. It is concluded that protease-activated receptors or trypsin-sensitive proteins in the basolateral membrane of toad skin epithelial cells interfere with regulative steps involved in the voltage-activation of GCl. This may be harmful for the segregation of epithelial cells using this enzyme.

Two K(+)-selective conductances in single proximal tubule cells isolated from frog kidney are regulated by ATP

1. The whole-cell and single channel patch clamp techniques were used to identify K(+)-selective conductances in single proximal tubule cells isolated from frog kidney and to examine their ATP sensitivity. Whole-cell currents were inhibited by the K+ channel inhibitors Ba2+ and quinidine in a dose-dependent manner. Addition of Ba2+ alone, quinidine alone, or both inhibitors together revealed two separate conductances, one of which was blocked by both Ba2+ and quinidine (GBa)1, the other being sensitive to quinidine alone (Gquin). 2. With Na(+)-containing Ringer solution in the bath and K(+)-containing Ringer solution in the pipette, both currents were selective for K+ over Na+. The K+ : Na+ selectivity ratio of GBa was around 50:1, while that of Gquin was 4:1. In symmetrical KCl solutions GBa showed inward rectification, while Gquin demonstrated outward rectification. 3. In the absence of pipette ATP, both GBa and Gquin ran down over 10 min. However, when 2 mM ATP was included in the pipette GBa increased, while Gquin remained unchanged. 4. Single channel studies demonstrated that a basolateral K+ channel shared several of the characteristics of GBa. It was inhibited by both Ba2+ and quinidine, underwent run-down in excised patches in the absence of ATP, and was activated by ATP. 5. We conclude that cells of the frog proximal tubule contain at least two distinct K(+)-selective conductances, both of which are regulated by ATP, and which may be involved in pump-leak coupling.

Activation of the plasma membrane chloride channel by protein kinase C in isolated guinea-pig hepatocytes

1. To assess the nature of the underlying mechanism of noradrenaline-induced increase of Cl- conductances in hepatocytes, macroscopic and unitary currents through noradrenaline-induced Cl- channels were examined in enzymatically isolated guinea-pig hepatocytes using whole-cell, cell-attached and excised inside-out configurations of the patch-clamp technique. 2. When K+ conductances were blocked and the intracellular Ca2+ concentration ([Ca2+]i) was set at 0.1 microM, bath application of noradrenaline activated the time-independent membrane currents under whole-cell voltage-clamp conditions. The current was similarly activated by phorbol ester (PMA), an activator of protein kinase C (PKC), while a specific protein kinase C inhibitor, H-9, reversed PMA activation of the current. The inactive phorbol ester, 4 alpha-phorbol 12-myristate, 13-acetate (alpha PMA), failed to activate the channel. 3. The reversal potential of the PMA-activated current shifted by approximately 60 mV per 10-fold change in the external Cl- concentration, indicating that the current was Cl- selective. Bath application of 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) partially inhibited both the noradrenaline- and PMA-induced currents. 4. In single channel recordings from cell-attached patches, bath application of noradrenaline or PMA induced unitary current activity, the averaged slope conductance of which was 10.1 +/- 1.5 pS (mean +/- S.D.; n = 12) in the noradrenaline-induced current and 9.7 +/- 1.3 pS (n = 7) in the PMA-induced current. The open time distribution was moderately well fitted by a single exponential function with mean open lifetime of 88.5 +/- 10.6 ms (n = 10), while at least two exponentials were required to fit the closed time distributions with a time constant for the fast component of 24.4 +/- 5.8 ms (n = 10) and for the slow component of 316.9 +/- 49.2 ms (n = 10). 5. Bath application of purified PKC to excised inside-out patches activated the channel. The PKC selective inhibitor, PKC(19-36), and DIDS inhibited the PKC-activated channel. 6. These results suggest that PKC can phosphorylate the channel protein or a related structure leading to the activation of Cl- channels in guinea-pig hepatocytes.

Inward-rectifier potassium channels in basolateral membranes of frog skin epithelium

Inward-rectifier K channel: using macroscopic voltage clamp and single-channel patch clamp techniques we have identified the K+ channel responsible for potassium recycling across basolateral membranes (BLM) of principal cells in intact epithelia isolated from frog skin. The spontaneously active K+ channel is an inward rectifier (Kir) and is the major component of macroscopic conductance of intact cells. The current-voltage relationship of BLM in intact cells of isolated epithelia, mounted in miniature Ussing chambers (bathed on apical and basolateral sides in normal amphibian Ringer solution), showed pronounced inward rectification which was K(+)-dependent and inhibited by Ba2+, H+, and quinidine. A 15-pS Kir channel was the only type of K(+)-selective channel found in BLM in cell-attached membrane patches bathed in physiological solutions. Although the channel behaves as an inward rectifier, it conducts outward current (K+ exit from the cell) with a very high open probability (Po = 0.74-1.0) at membrane potentials less negative than the Nernst potential for K+. The Kir channel was transformed to a pure inward rectifier (no outward current) in cell-attached membranes when the patch pipette contained 120 mM KCl Ringer solution (normal NaCl Ringer in bath). Inward rectification is caused by Mg2+ block of outward current and the single-channel current-voltage relation was linear when Mg2+ was removed from the cytosolic side. Whole-cell current-voltage relations of isolated principal cells were also inwardly rectified. Power density spectra of ensemble current noise could be fit by a single Lorentzian function, which displayed a K dependence indicative of spontaneously fluctuating Kir channels.

CONCLUSIONS

under physiological ionic gradients, a 15-pS inward-rectifier K+ channel generates the resting BLM conductance in principal cells and recycles potassium in parallel with the Na+/K+ ATPase pump.

Outwardly rectifying chloride current in rabbit osteoclasts is activated by hyposmotic stimulation

1. We characterized chloride currents in freshly isolated rabbit osteoclasts using whole-cell and single channel patch-clamp recording configurations. Depolarization activated an outwardly rectifying current in 40-50% of cells, distinct from the inwardly rectifying K+ current we have previously reported in osteoclasts. 2. The outwardly rectifying current persisted under conditions where all K+ currents were blocked. Furthermore, the outward current was reversibly inhibited by Cl- transport blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS); 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS); 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS); and niflumic acid. The blocked current had a reversal potential close to the predicted chloride equilibrium potential and was dependent on the chloride concentration gradient. 3. In those osteoclasts in which outwardly rectifying current was not initially apparent, exposure to hyposmotic extracellular solution resulted in its reversible activation. The induced current was due to Cl-, based on its reversal close to the chloride equilibrium potential and sensitivity to blockade by Cl- channel inhibitors. The hyposmotically induced current could be activated in Ca(2+)-free solutions containing 0.2 mM EGTA. 4. When studied in the current-clamp configuration, hyposmotic stimulation caused depolarization from -76 +/- 5 to -5 +/- 6 mV (mean +/- S.D., n = 7). 5. Unitary Cl- currents were recorded in the cell-attached patch configuration at positive potentials. Single channels had a slope conductance of 19 +/- 3 pS (n = 5). Reduction of the external [Cl-] shifted the current-voltage relationship in the positive direction, supporting the conclusion that these were Cl- currents. Like the whole-cell currents, single channel Cl- currents were activated by exposure of cells to hyposmotic bathing solution. 6. We conclude that rabbit osteoclasts express an outwardly rectifying Cl- current that can be activated by osmotic stress. Cl- channels may play a role in cell volume regulation and may also provide conductive pathways for dissipating the potential difference that arises from electrogenic proton transport during bone resorption.

Characterization of the outward rectifying potassium channel in a novel mouse intestinal smooth muscle cell preparation

1. The outward rectifying K+ conductance and underlying single channel behaviour in mouse small intestine (MSI) smooth muscle cells was studied using microelectrode impalement and the patch clamp technique. 2. At 37 degrees C, smooth muscle cells in MSI explants had a resting membrane potential around -65 mV and showed spontaneous electrical and mechanical activity. 3. Under whole-cell voltage clamp, depolarization of smooth muscle cells in the explants evoked a methoxyverapamil (D600)-sensitive, partially inactivating inward current and a non-inactivating outward current. The outward current was also observed in enzymatically dispersed cells from neonatal mouse small intestine. 4. The reversal potential of the outward current as established in tail current experiments was -70.2 mV. Tail currents could be fitted with a single exponential, suggesting the participation of only one population of channels. 5. The outward current was sensitive to 4-aminopyridine (10(-4) M), Ba2+ (1 mM) and to the presence of Cs+ in the pipette, but not to D600 (10(-6) M), or the presence of ATP (1 mM) in the pipette. 6. In the cell-attached patch configuration, a unitary outward current was observed that showed increased activity upon depolarization of the patch. The current-voltage relationship was close to linear with a slope conductance of 186 pS. 7. With normal K+ (6 mM) in the pipette, the extrapolated reversal potential for the unitary current was around -75 mV, while with high K+ (120 mM) the reversal potential was close to 0 mV. 8. Averaging single channel traces recorded under a depolarizing pulse protocol resulted in a trace with similar time characteristics as the outward current observed in the whole-cell configuration. 9. The burst behaviour of the channel was described by a simple model consisting of two closed states, Cf (intraburst closed state) and Cs (interburst closed state) and an open state (O). The rate constants in the model showed differential sensitivity to potential changes, channel blockade by Ba2+ and equimolar K+ conditions. 10. It was concluded that the outward rectifying potassium current in MSI smooth muscle cells is mediated by a 186 pS bursting channel. Voltage dependency and Ba2+ blockade are mainly reflected by changes in the transition rate from the open channel state to the interburst closed state.

Artifactual expression of maxi-K+ channels in basolateral membrane of gallbladder epithelial cells

To patch clamp the basolateral cell membrane, sheets of Necturus gallbladder epithelium were stripped of the subepithelial tissue layers and affixed apical side down on cover slips coated with Cell-Tak [F. Wehner, L. Garretson, K. Dawson, Y. Segal, and L. Reuss. Am. J. Physiol. 258 (Cell Physiol. 27): C1159-C1164, 1990]. In 90% of the patches we observed K+ channels identical to the maxi-K+ channels previously demonstrated in the apical membrane (Y. Segal and L. Reuss. J. Gen. Physiol. 95: 791-818, 1990). To ascertain whether these channels were present in the native tissue, we carried out intracellular-microelectrode studies. We tested for activation of basolateral membrane K+ conductance by depolarization or by elevation of intracellular Ca2+ and for tetraethylammonium sensitivity of the basolateral membrane voltage and fractional resistance. The results were negative, indicating that maxi-K+ channels are not expressed in the basolateral membrane of the "intact" epithelium. Using the same intracellular-microelectrode protocol on the apical membrane, we demonstrated the presence of an apical K+ conductance attributable to maxi-K+ channels. Additional experiments revealed a Ba(2+)-sensitive basolateral K+ conductance in the native epithelium. We conclude that in the stripped preparation there is artifactual expression of maxi-K+ channels. In addition, the native basolateral membrane K+ channels either are not expressed in this preparation or have a low conductance and cannot be discerned from the background noise.