KCl leakage from microelectrodes and its impact on the membrane parameters of a nonexcitable cell

Role of "active" potassium transport in the regulation of cytoplasmic pH by nonanimal cells

High-affinity potassium uptake in Neurospora occurs by symport with protons [Km (apparent) = 15 microM at pH 5.8], for which a large inward gradient (approximately 400 mV) is generated by the H+-extruding ATPase of the plasma membrane. Operating in parallel, the two transport systems yield a net 1:1 exchange of K+ for cytoplasmic H+. Since this exchange could play a role in cytoplasmic pH (pHi) regulation, the coordinated functioning of the K+-H+ symport and H+ pump has been examined during acid stress. Cytoplasmic acid loads were imposed by injection and by exposure to extracellular permeant weak acid. Multibarrelled microelectrodes were used to monitor membrane potential (Vm), pHi, and the current-voltage (I-V) characteristics of the cells. The behaviors of the H+ pump and K+-H+ symport were resolved, respectively, by fitting whole membrane I-V curves to an explicit kinetic model of the Neurospora membrane and by subtracting I-V curves obtained in the absence from those obtained in the presence of 5-200 microM K+ outside. Proton pumping accelerates nearly in proportion with the cytoplasmic H+ concentration, but pHi recovery from imposed acid loads is dependent on micromolar K+ outside. Potassium import via the symport leads to a measurable alkalinization of the cytoplasm in accordance with stoichiometric (1:1) K+/H+ exchange. Potassium transport is accelerated at low pHi, but in a manner consistent with its inherent voltage sensitivity and changes in Vm resulting from an increased rate of H+ extrusion by the pump. The primary response to acid stress thus rests with the H+ pump, but K+ transport introduces an essential kinetic "valve" that can regulate net H+ export.

Electrical characteristics of stomatal guard cells: The ionic basis of the membrane potential and the consequence of potassium chlorides leakage from microelectrodes

The membrane electrical characteristics of stomatal guard cells in epidermal strips from Vicia faba L. and Commelina communis L. were explored using conventional electrophysiological methods, but with double-barrelled microelectrodes containing dilute electrolyte solutions. When electrodes were filled with the customary 1-3 M KCl solutions, membrane potentials and resistances were low, typically decaying over 2-5 min to near-30 mV and <0.2 kω·cm(2) in cells bathed in 0.1 mM KCl and 1 mM Ca(2+), pH 7.4. By contrast, cells impaled with electrodes containing 50 or 200 mM K(+)-acetate gave values of-182±7 mV and 16±2 kω·cm(2) (input resistances 0.8-3.1 Gω, n=54). Potentials as high as (-) 282 mV (inside negative) were recorded, and impalement were held for up to 2 h without appreciable decline in either membrane parameter. Comparison of results obtained with several electrolytes indicated that Cl(-) leakage from the microelectrode was primarily responsible for the decline in potential and resistance recorded with the molar KCl electrolytes. Guard cells loaded with salt from the electrodes also acquired marked potential and conductance responses to external Ca(2+), which are tentatively ascribed to a K(+) conductance (channel) at the guard cell plasma membrane.Measurements using dilute K(+)-acetate-filled electrodes revealed, in the guard cells, electrical properties common to plant and fungal cell membranes. The cells showed a high selectivity for K(+) over Na(+) (permeability ratio PNa/PK=0.006) and a near-Nernstian potential response to external pH over the range 4.5-7.4 (apparent PH/PK=500-600). Little response to external Ca(2+) was observed, and the cells were virtually insensitive to CO2. These results are discussed in the context of primary, charge-carrying transport at the guard cell plasma membrane, and with reference to possible mechanisms for K(+) transport during stomatal movements. They discount previous notions of Ca(2+)-and CO2-mediated transport control. It is argued, also, that passive (diffusional) mechanisms are unlikely to contribute to K(+) uptake during stomatal opening, despite membrane potentials which, under certain, well-defined conditions, lie negative of the potassium equilibrium potential likely prevailing.

The plasma membrane ATPase of Neurospora: a proton-pumping electroenzyme

Probably the best marker enzyme for plasma membranes of eukaryotic cells is a magnesium-dependent, vanadate-inhibited ATPase whose primary function is the transmembrane transport of cations. In animal cells, different species of the enzyme transport different cations: sodium ions released in unequal exchange for potassium ions, calcium ions extruded alone (perhaps), or protons secreted in equal exchange for potassium ions. But in plants and fungi only proton secretion has been clearly demonstrated. A useful model cell for studying the proton-secreting ATPase has been the ascomycete fungus Neurospora, in which the enzyme drives an outward current of protons that can exceed 50 microA/cm2 and can support membrane potentials greater than 300 mV. Both thermodynamic and kinetic studies have shown that the proton-pumping ATPase of Neurospora normally transports only a single proton for each ATP molecule split; and kinetic modelling studies have suggested (contrary to conventional assumptions) that the fast steps in the overall reaction are transmembrane transit of the proton and its dissociation following transport, while the slow steps are the binding of protons and/or ATP. The primary structure of the Neurospora enzyme, recently deduced by gene sequencing, is very close to that of the yeast (Saccharomyces) enzyme, and the hydropathic patterns for both closely resemble those for the animal-cell plasma-membrane ATPases. All of these enzymes appear to have 6-10 membrane-spanning alpha-helices, plus a large cytoplasmic headgroup which bears the catalytic nucleotide-binding site. Structural data, taken together with the electrical-kinetic behavior, suggest that the catalytic headgroup functions as an energized gate for protons. From a geometric point of view, action of such a gate would transfer the membrane field across the "transported" ion, rather than vice versa.

Intracellular microelectrode measurements in small cells evaluated with the patch clamp technique

Microelectrode penetration of small cells leads to a sustained depolarization of the resting membrane potential due to a transmembrane shunt resistance (Rs) introduced by the microelectrode. This has led to underestimation of the resting membrane potential of various cell types. However, measurement of the fast potential transient occurring within the first few milliseconds after microelectrode penetration can provide information about pre-impalement membrane electrophysiological properties. We have analyzed an equivalent circuit of a microelectrode measurement to establish the conditions under which the peak of the impalement transients (Ep) approaches the pre-impalement resting membrane potential (Em) of small cells most closely. The simulation studies showed that this is the case when the capacitance of the microelectrode is low and the membrane capacitance of the cell high. In experiments performed to assess the reliability of Ep as a measure of Em, whole-cell patch clamp measurements were performed in the current clamp mode to monitor, free from the effects of Rs, Em in cultured human monocytes. Microelectrode impalement of such patch clamped cells and measurement of Ep made it possible to detect correlation between Ep and Em and showed that for small cells such as human monocytes Ep is on average 6 mV less negative than the resting membrane potential.

Effect of temperature on transmembrane potential of mouse liver cells

Mouse liver transmembrane potential (Vm), measured under steady-state conditions with conventional microelectrodes, was -40 +/- 0.6 mV, and intracellular Na+ and K+ activities, measured with liquid ion-exchanger ion-sensitive microelectrodes, were 17 +/- 2 and 104 +/- 4 mM, respectively. The corresponding K+ and Na+ equilibrium potentials (EK and ENa) were -88 and 48 mV. Vm also varied as a linear function of temperature. In the range of 37-27 degrees C, the temperature coefficient (Q10) of 1.61 was greater than the Q10 of 1.033 predicted for a direct proportion of absolute temperature. A decrease in hepatocyte EK accounted for only a small portion of the total decrease in Vm resulting due to cooling from 37 to 25 degrees C. In contrast, slopes of the linear portion of Vm versus log10 external K+ activity were -24 and -14 mV/10-fold change in external K+ activity at 37 and 25 degrees C, respectively. This is consistent with an increase of membrane Na+- to -K+ permeability ratio (PNa/PK) with cooling. Ba2+ and quinine, which block membrane K+ channels, reversibly inhibited increases in hepatocyte Vm resulting due to heating from 37 to 40 degrees C. This suggests that membrane PK varies directly with temperature. We postulate that effects of temperature on liver Vm result from temperature effects on membrane K+ channel conductance and on the Na+-K+ pump. The results also are consistent with temperature effects on kinetic parameters for opening and closing of membrane K+ channels.

K+, Na+, and Cl- activities in ventricular myocytes isolated from rabbit heart

Intracellular K+, Na+, and Cl- activities (aiK, aiNa and aiCl) were measured in ventricular myocytes enzymatically isolated from adult rabbit heart. The activities in normal Tyrode solution containing 2.5 mM Ca2+ were the following (in mM): aiK = 100.0 +/- 3.5 (n = 9); aiNa = 8.4 +/- 1.5 (n = 6); and aiCl = 17.9 +/- 1.5 (n = 11) (mean +/- SE). Membrane potential was -81.6 +/- 0.7 mV (n = 26). These values were determined after correction for changes of junction and tip potential at the reference electrode, estimated to be 4.9 +/- 0.6 mV (n = 7) for 0.15 M KCl-filled electrodes; and intracellular interference detected by the Cl- ion-selective electrode, 11.2 +/- 0.6 mM (n = 4). Extended-tip shunting was avoided by fabricating Na+ ion-selective microelectrodes from aluminosilicate rather than borosilicate glass. These results show that isolated cardiac cells can maintain normal intracellular ion activities. Diffusion of electrolyte from the reference electrode can rapidly alter the intracellular milieu, however. After 10 min of impalement with 0.15 M KCl-filled microelectrodes (resistance approximately equal to 25 M omega), aiK increased by 8.7 +/- 2.0 mM and aiCl by 10.3 +/- 3.1 mM. In contrast, aiNa did not significantly change during the double impalement.

A potassium-proton symport in Neurospora crassa

Combined ion flux and electrophysiological measurements have been used to characterized active transport of potassium by cells of Neurospora crassa that have been moderately starved of K+ and then maintained in the presence of millimolar free calcium ions. These conditions elicit a high-affinity (K1/2 = 1-10 microM) potassium uptake system that is strongly depolarizing. Current-voltage measurements have demonstrated a K+-associated inward current exceeding (at saturation) half the total current normally driven outward through the plasma membrane proton pump. Potassium activity ratios and fluxes have been compared quantitatively with electrophysiological parameters, by using small (approximately 15 micron diam) spherical cells of Neurospora grown in ethylene glycol. All data are consistent with a transport mechanism that carries K ions inward by cotransport with H ions, which move down the electrochemical gradient created by the primary proton pump. The stoichiometry of entry is 1 K ion with 1 H ion; overall charge balance is maintained by pumped extrusion of two protons, to yield a net flux stoichiometry of 1 K+ exchanging for 1 H+. The mechanism is competent to sustain the largest stable K+ gradients that have been measured in Neurospora, with no direct contribution from phosphate hydrolysis or redox processes. Such a potassium-proton symport mechanism could account for many observations reported on K+ movement in other fungi, in algae, and in higher plants.

Fusion of renal epithelial cells: a model for studying cellular mechanisms of ion transport

The investigation of epithelial ion transport at the cellular level by means of electrophysiological techniques is hampered by the small size of epithelial cells. Moreover, interpretation of experiments is complex due to poorly defined and highly variable paracellular leaks (shunt pathways). In search of a new experimental approach we developed a technique to isolate renal epithelial cells (diameter approximately equal to 10 micron) from diluting segments of the frog kidney and to fuse them to "giant" cells (diameter approximately equal to 100 micron). These cells generate membrane potentials of -54.1 +/- 1.6 mV (mean +/- SEM; n = 40). They are sensitive to the diuretic drugs furosemide and amiloride and to the K+- and Cl- -permeability blockers Ba2+ and anthracene-9-carboxylic acid. The experiments demonstrate membrane potential measurements in cells isolated from renal epithelium and fused to giant cells. The cells retain their specific membrane properties and could serve as a valuable experimental model in epithelial research.

Oscillations of cytoplasmic concentrations of Ca2+ and K+ in fused L cells

Using Ca2+- and K+-selective microelectrodes, the cytosolic free Ca2+ and K+ concentrations were measured in mouse fibroblastic L cells. When the extracellular Ca2+ concentration exceeded several micromoles, spontaneous oscillations of the intracellular free Ca2+ concentration were observed in the submicromolar ranges. During the Ca2+ oscillations, the membrane potential was found to oscillate concomitantly. The peak of cyclic increases in the free Ca2+ level coincided in time with the peak of periodic hyperpolarizations. Both oscillations were abolished by reducing the extracellular Ca2+ concentration down to 10(-7) M or by applying a Ca2+ channel blocker, nifedipine (50 microM). In the presence of 0.5 mM quinine, an inhibitor of Ca2+-activated K+ channel, sizable Ca2+ oscillations still persisted, while the potential oscillations were markedly suppressed. Oscillations of the intracellular K+ concentration between about 145 and 140 mM were often associated with the potential oscillations. The minimum phase of the K+ concentration was always 5 to 6 sec behind the peak hyperpolarization. Thus, it is concluded that the oscillation of membrane potential results from oscillatory increases in the intracellular Ca2+ level, which, in turn, periodically stimulate Ca2+-activated K+ channels.

Voltage clamp of bull-frog cardiac pace-maker cells: a quantitative analysis of potassium currents

Spontaneously active single cells have been obtained from the sinus venosus region of the bull-frog, Rana catesbeiana, using an enzymic dispersion procedure involving serial applications of trypsin, collagenase and elastase in nominally 0 Ca2+ Ringer solution. These cells have normal action potentials and fire spontaneously at a rate very similar to the intact sinus venosus. A single suction micro-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981; Hume & Giles, 1983) has been used to record the spontaneous diastolic depolarizations or pace-maker activity as well as the regenerative action potentials in these cells. This electrophysiological activity is completely insensitive to tetrodotoxin (TTX; 3 X 10(-6) M) and is very similar to that recorded from an in vitro sinus venosus preparation. The present experiments were aimed at identifying the transmembrane potassium currents, and analysing their role(s) in the development of the pace-maker potential and the repolarization of the action potential. Depolarizing voltage-clamp steps from the normal maximum diastolic potential (-75 mV) elicit a time- and voltage-dependent activation of an outward current. The reversal potential of this current in normal Ringer solution [( K+]0 2.5 mM) is near -95 mV; and it shifts by 51 mV per tenfold increase in [K+]0, which strongly suggests that this current is carried by K+. We therefore labelled it IK. The reversal potential of IK did not shift in the positive direction following very long (20 s) depolarizing clamp steps to +20 mV, indicating that 'extracellular' accumulation of [K+]0 does not produce any significant artifacts. The fully activated instantaneous current-voltage (I-V) relationship for IK is approximately linear over the range of potentials -130 to -30 mV. Thus, the ion transfer mechanism of IK may be described as a simple ohmic conductance in this range of potentials. Positive relative to -30 mV, however, the I-V exhibits significant inward rectification. A Hodgkin-Huxley analysis of the kinetics of IK, including a demonstration that the envelope of tails quantitatively matches the time course of the onset of IK during a prolonged depolarizing clamp step has been completed. The steady-state activation variable (n infinity) of IK spans the voltage range approximately -40 to +10 mV. It is well-fitted by a Boltzmann distribution function with half-activation at -20 mV. The time course of decay of IK is a single exponential. However, the activation or onset of IK shows clear sigmoidicity in the range of potentials from the activation threshold (-40 mV) to 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane voltage, resistance, and channel switching in isolated mouse fibroblasts (L cells): a patch-electrode analysis

The whole-cell patch-electrode technique of Fenwick, Marty & Neher (1982) has been applied to single suspension-cultured mouse fibroblasts. Seals in the range of 10-50 G omega were obtained without special cleaning of the cell membranes. Rupture of the membrane patch inside the electrode was accompanied by a shift of measured potential into the range -10 to -25 mV, but in most cases with little change in the recorded resistance. The latter fact implied that the absolute resistance of the cell membrane must be in the same range as the seal resistance and the recorded potential is a poor measure of actual cell membrane potential. Steady-state current-voltage curves (range -160 mV to +80 mV) were generated before and after rupture of the membrane patch, and the difference between these gave (zero-current) membrane potentials of -50 to -75 mV, which represents a leak-corrected estimate of the true cell-membrane potential. The associated slope conductivity of the cell membrane was 5-15 microS/cm2 (assumed smooth-sphere geometry, cells 13-15 microns in diameter) and was K+-dominated. With 0.1 mM (or more) free Ca2+ filling the patch electrode, membrane potentials in the range -60 to -85 mV were observed following patch rupture, with associated slope conductivities of 200-400 microS/cm2, also K+-dominated. Similar voltages and conductivities were observed at the peak of pulse-induced 'hyperpolarizing activation' (Nelson, Peacock, & Minna, 1972), and the two phenomena probably reflect the behaviour of Ca2+-activated K+ channels. Both the pulse-induced conductance and the Ca2+-activated conductance spontaneously decayed, the latter over periods of 5-15 min following patch rupture. Sr2+, Ba2+, and Co2+ could also activate the putative K+ channels, but only Sr2+ really mimicked Ca2+. Co2+ and Ba2+ activated with a delay of several minutes following patch rupture, and deactivated quickly with a small decrease of conductance and a large decrease of membrane potential. Evidently, Co2+ and Ba2+ affect channel specificity as well as channel opening and closing kinetics.

Electrical properties of Madin-Darby canine kidney cells. Effects of extracellular potassium and bicarbonate

To gain some insight into electrogenic transport processes across the plasma membrane of Madin-Darby canine kidney (MDCK)-cells, continuous measurements of the potential difference across the plasma measurements of the potential difference across the plasma membrane (PD) were made during step changes of extracellular ion composition as well as application of barium or valinomycin. During control conditions mimicking in vivo extracellular fluid, PD approaches -51.5 +/- 0.8 mV (n = 62). Step increase of extracellular potassium concentration from 5.4 to 10, to 20 or to 35 mmol/l, depolarizes PD by +5.5 +/- 0.8 mV (n = 7), by +15.8 +/- 0.5 mV (n = 64) and by +23.8 +/- 1.2 mV (n = 12), respectively. 1 mmol/l barium depolarizes PD by +19.8 +/- 0.6 mV (n = 38) and abolishes the effect of increasing extracellular potassium from 5.4 to 10 mmol/l but not to 35 mmol/l. Ten mumol/l valinomycin hyperpolarizes PD to -69.3 +/- 2.9 mV (n = 7). In the presence of valinomycin, increase of extracellular potassium from 5.4 to 20 mmol/l depolarizes PD by +31.0 +/- 1.0 mV (n = 7). Ouabain depolarizes PD and reduces the sensitivity of PD to extracellular potassium concentration. Omission of extracellular bicarbonate and carbon dioxide as well as increase of extracellular bicarbonate at constant carbon dioxide lead to a hyperpolarization and enhanced sensitivity of PD to extracellular potassium. In the presence of barium, the effects of omitted bicarbonate and carbon dioxide of MDCK-cells is highly conductive to potassium.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane potentials of individual cells of isolated gastric glands of rabbit

Individual glands of rabbit gastric mucosa were prepared for measurements of cell membrane potentials. In the first experiments a collagenase isolation technique was used which produced gland fragments that were fixed on agarose. In later experiments a microdissection technique was used which allowed whole glands to be isolated that were held in suction pipettes. Individual parietal or chief cells could be recognized and impaled with microelectrodes, however, the yield of reliable recordings was small and the distinction from artifacts sometimes difficult. In acceptable recordings the membrane potentials of both cell types varied between around -20 and -35 mV or exceptionally -50 mV in both preparations, with mean values being around -26 mV. The significance of the recordings was tested by ion substitution experiments. Substitution of all chloride by sulfate increased the membrane potential to values ranging up to -60 and -80 mV that are commonly observed in other cells.

Thermosynthesis by biomembranes: energy gain from cyclic temperature changes

A theoretical mechanism is described allowing biomembranes to convert heat into electrical energy during temperature cycling (thermosynthesis). Necessary conditions for thermosynthesis are a temperature dependent electrical capacity and a conductivity as low as that of artificial lipid bilayers. Temperature cycling, and consequently thermosynthesis, can take place in leaves during cyclic transpiration and in organisms in natural waters that are carried along by convection currents. Electrogenic ATPases can convert the electrical energy gained by thermosynthesis into ATP if their activity and stoichiometry are properly regulated. The power of thermosynthesis is discussed and its possible value compared with the power of respiration. Environments where thermosynthesis may occur are listed. Thermosynthesis is a plausible energy source for the first living organisms.

Cell K activity in frog skin in the presence and absence of cell current

Cell K activity, acK, was measured in the short-circuited frog skin by simultaneous cell punctures from the apical surface with open-tip and K-selective microelectrodes. Strict criteria for acceptance of impalements included constancy of the open-tip microelectrode resistance, agreement within 3% of the fractional apical voltage measured with open-tip and K-selective microelectrodes, and constancy of the differential voltage recorded between the open-tip and the K microelectrodes 30-60 sec after application of amiloride or substitution of apical Na. Skins were bathed on the serosal surface with NaCl Ringer and, to reduce paracellular Cl conductance and effects of amiloride on paracellular conductance, with NaNO3 Ringer on the apical surface. Under control conditions acK was nearly constant among skins (mean +/- SD = 92 +/- 8 mM, 14 skins) in spite of a wide range of cellular currents (5 to 70 microA/cm2). Cell current (and transcellular Na transport) was inhibited by either apical addition of amiloride or substitution of Na by other cations. Although in some experiments the expected small increase in acK after inhibition of cell current was observed, on the average the change was not significant (98 +/- 11 mM after amiloride, 101 +/- 12 mM after Na substitution), even 30 min after the inhibition of cell current. The membrane potential, which in the control state ranged from -42 to -77 mV, hyperpolarized after inhibition of cell current, initially to -109 +/- 5 mV, then depolarizing to a stable value (-88 +/- 5 mV) after 15-25 min. At this time K was above equilibrium (EK = 98 +/- 2 mV), indicating that the active pump mechanism is still operating after inhibition of transcellular Na transport. The measurement of acK permitted the calculation of the passive K current and pump current under control conditions, assuming a "constant current source" with almost all of the basolateral conductance attributable to K. We found a significant correlation between pump current and cell current with a slope of 0.31, indicating that about one-third of the cell current is carried by the pump, i.e., a pump stoichiometry of 3Na/2K.

Effects of intracellular sodium and potassium iontophoresis on membrane potentials and resistances in toad urinary bladder

Glass microelectrodes were used to measure membrane potentials and the ratio of apical to basolateral membrane resistances before and after the passage of current from the potential-recording microelectrode to ground, in toad urinary bladder epithelium, in order to iontophorese cations into the cell. After application of the current, there was a transient change in the tip potential of the microelectrode. This artifact was measured with the microelectrode in the mucosal medium and was subtracted from the potential recorded in the cell. The serosal medium was bathed by Ringer's solution containing 51.5 mM K+ to minimize any current-induced increase of K+ in the unstirred layer. Under those conditions, both Na+ and K+ iontophoresis caused a significant hyperpolarization of basolateral membrane potential (Vcs) and a significant increase in the ratio of apical to basolateral membrane resistances (Ra/Rb). When bladders were exposed to amiloride in the mucosal solution, Na+ iontophoresis caused the basolateral membrane to hyperpolarize, but no significant changes were observed in Ra/Rb. When Na+ was injected in the presence of serosal ouabain, Vcs depolarized and Ra/Rb increased. K+ iontophoresis caused the basolateral membrane potential to hyperpolarize in the presence of ouabain but Ra/Rb did not change significantly. These results indicate that the Na+ pump in toad bladder is rheogenic, that apical Na+ conductance is sensitive to the cell levels of Na+ and K+ and that the basolateral membrane is K+ permeable.

Electrogenic active proton pump in Rana esculenta skin and its role in sodium ion transport

Kinetic and electrophysiological studies were carried out in the in vitro Rana esculenta skin, bathed in dilute sodium solution, to characterize the proton pump and coupling between sodium absorption (JNa+n) and proton excretion (JH+n). JNa+n and JH+n were both dependent on transepithelial potential (psi ms); hyperpolarizing the skin decreased JNa+n and increased JH+n; depolarization produced the opposite effects. Amiloride (5 X 10(-5) M) at a clamped psi ms of +50 mV inhibited JNa+n without affecting JH+n. Variations of psi ms or pH had identical effects on JH+n. Ethoxzolamide inhibited JH+n and simultaneously increased psi ms by 15-30 mV. These changes were accompanied by depolarization of the apical membrane potential psi mc from -47 to -25 mV and an increase in apical membrane resistance of 30%; no significant effects on basolateral membrane potential (psi cs) and resistance (Rb) nor on shunt resistance (Rj) were observed. The proton pump appears to be localized at the apical membrane. The proton pump was also inhibited by deoxygenation, oligomycin, dicyclohexylcarbodiimide and vanadate (100, 78, 83 and 100% inhibition respectively). The variations of JH+n and of the measured electrical currents were significantly correlated. These findings are supportive evidence of a primary active proton pump, electrogenic and strictly linked to aerobic metabolism. The current-voltage (I-V) relation of the proton pump was obtained as the difference in the I-V curves of the apical membrane extracted before and after proton-pump inhibition by ethoxzolamide during amiloride block of sodium transport. The proton-pump current (IP) was best described by a saturable exponential function of psi mc. Maximal pump current (ImaxP) was calculated to be 200 nequiv h-1 cm-2 at a psi mc of +50 mV and the pump reversal potential ERP was -130 mV. The effect of ethoxzolamide to depolarize psi mc was dependent on the relation between psi mc and ERP. Maximal induced depolarization occurred at a psi mc of +50 mV whereas ethoxzolamide exerted minimal effect on psi mc when the ERP was approached either by voltage clamping the apical membrane or by the addition of amiloride. We show that electroneutral sodium-proton countertransport is not the mechanism of active proton excretion in frog skin but that it is the proton excretion which provides a favourable electrical driving force for passive apical sodium entry.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of intracellular pH in vertebrate central neurons

The regulation of intracellular pH (pHi) was investigated in reticulospinal neurons of the lamprey using ion-selective microelectrodes. Steady-state pHi in 23 mM HCO-3-buffered Ringer was 7.44 +/- 0.03 with a membrane potential of 54 +/- 4 mV (mean +/- S.E.M., n = 6). In nominally HCO-3-free solutions, pHi recovery from acid loading was blocked by 10(-3)M amiloride. Recovery was stimulated by transition to HCO-3-containing solutions. Results suggest that pHi regulation in lamprey reticulospinal neurons is mediated by a Na+-H+ exchanger. The presence of a distinct, HCO-3-dependent pHi regulatory mechanism is postulated.

Transient and sustained effects of hormones and calcium on membrane potential in a bone cell clone

Measurements were made of the electrophysiological and cAMP response to changes in extracellular [Ca2+] and to hormone application in a bone cell clone. Both transient and long-term electrophysiological responses were studied. An increase in extracellular [Ca2+] usually resulted in a transient hyperpolarization of about 60-sec duration. In addition, increases in extracellular [Ca2+] from 0.9 to 1.8 mM and from 1.8 to 3.6 mM resulted in long-term hyperpolarization and increased potential fluctuations. Increasing bathing [Ca2+] until the membrane potential reached the K+ equilibrium level resulted in a significant decrease in fluctuations. Addition to the bathing medium of quinine, a putative blocker of the Ca2+-dependent K+ channel, resulted in long-term depolarization of the mean membrane potential, and a long-term decrease in potential fluctuations. Addition of Mg2+, a mild antagonist of Ca2+ entry into the cell, produced transient depolarization and reduction of potential fluctuations. These effects suggest that the potential fluctuations reflect cytoplasmic [Ca2+] fluctuations via Ca2+-dependent K+ membrane channels. Under an extracellular [Ca2+] of 1.8 mM, the application of prostaglandin E2 (PGE2), isoproterenol, and parathyroid hormone produced no significant effect on mean membrane potential or on the sustained potential fluctuations, but PGE2 did significantly raise intracellular cAMP. Under an increased bathing [Ca2+], significant changes in mean potential and fluctuations did occur in response to PGE2, but not in response to the other hormones, while the PGE2 effect on cAMP was not greatly changed. Hyperpolarizing transients of about 30-sec duration occurred in response to all of the hormones, particularly at an extracellular [Ca2+] of 3.6 mM. Thus, there are both transient and long-term electrophysiological responses to hormone application, with only the long-term response correlated with the production of cAMP. These electrophysiological responses may represent separate transient and long-term calcium transport responses to hormone application.

Effects of antidiuretic hormone on cellular conductive pathways in mouse medullary thick ascending limbs of Henle: II. determinants of the ADH-mediated increases in transepithelial voltage and in net Cl-absorption

Cellular impalements were used in combination with standard transepithelial electrical measurements to evaluate some of the determinants of the spontaneous lumen-positive voltage, Ve, which attends net Cl- absorption, JnetCl, and to assess how ADH might augment both JnetCl and Ve in the mouse medullary thick ascending limb of Henle microperfused in vitro. Substituting luminal 5 mM Ba++ for 5 mM K+ resulted in a tenfold increase in the apical-to-basal membrane resistance ratio, Ra/Rbl, and increasing luminal K+ from 5 to 50 mM in the presence of luminal 10(-4)M furosemide resulted in a 53-mV depolarization of apical membrane voltage, Va. Thus K+ accounted for at least 85% of apical membrane conductance. Either with or without ADH, 10(-4) M luminal furosemide reduced Ve and JnetCl to near zero values and hyperpolarized both Va and Vbl, the voltage across basolateral membranes; however, the depolarization of Vbl was greater in the presence than in the absence of hormone while the hormone had no significant effect on the depolarization of Va. Thus ADH-dependent increases in Ve were referable to greater depolarizations of Vbl in the presence of ADH than in the absence of ADH. 68% of the furosemide-induced hyperpolarization of Va was referable to a decrease in the K+ current across apical membranes, but, at a minimum, only 19% of the hyperpolarization of Vbl could be accounted for by a furosemide-induced reduction in basolateral membrane Cl- current. Thus an increase in intracellular Cl- activity may have contributed to the depolarization of Vbl during net Cl- absorption, and the intracellular Cl- activity was likely greater with ADH than without hormone. Since ADH increases apical K+ conductance and since the chemical driving force for electroneutral Na+, K+, 2Cl- cotransport from lumen to cell may have been less in the presence of ADH than in the absence of hormone, the cardinal effects of ADH may have been to increase the functional number of both Ba++-sensitive conductance K+ channels and electroneutral Na+, K+, 2Cl- cotransport units in apical plasma membranes.

Isolated rat hepatocyte couplets: a primary secretory unit for electrophysiologic studies of bile secretory function

Hepatocyte couplets were isolated by collagenase perfusion from rat liver. Between adjacent cells, the bile canaliculus forms a closed space into which secretion occurs. As in intact liver, Mg2+-ATPase is localized at the canalicular lumen, the organic anion fluorescein is excreted, and secretion is modified by osmotic gradients. By passing a microelectrode through one cell into the canalicular vacuole, a transepithelial potential profile was obtained. In 27 cell couplets the steady-state intracellular (-26.3 +/- 5.3 mV) and intracanalicular (-5.9 +/- 3.3 mV) potentials were recorded at 37 degrees C with reference to the external medium. Input resistances were determined within the cell (86 +/- 23 M omega) and in the bile canalicular lumen (32 +/- 17 M omega) by passing current pulses through the microelectrode. These data define electrical driving forces for ion transport across the sinusoidal, canalicular, and paracellular barriers and indicate ion permeation across a leaky paracellular junctional pathway. These findings indicate that the isolated hepatocyte couplet is an effective model for electrophysiologic studies of bile secretory function.

The effects of ryanodine, EGTA and low-sodium on action potentials in rat and guinea-pig ventricular myocytes: evidence for two inward currents during the plateau

Action potentials were recorded from single cells isolated from rat and guinea-pig ventricular muscle. In rat cells the repolarization showed two distinct phases, referred to as the early and late phases. In guinea-pig cells there was a maintained plateau. Reducing external sodium by replacement with lithium or choline suppressed the late phase of the action potential in rat cells, and shortened the plateau of the action potential in guinea-pig cells. Intracellular EGTA abolished contraction while suppressing the late phase of the action potential in rat cells, and shortening the plateau in guinea-pig cells. Ryanodine (1 microM), which is thought to inhibit the release of calcium from internal stores, suppressed contraction and the late phase of the action potential in rat cells. In guinea-pig cells, there was no substantial effect of ryanodine (1 microM) on either contraction or the time course of the action potential. The late phase of the action potential in rat cells was suppressed by increasing the external potassium concentration to 12 mM, and enhanced by reducing external potassium to 1.2 mM. It is concluded that an inward current activated by internal calcium contributes to the late phase of the action potential in rat cells, and to the plateau in guinea-pig cells. Two possibilities are a current arising from electrogenic sodium-calcium exchange, and a current through ion channels activated by calcium. The effects of reducing external sodium would be consistent with either mechanism. The contribution of such an inward current would be expected to be modified by outward currents through a rectifying potassium conductance which varies with external potassium concentration. In the rat, but not the guinea-pig, the rise in internal calcium which activates the inward current seems to be largely dependent on ryanodine-sensitive release of calcium from internal stores.

Stoichiometry of H+/amino acid cotransport in Neurospora crassa revealed by current-voltage analysis

Coupling of ions to the uptake of neutral and basic amino acids via a general amino acid transport system (System II), was studied in a mutant of Neurospora crassa (bat mtr) which lacks other transport systems for these solutes. All amino acids tested--including ones bearing no net charge--elicited rapid membrane depolarization, as expected for ion-coupled transport. (Since amino acid transport in Neurospora is not dependent on extracellular Na+ or K+, the associated ion is presumed to be H+.) Although the 14C-labeled amino acid fluxes through System II are largely independent of the identity of the amino acid, the depolarization caused by basic amino acids (L-lysine and L-ornithine) is 60-70% greater than that for neutral amino acids (e.g. L-leucine). This difference is consistent with a constant H+/amino acid stoichiometry of 2, the extra charge for lysine and ornithine being that on the amino acid itself, so that the charge ratio basic:neutral amino acids is 3:2. When actual membrane charge flow associated with amino acid uptake was compared with measured 14C-labeled amino acid influx, ratios of 2.07 charges/mol L-leucine and 3.40 charges/mol L-lysine were obtained, again in accord with a constant translocation stoichiometry of 2H+/amino acid. The advantages of this electrical method for estimating H+/solute stoichiometry in cotransport are discussed in relation to more familiar methods.