Conduction and selectivity in potassium channels

Single-channel properties of the reconstituted voltage-regulated Na channel isolated from the electroplax of Electrophorus electricus

The tetrodotoxin-binding protein purified from electroplax of Electrophorus electricus has been reincorporated into multilamellar vesicles that were used for patch recording. When excised patches of these reconstituted membranes were voltage clamped in the absence of neurotoxins, voltage-dependent single-channel currents were recorded. These displayed properties qualitatively and quantitatively similar to those reported for Na channels from nerve and muscle cells, including uniform single-channel conductances of the appropriate magnitude (approximately equal to 11 pS in 95 mM Na+), mean open times of approximately equal to 1.9 msec, and 7-fold selectively for Na+ over K+. Currents averaged from many depolarizations showed initial voltage-dependent activation and subsequent inactivation. In the presence of batrachotoxin, channels were observed with markedly different properties, including conductances of 20-25 pS (95 mM Na+), mean open times of approximately equal to 28 msec, and no indication of inactivation. Collectively, these findings indicate that the tetrodotoxin-binding protein of electroplax is a voltage-regulated sodium channel.

Patch-clamp study of rubidium and potassium conductances in single cation channels from mammalian exocrine acini

Single-channel current recordings were carried out on excised inside-out patches of baso-lateral plasma membrane from exocrine acinar cells. The mouse pancreas and submandibular gland as well as the pig pancreas were investigated. In the mouse pancreas the voltage-insensitive Ca2+-activated cation channel was studied. Single-channel current-voltage (i/v) relationships were studied in symmetrical Rb+-rich solutions and in asymmetrical Rb+/Na+ and Na+/Rb+ solutions. In all cases the i/v relations were linear and had the same slope representing a single-channel conductance of about 33 pS which is identical to that previously obtained with symmetrical Na+ solutions or asymmetrical Na+/K+ solutions. In the mouse submandibular gland and the pig pancreas the voltage and Ca2+-activated K+ channel was studied. The outward currents observed after depolarization in the presence of quasi-physiological Na+/K+ gradients were immediately abolished when all the K+ in the bath fluid was replaced by Rb+ (bath fluid in contact with inside of plasma membrane). This effect was immediately and fully reversible upon return to the high K+ solution. The voltage and Ca2+-activated K+ channel was also studied in asymmetrical K+/Rb+ and Rb+/K+ solutions. In the first case inward (K+) currents could be observed but not outward (Rb+) currents, while in the other case inward (Rb+) currents could not be seen whereas outward (K+) currents were measured. The current-voltage relationships were approximately linear and the null potential was close to 0 mV in both situations.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-stationary fluctuations of the potassium conductance at the node of ranvier of the frog

Potassium currents were recorded from voltage-clamped nodes of isolated, myelinated axons of Rana pipiens. Nodes were maintained in a modified Ringer solution containing tetrodotoxin to block sodium current and 47.5 mM-potassium to minimize effects of extracellular potassium accumulation. Voltage protocols included depolarizing pulses lasting a few milliseconds to several seconds. Fluctuations about the ensemble average of the current were characterized in terms of non-stationary variance and autocovariance. The fluctuations had a Gaussian amplitude distribution and were virtually free of contaminations from systematic variations of the membrane current. Corrections for background noise were based on measurements done while potassium current was blocked with tetraethylammonium, and on simulations of extrinsic current fluctuations expected to arise from noise in the actual membrane voltage. The fluctuations were attributed to variations of nodal potassium conductance, since they were absent at the reversal potential of the potassium current and at membrane voltages that do not activate potassium current. Covariances indicated that voltage steps that reversed a macroscopic potassium current also reversed the sign of the fluctuation. Plots of the conductance variance versus the mean potassium conductance were generated from both the activation and deactivation (tail) phases of the potassium currents at various voltages between -80 and +70 mV. When the current was activated by a small depolarization (-50 mV) the trajectories from both phases were indistinguishable and were fitted by the parabola expected for a single population of channels with only one open-channel conductance. Apparent single-channel conductance from the early activation phase averaged 24 pS and was not significantly voltage dependent. In contrast, experiments with large depolarizations (+10 to +70 mV) gave significantly different variance--mean trajectories during activation and deactivation and these trajectories were poorly fitted by parabola. This result indicates that the fluctuations reflect several populations of channels and/or a population of channels that can have several levels of non-zero conductance. Projections of the fluctuation covariance showed long correlations, as well as the rapidly decaying component expected from the activation gating of channels. A slow fluctuation arose at a time slightly later than the rise of potassium current, spanned the entire length of brief depolarizations, and extended up to 880 ms during long depolarizations.(ABSTRACT TRUNCATED AT 400 WORDS)

Single channel recordings of calcium-activated potassium channels in the apical membrane of rabbit cortical collecting tubules

Recordings of single potassium channels from the apical membrane of rabbit cortical collecting tubule have been achieved using the patch-clamp technique. The conductive properties of the channel have been studied in inside-out patches. The slope conductance of the open channel is approximately equal to 90 pS. The channel is selective to potassium over sodium, with a selectivity ratio of 9:1. Decreasing the calcium concentration of the solution bathing the cytoplasmic face of the patch results in a decrease of the open-channel probability. Decreasing the calcium concentration to 10 nM or less completely inhibited channel activity. The channel is also inhibited by barium in a dose-dependent fashion.

Cytotoxic effect of choline, abolished by furosemide, in the diluting segment of frog kidney

Previous observations suggest that luminal application of tetra-N-alkylammonium ions may impair ion transport in the amphibian diluting segment. To investigate this question conventional KCl-filled and Cl- sensitive microelectrodes were applied in diluting segments of the isolated perfused kidney of rana esculenta to evaluate transepithelial electrical and chloride electrochemical (PDte, EClte) as well as peritubular cell membrane potential difference (PDpt), measured at static head conditions. After determination of control values the tubule lumen was exposed to choline (95 mmol/l, substituted for Na+) both in presence or absence of furosemide (5 X 10(-5) mol/l). Then, the lumen was again perfused with control solution and the measurements were repeated. Thus, a time course for possible choline induced effects was obtained both in the presence and absence of furosemide. The lumen positive PDte decreased from 11.2 +/- 1.0 mV to 6.3 +/- 0.8 mV after 2 min and to 1.9 +/- 0.4 mV after 30 min exposure to choline. PDpt (cell interior negative) decreased from 70 +/- 2 mV to 58 +/- 3 mV and to 42 +/- 5 mV after 2 and 30 min, respectively. Intraluminal Cl- activity increased from its initial steady state value of 20 +/- 2 mmol/l to 39 +/- 2 mmol/l after 30 min exposure to choline. However, if the tubule lumen was exposed to choline in presence of furosemide (5 X 10(-5) mol/l), all the above described choline-induced effects did not become apparent.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent block by amiloride and other monovalent cations of apical Na channels in the toad urinary bladder

Inhibition of the Na conductance of the apical membrane of the toad urinary bladder by amiloride, alkali cations and protons was voltage dependent. Bladders were bathed with a high K-sucrose serosal medium to reduce series basal-lateral resistance and potential difference. Transepithelial current-voltage relationships were measured over a voltage range of +/- 200 mV with a voltage ramp of frequency 0.5 to 1 Hz. Na channel I-V relationships were obtained by subtraction of currents measured in the presence of maximal doses of amiloride (10 to 20 microM). With submaximal doses of amiloride (0.05 to 0.5 microM), the degree of inhibition of the Na channel current (INa) increased as the mucosal potential was made more positive. The data can be reasonably well explained by assuming that amiloride blocks Na transport by binding to a site which senses approximately 12% of the transmembrane voltage difference. INa was reduced in a qualitatively similar voltage-dependent manner by mucosal K, Rb, Cs and Tl (approximately 100 mM) and by mucosal H (approximately 1 mM). Block by these cations cannot be explained in terms of interactions with a single membrane-voltage-sensing site; a model in which there are two or more blocking sites in series provides a better description of the data. On the other hand, amiloride block was reduced competitively by mucosal Na and K, suggesting that occupation of the channel by one cation excludes occupancy by the others. ADH and ouabain also reduce the apparent affinity of amiloride for its blocking site. Thus, intracellular Na may also compete with amiloride for occupancy of the channel.

Intracellular Na+ activates a K+ channel in mammalian cardiac cells

In a wide variety of cells, various intracellular agents, such as Ca2+, ATP and cyclic nucleotides, regulate ionic conductances of the membrane. In cardiac cells, the intracellular Na+ concentration [( Na+]i) frequently increases when a disturbance occurs in the electrogenic Na-K pump activity or the Na-Ca exchange mechanism. We have investigated a possible role of [Na+]i in controlling ion channels by using a patch-clamp method, and have found a K+ channel that is gated by [Na+]i greater than 20 mM, but not by the intracellular Ca2+ concentration (approximately 10(-4) M). We report here that the channel has a unitary conductance of 207 +/- 19 pS (n = 16) with K+ concentrations of 150 mM outside and 49 mM inside, and shows no detectable voltage-dependent kinetics. The Na+-activated K+ channel represents a novel class of ionic channel.

A patch-clamp study of potassium channels and whole-cell currents in acinar cells of the mouse lacrimal gland

Individual acinar cells were isolated enzymatically from the mouse exorbital lacrimal gland. Their electrical characteristics were studied by the patch-clamp methods of single-channel and whole-cell recording as described by Hamill, Marty, Neher, Sakmann & Sigworth (1981). Recording from cell-attached and excised inside-out patches of acinar membrane with quasi-physiological ion gradients demonstrated large outward current events that correspond to single-channel openings. The amplitude, frequency and duration of channel events increased as the membrane patch was depolarized and were reduced by hyperpolarization of the patch membrane. The reversal potential for these channel events is more negative than -40 mV. In excised inside-out patches exposed to quasi-physiological ion gradients single-channel events were abolished when K+ was replaced by Rb+. Since there was no Cl- gradient the channel is clearly highly selective for K+. In excised inside-out patches, when the free Ca2+ concentration bathing the physiological inside of the membrane was raised from less than 10(-9) M to 10(-8) M the frequency and duration of opening of the K+ channel was increased. The channel was almost continuously open when the membrane was exposed to 10(-7) M-free Ca2+. 'Whole cell' recording of lacrimal acinar cells containing 140 mM-KCl and 1 mM-EGTA (with no added Ca2+) provided cell resting membrane potentials of -30 to -40 mV. Depolarizing voltage jumps from the resting membrane potential evoked large outward currents. Hyperpolarizing voltage jumps only evoked small inward currents. Whole-cell recording where RbCl replaced KCl in the pipette provided resting membrane potentials of -20 to -30 mV, reduced the amplitude of outward currents evoked by cell-depolarizing voltage steps by 60% and slowed the time course of the currents. Isolated cells containing 140 mM-KCl and 1 mM-EGTA were voltage clamped at their resting membrane potentials. Acetylcholine (ACh) was applied locally and immediately evoked a strong outward current which rapidly declined to a steady-state level. Sustained agonist responses were obtained by exposing the isolated cell to a solution containing 10(-6) M-ACh. In both K+- and Rb+-filled cells, where the intracellular Ca2+ concentration was buffered by the inclusion of 1 mM-EGTA, 10(-6) M-ACh evoked sustained outward currents that corresponded to cell hyperpolarizations of 5-15 and 10-20 mV, respectively. Increasing intracellular Ca2+ buffering by including 10 mM-EGTA abolished secretagogue-induced outward current in both K+- and Rb+-filled cells. It is concluded that the lacrimal acinar cell membrane contains voltage- and Ca2+-activated K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of a delayed outward-rectifying K+ conductance in cultured mouse peritoneal macrophages

Patch clamp techniques were used to study ionic currents in cultured mouse peritoneal macrophages. Whole-cell voltage clamp studies of cells 1-5 hr after isolation showed only a high-resistance linear membrane. After 1 day in culture, 82 of 85 cells studied had developed a voltage- and time-dependent potassium (K+) conductance similar to the delayed outward rectifier in nerve and muscle cells. The current activated when the membrane was depolarized above -50 mV. The sigmoidally rising current rose to a peak at a rate that increased with depolarization. Inactivation proceeded exponentially with a time constant of approximately equal to 450 ms. Recovery from inactivation was slow (tau = 12 s). The reversal potentials for varying extracellular K+ concentrations followed the Nernst predictions for a K+ -specific channel. The conductance was blocked by extracellular 4-aminopyridine and by intracellular tetraethylammonium chloride, barium, and cesium. Single-channel K+ currents comprising this net current had a conductance of 16 pS, exhibited bursting behavior, and inactivated with time. No inward currents were ever detected in macrophages cultivated for up to 4 days. Short-term exposure to chemoattractant and transmitter agents failed to activate an inward current. Macrophages may change their membrane electrophysiological properties depending on their state of functional activation. We postulate that the K+ conductance develops prior to depolarizing conductances involved in the macrophage's immunological functions.

Delayed rectification in the cardiac Purkinje fiber is not activated by intracellular calcium

This study was designed to test the hypothesis that an outward current (Ix) responsible for action potential repolarization in the cardiac Purkinje fiber is activated by intracellular calcium (Cai). Pharmacological probes were combined with the measurement of membrane current and contractile activity under voltage clamp conditions. Experiments were designed to examine properties of Ix that have previously linked activation of this current to changes in Cai. The independence of Ix from Cai was demonstrated for each case tested. Thus, the results of these experiments support the view that Ix is not a calcium-activated current.

Ca2+-activated K+ channels in human red cells. Comparison of single-channel currents with ion fluxes

Exposure of the inner surface of intact red cells or red cell ghosts to Ca2+ evokes unitary currents that can be measured in cell-attached and cell-free membrane patches. The currents are preferentially carried by K+ (PK/PNa 17) and show rectification. Increasing the Ca2+ concentration from 0 to 5 microM increases the probability of the open state of the channels parallel to the change of K+ permeability as observed in suspensions of red cell ghosts. Prolonged incubation of red cell ghosts in the absence of external K+ prevents the Ca2+ from increasing K+ permeability. Similarly, the probability to find Ca2+-activated unitary currents in membrane patches is drastically reduced. These observations suggest that the Ca2+-induced changes of K+ permeability observed in red cell suspensions are causally related to the appearance of the unitary K+ currents. Attempts to determine the number of K+ channels per cell were made by comparing fluxes measured in suspensions of red cells with the unitary currents in membrane patches as determined under comparable ionic conditions. At 100 mM KCl in the external medium, where no net movements of K+ occur, the time course of equilibration of 86Rb+ does not follow a single exponential. This indicates a heterogeneity of the response to Ca2+ of the cells in the population. The data are compatible with the assumption that 25% of the cells respond with Pk = 33.2 X 10(-14)cm3/s and 75% with Pk = 3.1 X 10(-14)cm3/s. At 100 mM external K+ the zero current permeability of a single channel is 6.1 X 10(-14)cm3/s (corresponding to a conductance of 22 pS). Using appropriate values for the probability of a channel in the open state, we estimated that 25% of the cells in the population contain 11-55, and 75% of the cells 1-5 channels per cell that are activated in the time average (20 degrees C, pH 7.6).

Ionic events during the volume response of human peripheral blood lymphocytes to hypotonic media. II. Volume- and time-dependent activation and inactivation of ion transport pathways

Hypotonic dilution of human peripheral blood lymphocytes (PBL) induces large conductive permeabilities for K+ and Cl-, associated with the capacity of the cells to regulate their volumes. When rapid cation leakage is assured by the addition of the ionophore gramicidin, the behavior of the anion conductance pathway can be independently examined. Using this technique it is demonstrated that the volume-induced activation of Cl- transport is triggered at a threshold of approximately 1.15 X isotonic cell volume. If the volume of a cell is increased to this level or above, the Cl- transport system is activated, whereas if the volume of a swollen cell is decreased below the threshold value, the Cl- transport is inactivated. Activation and inactivation are independent of the relative volume changes and of the actual cellular Na+, K+, or Cl- concentrations, as well as of the changes in membrane potential in PBL. When net salt movement and thus volume change are inhibited by specific blockers of K+ transport (e.g., quinine, or Ca2+ depletion), volume-induced Cl- conductance shows a time-dependent inactivation, with a half-time of 5-8 min. The Cl- conductance, when activated, appears to involve an all-or-none response. In contrast, volume-induced K+ conductance is a graded response, with the increase in K+ flux being roughly proportional to the hypotonicity-induced increase in cell volume. The data indicate that during lymphocyte volume response in hypotonic media, anion conductance increases by orders of magnitude, exceeding the K+ conductance, so that the rate of the volume decrease (KCl efflux) is determined by a graded alteration in K+ conductance. When the cell volume approaches the isotonic value, it is stabilized by the inactivation of the anion conductance pathway.

Calcium-activated potassium channels and their role in secretion

Single-channel recording by the patch-clamp technique has now characterized three kinds of membrane potassium channels activated by intracellular calcium ions in animal cells. These play a crucial part in the regulation of membrane potential and of secretion.

Voltage-gated K+ channels in human T lymphocytes: a role in mitogenesis?

Membrane receptors and ion transport mechanisms probably have an important role in lymphocyte activation leading to T-lymphocyte proliferation in the immune response. Here we have applied a gigaohm-seal patch clamp technique to reveal the identity and properties of ion channels in human T lymphocytes. A voltage-dependent potassium channel bearing a resemblance to the delayed rectifier of nerve and muscle cells was found to be the predominant ion channel in these cells. In the whole cell recording conformation, the channels open with sigmoid kinetics during depolarizing voltage steps, reaching a maximum K+ conductance of 3-5 nS. The current subsequently becomes almost completely inactivated during a long-lasting depolarization. Currents through single K+ channels recorded in whole cell and outside-out patch recording conformations reveal a unitary channel conductance of about 16 pS in normal Ringer solution. Thus, the peak current corresponds to approximately 200-300 conducting K+ channels per cell. Phytohaemagglutinin (PHA), at concentrations that produce mitogenesis, alters K+ channel gating within 1 min of addition to the bathing solution, causing channels to open more rapidly and at more negative membrane potentials. 3H-thymidine incorporation by T lymphocytes following PHA stimulation is inhibited by the 'classical' K+ channel blockers tetraethylammonium and 4-aminopyridine, and also by quinine, at doses found to block the K+ channel in voltage-clamped T lymphocytes, suggesting that K+ channels may play a part in mitogenesis.

Barium-induced electro-mechanical uncoupling in myocardial cells

The heart of the adult moth Hyalophora cecropia requires extracellular calcium to maintain electrogenesis as well as tension development. In this study we ask whether the processes of autorhythmicity, driven electrogenesis and tension development require calcium specifically or whether the divalent cation Ba2+ can be substituted for calcium to support these activities. Ba2+ substituted for Ca2+ in equimolar amounts caused a marked (25 mV) hyperpolarization, suppression both of pacemaker activity and of tension development in spontaneously beating semi-isolated heart cells. Heart cells bathed in Ba2+ saline and paced by action potentials (produced by external stimuli) of greatly increased amplitude, prolonged phase 2 (plateau) and increased latency, and after 30 min, no mechanical activity was observed. These changes were completely reversible when calcium was reintroduced. We conclude that Ba2+ substitution for Ca2+ is an effective electromechanical uncoupler in moth heart cells. Although Ba2+ can support electrogenesis, it cannot replace 'trigger'-Ca2+ needed to release calcium from sarcoplasmic stores to effect tension development.

Single potassium channels with delayed rectifier behavior from lobster axon membranes

Single-channel potassium currents from lobster axon membranes were studied in planar bilayers made from monolayers. Channel-opening events are grouped by time, forming bursts with an average duration of 4.5 ms. The mean open time at 0 mV is 1.8 ms. The frequency of bursts is voltage dependent, increasing e-fold per 12-16 mV. At sufficiently high positive voltages, channels inactivate. Measured from reversal potentials, channels discriminate against Na+ by a permeability ratio PNa/PK of 1:30. The channel is blocked by tetraethylammonium and nonyltrimethylammonium in a voltage-dependent manner and at concentrations similar to those used in whole-axon experiments. Voltage-dependent block by Cs+ suggests that more than one ion may occupy the channel simultaneously. The kinetics and selectivity of this channel suggest that purified axolemma contains active K+ channels that are likely to participate in delayed rectification in the lobster axon membrane.

Single channel recordings from calcium-activated potassium channels in cultured rat muscle

Single channel recordings from cultured rat skeletal muscle have revealed a large conductance (230 pS) channel with a high selectivity for K+ over Na+. In excised patches of membrane, the probability of channel opening is sensitive to micromolar concentrations of calcium ions at the intracellular surface of the patch. Channel openings appear grouped together into bursts whose duration increases with Ca2+ and membrane depolarization. Statistical analysis of the individual open times during each burst showed that there are two distinct open states of similar conductance but dissimilar average lifetimes. These channels might contribute to a macroscopic calcium-activated potassium conductance in rat skeletal muscle and other preparations.

Properties of an inward rectifying K channel in the membrane of guinea-pig atrial cardioballs

Single channel outward current fluctuations are recorded in excised (outside-out) membrane patches of isolated atrial cells in culture (cardioballs) from hearts of adult guinea-pigs. The ionic channel displays a high selectivity to K ions. Accordingly the reversal potential of the single channel current is close to the K equilibrium potential. The open channel conductance is unaffected by the membrane potential but depends on the K concentration of the outside solution (19.7pS at 2 mM Ko to 30.7pS at 20 mM Ko). The open state probability (Po) of the channel shows a marked voltage dependence. Po amounts to c.0.9 at -40 mV and decreases to c.0.1 at +40 mV. Under the assumption of no channel interaction a macroscopic steady state current voltage relationship is reconstructed from the single channel data. The relationship displays inward-going rectification. The rectification is due to the voltage dependence of Po. The I-V curve displays a negative slope at membrane potentials positive to -15 mV. In bathing solutions containing Ba ions (0.2 mM) Po is reduced by rapid closures which interrupt the open state events. The unit channel conductance is unaffected by Ba ions. The channel block exerted by Ba ions is augmented with increasing membrane hyperpolarization. The results suggest that the channel studied may represent a background K conductance.

Interaction of intracellular electrolytes and tubular transport

To disclose possible regulatory mechanisms, the potential difference across the peritubular cell membrane (PDpt) and intracellular activities of sodium (Nai+), potassium (Ki+), calcium (Cai2+), bicarbonate (HCO3i-) and chloride (Cli-) have been traced continuously during inhibition of Na+/K+-ATPase with ouabain. Within 31 +/- 4 min following application of ouabain, PDpt decreases (from 57 +/- 2 mV) to half and Ki+ by 37.7 +/- 2.2 mmol/l (from 63.5 +/- 1.9 mmol/l), Nai+ increases by 35.1 +/- 4.1 mmol/l (from 13.2 +/- 2.4 mmol/l), Cai2+ by 0.17 +/- 0.2 mumol/l (from 0.09 mumol/l), HCO3i-) by 3.0 +/- 1.1 mmol/l (from 15.3 +/- 2.0 mmol/l) and Cli- by 6.2 +/- 1.0 mmol/l (from 14.4 +/- 1.6 mmol/l). Within the same time the luminal and peritubular cell membrane resistances increase 45 +/- 15% and 53 +/- 17%, respectively. The increase of the resistances is mainly due to a decrease of K+ conductance, which in turn mainly accounts for the depolarisation of PDpt. Additional experiments demonstrate that the K+ conductance of the peritubular cell membrane is sensitive to the cell membrane potential difference and possibly linked to Na+/K+-ATPase activity. The decline of PDpt probably accounts for intracellular alkalinisation which in turn reduces Na+/H+ exchange. Na+-coupled transport of glucose and phenylalanine decrease in linear proportion to PDpt. The transport of these and probably of similar substances represents the main threat to electrolyte homeostasis of the cells.

Quantification of Ca2+-activated K+ channels under hormonal control in pig pancreas acinar cells

Ca2+- and voltage-activated K+ channels are found in many electrically excitable cells and have an important role in regulating electrical activity. Recently, the large K+ channel has been found in the baso-lateral plasma membranes of salivary gland acinar cells, where it may be important in the regulation of salt transport. Using patch-clamp methods to record single-channel currents from excised fragments of baso-lateral acinar cell membranes in combination with current recordings from isolated single acinar cells and two- and three-cell clusters, we have now for the first time characterized the K+ channels quantitatively. In pig pancreatic acini there are 25-60 K+ channels per cell with a maximal single channel conductance of about 200 pS. We have quantified the relationship between internal ionized Ca2+ concentration [( Ca2+]i) membrane potential and open-state probability (p) of the K+ channel. By comparing curves obtained from excised patches relating membrane potential to p, at different levels of [Ca2+]i, with similar curves obtained from intact cells, [Ca2+]i in resting acinar cells was found to be between 10(-8) and 10(-7) M. In microelectrode experiments acetylcholine (ACh), gastrin-cholecystokinin (CCK) as well as bombesin peptides evoked Ca2+-dependent opening of the K+ conductance pathway, resulting in membrane hyperpolarization. The large K+ channel, which is under strict dual control by internal Ca2+ and voltage, may provide a crucial link between hormone-evoked increase in internal Ca2+ concentration and the resulting NaCl-rich fluid secretion.