Effects of millimeter waves on ionic currents of Lymnaea neurons

The effects of mm‐waves 60.22–62.22 GHz and 75 GHz on A‐type K⁺ currents and the effects of 61.22 GHz on Ca²⁺ currents of Lymnaea neurons were investigated using a whole‐cell voltage‐clamp technique. The open end of a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates at the waveguide outlet, inserted into physiological solution, were in the range of 0–2400 W/kg. Millimeter wave irradiation increased the peak amplitudes, activation rates, and inactivation rates of both ion currents. The changes in A‐type K⁺ current were not dependent on the irradiation frequency. It was shown that the changes in the amplitudes and kinetics of both currents resulted from the temperature rise produced by irradiation. No additional effects of irradiation on A‐type K⁺ current other than thermal were found when tested at the phase transition temperature or in the presence of ethanol. Ethanol reduced the thermal effect of irradiation. Millimeter waves had no effect on the steady‐state activation and inactivation curves, suggesting that the membrane surface charge and binding of calcium ions to the membrane in the area of channel locations did not change. Bioelectromagnetics 20:24–33, 1999. © 1999 Wiley‐Liss, Inc.     

Sodium currents in toad cardiac pacemaker cells

Cells in the pacemaker region of toad (Bufo marinus) sinus venosus had spontaneous rhythmic action potentials. The rate of firing of action potentials, the rate of diastolic depolarization and the maximum rate of rise of action potentials were reduced by TTX (10 nm to 1 m). Currents were recorded with the whole cell, tight seal technique from cells enzymatically dissociated from this region. Cells studied were identified as pacemaker cells by their characteristic morphology, spontaneous rhythmic action potential activity that could be blocked by cobalt but not by TTX and lack of inward rectification. When calcium, potassium and nonselective cation currents (I_f) activated by hyperpolarization were blocked, depolarization was seen to generate transient and persistent inward currents. Both were sodium currents: they were abolished by tetrodotoxin (10 to 100 nm), their reversal potential was close to the sodium equilibrium potential and their amplitude and reversal potential were influenced as expected for sodium currents when extracellular sodium ions were replaced with choline ions. The transient sodium current was activated at potentials more positive than –40 mV while the persistent sodium current was obvious at more negative potentials. It was concluded that, in toad pacemaker cells, TTX-sensitive sodium currents contributing both to the upstroke of action potentials and to diastolic depolarization may play an important role in setting heart rate.We thank the Australian National Heart Foundation for their support. D.A.S. is an NHMRC Senior Research Officer.     

Ionic currents in cardiac myocytes of squid, Alloteuthis subulata

This paper provides the first study of voltage-sensitive membrane currents present in heart myocytes from cephalopods. Whole cell patch clamp recordings have revealed six different ionic currents in myocytes freshly dissociated from squid cardiac tissues (branchial and systemic hearts). Three types of outward potassium currents were identified: first, a transient outward voltage-activated A-current (I_A), blocked by 4-aminopyridine, and inactivated by holding the cells at a potential of −40 mV; second, an outward, voltage-activated, delayed rectifier current with a sustained time course (I_K); and third, an outward, calcium-dependent, potassium current (I_K(Ca)) sensitive to Co²⁺ and apamin, and with the characteristic N-shaped current voltage relationship. Three inward voltage-activated currents were also identified. First, a rapidly activating and inactivating, sodium current (I_Na), blocked by tetrodotoxin, inactivated at holding potentials more positive than −40 mV, and abolished when external sodium was replaced by choline. Second, an L-type calcium current (I_Ca,L) with a sustained time course, suppressed by nifedipine or Co²⁺, and enhanced by substituting Ca²⁺ for Ba²⁺ in the external medium. The third inward current was also carried by calcium ions, but could be distinguished from the L-type current by differences in its voltage dependence. It also had a more transient time course, was activated at more negative potentials, and resembled the previously described low-voltage-activated, T-type calcium current. Accepted: 24 September 1999     

Sodium-dependent plateau potentials in electrocytes of the electric fish <Emphasis Type="Italic">Gymnotus carapo</Emphasis>

The weakly electric fish Gymnotus carapo emits a triphasic electric organ discharge generated by muscle-derived electrocytes, which is modified by environmental and physiological factors. Two electrode current clamp recordings in an in vitro preparation showed that Gymnotus electrocytes fired repetitively and responded with plateau potentials when depolarized. This electrophysiological behavior has never been observed in electrocytes from related species. Two types of plateaus with different thresholds and amplitudes were evoked by depolarization when Na⁺-dependent currents were isolated in a K⁺- and Ca²⁺-free solution containing TEA and 4-AP. Two electrode voltage clamp recordings revealed a classical fast activating–inactivating Na⁺ current and two persistent Na⁺-dependent currents with voltage-dependencies consistent with the action potential (AP) and the two plateaus observed under current clamp, respectively. The three currents, the APs and the plateaus were reduced by TTX, and were absent in Na⁺-free solution. The different Na⁺-dependent currents in Gymnotus electrocytes may be targets for the modifications of the electric organ discharge mediated by environmental and physiological factors.     

Differential contributions of somatic and dendritic calcium-dependent potassium currents to the control of motoneuron excitability following spinal cord injury

The hyperexcitability of alpha-motoneurons and accompanying spasticity following spinal cord injury (SCI) have been attributed to enhanced persistent inward currents (PICs), including L-type calcium and persistent sodium currents. Factors controlling PICs may offer new therapies for managing spasticity. Such factors include calcium-activated potassium (KCa) currents, comprising in motoneurons an after-hyperpolarization-producing current (I _KCaN) activated by N/P-type calcium currents, and a second current (I _KCaL) activated by L-type calcium currents (Li and Bennett in J neurophysiol 97:767–783, 2007). We hypothesize that these two currents offer differential control of PICs and motoneuron excitability based on their probable somatic and dendritic locations, respectively. We reproduced SCI-induced PIC enhancement in a two-compartment motoneuron model that resulted in persistent dendritic plateau potentials. Removing dendritic I _KCaL eliminated primary frequency range discharge and produced an abrupt transition into tertiary range firing without significant changes in the overall frequency gain. However, I _KCaN removal mainly increased the gain. Steady-state analyses of dendritic membrane potential showed that I _KCaL limits plateau potential magnitude and strongly modulates the somatic injected current thresholds for plateau onset and offset. In contrast, I _KCaN had no effect on the plateau magnitude and thresholds. These results suggest that impaired function of I _KCaL may be an important intrinsic mechanism underlying PIC-induced motoneuron hyperexcitability following SCI.     

Effects of Inorganic Mercury and Methylmercury on the Ionic Currents of Cultured Rat Hippocampal Neurons

1. The effects of inorganic Hg²⁺ and methylmercuric chloride on the ionic currents of cultured hippocampal neurons were studied and compared. We examined the effects of acute exposure to the two forms of mercury on the properties of voltage-activated Ca²⁺ and Na⁺ currents and N-methyl-D-aspartate (NMDA)-induced currents.2. High-voltage activated Ca²⁺ currents (L type) were inhibited by both compounds at low micromolar concentrations in an irreversible manner. Mercuric chloride was five times as potent as methylmercury in blocking L-channels.3. Both compounds caused a transient increase in the low-voltage activated (T-type) currents at low concentrations (1 M) but blocked at higher concentrations and with longer periods of time.4. Inorganic mercury blockade was partially use dependent, but that by methylmercury was not. There was no effect of exposure of either form of mercury on the I–V characteristics of Ca²⁺ currents.5. Na⁺- and NMDA-induced currents were essentially unaffected by either mercury compound, showing only a delayed nonspecific effect at a time of overall damage of the membrane.6. We conclude that both mercury compounds show a relatively selective blockade of Ca²⁺ currents, but inorganic mercury is more potent than methylmercury.     

Ecdysteroid control of ionic current development in Manduca sexta motoneurons

The steroid hormone 20-hydroxyecdysone (20-HE) regulates several processes during insect metamorphosis. We studied the effects of 20-HE on the development of voltage-sensitive ionic currents of thoracic leg motoneurons of Manduca sexta. The larval leg motoneurons persist throughout metamorphosis but undergo substantial morphological reorganization, which is under the control of 20-HE and accompanied by changes in Ca²⁺ and K⁺ current densities. To determine whether 20-HE controls the changes in Ca²⁺ and K⁺ current levels during postembryonic development, identified thoracic leg motoneurons isolated from late larval and early pupal stages were taken into primary cell culture. Whole-cell Ca²⁺ and K⁺ currents were measured after 1–4 days of steroid hormone incubation. In the presence of 20-HE, peak Ca²⁺ currents of pupal leg motoneurons increased from day 1 to day 4 in vitro. Thus, at culture day 4 the pupal Ca²⁺ current levels were larger in 20-HE–treated than in untreated cells. By contrast, 20-HE did not affect the Ca²⁺ current amplitudes of larval leg motoneurons. Whole-cell K⁺ currents, measured at 4 days in pupal motoneurons, consisted of a fast-activating transient current and a sustained, slowly inactivating current. 20-HE did not affect the amplitude of the transient or sustained currents after 4 days in vitro. Thus, a direct steroid hormone effect may control the proper maturation of voltage-sensitive Ca²⁺ currents in leg motoneurons. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 211–223, 1998     

Ca2+ and Na+ current patterns during oocyte maturation, fertilization, and early developmental stages of Ciona intestinalis

Using the whole-cell voltage clamp technique, the electrical changes in oocyte and embryo plasma membrane were followed during different meiotic and developmental stages in Ciona intestinalis. We show, for the first time, an electrophysiological characterization of the plasma membrane in oocytes at the germinal vesicle (GV) stage with high L-type calcium (Ca2+) current activity that decreased through meiosis. Moreover, the absence of Ca2+ reduced germinal vesicle breakdown (GVBD), which is consistent with a role of Ca2+ currents in the prophase/metaphase transition. In mature oocytes at the metaphase I (MI) stage, Ca2+ currents decreased and then disappeared and sodium (Na+) currents first appeared remaining high up to the zygote stage. Intracellular Ca2+ release was higher in MI than in GV, indicating that Ca2+ currents in GV may contribute to fill the stores which are essential for oocyte contraction at fertilization. The fertilization current generated in Na+ free sea water was significantly lower than the control; furthermore, oocytes fertilized in the absence of Na+ showed high development of anomalous "rosette" embryos. Current amplitudes became negligible in embryos at the 2- and 4-cell stage, suggesting that signaling pathways that mediate first cleavage do not rely on ion current activities. At the 8-cell stage embryo, a resumption of Na+ current activity and conductance occurred, without a correlation with specific blastomeres. Taken together, these results imply: (i) an involvement of L-type Ca2+ currents in meiotic progression from the GV to MI stage; (ii) a role of Na+ currents during electrical events at fertilization and subsequent development; (iii) a major role of plasma membrane permeability and a minor function of specific currents during initial cell line segregation events.     

Effects of SDPNFLRF-amide (PF1) on voltage-activated currents in Ascaris suum muscle

Helminth infections are of significant concern in veterinary and human medicine. The drugs available for chemotherapy are limited in number and the extensive use of these drugs has led to the development of resistance in parasites of animals and humans ( [Geerts and Gryseels, 2000], [Kaplan, 2004] and [Osei-Atweneboana et al., 2007]). The cyclooctadepsipeptide, emodepside, belongs to a new class of anthelmintic that has been released for animal use in recent years. Emodepside has been proposed to mimic the effects of the neuropeptide PF1 on membrane hyperpolarization and membrane conductance (Willson et al., 2003). We investigated the effects of PF1 on voltage-activated currents in Ascaris suum muscle cells. The whole cell voltage-clamp technique was employed to study these currents. Here we report two types of voltage-activated inward calcium currents: transient peak (I_peak) and a steady-state (I_ss). We found that 1 μM PF1 inhibited the two calcium currents. The I_peak decreased from −146 nA to −99 nA (P = 0.0007) and the I_ss decreased from −45 nA to −12 nA (P = 0.002). We also found that PF1 in the presence of calcium increased the voltage-activated outward potassium current (from 521 nA to 628 nA (P = 0.004)). The effect on the potassium current was abolished when calcium was removed and replaced with cobalt; it was also reduced at a higher concentration of PF1 (10 μM). These studies demonstrate a mechanism by which PF1 decreases the excitability of the neuromuscular system by modulating calcium currents in nematodes. PF1 inhibits voltage-activated calcium currents and potentiates the voltage-activated calcium-dependent potassium current. The effect on a calcium-activated-potassium channel appears to be common to both PF1 and emodepside (Guest et al., 2007). It will be of interest to investigate the actions of emodepside on calcium currents to further elucidate the mechanism of action.     

Calibration of ring-current effects in proteins and nucleic acids

Summary Density functional chemical shielding calculations are reported for methane molecules placed in a variety of positions near aromatic rings of the type found in proteins and nucleic acids. The results are compared to empirical formulas that relate these intermolecular shielding effects to magnetic anisotropy (ring-current) effects and to electrostatic polarization of the C–H bonds. Good agreement is found between the empirical formulas and the quantum chemistry results, allowing a reassessment of the ring-current intensity factors for aromatic amino acids and nucleic acid bases. Electrostatic interactions contribute significantly to the computed chemical shift dispersion. Prospects for using this information in the analysis of chemical shifts in proteins and nucleic acids are discussed.     

Is ZD7288 a selective blocker of hyperpolarization-activated cyclic nucleotide-gated channel currents?

ZD7288 has been widely used as a tool in the study of hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels), and to test the relationships between HCN channels and heart and brain function. ZD7288 is widely considered a selective blocker of HCN currents. Here we show that ZD7288 inhibits not only HCN channel currents, but also Na⁺ currents in DRG neurons and ZD7288 was confirmed to inhibit Na⁺ current in HEK293 cells transfected with Na_v1.4 plasmids. Thus our findings challenge the view that ZD7288 is a selective blocker of HCN channels. Conclusions about the role of NCN channels in neuronal function should be re-evaluated if based exclusively on the effect of ZD7288.     

Electrophysiology of the marine diatom Coscinodiscus wailesii IV: types of non-linear current-voltage-time relationships recorded with single saw-tooth voltage-clamp experiments

Electrophysiological states of the marine diatom Coscinodiscus wailesii are known to change spontaneously in the temporal range of seconds. In order to assess the genuine current-voltage-time relationships of individual states in less than a second, voltage-clamp experiments have been carried out using single sweeps of saw-tooth shaped command voltages. This method is introduced with model calculations. Plotting the results in current-voltage coordinates provides convenient access to several electrophysiological entities, such as absence of drift (smoothly closed IV loops), membrane capacitance (by I jump at sign reversal of dV/dt), and ohmic conductances (in linear regions of the current-voltage relationship), as well as equilibrium voltage (internal intersection of capacitance-corrected, 8-shaped tracings) and coarse gating kinetics (rise or fall of capacitance-corrected I at sign reversal of dV/dt) of a voltage-sensitive ion conductance. From electrophysiological measurements with double-barreled glass-microelectrodes on C. wailesii, several distinct types of current-voltage loops are presented. Most of the data, including recordings from electrical excitation, can be interpreted as temporal relaxations of voltage-sensitive conductances for K⁺ and Cl⁻. A more detailed analysis of the effect of tetraethylammonium (TEA⁺) shows that 10 and 20 mM TEA⁺ inhibit the K⁺ conductance in C. wailesii only by up to about 20% but predominantly via a K⁺ outward rectifier. Received: 23 December 1998 / Revised version: 1 June 1999 / Accepted: 1 June 1999     

Is ZD7288 a selective blocker of hyperpolarization-activated cyclic nucleotide-gated channel currents?

ZD7288 has been widely used as a tool in the study of hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels), and to test the relationships between HCN channels and heart and brain function. ZD7288 is widely considered a selective blocker of HCN currents. Here we show that ZD7288 inhibits not only HCN channel currents, but also Na⁺ currents in DRG neurons and ZD7288 was confirmed to inhibit Na⁺ current in HEK293 cells transfected with Na_v1.4 plasmids. Thus our findings challenge the view that ZD7288 is a selective blocker of HCN channels. Conclusions about the role of NCN channels in neuronal function should be re-evaluated if based exclusively on the effect of ZD7288.     

Aggregation and spawning of the humphead wrasse Cheilinus undulatus (Pisces: Labridae): general aspects of spawning behaviour

The humphead wrasse Cheilinus undulatus formed resident spawning aggregations daily after high tide at specific locations along the seaward edge of the Palau barrier reef. The location and extent of one aggregation site remained consistent for 6 years with no physical features distinguishing it from adjacent areas. Spawning was documented most months and probably occurred year round with possible seasonal and lunar variation. Spawning males arrived first at the site, followed by females and potentially small primary males. The aggregation female to male sex ratio was estimated to be between 6:1 and 10:1. A maximum of 15 males and 100–150 females were observed at the site. A male courtship posture with the anal fin pointed, the caudal fin folded down and the dorsal fin folded against the body was maintained while swimming a few metres off the bottom in view of females. When ready to spawn females rose up as the posturing male passed and the pair released gametes in a relatively sedate fashion near the surface along the shelf break. No attempted predation on spawning adults was seen. Egg predation after spawning was uncommon. On days with early to mid‐day high tides the spawning period started 2·0–2·5 h after high tide when the speed of lagoon–ocean tidal currents peaked and lasted c. 1 h. On days with later afternoon high tides, spawning occurred sooner after high tide and before current speeds peaked. Other fishes with planktonic eggs spawned at the site as pairs or small groups in a rough succession after high tide with C. undulatus, the last species to spawn.     

Effects of chronic exposure to cadmium- or lead-enriched environments on ionic currents of identified neurons inLymnaea stagnalis L.

Summary 1. Voltage-activated ionic currents of three identified neurons ofLymnaea stagnalis L. were compared in control snails and in animals having been exposed to a cadmium- or lead-enriched environment for 2 weeks. We determined the presence, amplitude, and changes, if any, in the current-voltage characteristics of calcium and potassium currents in each of the three neurons from each of the three groups of animals. Finally, we have compared the effects of acute administration of Cd²⁺ or Pb²⁺ on neurons from control and chronically exposed animals.2. Chronic exposure to cadmium resulted in a near doubling of the high voltage-activated calcium current.3. No differences were found in the effects of acute application of Cd²⁺ or Pb²⁺ on neurons of pretreated and control animals. Cadmium was a potent blocker of the Ca current in either case, while lead caused only a 20% inhibition of the Ca current in neurons of both control and lead-exposed animals.4. Potassium currents were affected in both Cd²⁺- and Pb²⁺-exposed animals. While the sustained outward current was not influenced noticeably, the fast K current was affected in different ways in different neurons. Some did not show this current in the controls but expressed it in neurons from the exposed animals. Other neurons showed the current in the controls and its depression in exposed animals. Acute application of cadmium did not modulate the K current, but lead enhanced the peak amplitude of the transient K current in neurons of both exposed and control snails.     

Childhood cancer in relation to indicators of magnetic fields from ground current sources

This study examines childhood cancer risk in relation to certain factors likely to indicate magnetic field exposure from ground currents in the home. Substantial ground currents are most often found in homes having conductive plumbing, in which an uninterrupted metallic path in the water pipes and water main connects the grounding systems of neighboring houses. Information on plumbing conductivity was obtained from water suppliers for the homes of 347 cases and 277 controls identified in an earlier study of magnetic field exposure and childhood cancer in the Denver area. An increased cancer risk was observed for children in homes with conductive plumbing: The matched odds ratio was 1.72 (1.03–2.88) and increased to 3.00 (1.33–6.76) when analysis was limited to cases and controls who were residentially stable from the reference date to the study date. A measurement metric likely to indicate active ground currents (measurements having above-median intensity and a nonvertical orientation of <55° from the horizontal) was identified. In contrast to measured field intensity alone, for which only modest associations with cancer have been reported, this metric shows a high and significant cancer risk [matched O.R. = 4.0 (1.6–10.0)] consistent over a range of intensity and angle cutpoints. Such elevated nonvertical fields were also associated with cancer in an independent data set, which was gathered to study adult nonlymphocytic leukemia in the Seattle area. The associations of cancer with conductive plumbing and with this exposure metric both suggest that cancer risk is increased among persons with elevated magnetic field exposure from residential ground currents. © 1995 Wiley-Liss, Inc.     

Pharmacological characterization of ionic currents that regulate high‐frequency spontaneous activity of electromotor neurons in the weakly electric fish,Apteronotus leptorhynchus

The neural circuit that controls the electric organ discharge (EOD) of the brown ghost knifefish (Apteronotus leptorhynchus) contains two spontaneous oscillators. Both pacemaker neurons in the medulla and electromotor neurons (EMNs) in the spinal cord fire spontaneously at frequencies of 500–1000 Hz to control the EOD. These neurons continue to fire in vitro at frequencies that are highly correlated with in vivo EOD frequency. Previous studies used channel blocking drugs to pharmacologically characterize ionic currents that control high‐frequency firing in pacemaker neurons. The goal of the present study was to use similar techniques to investigate ionic currents in EMNs, the other type of spontaneously active neuron in the electromotor circuit. As in pacemaker neurons, high‐frequency firing of EMNs was regulated primarily by tetrodotoxin‐sensitive sodium currents and by potassium currents that were sensitive to 4‐aminopyridine and κA‐conotoxin SIVA, but resistant to tetraethylammonium. EMNs, however, differed from pacemaker neurons in their sensitivity to some channel blocking drugs. Alpha‐dendrotoxin, which blocks a subset of Kv1 potassium channels, increased firing rates in EMNs, but not pacemaker neurons; and the sodium channel blocker μO‐conotoxin MrVIA, which reduced firing rates of pacemaker neurons, had no effect on EMNs. These results suggest that similar, but not identical, ionic currents regulate high‐frequency firing in EMNs and pacemaker neurons. The differences in the ionic currents expressed in pacemaker neurons and EMNs might be related to differences in the morphology, connectivity, or function of these two cell types. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2006     

Characteristics of paxilline-sensitive calcium-dependent potassium current in isolated intestine myocytes

An inward current in smooth muscle cells (SMCs) of the taenia coli is known to be transferred via potassium channels and nonselective cation channels. The outward current is of a potassium nature and includes several components, Ca-dependent potassium current (I _K(Ca)) and delayed rectifying potassium current (I _K(V)) in particular. Applications of 100 nM paxilline to SMCs of the guinea-pig taenia coli suppressed considerably the outward current and decreased its oscillations; the effect of paxilline reached its maximum in 2 to 3 min from the beginning of application. Analysis of the current-voltage (I-V) relationship observed under conditions of such applications showed that the paxilline-sensitive current is highly dependent on the intracellular Ca²⁺ concentration; a change in the I-V slope within a segment of the maximum activation of the calcium current is indicative of this peculiarity. Application of paxilline against the background of the action of 1 mM tetraethylammonium (a nonselective blocker of potassium channels) evoked no additional suppression of the outward current. In most cells, we observed spontaneous outward currents (SOCs). Application of 100 nM paxilline nearly completely blocked high-amplitude SOCs (>10 pA) formed due to activation of big-conductance Ca-dependent potassium channels. At the same time, the frequency of small-amplitude SOCs (<10 pA) practically did not change. Thus, according to the pharmacological and time characteristics, voltage dependence, and sensitivity to the intracellular Ca²⁺ concentration, we identified a voltage-operated paxilline-sensitive component in I _K(Ca) that is transferred via big-conductance Ca-dependent potassium channels. Neirofiziologiya/Neurophysiology, Vol. 39, No. 3, pp. 201–207, May–June, 2007.     

Ca2+-activated K+ currents inNecturus choroid plexus

Summary The tight-seal whole-cell recording method has been used to studyNecturus choroid plexus epithelium. A cell potential of –59±2 mV and a whole cell resistance of 56±6 M were measured using this technique. Application of depolarizing step potentials activated voltage-dependent outward currents that developed with time. For example, when the cell was bathed in 110mm NaCl Ringer solution and the interior of the cell contained a solution of 110mm KCl and 5nm Ca²⁺, stepping the membrane potential from a holding value of –50 to –10 mV evoked outward currents which, after a delay of greater than 50 msec, increased to a steady state in 500 msec. The voltage dependence of the delayed currents suggests that they may be currents through Ca²⁺-activated K^_ channels. Based on the voltage dependence of the activation of Ca²⁺-activated K⁺ channels, we have devised a general method to isolate the delayed currents. The delayed currents were highly selective for K⁺ as their reversal potential at different K⁺ concentration gradients followed the Nernst potential for K⁺. These currents were reduced by the addition of TEA⁺ to the bath solution and were eliminated when Cs⁺ or Na⁺ replaced intracellular K⁺. Increasing the membrane potential to more positive values decreased both the delay and the half-times (t _1/2) to the steady value. Increasing the pipette Ca²⁺ also decreased the delay and decreasedt _1/2. For instance, when pipette Ca²⁺ was increased from 5 to 500nm, the delay andt _1/2 decreased from values greater than 50 and 150 msec to values less than 10 and 50 msec. We conclude that the delayed currents are K⁺ currents through Ca²⁺-activated K⁺ channels.At the resting membrane potential of –60 mV, Ca²⁺-activated K⁺ channels contribute between 13 to 25% of the total conductance of the cell. The contribution of these channels to cell conductance nearly doubles with membrane depolarization of 20–30 mV. Such depolarizations have been observed when cerebrospinal fluid (CSF) secretion is stimulated by cAMP and with intracellular Ca²⁺. Thus the Ca²⁺-activated K⁺ channels may play a specific role in maintaining intracellular K⁺ concentrations during CSF secretion.