血小板活化因子对豚鼠心室肌细胞动作电位和钾通道的影响

应用全细胞膜片钳技术研究了血小板活化因子(platelet activatingfactor,PAF)对豚鼠心室肌细胞动作电位和钾电流的影响.结果发现,当电极内液ATP浓度为5 mmol/L(模拟正常条件)时,1 μmol/L PAF使APD90由对照的225.8±23.3 ms延长至352.8±29.8ms(n=5,P＜0.05);使IK尾电流在指令电压 30 mV由对照的173.5±16.7 pA降至152.1±11.5 pA(P＜0.05,n=4);使Ikl在指令电压为-120 mV时由对照组的-6.1±1.3 nA降至-5.6±1.1 nA(P＜0.05,n=5);但PAF在生理膜电位范围(-90mV～ 20mV)对IK1没有影响.当电极内液ATP浓度为0mmol/L时,IK·ATP开放(模拟缺血条件),1 μmol/LPAF却显著缩短APD90,由对照的153±24.6 ms缩短至88.2±19.4 ms(n=5,P＜0.01).而用1 μmol/L格列本脲(IK·ATP的特异阻断剂)预处理后,恢复了PAF可显著延长动作电位时程的作用.结果提示,PAF可能扩大缺血心肌和正常心肌细胞动作电位时程的不均一性,是缺血/再灌注性心律失常发生的重要原因.     

Voltage-dependent and odorant-regulated currents in isolated olfactory receptor neurons of the channel catfish.

The electrical properties of olfactory receptor neurons, enzymatically dissociated from the channel catfish (Ictalurus punctatus), were studied using the whole-cell patch-clamp technique. Six voltage-dependent ionic currents were isolated. Transient inward currents (0.1-1.7 nA) were observed in response to depolarizing voltage steps from a holding potential of -80 mV in all neurons examined. They activated between -70 and -50 mV and were blocked by addition of 1 microM tetrodotoxin (TTX) to the bath or by replacing Na+ in the bath with N-methyl-D-glucamine and were classified as Na+ currents. Sustained inward currents, observed in most neurons examined when Na+ inward currents were blocked with TTX and outward currents were blocked by replacing K+ in the pipette solution with Cs+ and by addition of 10 mM Ba2+ to the bath, activated between -40 and -30 mV, reached a peak at 0 mV, and were blocked by 5 microM nimodipine. These currents were classified as L-type Ca2+ currents. Large, slowly activating outward currents that were blocked by simultaneous replacement of K+ in the pipette with Cs+ and addition of Ba2+ to the bath were observed in all olfactory neurons examined. The outward K+ currents activated over approximately the same range as the Na+ currents (-60 to -50 mV), but the Na+ currents were larger at the normal resting potential of the neurons (-45 +/- 11 mV, mean +/- SD, n = 52). Four different types of K+ currents could be differentiated: a Ca(2+)-activated K+ current, a transient K+ current, a delayed rectifier K+ current, and an inward rectifier K+ current. Spontaneous action potentials of varying amplitude were sometimes observed in the cell-attached recording configuration. Action potentials were not observed in whole-cell recordings with normal internal solution (K+ = 100 mM) in the pipette, but frequently appeared when K+ was reduced to 85 mM. These observations suggest that the membrane potential and action potential amplitude of catfish olfactory neurons are significantly affected by the activity of single channels due to the high input resistance (6.6 +/- 5.2 G omega, n = 20) and low membrane capacitance (2.1 +/- 1.1 pF, n = 46) of the cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

SEA0400, a novel Na+/Ca2+ exchanger inhibitor, reduces calcium overload induced by ischemia and reperfusion in mouse ventricular myocytes

Given the potential clinical benefit of inhibiting Na+/Ca2+ exchanger (NCX) activity during myocardial ischemia reperfusion (I/R), pharmacological approaches have been pursued to both inhibit and clarify the importance of this exchanger. SEA0400 was reported to have a potent NCX selectivity. Thus, we examined the effect of SEA0400 on NCX currents and I/R induced intracellular Ca2+ overload in mouse ventricular myocytes using patch clamp techniques and fluorescence measurements. Ischemia significantly inhibited inward and outward NCX current (from -0.04+/-0.01 nA to 0 nA at -100 mV; from 0.23+/-0.08 nA to 0.11+/-0.03 nA at +50 mV, n=7), Subsequent reperfusion not only restored the current rapidly but enhanced the current amplitude obviously, especially the outward currents (from 0.23+/-0.08 nA to 0.49+/-0.12 nA at +50 mV, n=7). [Ca2+]i, expressed as the ratio of Fura-2 fluorescence intensity, increased to 138+/-7% (P<0.01) during ischemia and to 210+/-11% (P<0.01) after reperfusion. The change of NCX current and the increase of [Ca2+]i during I/R can be blocked by SEA0400 in a dose-dependent manner with an EC50 value of 31 nM and 28 nM for the inward and outward NCX current, respectively. The results suggested that SEA0400 is a potent NCX inhibitor, which can protect mouse cardiac myocytes from Ca2+ overload during I/R injuries.     

Patch-clamp study of isolated taste receptor cells of the frog

Summary Taste discs were dissected from the tongue ofR. ridibunda and their cells dissociated by a collagenase/low Ca/mechanical agitation protocol. The resulting cell suspension contained globular epithelial cells and, in smaller number, taste receptor cells. These were identified by staining properties and by their preserved apical process, the tip of which often remained attached to an epithelial (associated) cell. When the patch pipette contained 110mm KCl and the cells were superfused with NaCl Ringer's during whole-cell recording, the mean zero-current potential of 22 taste receptor cells was –65.2 mV and the slope resistance 150 to 750 M. Pulse-depolarization from a holding voltage of –80 mV activated a transient TTX-blockable inward Na current. Activation became noticeable at –25 mV and was half-maximal at –8 mV. Steady-state inactivation was half-maximal at –67 mV and complete at –50 mV. Peak Na current averaged –0.5 nA/cell. The Ca-ionophore A23187 shifted the activation and inactivation curve to more negative voltages. Similar shifts occurred when the pipette Ca was raised. External Ni (5mm) shifted the activation curve towards positive voltages by 10 mV. Pulse depolarization also activated outward K currents. Activation was slower than that of Na current and inactivation slower still. External TEA (7.5mm) and 4-aminopyridine (1mm) did not block, but 5mm Ba blocked the K currents. K-tail currents were seen on termination of depolarizing voltage pulses. A23187 shifted theI _K(V)-curve to more negative voltages. Action potentials were recorded when passing pulses of depolarizing outward current. Of the frog gustatory stimulants, 10mm Ca caused a reversible 5-to 10-mV depolarization in the current-clamp mode. Quinine (0.1mm, bitter) produced a reversible depolarization accompanied by a full block of Na current and, with slower time-course, a partial block of K currents. Cyclic AMP (5mm in the external solution or 0.5 m in the pipette) caused reversible depolarization (to –40 to –20 mV) due to partial blockage of K currents, but only if ATP was added to the pipette solution. Similar responses were elicited by stimulating the adenylate cyclase with forskolin. Blockage of cAMP-phosphodiesterase enhanced the response to cAMP. These results suggest that cAMP may be one of the cytosolic messengers in taste receptor cells. Replacement of ATP by AMP-PNP in the pipette abolished the depolarizing response to cAMP. Inclusion of ATP--S in the pipette caused slow depolarization to –40 to –20 mV, due to partial blockage of K currents. Subsequently, cAMP was without effect. The remaining K currents were blockable by Ba. These results suggest that cAMP initiates phosphorylation of one set of K channels to a nonconducting conformation.     

Nonselective cationic currents activated by acetylcholine in swine tracheal smooth muscle cells

Membrane currents in isolated swine tracheal smooth muscle cells were investigated using a pipette solution containing BAPTA-Ca2+ buffer and Cs+ as the major cation. With a pipette solution containing 100 nM free Ca2+, acetylcholine (ACh; 1-100 microM), in a concentration-dependent manner, activated a current without inducing shortening of cells, although neither 1 mM histamine nor 1 microM leukotriene D4 activated the current (n = 7, n is the number of cells). The effect of 100 microM ACh was suppressed by pretreatment with 100 microM atropine (n = 6) or intracellular application of preactivated pertussis toxin at a concentration of 0.1 microg x mL(-1) (n = 8). Genistein (0.1-100 microM), in a concentration-dependent manner, suppressed the activation of the inward current by 100 microM ACh, whereas it did not significantly suppress that of the outward current (n = 6-8). With a pipette solution containing 50 nM free Ca2+, outward current, but not inward current, was activated by 100 microM ACh (n = 10). When the pipette solution had free Ca2+ concentrations greater than 50 nM, the inward current together with the outward current was activated. The ratio between the amplitude of the inward and outward currents was significantly increased as the free Ca2+ concentration in the pipette solution increased. The steady-state activation curve of the ACh-activated current with the 50 nM free Ca2+ pipette solution was fitted by a single Boltzmann distribution (Vh = +69.8 mV, k = -11.9 mV, n = 10). The activation time constant became smaller as the membrane potential was more depolarized (164.3+/-5.9 ms at +40 mV to 92.4+/-6.3 ms at +120 mV, n = 10). The reversal potential was not significantly changed by reducing extracellular Cl- concentration to one-tenth of the control (n = 8), suggesting that the current is a nonselective cationic current. These results suggest that ACh activates an outward nonselective cationic current via pertussis toxin-sensitive G-protein(s) coupled with muscarinic receptors. Involvement of genistein-sensitive tyrosine kinase in the activation process of the current is unlikely.     

Mechanism of rat mesenteric arterial KATP channel activation by 14,15-epoxyeicosatrienoic acid

Recently, we reported that 11,12-epoxyeicosatrienoic acid (11,12-EET) potently activates rat mesenteric arterial ATP-sensitive K(+) (K(ATP)) channels and produces significant vasodilation through protein kinase A-dependent mechanisms. In this study, we tried to further delineate the signaling steps involved in the activation of vascular K(ATP) channels by EETs. Whole cell patch-clamp recordings [0.1 mM ATP in the pipette, holding potential (HP) = 0 mV and testing potential (TP) = -100 mV] in freshly isolated rat mesenteric smooth muscle cells showed small glibenclamide-sensitive K(ATP) currents (19.0 +/- 7.9 pA, n = 5) that increased 6.9-fold on exposure to 5 microM 14,15-EET (132.0 +/- 29.0 pA, n = 7, P < 0.05 vs. control). With 1 mM ATP in the pipette solution, K(ATP) currents (HP = 0 mV and TP = -100 mV) were increased 3.5-fold on exposure to 1 microM 14,15-EET (57.5 +/- 14.3 pA, n = 9, P < 0.05 vs. baseline). In the presence of 100 nM iberiotoxin, 1 microM 14,15-EET hyperpolarized the membrane potential from -20.5 +/- 0.9 mV at baseline to -27.1 +/- 3.0 mV (n = 6 for both, P < 0.05 vs. baseline), and the EET effects were significantly reversed by 10 microM glibenclamide (-21.8 +/- 1.4 mV, n = 6, P < 0.05 vs. EET). Incubation with 5 microM 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE), a 14,15-EET antagonist, abolished the 14,15-EET effects (31.0 +/- 11.8 pA, n = 5, P < 0.05 vs. 14,15-EET, P = not significant vs. control). The 14,15-EET effects were inhibited by inclusion of anti-G(s)alpha antibody (1:500 dilution) but not by control IgG in the pipette solution. The effects of 14,15-EET were mimicked by cholera toxin (100 ng/ml), an exogenous ADP-ribosyltransferase. Treatment with the ADP-ribosyltransferase inhibitors 3-aminobenzamide (1 mM) or m-iodobenzylguanidine (100 microM) abrogated the effects of 14,15-EET on K(ATP) currents. These results were corroborated by vasodilation studies. 14,15-EET dose-dependently dilated isolated small mesenteric arteries, and this was significantly attenuated by treatment with 14,15-EEZE or 3-aminobenzamide. These results suggest that 14,15-EET activates vascular K(ATP) channels through ADP-ribosylation of G(s)alpha.     

Properties of internally perfused, voltage-clamped, isolated nerve cell bodies

The membrane properties of isolated neurons from Helix aspersa were examined by using a new suction pipette method. The method combines internal perfusion with voltage clamp of nerve cell bodies separated from their axons. Pretreatment with enzymes such as trypsin that alter membrane function is not required. A platinized platinum wire which ruptures the soma membrane allows low resistance access directly to the cell's interior improving the time resolution under voltage clamp by two orders of magnitude. The shunt resistance of the suction pipette was 10-50 times the neuronal membrane resistance, and the series resistance of the system, which was largely due to the tip diameter, was about 10(5) omega. However, the peak clamp currents were only about 20 nA for a 60-mV voltage step so that measurements of membrane voltage were accurate to within at least 3%. Spatial control of voltage was achieved only after somal separation, and nerve cell bodies isolated in this way do not generate all-or-none action potentials. Measurements of membrane potential, membrane resistance, and membrane time constant are equivalent to those obtained using intracellular micropipettes, the customary method. With the axon attached, comparable all-or-none action potentials were also measured by either method. Complete exchange of Cs+ for K+ was accomplished by internal perfusion and allowed K+ currents to be blocked. Na+ currents could then be blocked by TTX or suppressed by Tris-substituted snail Ringer solution. Ca2+ currents could be blocked using Ni2+ and other divalent cations as well as organic Ca2+ blockers. The most favorable intracellular anion was aspartate-, and the sequence of favorability was inverted from that found in squid axon.     

Membrane currents in taste cells of the rat fungiform papilla. Evidence for two types of Ca currents and inhibition of K currents by saccharin

Taste buds were isolated from the fungiform papilla of the rat tongue and the receptor cells (TRCs) were patch clamped. Seals were obtained on the basolateral membrane of 281 TRCs, protruding from the intact taste buds or isolated by micro-dissection. In whole-cell configuration 72% of the cells had a TTX blockable transient Na inward current (mean peak amplitude 0.74 nA). All cells had outward K currents. Their activation was slower than for the Na current and a slow inactivation was also noticeable. The K currents were blocked by tetraethylammonium, Ba, and 4-aminopyridine, and were absent when the pipette contained Cs instead of K. With 100 mM Ba or 100 mM Ca in the bath, two types of inward current were observed. An L-type Ca current (ICaL) activated at -20 mV had a mean peak amplitude of 440 pA and inactivated very slowly. At 3 mM Ca the activation threshold of ICaL was near -40 mV. A transient T-type current (ICaT) activated at -50 mV had an average peak amplitude of 53 pA and inactivated with a time constant of 36 ms at -30 mV. ICaL was blocked more efficiently by Cd and D600 than ICaT. ICaT was blocked by 0.2 mM Ni and half blocked by 200 microM amiloride. In whole-cell voltage clamp, Na-saccharin caused (in 34% of 55 cells tested) a decrease in outward K currents by 21%, which may be expected to depolarize the TRCs. Also, Na-saccharin caused some taste cells to fire action potentials (on-cell, 7 out of 24 cells; whole-cell, 2 out of 38 cells responding to saccharin) of amplitudes sufficient to activate ICaL. Thus the action potentials will cause Ca inflow, which may trigger release of transmitter.     

The isolated anterior stomach of larval mosquitoes (Aedes aegypti): voltage-clamp measurements with a tubular epithelium

The anterior stomach of larval Aedes aegypti was isolated and perfused via two pipettes. For transepithelial voltage (V(te)) measurement, the inflow pipette and the bath were connected via agar bridges to calomel electrodes. For voltage-clamping, the lumen of the tissue contained an Ag/AgCl wire held by the outflow pipette, and the preparation was placed in a bath within a spiral of Ag/AgCl wire. After equilibrating the tissue in mosquito saline on both sides, a V(te) of -8+/-1 mV was measured (+/-S.E.M., N=32). Current-voltage curves (+/-100 mV) demonstrated ohmic behaviour of the epithelium. Short-circuiting resulted in a current (I(sc)) of 103+/-16 microA cm(-2) and a mean transepithelial conductance (G(te)) of 11.8+/-1.3 mS cm(-2) (+/-S.E.M., N=32). A Yonath-Civan plot of G(te) of individual preparations over the corresponding I(sc) resulted in a straight line (r(2)=0.8422), indicating that the difference in I(sc) of individual preparations is mainly based on different transcellular conductances (G(c)). This analysis allowed to estimate the mean leak conductance (G(l) approximately 3.9 mS cm(-2)) and the mean transcellular electromotive force (E(c) approximately 13 mV). After administering 0.2 micromol L(-1) serotonin, I(sc) and G(te) significantly increased, to 457+/-49 microA cm(-2) and to 21.3+/-2.3 mS cm(-2) (+/-S.E.M., N=31, P<0.05), respectively. The Yonath-Civan plot after serotonin resulted again in a straight line (r(2)=0.8219), indicating a mean G(l) of about 1 mS cm(-2) and a mean E(c) of about 22 mV. Dinitrophenol (2.5 mmol L(-1)) almost abolished I(sc) and significantly reduced G(te) (N=6). Concanamycin A (100 micromol L(-1)) reduced I(sc) by more than 90% without significantly affecting G(te).     

A hypertonicity-activated nonselective conductance in single proximal tubule cells isolated from mouse kidney

The whole-cell patch-clamp technique was used to examine nonselective conductances in single proximal tubule cells isolated from mouse kidney. Single cells were isolated in either the presence or absence of a cocktail designed to stimulate cAMP. Patches were obtained with Na+ Ringer in the bath and Cs+ Ringer in the pipette. On initially achieving the whole-cell configuration, whole-cell currents were small. In cAMP-stimulated cells, with 5 mM ATP in the pipette solution, whole-cell currents increased with time. The activated current was linear, slightly cation-selective, did not discriminate between Na+ and K+ and was inhibited by 100 microM gadolinium. These properties are consistent with the activation of a nonselective conductance, designated G(NS). Activation of G(NS) was abolished with pipette AMP-PNP, ATP plus alkaline phosphatase or in the absence of ATP. In unstimulated cells G(NS) was activated by pipette ATP together with PKA. These data support the hypothesis that G(NS) is activated by a PKA-mediated phosphorylation event. G(NS) was also activated by a hypertonic shock. However, G(NS) does not appear to be involved in regulatory volume increase (RVI), as RVI was unaffected in the presence of the G(NS) blocker gadolinium. Instead, the ATP sensitivity of G(NS) suggests that it may be regulated by the metabolic state of the renal proximal tubule cell.     

Channels involved in transient currents unmasked by removal of extracellular calcium in cardiac cells

In cardiac cells that lack macroscopic transient outward K(+) currents (I(to)), the removal of extracellular Ca(2+) can unmask "I(to)-like" currents. With the use of pig ventricular myocytes and the whole cell patch-clamp technique, we examined the possibility that cation efflux via L-type Ca(2+) channels underlies these currents. Removal of extracellular Ca(2+) and extracellular Mg(2+) induced time-independent currents at all potentials and time-dependent currents at potentials greater than -50 mV. Either K(+) or Cs(+) could carry the time-dependent currents, with reversal potential of +8 mV with internal K(+) and +34 mV with Cs(+). Activation and inactivation were voltage dependent [Boltzmann distributions with potential of half-maximal value (V(1/2)) = -24 mV and slope = -9 mV for activation; V(1/2) = -58 mV and slope = 13 mV for inactivation]. The time-dependent currents were resistant to 4-aminopyridine and to DIDS but blocked by nifedipine at high concentrations (IC(50) = 2 microM) as well as by verapamil and diltiazem. They could be increased by BAY K-8644 or by isoproterenol. We conclude that the I(to)-like currents are due to monovalent cation flow through L-type Ca(2+) channels, which in pig myocytes show low sensitivity to nifedipine.     

Odorant-induced currents in intact patches from rat olfactory receptor neurons: theory and experiment.

Odorant-induced currents in mammalian olfactory receptor neurons have proved difficult to obtain reliably using conventional whole-cell recording. By using a mathematical model of the electrical circuit of the patch and rest-of-cell, we demonstrate how cell-attached patch measurements can be used to quantitatively analyze responses to odorants or a high (100 mM) K+ solution. High K+ induced an immediate current flux from cell to pipette, which was modeled as a depolarization of approximately 52 mV, close to that expected from the Nernst equation (56 mV), and no change in the patch conductance. By contrast, a cocktail of cAMP-stimulating odorants induced a current flux from pipette into cell following a significant (4-10 s) delay. This was modeled as an average patch conductance increase of 36 pS and a depolarization of 13 mV. Odorant-induced single channels had a conductance of 16 pS. In cells bathed with no Mg2+ and 0.25 mM Ca2+, odorants induced a current flow from cell to pipette, which was modeled as a patch conductance increase of approximately 115 pS and depolarization of approximately 32 mV. All these results are consistent with cAMP-gated cation channels dominating the odorant response. This approach, which provides useful estimates of odorant-induced voltage and conductance changes, is applicable to similar measurements in any small cells.     

Voltage-activation of high-conductance K+ channel in the insulin-secreting cell line RINm5F is dependent on local extracellular Ca2+ concentration

Patch-clamp single-channel current recording experiments have been carried out on intact insulin-secreting RINm5F cells. Voltage-activation of high-conductance K+ channels were studied by selectively depolarizing the electrically isolated patch membrane under conditions with normal Ca2+ concentration in the bath solution but with or without Ca2+ in the patch pipette solution. When Ca2+ was present in the pipette, 40 mV to 120 mV depolarizing pulses (100 ms) from the normal resting potential (-70 mV) regularly evoked tetraethylammonium-sensitive large outward single-channel currents and the average open state probability during the pulses varied from about 0.015 (40 mV pulses) to 0.1 (120 mV pulses). In the absence of Ca2+ in the pipette solution the same protocol resulted in fewer and shorter K+ channel openings and the open-state probability varied from about 0.0015 (40 mV pulses) to about 0.03 (120 mV pulses). It is concluded that Ca2+ entering voltage-gated channels raises [Ca2+]i locally and thereby markedly enhances the open-state probability of tetraethylammonium-sensitive voltage-gated high-conductance K+ channels.     

CFTR Channels Expressed in CHO Cells Do Not Have Detectable ATP Conductance

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated, ATP-dependent chloride channel which may have additional functions. Recent reports that CFTR mediates substantial electrodiffusion of ATP from epithelial cells have led to the proposal that CFTR regulates other ion channels through an autocrine mechanism involving ATP. The aim of this study was to determine the ATP conductance of wild-type CFTR channels stably expressed in Chinese hamster ovary cells using patch clamp techniques. In the cell-attached configuration with 100 mm Mg · ATP or Tris · ATP solution in the pipette and 140 mm NaCl in the bath, exposing cells to forskolin caused the activation of a low-conductance channel having kinetics resembling those of CFTR. Single channel currents were negative at the resting membrane potential (V _m ), consistent with net diffusion of Cl from the cell into the pipette. The transitions decreased in amplitude, but did not reverse direction, as V _m was clamped at increasingly positive potentials to enhance the driving force for inward ATP flow (>+80 mV). In excised patches, single channel currents did not reverse under essentially biionic conditions (Cl_in/ATP_out or ATP_in/Cl_out), although PKA-activated currents were clearly visible in the same patches at voltages where they would be carried by chloride ions. Moreover, with NaCl solution in the bath and a mixture of ATP and Cl in the pipette, the single channel I/V curve reversed at the predicted equilibrium potential for chloride. CFTR channel currents disappeared when patches were exposed to symmetrical ATP solutions and were restored by reexposure to Cl solution. Finally, in the whole-cell configuration with NaCl in the bath and 100 mm MgATP or TrisATP in the pipette, cAMP-stimulated cells had time-independent, outwardly rectifying currents consistent with CFTR selectivity for external Cl over internal ATP. Whole-cell currents reversed near V _m =−55 mV under these conditions, however the whole cell resistance measured at −100 mV was comparable to that of the gigaohm seal between the plasma membrane and glass pipette (7 GΩ). We conclude that CFTR does not mediate detectable electrodiffusion of ATP. Received: 8 November 1995/Revised: 23 January 1996     

Alterations in outward K(+) currents on removal of external Ca(2+) in human atrial myocytes

External divalent cations are known to play an important role in the function of voltage-gated ion channels. The purpose of this study was to examine the sensitivity of the voltage-gated K(+) currents of human atrial myocytes to external Ca(2+) ions. Myocytes were isolated by collagenase digestion of atrial appendages taken from patients undergoing coronary artery-bypass surgery. Currents were recorded from single isolated myocytes at 37 degrees C using the whole-cell patch-clamp technique. With 0.5 mM external Ca(2+), voltage pulses positive to -20 mV (holding potential = -60 mV) activated outward currents which very rapidly reached a peak (I(peak)) and subsequently inactivated (tau = 7.5 +/- 0.7 msec at +60 mV) to a sustained level, demonstrating the contribution of both rapidly inactivating transient (I(to1)) and non-inactivating sustained (I(so)) outward currents. The I(to1) component of I(peak), but not I(so), showed voltage-dependent inactivation using 100 msec prepulses (V(1/2) = -35.2 +/- 0.5 mV). The K(+) channel blocker, 4-aminopyridine (4-AP, 2 mM), inhibited I(to1) by approximately 76% and reduced I(so) by approximately 33%. Removal of external Ca(2+) had several effects: (i) I(peak) was reduced in a manner consistent with an approximately 13 mV shift to negative voltages in the voltage-dependent inactivation of I(to1). (ii) I(so) was increased over the entire voltage range and this was associated with an increase in a non-inactivating 4-AP-sensitive current. (iii) In 79% cells (11/14), a slowly inactivating component was revealed such that the time-dependent inactivation was described by a double exponential time course (tau(1) = 7.0 +/- 0.7, tau(2) = 90 +/- 21 msec at +60 mV) with no effect on the fast time constant. Removal of external Ca(2+) was associated with an additional component to the voltage-dependent inactivation of I(peak) and I(so) (V(1/2) = -20.5 +/- 1.5 mV). The slowly inactivating component was seen only in the absence of external Ca(2+) ions and was insensitive to 4-AP (2 mM). Experiments with Cs(+)-rich pipette solutions suggested that the Ca(2+)-sensitive currents were carried predominantly by K(+) ions. External Ca(2+) ions are important to voltage-gated K(+) channel function in human atrial myocytes and removal of external Ca(2+) ions affects I(to1) and 4-AP-sensitive I(so) in distinct ways.     

Short-term exposure to waterborne free silver has acute effects on membrane current of Xenopus oocytes

Waterborne free silver can cause osmo- and ionoregulatory disturbances in freshwater organisms. The effects of a short-term exposure to extracellular Ag+ ions on membrane currents were investigated in voltage-clamped defolliculated Xenopus oocytes. At a holding potential of -60 mV, ionic silver (1 microM Ag+) increased inward currents (=I(Ag)) from -8+/-2 nA to -665+/-41 nA (n=74; N=27). I(Ag) activated within 2 min of silver exposure and then rose impetuously. This current was largely reversible by washout and repeatable. I(Ag) reversed around -30 mV and rectified slightly at more positive potentials. Na+-free bath conditions reduced the silver-induced current to a smaller but sustained current. The response to silver was abolished by the Cl- channel blockers DIDS and SITS, whereas niflumic acid strongly potentiated I(Ag). Intraoocyte injection of AgNO3 to about 1 mM [Ag]i strongly potentiated I(Ag). Extracellular application of either dithiothreitol (DTT), a compound known to reduce disulfide bridges, or L-cysteine abolished Ag+-activated increase of membrane current. In contrast, n-ethylmaleimide (NEM) which oxidizes SH-groups potentiated I(Ag). Hypoosmotic bath solution significantly increased I(Ag) whereas hyperosmolar conditions attenuated I(Ag). The activation of I(Ag) was largely preserved after chelation of cytosolic Ca2+ ions with BAPTA/AM. Taken together, these data suggest that Xenopus oocytes are sensitive to short-term exposure to waterborne Ag+ ions and that the elicited membrane currents result from extra- and intracellular action of Ag+ ions on peptide moieties at the oocyte membrane but may also affect conductances after internalization.     

Properties of an early outward current in single cells of the mouse ventricle

Single ventricular myocytes of adult mice were prepared by enzymatic dissociation for voltage clamp experiments with the one suction pipette dialysis method. After blocking the Na current by 10(-4) mol/l TTX early outward currents (IEO) with incomplete inactivation could be elicited by clamping from -50 mV to test potentials (VT) positive to -30 mV. Interfering Ca currents were very small (less than 0.6 nA at VT = 0 mV). The approximation of IEO by the q4r-model showed a pronounced decrease in the time constant of activation (tau q) to more positive potentials. At 50 ms test pulses the time course of the incomplete inactivation could be described by two exponentials and a constant. The time constant of the fast exponential (tau r1) showed a slight decline towards more positive test potentials (8.1 +/- 1.0 ms at -10 mV; 5.8 +/- 1.2 ms at +50 mV, mean +/- SD, n = 5) whereas the time constant of the slow exponential (tau r2) was voltage independent (41.1 +/- 7.9 ms, mean +/- SD, n = 5). The contributions of the fast exponential and the pedestal increased towards positive test potentials. The Q10 value for the time constants of activation and fast inactivation was 2.36 +/- 0.19 and 2.51 +/- 0.09 (mean +/- SD, n = 3), respectively. After an initial delay the recovery of IEO at a recovery potential of -50 mV could be fitted monoexponentially with a time constant of 16.3 +/- 2.9 ms (mean +/- SD, n = 3). The time course of the onset of inactivation determined with the double pulse protocol was slower than the decay at the same potential, and could be described as sum of a fast (tau = 18.4 +/- 6.0 ms) and a slow (tau = 62.1 +/- 19.9ms, mean +/- SD, n = 3) exponential. IEO could be blocked completely by 1 mmol/l 4-aminopyridine at potentials up to +20 mV. Stronger depolarizations had an unblocking effect.     

A transient voltage-dependent outward current in cultured cerebellar granules

Granule cells were dissociated from rat cerebella with a procedure that yields a 98% pure cell population. Potassium currents in these cells were studied using the patch-clamp technique. Depolarizing pulses of 10 mV step and 100 ms duration from a holding potential of –80 mV elicited two different potassium outward currents: a transient, low-voltage activated component and a long lasting, high-voltage activated component. At +30 mV, the total current reached an amplitude of 2 nA (mean value of 15 experiments). The reversal potential of the transient current, estimated by measuring tail currents, was –77 mV, close to that predicted by the Nernst equation. The transient current was half inactivated with a holding potential of –78 mV and completely inactivated with –50 mV or more positive holding potentials. Finally, the current decay could be fitted by the sum of two exponentials with time constants of about 20 and 250 ms.     

Properties of a potassium channel in the basolateral membrane of renal proximal convoluted tubule and the effect of cyclosporine on it

We studied the potassium channel in the basolateral membrane of the rat proximal convoluted tubule as affected by cyclosporine A. Proximal convoluted tubules were dissected from the rat kidney under a stereoscopic microscope, without a preliminary enzyme treatment. The standard configuration for single-channel tight seal patch-clamp technique was used to record channel currents. A small conductance, stretch-sensitive potassium channel could be observed spontaneously in most of the cell-attached patches as the gigaohm seal was formed. In the inside-out configuration, channel activity was diminished. The K(+) channel appeared to be an inward rectifier. The limiting inward slope conductance was 28.3+/-1.7 pS (Vp was between 40 mV and 80 mV, n=6) and the outward chord conductance was 5.6+/-0.3 pS (Vp was between -40 and -60 mV, n=5). The open dwell time constants of the potassium channel were 0.524 ms and 5.087 ms, while the closed dwell time constants were 1.029 ms and 16.500 ms. The opening probability of the channel decreased when the extracellular fluid was acidified. Cyclosporine A had no significant effect on the potassium channel of the proximal tubular cell in the basolateral membrane at concentrations of 10 and 50 microg/ml, while at 100 microg/ml, it decreased the opening probability.     

Swelling-activated chloride channels in multidrug-sensitive and - resistant cells

Resistance to chemotherapeutic agents in neoplastic cells is often mediated by expression of P-glycoprotein, which functions as a drug- efflux pump for a broad range of substrates. We have used a combination of patch clamp and video-imaging techniques to examine the expression and drug-efflux function of P-glycoprotein and to determine the possible correlation with swelling-activated chloride channels in drug- sensitive and -resistant cell lines. Two pairs of cell lines were used in these experiments: (a) control NIH-3T3 cells and a corresponding MDR1-transfectant; and (b) control 8226 myeloma cells and a derivative cell line selected for resistance to chemotherapeutic agents. Control cells lacked detectable P-glycoprotein expression based on Western blotting, immunofluorescence staining with a specific monoclonal antibody, and a functional assay of rhodamine-123 (R123) efflux. Resistant cells expressed P-glycoprotein at high levels and rapidly exported R123. During whole-cell recording using either hyperosmotic pipette solution or hypoosmotic Ringer solution, cell swelling was accompanied by Cl- channel opening in all four cell lines. The rates of induction, biophysical properties and magnitudes of Cl conductance (gCl) were indistinguishable between control and corresponding multidrug-resistant cells: gCl reached 0.96 +/- 0.31 (n = 14) and 0.83 +/- 0.31 nS/pF (mean +/- SD; n = 31) in NIH-3T3 and NIH-3T3/MDR cells, respectively; and 0.31 +/- 0.20 (n = 9) and 0.37 +/- 0.22 nS/pF (n = 7) in 8226 and 8226/Dox40 cells, respectively. gCl exhibited moderate outward rectification in symmetrical Cl- solutions, with a rectification ratio of 1.4 at +/- 50 mV. Cl- channels slowly closed during strong depolarization beyond +60 mV. Using video-imaging techniques with SPQ as a fluorescent probe, we monitored Cl(-)-channel opening in intact drug-sensitive and -resistant cells. gCl, measured either with whole-cell recording or SPQ imaging, was blocked by DIDS (voltage-dependent Kd < 50 microM at +40 mV), NPPB (Kd approximately 30 microM), and tamoxifen (complete and irreversible block approximately 10 microM). None of these blockers inhibited R123 efflux. NPPB accelerated R123 efflux, an effect that was mimicked by CCP, a mitochondrial uncoupler. In contrast, verapamil selectively blocked R123 efflux (Kd = 0.3 to 0.5 microM); 10 microM left gCl unaltered. Induction of gCl was not affected by vincristine or doxorubicin in the pipette solution. Moreover, the rate of R123 efflux did not change during cell swelling. We conclude that P-glycoprotein and swelling- activated chloride channels function independently and are separable by expression and by pharmacological sensitivities.