Developmental acquisition of Ca2+-sensitivity by K+ channels in spinal neurones

Molecular Determinants of BK Channel Functional Diversity and Functioning

Large-conductance Ca2+- and voltage-activated K+ (BK) channels play many physiological roles ranging from the maintenance of smooth muscle tone to hearing and neurosecretion. BK channels are tetramers in which the pore-forming α subunit is coded by a single gene ( Slowpoke, KCNMA1). In this review, we first highlight the physiological importance of this ubiquitous channel, emphasizing the role that BK channels play in different channelopathies. We next discuss the modular nature of BK channel-forming protein, in which the different modules (the voltage sensor and the Ca2+ binding sites) communicate with the pore gates allosterically. In this regard, we review in detail the allosteric models proposed to explain channel activation and how the models are related to channel structure. Considering their extremely large conductance and unique selectivity to K+, we also offer an account of how these two apparently paradoxical characteristics can be understood consistently in unison, and what we have learned about the conduction system and the activation gates using ions, blockers, and toxins. Attention is paid here to the molecular nature of the voltage sensor and the Ca2+ binding sites that are located in a gating ring of known crystal structure and constituted by four COOH termini. Despite the fact that BK channels are coded by a single gene, diversity is obtained by means of alternative splicing and modulatory β and γ subunits. We finish this review by describing how the association of the α subunit with β or with γ subunits can change the BK channel phenotype and pharmacology.

Action of propranolol in the reaction of smooth musculature of tracheal rings induced with acetylcholine, histamine, serotonin (5-HT) and prostaglandin (PGF2-alfa) in vitro and in vivo

Actions of acetylcholine (ACh), histamines, serotonins (5-HT) and prostaglandins (PGF2-alfa) in concentrations of 10(-4), 10(-3), 10(-2) and 10(-1) mol/dm(3) were analyzed in vitro conditions in isolated specimens of tracheas of 24 pigs, 7 guinea pigs, and dead persons for different reasons (8), in the presence and without presence of propranolol. Whilst, research regarding actions of aerosolized histamines (10 mg, 1%, 2 min), in the presence and without the presence of aerosolized propranolol (20 mg, 2%, 2 min) was done in vivo in 6 healthy persons. Study results show that propranolol does not emphasize contraction of the airways smooth musculature as induced by ACh, histamine, 5-HT and PGF2-alfa in vitro conditions (p>0,1). Also, in vivo we found a non-significance of tracheal smooth musculature constriction (p>0,1).

Increased large conductance calcium-activated potassium (BK) channel expression accompanied by STREX variant downregulation in the developing mouse CNS

Background

Large conductance calcium- and voltage activated potassium (BK) channels are important determinants of neuronal excitability through effects on action potential duration, frequency and synaptic efficacy. The pore- forming subunits are encoded by a single gene, KCNMA1, which undergoes extensive alternative pre mRNA splicing. Different splice variants can confer distinct properties on BK channels. For example, insertion of the 58 amino acid stress-regulated exon (STREX) insert, that is conserved throughout vertebrate evolution, encodes channels with distinct calcium sensitivity and regulation by diverse signalling pathways compared to the insertless (ZERO) variant. Thus, expression of distinct splice variants may allow cells to differentially shape their electrical properties during development. However, whether differential splicing of BK channel variants occurs during development of the mammalian CNS has not been examined.

Results

Using quantitative real-time polymerase chain reaction (RT-PCR) Taqman™ assays, we demonstrate that total BK channel transcripts are up regulated throughout the murine CNS during embryonic and postnatal development with regional variation in transcript levels. This upregulation is associated with a decrease in STREX variant mRNA expression and an upregulation in ZERO variant expression.

Conclusion

As BK channel splice variants encode channels with distinct functional properties the switch in splicing from the STREX phenotype to ZERO phenotype during embryonic and postnatal CNS development may provide a mechanism to allow BK channels to control distinct functions at different times of mammalian brain development.

Development of Ca2+-channel and BK-channel expression in embryos and larvae of Xenopus laevis

The patterns and density of channels expressed in neurons critically determine their electrical properties. We have examined developmental regulation of Ca2+-channel expression during the maturation of the spinal motor circuits in Xenopus as it develops from an embryo to a larva. In embryonic neurons approximately 60% of the current is carried by N-type channels, 8% by l-type channels and the remainder by an unidentified channel. As the embryo matures, omega-agatoxin-sensitive P/Q channels are gradually expressed and replace the unidentified HVA channel such that at stage 42 approximately 25% of the current is carried by P/Q channels. We have used fluorescent labelling of selective channel toxins to directly observe the distribution of P/Q, N and BK channels. The P/Q channel distribution was most prevalent on the cell surface proximal to the areas of the soma where processes emerge. Both N and BK channels were distributed throughout the soma but still exhibited concentration around the areas adjacent to the emergence of processes from the soma. The patterns of fluorescence labelling during development mirrored the development of the respective ionic currents. Both N and P/Q channels contribute roughly equally to activation of the BK current, suggesting that overlap in the distribution of the N, P/Q and BK channels is important in their functional interdependence. The newly expressed P/Q channels play a role in spike initiation and repetitive firing in larval spinal neurons and contribute to burst generation during swimming in the larva.

Selective regulation of xSlo splice variants during Xenopus embryogenesis

Calcium-activated potassium channels regulate excitability of the adult nervous system. In contrast, little is known about the contribution of calcium-activated potassium channels to excitability of the embryonic nervous system when electrical membrane properties and intracellular calcium levels show dramatic changes. Embryonic Xenopus spinal neurons exhibit a well-characterized developmental program of excitability that involves several different currents including calcium-activated ones. Here, we show that a molecular determinant of calcium-activated potassium channels, xSlo, is expressed during Xenopus embryogenesis even prior to differentiation of excitable tissues. Five different xSlo variants are expressed in embryonic tissues as a consequence of alternative exon usage at a single splice site. One of these variants, xSlo59, is neural-specific, and its expression is limited to late stages of neuronal differentiation. However, expression of the four other variants occurs in both muscle and neurons at all stages of development examined. Electrophysiological analysis of recombinant xSlo channels reveals that the xSlo59 exon serves as a gain-of-function module and allows physiologically relevant levels of membrane potential and intracellular calcium to activate effectively the resultant channel. These results suggest that xSlo59 channels play a unique role in sculpting the excitable membrane properties of Xenopus spinal neurons.

Contribution of presynaptic calcium-activated potassium currents to transmitter release regulation in cultured Xenopus nerve-muscle synapses

Using Xenopus nerve-muscle co-cultures, we have examined the contribution of calcium-activated potassium (K(Ca)) channels to the regulation of transmitter release evoked by single action potentials. The presynaptic varicosities that form on muscle cells in these cultures were studied directly using patch-clamp recording techniques. In these developing synapses, blockade of K(Ca) channels with iberiotoxin or charybdotoxin decreased transmitter release by an average of 35%. This effect would be expected to be caused by changes in the late phases of action potential repolarization. We hypothesize that these changes are due to a reduction in the driving force for calcium that is normally enhanced by the local hyperpolarization at the active zone caused by potassium current through the K(Ca) channels that co-localize with calcium channels. In support of this hypothesis, we have shown that when action potential waveforms were used as voltage-clamp commands to elicit calcium current in varicosities, peak calcium current was reduced only when these waveforms were broadened beginning when action potential repolarization was 20% complete. In contrast to peak calcium current, total calcium influx was consistently increased following action potential broadening. A model, based on previously reported properties of ion channels, faithfully reproduced predicted effects on action potential repolarization and calcium currents. From these data, we suggest that the large-conductance K(Ca) channels expressed at presynaptic varicosities regulate transmitter release magnitude during single action potentials by altering the rate of action potential repolarization, and thus the magnitude of peak calcium current.

Enhancement in activities of large conductance calcium-activated potassium channels in CA1 pyramidal neurons of rat hippocampus after transient forebrain ischemia

It has been reported previously that the neuronal excitability persistently suppresses and the amplitude of fast afterhyperpolarization (fAHP) increases in CA1 pyramidal cells of rat hippocampus following transient forebrain ischemia. To understand the conductance mechanisms underlying these post-ischemic electrophysiological alterations, we compared differences in activities of large conductance Ca(2+)-activated potassium (BK(Ca)) channels in CA1 pyramidal cells acutely dissociated from hippocampus before and after ischemia by using inside-out configuration of patch clamp techniques. (1) The unitary conductance of BK(Ca) channels in post-ischemic neurons (295 pS) was higher than that in control neurons (245 pS) in symmetrical 140/140 mM K(+) in inside-out patch; (2) the membrane depolarization for an e-fold increase in open probability (P(o)) showed no significant differences between two groups while the membrane potential required to produce one-half of the maximum P(o) was more negative after ischemia, indicating no obvious changes in channel voltage dependence; (3) the [Ca(2+)](i) required to half activate BK(Ca) channels was only 1 microM in post-ischemic whereas 2 microM in control neurons, indicating an increase in [Ca(2+)](i) sensitivity after ischemia; and (4) BK(Ca) channels had a longer open time and a shorter closed time after ischemia without significant differences in open frequency as compared to control. The present results indicate that enhanced activity of BK(Ca) channels in CA1 pyramidal neurons after ischemia may partially contribute to the post-ischemic decrease in neuronal excitability and increase in fAHP.

Dynamics of intrinsic electrophysiological properties in spinal cord neurones

The spinal cord is engaged in a wide variety of functions including generation of motor acts, coding of sensory information and autonomic control. The intrinsic electrophysiological properties of spinal neurones represent a fundamental building block of the spinal circuits executing these tasks. The intrinsic response properties of spinal neurones--determined by the particular set and distribution of voltage sensitive channels and their dynamic non-linear interactions--show a high degree of functional specialisation as reflected by the differences of intrinsic response patterns in different cell types. Specialised, cell specific electrophysiological phenotypes gradually differentiate during development and are continuously adjusted in the adult animal by metabotropic synaptic interactions and activity-dependent plasticity to meet a broad range of functional demands.

Control of spontaneous activity during development

Electrical activity participates in the development of the nervous system and comes in two general forms. Use-dependent or experience-driven activity occurs relatively late in development, and is important in events of terminal nervous system differentiation, such as stabilization of synaptic connections. Earlier in development, activity is spontaneous, occurring independently of normal sensory input and motor output. Spontaneous activity participates in many of the initial events of axon outgrowth, pruning of synaptic connections, and maturation of neuronal signaling properties. Despite its importance, the genesis of spontaneous activity is poorly understood. What is clear is that spontaneous activity must be regulated by the patterns with which voltage- and ligand-gated ion channels develop in individual neurons. This review explores how that regulation most likely occurs.

Sustained potassium currents in maturing CA1 hippocampal neurones

Whole-cell voltage clamp techniques were used to characterize sustained outward currents in maturing (P4 to P48) acutely isolated rat CA1 hippocampal neurones. Sodium removal and signal subtraction were used to isolate a sodium dependent sustained potassium current (IKNa). Calcium blockade (Co2+), sensitivity to a low TEA dose (0.5 mM) and sensitivity to Charibdotoxin (CTX 25 nM) and Iberiotoxin (IbTX 25 nM), in conjunction with signal subtraction, were used to isolate a sustained current with the characteristics of IC (IKCa). IKNa was found in both immature (P4-5) and older (P > 21) cells; this corresponded, respectively, to 56 +/- 5% and 36 +/- 6% of the outward current in younger and older cells. In the course of maturation, the voltage dependence of activation of IKNa shifted to more hyperpolarized values by approximately 20 mV. In the younger cells (P5-18) there was no evidence for sensitivity to CTX or IbTX. In 55 out of 77 older cells we found a component sensitive to CTX, IbTX, 0.5 mM TEA and Co2+.

Developmental changes in expression of ion currents accompany maturation of locomotor pattern in frog tadpoles

1. The K+ currents of spinal neurons acutely dissociated from Xenopus larvae were studied and compared with those of neurons dissociated from Xenopus embryos. 2. The density of total outward current in the larval and embryonic neurons remained the same from stage 37/38 to stage 42. 3. Almost all neurons at stage 42 expressed a fast activating Ca2+-dependent K+ current (IKCa) that was largely absent from embryonic neurons. Whereas IKCa became larger and more prevalent during development, the delayed rectifier K+ currents were down-regulated. 4. About 53 % of IKCa was selectively blocked by iberiotoxin which had no effect on the delayed rectifier K+ currents or the K+ currents of embryonic neurons. 5. The firing properties of neurons isolated from embryos were unchanged by iberiotoxin. However, the toxin greatly increased the frequency of firing in larval neurons. 6. Iberiotoxin extended the duration of ventral root bursts during fictive swimming in larvae at stages 41 and 42 but had no effect at stage 40. The progressive expression of IKCa thus contributed to burst termination. 7. We have found that changes in expression of outward current closely correlate with the maturation of the motor pattern during development. At a time when the motor pattern has a need for a burst-terminating mechanism, the larval neurons express a channel with properties appropriate for such a role.

Allosteric gating of a large conductance Ca-activated K+ channel

Large-conductance Ca-activated potassium channels (BK channels) are uniquely sensitive to both membrane potential and intracellular Ca2+. Recent work has demonstrated that in the gating of these channels there are voltage-sensitive steps that are separate from Ca2+ binding steps. Based on this result and the macroscopic steady state and kinetic properties of the cloned BK channel mslo, we have recently proposed a general kinetic scheme to describe the interaction between voltage and Ca2+ in the gating of the mslo channel (Cui, J., D.H. Cox, and R.W. Aldrich. 1997. J. Gen. Physiol. In press.). This scheme supposes that the channel exists in two main conformations, closed and open. The conformational change between closed and open is voltage dependent. Ca2+ binds to both the closed and open conformations, but on average binds more tightly to the open conformation and thereby promotes channel opening. Here we describe the basic properties of models of this form and test their ability to mimic mslo macroscopic steady state and kinetic behavior. The simplest form of this scheme corresponds to a voltage-dependent version of the Monod-Wyman-Changeux (MWC) model of allosteric proteins. The success of voltage-dependent MWC models in describing many aspects of mslo gating suggests that these channels may share a common molecular mechanism with other allosteric proteins whose behaviors have been modeled using the MWC formalism. We also demonstrate how this scheme can arise as a simplification of a more complex scheme that is based on the premise that the channel is a homotetramer with a single Ca2+ binding site and a single voltage sensor in each subunit. Aspects of the mslo data not well fitted by the simplified scheme will likely be better accounted for by this more general scheme. The kinetic schemes discussed in this paper may be useful in interpreting the effects of BK channel modifications or mutations.

Intrinsic voltage dependence and Ca2+ regulation of mslo large conductance Ca-activated K+ channels

The kinetic and steady-state properties of macroscopic mslo Ca-activated K+ currents were studied in excised patches from Xenopus oocytes. In response to voltage steps, the timecourse of both activation and deactivation, but for a brief delay in activation, could be approximated by a single exponential function over a wide range of voltages and internal Ca2+ concentrations ([Ca]i). Activation rates increased with voltage and with [Ca]i, and approached saturation at high [Ca]i. Deactivation rates generally decreased with [Ca]i and voltage, and approached saturation at high [Ca]i. Plots of the macroscopic conductance as a function of voltage (G-V) and the time constant of activation and deactivation shifted leftward along the voltage axis with increasing [Ca]i. G-V relations could be approximated by a Boltzmann function with an equivalent gating charge which ranged between 1.1 and 1.8 e as [Ca]i varied between 0.84 and 1,000 microM. Hill analysis indicates that at least three Ca2+ binding sites can contribute to channel activation. Three lines of evidence indicate that there is at least one voltage-dependent unimolecular conformational change associated with mslo gating that is separate from Ca2+ binding. (a) The position of the mslo G-V relation does not vary logarithmically with [Ca]i. (b) The macroscopic rate constant of activation approaches saturation at high [Ca]i but remains voltage dependent. (c) With strong depolarizations mslo currents can be nearly maximally activated without binding Ca2+. These results can be understood in terms of a channel which must undergo a central voltage-dependent rate limiting conformational change in order to move from closed to open, with rapid Ca2+ binding to both open and closed states modulating this central step.

Immature properties of large-conductance calcium-activated potassium channels in rat neuroepithelium

The pharmacological and biophysical properties of large-conductance Ca-activated K (BK) channels from embryonic rat telencephalic neuroepithelium were investigated with in situ patch-clamp techniques. A fraction of these channels exhibited properties characteristic of BK channels recorded in well differentiated cells, including normal gating mode (BKN channels). The vast majority of BK channels expressed distinctive properties, the most conspicuous being their buzz gating mode (BKB channels). BKB channels were insensitive to a concentration of charybdotoxin that completely and reversibly blocked BKN channels. In contrast with the strict dependence of BKN channel activation on cytoplasmic Ca, BKB channels displayed substantially high open probability (Po) after inside-out patch excision in a Ca-free medium. Intracellular trypsin down-regulated the Po of BKB channels, which then exhibited a greater sensitivity to cytoplasmic Ca, mainly in the positive direction (increased Po with increased Ca). This suggested a modulatory role for Ca as opposed to its gating role in BKN channels. Ca ions also reduced current amplitude of both types of channels. BKB channels were less voltage sensitive than BKN channels, but this was not correlated with their lower Ca sensitivity. We speculate that BKB channels may represent immature forms in the developmental expression of BK channels.

Calcium-activated potassium channels in adrenal chromaffin cells

Rat chromaffin cells express an interesting diversity of Ca(2+)-dependent K+ channels, including a voltage-independent, small-conductance, apamin-sensitive SK channel and two variants of voltage-dependent, large-conductance BK channels. The two BK channel variants are differentially segregated among chromaffin cells, such that BK current is completely inactivating in about 75-80% of rat chromaffin cells, while the remainder express a mix of inactivating and non-inactivating current or mostly non-inactivating BKs current. The single-channel conductance of BKi channels is identical to that of BKs channels. Although rates of current activation are similar in the two variants, the deactivation kinetics of the two channels also differ. Furthermore, BKi channels are somewhat less sensitive to scorpion toxins than BKs channels. The slow component of BKi channel deactivation may be an important determinant of the functional role of these channels. During blockade of SK current, cells with BKi current fire tonically during sustained depolarizing current injection, whereas cells with BKs current tend to fire only a few action potentials before becoming quiescent. The ability to repetitively fire requires functional BKi channels, since partial blockade of BKi channels by CTX makes a BKi cell behave much like a BKs cell. In contrast, the physiological significance of BKi inactivation may arise from the ability of secretagogue-induced [Ca2+]i elevations to regulate the availability of BKi channels during subsequent action potentials (Herrington et al., 1995). By reducing the number of BK channels available for repolarization, the time course of action potentials may be prolonged. This possibility remains to be tested directly. These results raise a number of interesting questions pertinent to the control of secretion in rat adrenal chromaffin cells. An interesting hypothesis is that cells with a particular kind of BK current may reflect particular subpopulations of chromaffin cells. These subpopulations might differ either in the nature of the material secreted from the cell (e.g., Douglass and Poisner, 1965) or in the responsiveness to particular secretagogues. The differences in electrical behavior between cells with BKi and BKs current suggest that the pattern of secretion that might be elicited by a single type of stimulus could differ. For BKi cells, secretion may occur in a tonic fashion during sustained depolarization, while secretion from cells with BKs current may be more phasic. In the absence of specific structural information about the domains responsible for inactivation of BKi channels, our understanding of the mechanism of inactivation remains indirect. BKi inactivation shares many features with N-terminal inactivation of voltage-dependent K+ channels. However, there are provocative differences between the two types of inactivation which require us to propose that the native inactivation domain of BKi channels may occlude access of permeant ions to the BK channel permeation pathway in a position at some distance from the actual mouth of the channel. Further understanding of the structural and mechanistic basis of inactivation of BKi channels promises to provide new insights into both the cytoplasmic topology of BK channels and the Ca(2+)- and voltage-dependent steps involved in channel activation.

Target-derived factors regulate the expression of Ca(2+)-activated K+ currents in developing chick sympathetic neurones

1. The functional expression of Ca(2+)-activated K+ currents (IK(Ca)) and voltage-activated Ca2+ currents (ICa) was examined using whole-cell recordings from chick lumbar sympathetic neurones developing in situ and under various conditions in vitro. 2. Macroscopic IK(Ca) was expressed at low current density (< 0.01 mA cm-2) in neurones isolated at embryonic days 9-16 (E9-16). IK(Ca) was expressed at high densities (> 0.04 mA cm-2) at E17-19. By contrast, there was no significant difference in ICa density between sympathetic neurones isolated at E13 and E18. 3. When sympathetic neurones were isolated at E13 and maintained in vitro for 5 days, IK(Ca) was expressed at a significantly lower density (< 0.01 mA cm-2) than in neurones isolated acutely at E18 (> 0.04 mA cm-2). There was no difference in ICa density between neurones that developed in vitro and in situ. 4. When E13 sympathetic neurones were cultured for 5 days in the presence of a confluent layer of ventricular myocytes, they expressed IK(Ca) at a high density (> 0.04 mA cm-2), similar to that of E18 neurones that developed entirely in situ. Cardiac cell-conditioned medium produced similar effects. However, co-culture of sympathetic neurones with spinal cord explants did not allow for normal IK(Ca) expression in vitro. 5. Culturing sympathetic neurones in the presence of 5 ng ml-1 nerve growth factor (NGF) caused a significant increase in IK(Ca) density but this effect was only seen in 50% of cells examined. 6. The largest developmental changes in macroscopic IK(Ca) occur several days after other K+ currents and ICa are expressed at maximal density. The normal developmental expression of IK(Ca) is dependent upon extrinsic factors, including target-derived differentiation factors.

Gating mode conversion by proteolysis in a large-conductance K+ channel from embryonic rat telencephalon

1. In situ patch-clamp recordings of early embryonic large-conductance K+ channels consistently revealed (86% of patches) a complex behaviour characterized by noisy fluctuations between open and closed states of relatively short duration. 2. This behaviour is similar to the buzz mode, a type of gating observed only very rarely in some channels. Its prevalence in proliferative neuroepithelium may thus constitute a criterion of immaturity. 3. In 89% of patches, the buzz mode was converted to a 'normal' mode by intracellular exposure to trypsin. 4. These observations suggest that the immature channel protein includes (or is affected by) a cytoplasmic component, the presence or absence of which determines certain sets of conformational transitions.

Transient outward currents in cochlear ganglion neurons of the chick embryo

Cochlear ganglion neurons were isolated from chick embryos and membrane currents recorded using the patch-clamp technique. Depolarizing voltage steps elicited transient outward currents whose inactivation was best fitted by a double-exponential function with time constants < 30 ms and > 100 ms. The fast inactivating transient outward current (Ito,f) had a threshold for activation of -61 +/- 5.5 mV; steady-state inactivation was voltage-dependent between -90 and -60 mV, with half-inactivation near -75 mV. The slowly inactivating outward current (Ito,s) showed an activation threshold of 34 +/- 4 mV. Half-inactivation was at -67 +/- 3 mV. Ito,f was blocked by 4-aminopyridine which did not affect Ito,s. The effect was concentration- and voltage-dependent. Tetraethylammonium had no effect on either fast or slow transient currents but reduced the amplitude of the non-inactivating outward current in a dose-dependent manner. Ito,f was strongly inhibited by removing Ca2+ from the extracellular bathing solution. Cobalt ions inhibited Ito,f in a dose-dependent manner between 2 and 20 mM. The inhibitory effect of Co2+ was voltage-dependent, displaying a bell-shaped inhibition curve as a function of membrane voltage, maximal inhibition occurring between -20 and 0 mV. Ca2+ removal did not affect Ito,s and partially reduced the amplitude of the steady-state current. These results provide kinetic and pharmacological evidence for the presence of two distinct transient outward currents in cochlear neurons. These currents may play a role in the first synaptic relay of sound transmission.

An analysis of Na+ currents in rat olfactory receptor neurons

Na+ currents were observed in acutely-dissociated adult rat olfactory receptor neurons using the whole-cell recording techniques. The threshold for current activation was near -70 mV and currents were fully activated by -10 mV (midpoint: -45 mV). Steady-state inactivation was complete at potentials more positive than -70 mV and half complete at -110 mV (+/- less than 1, n = 8). Complete recovery from inactivation required one second at -100 mV (n = 7). The addition of 10 microM tetrodotoxin or 1 mM Zn2+ to the external solution was required to completely block the current. The current differs from those in amphibian and cultured neonatal rat olfactory neurons in its unusually negative voltage-dependence and slow recovery. Since mammalian olfactory neurons have very high input resistances, physiological resting potentials cannot usually be measured using whole-cell recording techniques. However, predominantly-capacitatively-coupled spikes activated by depolarisation were frequently observed in cell-attached patches. This indicates that the cells were excitable and implies that they must have had resting potentials more negative than -90 mV in order for this current to underlie the action potential.

Differential responses of Ca-activated K channels to bradykinin in sensory neurons and F-11 cells

The nonapeptide bradykinin (BK) excites a subset of dorsal root ganglion (DRG) neurons with putative nociceptive functions by stimulating an inward cation current. In addition, BK stimulates various intracellular signaling pathways including an elevation of intracellular Ca2+. In a DRG neuron x neuroblastoma hybrid cell (F-11), BK stimulates similar increases in intracellular [Ca2+] and inward current but also elicits a large transient outward current through Ca(2+)-activated K channels. We have investigated the mechanisms underlying differential expression of outward current responses in the two cell types at the single channel level. Although K(Ca) channel activity appears in inside-out patches from both cells exposed to Ca2+, BK applied to the extrapatch membrane of cell-attached patches activates K(Ca) channels in F-11 but not DRG neurons. Whereas single K(Ca) channels are quantitatively similar in terms of conductance, voltage-dependence, and sensitivity to tetraethylammonium, they differ in sensitivity to intracellular Ca2+. Channel activation in both cells requires at least four Ca2+ ions, but half-maximal activation occurs at slightly higher [Ca2+] for DRG neurons. The shift in the Ca2+ dose-response curve combined with the steep [Ca2+] dependence of channel open probability makes it less likely that a BK-induced rise in internal [Ca2+] induced will trigger a transient outward current and resultant hyperpolarization in a DRG neuron.

Developmental change in calcium-activated chloride current during the differentiation of Xenopus spinal neurons in culture

The duration and ionic dependence of action potentials change during the differentiation of embryonic amphibian spinal neurons both in vivo and in culture. The development of sodium, calcium, and potassium currents has been characterized in these cells and the shortening of the action potential has been shown to depend to a great extent on developmental changes of potassium currents. Previous evidence suggests that a chloride current may also be present in these embryonic neurons. Chloride currents were investigated with intracellular current-clamp and single-electrode and whole-cell voltage-clamp techniques. Most neurons exhibited a calcium-activated chloride current (ICl(Ca] that contributed to the postdepolarization following the action potential recorded in the absence of sodium and potassium currents. This current appeared to decrease in density and its deactivation rate increased during the first day in culture. Its incidence also declined during this period. A much larger Ca(2+)-dependent Cl- current was also observed in a subset of neurons after 24 hr, but was absent at earlier stages of development. The results suggest the presence of two Cl- currents with different developmental fates. The early current probably contributes to the repolarization of long calcium-dependent action potentials at initial stages of neuronal development, when potassium currents are small, and may serve to reduce the extent of repetitive firing.

Calcium-activated potassium channels: regulation by calcium

A wide variety of calcium-activated K channels has been described and can be conveniently separated into three classes based on differences in single-channel conductance, voltage dependence of channel opening, and sensitivity to blockers. Large-conductance calcium-activated K channels typically require micromolar concentrations of calcium to open, and their sensitivity to calcium increases with membrane depolarization, suggesting that they may be involved in repolarization events. Small-conductance calcium-activated K channels are generally more sensitive to calcium at negative membrane potentials, but their sensitivity to calcium is independent of membrane potential, suggesting that they may be involved in regulating membrane properties near the resting potential. Intermediate-conductance calcium-activated K channels are a loosely defined group, where membership is determined because a channel does not fit in either of the other two groups. Within each broad group, variations in calcium sensitivity and single-channel conductance have been observed, suggesting that there may be families of closely related calcium-activated K channels. Kinetic studies of the gating of calcium-activated potassium channels have revealed some basic features of the mechanisms involved in activation of these channels by calcium, including the number of calcium ions participating in channel opening, the number of major conformations of the channels involved in the gating process, and the number of transition pathways between open and closed states. Methods of analysis have been developed that may allow identification of models that give accurate descriptions of the gating of these channels. Although such kinetic models are likely to be oversimplifications of the behavior of a large macromolecule, these models may provide some insight into the mechanisms that control the gating of the channel, and are subject to falsification by new data.

Calcium-activated potassium channels in rat dissociated ventromedial hypothalamic neurons

Abstract A potassium-selective channel, characterized by a single channel conductance of 160 pS was demonstrated to be present in rat freshly dispersed ventromedial hypothalamic nucleus neurons. The single channel activity was shown to be dependent, using inside-out membrane patches, upon the presence of intracellular calcium ions, with maximal sensitivity between 10(-6) and 10(-6) M[Ca(2+)], and to be modulated by membrane voltage, depolarization causing an increase in open-state probability in the presence of an activating concentration of calcium. Therefore these properties place this channel into the category of a large conductance (maxi-K(+)) calcium-activated potassium (Ca(2+)-K(+)) channel. This channel is active in cell-attached recordings from glucoreceptive cells when depolarized by glucose or tolbutamide with openings often associated with action current repolarization. These openings were shown to be abolished in the presence of extracellular Cd(2+) and La(3+) ions, which block calcium channels, suggesting that extracellular calcium entry upon cell depolarization is responsible for their activation. On a few occasions, a larger conductance (250 pS) Ca(2+)-K(+) channel was observed in inside-out membrane patches isolated from ventromedial hypothalamic nucleus neurons. In contrast to the 160 pS channel, the presence of intracellularly-applied ATP caused a concentration-dependent, reversible inhibition of its open-state probability.

Maturation of a transient outward potassium current in mouse fetal hypothalamic neurons in culture

The whole-cell voltage clamp technique was used to record potassium currents in mouse fetal hypothalamic neurons developing in culture medium from days 1 to 17. The neurons were derived from fetuses of IOPS/OF1 mice on the 14th day of gestation. The mature neurons (greater than six days in culture) showed both a transient potassium current and a non-inactivating delayed rectifier potassium current. These were identified pharmacologically by using the potassium channel blockers tetraethyl ammonium chloride and 4-aminopyridine, and on the basis of their kinetics and voltage sensitivities. The delayed rectifier potassium current had a threshold of-20 mV, a slow time-course of activation, and was sustained during the voltage pulse. The 4-aminopyridine-sensitive current was transient, and was activated from a holding potential more negative (-80 mV) than that required for evoking the delayed rectifier potassium current (-40 mV). The delayed rectifier potassium current was detectable from day 1 onwards, while the transient potassium current showed a distinct developmental trend. The time-constant of inactivation became faster with age in culture. The half steady-state inactivation potential showed a shift towards less negative membrane potentials with age, and the relationship was best described by a logarithmic regression equation. The developmental trend of the transient potassium current may relate functionally to the progressive morphological changes, and the appearance of synaptic connections during ontogenesis.

Identification of Ca(2+)-activated K+ channels in cells of embryonic chick osteoblast cultures

Primary cultures of embryonic chick osteoblasts consist of a heterogeneous cell population. Patch clamp measurements were done on 1- to 5-day-old osteoblasts, osteocytes, fibroblastlike cells, and cells that could not be classified on morphologic criteria. The measurements showed the omnipresence of depolarization-activated high-conductance channels in cell-attached patches. The whole-cell experiments showed an outward rectifying conductance activating at positive membrane potentials. Channels underlying the latter conductance were found to be K+ conducting in outside-out membrane patches. The activation potential of the outward rectifying K+ conductance shifted to negative membrane potentials upon increasing the intracellular Ca2+ concentration within the range of 10(-8)-10(-3.2) M. The same happened with the activation potential of the K+ channels found in outside-out patches. Finally, inside-out patch experiments directly demonstrated the dependency of the activation potential of K+ channels on Ca2+ ions. Thus the identity and main characteristics of Ca2(+)-activated K+ channels expressed by the various cell types present in chick osteoblast cultures have now been established. Decreased input resistances were found in cells of cultures more than 2 days old. This is consistent with the establishment of electrical coupling between the cells. Functions in which Ca2(+)-activated K+ channels could play a role are discussed.

Elevated potassium shortens action potential duration by altering outward currents in chick dorsal root ganglia neurons

Dissociated embryonic chick dorsal root ganglion (DRG) neurons maintained in culture exhibit a mixed Na+/Ca2+ action potential. The characteristic "shoulder" on the repolarizing phase is due to the relatively prolonged inward Ca2+ current. DRG neurons grown in an elevated K+ medium (25 versus. 5 mM) lack the plateau phase of the action potential. Voltage-clamp analysis showed that this plastic change in action potential duration is not due to the loss of the inward Ca2+ current but is partly due to the appearance of a Ca2(+)-dependent, 4-aminopyridine-(4-AP)-sensitive transient outward current. Faster activation of the purely voltage-dependent delayed rectifier outward current also contributes to the rapid repolarization observed in neurons cultured in elevated K+ medium.

Chloride action potentials and currents in embryonic skeletal muscle of the chick

Chloride-dependent action potentials were elicited from embryonic skeletal muscle fibers of the chick during the last week of in ovo development. The duration of the action potentials was extremely long (greater than 8 sec). The action potentials were reversibly blocked by the stilbene derivative, SITS, a specific blocker of chloride permeability. Using patch clamp pipettes, in which the intracellular chloride concentration was controlled and with other types of ion channels blocked, the membrane potential at the peak of the action potential closely coincided with the chloride equilibrium potential calculated from the Nernst equation. These data indicate that activation of a chloride-selective conductance underlies the long duration action potential. The occurrence of the chloride-dependent action potential was found to increase during embryonic development. The percentage of fibers that displayed the action potential increased from approximately 20% at embryonic day 13 to approximately 70% at hatching. Chloride-dependent action potentials were not found in adult fibers. The voltage and time-dependent currents underlying the action potential were recorded under voltage clamp using the whole-cell version of the patch pipette technique. The reversal potential of the currents was found to shift with the chloride concentration gradient in a manner predicted by the Nernst equation, and the currents were blocked by SITS. These data indicate that chloride ions were the charge carriers. The conductance was activated by depolarization and exhibited very slow activation and deactivation kinetics.

Ca2+-activated K+ channels from cultured renal medullary thick ascending limb cells: effects of pH

Ca2+-activated K+ channels were studied in cultured medullary thick ascending limb cells (MTAL) using the patch-clamp technique. The purpose was to determine the effect of acidic pH on channel properties in excised patches of apical cell membrane. At pH 7.4, increasing Ca2+ on the intracellular side or applying positive voltages increases channel open probability. Reducing pH to 5.8 on the intracellular face of the channel decreases channel open probability at each voltage and Ca2+ concentration. Channel mean open times display two distributions and mean closed times display three distributions. Increasing Ca2+ or applying depolarizing voltages lengthens each of the mean open times and shortens each of the closed times. Lowering pH to 5.8 decreases the mean open times and increases mean closed times at each Ca2+ and voltage with the greatest effect on the mean closed times. In contrast, both single-channel conductance and channel kinetics are unaffected when pH is reduced to 5.8 on the extracellular face of the membrane. We conclude that protons interfere with Ca2+ binding to the gate of Ca2+-activated K+ channels reducing the probability of channel opening.

Comparative study of K channel behavior in beta cell lines with different secretory responses to glucose

The patch-clamp technique was used to identify and investigate two K channels in the cell membrane of the HIT cell, an insulin secreting cell line with glucose-sensitive secretion. Channel characteristics were compared with those of glucose-modulated K channels in the RINm5F cell, an insulin secreting cell line in which secretion is largely glucose insensitive. A 65.7 pS channel, identified with the ATP-sensitive K channel was seen in HIT cell-attached patches. Channel activity was dose-dependently inhibited by glucose, by 50 and 100% at 450 microM and 8 mM glucose, respectively, similar to the values previously reported for RIN cells. In inside-out patches channel activity was 50% inhibited by 56 microM ATP and completely blocked between 500 microM and 1 mM, again, similar to the values reported for RIN cells. As in RIN cells a second, considerably larger (184 pS), K channel was glucose sensitive; the glucose sensitivity was similar to that in RIN cells with 50 and 100% channel inhibition at 7.5 and 25 mM, respectively. After patch excision the mean channel conductance increased from 184 to 215 pS. Under these conditions activity was strongly calcium dependent in the range pCa 5-7, identifying this as a calcium- and voltage-dependent K (K(Ca, V] channel; the calcium sensitivity was similar to that of the adult rat beta cell K(Ca, V) channel. In inside-out RIN cell patches, the large K channel was less abundant but displayed a similar conductance (223 pS). However, its calcium sensitivity was more than 10 times lower than in HIT cells, similar to that of the K(Ca, V) channel in the neonatal rat beta cell, which also displays a reduced secretory response to glucose. Based on these observations, it is proposed that the low calcium sensitivity of the K(Ca, V) channel may be causally associated with secretory deficiency in RIN cells and the immature secretory response of the neonatal beta cell.

Development alters the expression of calcium currents in chick limb motoneurons

Calcium current types expressed in vertebrate spinal motoneurons have not been previously resolved. We have resolved three types in chick limb motoneurons identified by retrograde labeling and report that dramatic changes in their expression take place during development in vivo. T-, N-, and L-type calcium currents were distinguished on the basis of kinetics, voltage dependence, and unique pharmacological sensitivities. Developmental changes were characterized by studying motoneurons isolated from embryos at three stages spanning neuromuscular system development. T currents were dominant at the earliest stage. Motoneurons from embryos 2 days older showed much reduced T currents and much increased N and L currents. We suggest that mature motoneurons will be dominated by N- and L-type calcium currents and that T current may serve developmental roles.

Action potentials and sodium inward currents of developing neurons in Xenopus nerve-muscle cultures

Action potentials and voltage-gated Na+ inward currents from cultured embryonic neurons of Xenopus laevis were recorded using the patch-clamp technique in the whole cell configuration. Neurons together with muscle cells were dissociated from embryos shortly after completion of gastrulation. Under the voltage-clamp condition the voltage-gated Na+ inward current was isolated from other currents by pharmacological means and by ion substitution. A small Na+ current was observed in round cells without neurites (presumptive neurons). The mean amplitude of the peak Na+ current was 2.5 times larger in neurons with short processes than in presumptive neurons. As they developed further by extending longer processes, the maximum amplitude of the Na+ inward current recorded at the soma decreased. In varicosities, the Na+ inward current density was greater than that at the soma of neurons with extended neurites but kinetic properties and voltage-dependency were similar.

Postnatal development of cat hind limb motoneurons. III: Changes in size of motoneurons supplying the triceps surae muscle

The postnatal changes of neuronal dimensions were studied in cat triceps surae motoneurons intracellularly labeled with horseradish peroxidase. Systematic correlations were observed in the analysis of single dendrites at each studied stage, from birth to 44-46 days post natum (d.p.n.) age, between size parameters intrinsic to the dendrites as the diameter of a 1st-order dendrite, the combined dendritic length, the dendritic membrane area, and the degree of branching. Some variability among samples was evident in each studied age group. The correlations were, however, sufficiently close to permit indirect estimations of both combined dendritic length and dendritic membrane area for larger samples of neurons from data on dendritic stem caliber. The total postnatal increase in dendritic membrane area was, on the average, 400%, i.e., from close to 100 X 10(3) microns2 to about 500 X 10(3) microns2. The corresponding increase in soma area amounted to 100%. Analysis revealed that there was a time lag between the increase in somatic and dendritic size. Thus, adult somatic dimensions were attained at age 44-46 d.p.n.; however, at this stage, the mean total dendritic membrane area was only about half of the adult value. The postnatal increase in size appeared to vary among neurons, yielding a wider neuronal size spectrum in the adult cat than that observed in kittens. The measured increase in size corresponded to a calculated average addition of dendritic membrane area of 3700 microns2/day from birth to 22-24 d.p.n. and from that stage to 44-46 d.p.n. of 2700 microns2 per day. Likewise, the increase in combined dendritic length could initially be as large as 1 mm/day down to 0.4 mm/day between 22-24 and 44-46 d.p.n., with a mean growth during the first 44-46 d.p.n. of 0.5 to 0.6 mm/day. The ratios of daughters to parent branch diameters (sigmadd1.5: dp1.5) and the dendritic trunk parameter (sigma d1.5) recorded along the proximodistal dendritic path distance revealed transient changes that might impact on the electrotonic properties of the dendrites during postnatal development. Computations from the measured changes in dendritic branch lengths and calibers indicated that if membrane and internal resistivity remain unaltered during postnatal development, the dendritic domain is electrotonically more compact in the newborn kitten than in the adult cat.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of Ca2+-dependent K+ channels in the membrane of smooth muscle cells isolated from adult and foetal human aorta

Ca2+-activated K+ ionic currents in the membrane of cultured smooth muscle cells isolated from foetal and adult human aorta were studied using whole cell and single-channel patch-clamp techniques. Whole cell currents in adult smooth muscle cells were 3-8 times larger than in foetal cells of similar sizes. The elementary conductance and ionic selectivity of single Ca2+-activated K+ were identical for both types of cells. Channel openings occurred in burst, the duration of which was 3-5-fold longer in adult than in foetal cells. The voltage dependency of the channel activating mechanism and the dependency of the mean open time on the Ca2+ concentration on the inner side of the membrane were similar for both types of cells. These results suggest that the main reason for the increase in potassium conductance during development is an alteration in the open time of the Ca2+-activated K+ channels.

Changes in densities and kinetics of delayed rectifier potassium channels during neuronal differentiation

Single-channel K+ currents were recorded from young and mature spinal neurons cultured from Xenopus embryos to examine the bases of the developmental increases in density and in rate of activation of the macroscopic voltage-dependent delayed rectifier K+ current (IKv). K+ channels of three conductance classes (integral of 80, 30, and 15 pS) are present at both ages, but only the intermediate and small conductance classes are voltage-dependent and thus underlie IKv. The increase in the density of IKv is due to increases in the numbers of intermediate and small channels per cell, but not to changes in their open probabilities. The increase in rate of activation of IKv results from a change in the activation kinetics of the intermediate channel class alone.

Calcium and voltage dependence of single Ca2+-activated K+ channels from cultured hippocampal neurons of rat

Calcium and voltage dependence of the Ca2+-activated K+ channel, K(Ca), was studied at the single-channel level in cultured hippocampal neurons from rat. The K(Ca) channel has approx. 220 pS conductance in symmetrical 150 mM K+, and is gated both by voltage and by Ca2+ ions. For a fixed Ca2+ concentration at the inner membrane surface, [Ca]i, channel open probability, Po, increases e-fold for 14 mV positive change in membrane potential. At a fixed membrane potential (0 mV), channel activity is first observed at [Ca]i = 10(-6) M, and increases with Ca2+ concentration approximating an absorption isotherm with power 1.4. The [Ca]i required to half activate (Po = 0.5) the channel is 4.10(-6) M. When compared to other preparations, the K(Ca) channel from hippocampal neurons reported here shows the lowest Ca2+ sensitivity and the highest voltage sensitivity. These findings are interpreted in evolutionary terms.

Sensory transmitters regulate intracellular calcium in dorsal horn neurons

Primary afferent terminals in the dorsal horn of the spinal cord release excitatory amino acid and peptide transmitters that initiate the central processing of nociceptive information. The postsynaptic actions of amino acid transmitters on spinal neurons have been well characterized, but the cellular basis of peptide actions remains unclear. Substance P is the best characterized of the peptides present in sensory neurons and has been shown to depolarize dorsal horn neurons and to facilitate nociceptive reflexes. To determine the mechanisms by which substance P contributes to afferent synaptic transmission, we have monitored the levels of intracellular calcium in single isolated rat dorsal horn neurons and report that substance P can produce a prolonged elevation in calcium concentration by mobilizing its release from intracellular stores. This elevation may contribute to the long-term changes in the excitable properties of dorsal horn neurons that occur following afferent fibre stimulation. We have also found that L-glutamate elevates intracellular calcium in substance P-sensitive dorsal horn neurons by increasing calcium influx. These results provide a direct demonstration of intracellular calcium changes in response to neuropeptides in mammalian central neurons. They also indicate that there is convergent regulation of intracellular calcium in dorsal horn neurons by two different classes of sensory transmitters that are co-released from the same afferent terminals.

Expression of membrane currents in rat neocortical neurons in serum-free culture. II. Outward currents

The timing of expression and properties of outward membrane currents in cultured neocortical pyramidal-shaped neurons have been investigated using the gigaseal whole-cell voltage clamp and single-channel recording techniques. Dissociated primary cultures of synchronized (same cell cycle), growth arrested (G1 phase) and birth-dated cells from fetal rat (E18) were maintained in a serum-free medium. The earliest appearing membrane current in the soma is a voltage-dependent outward current carried by K+. The current consists of two components, one rapidly rising component, resembling those associated with the transient outward current (IA) and the other similar to the delayed rectifier current (IK). The ratio between the peak IA and IK was about 0.3 at all membrane voltages. The magnitude of both IA and IK increased with time in culture but the ratio remained unchanged. Direct measurements of unitary currents showed the presence of two voltage-activated outward conductances, 32 pS and 120 pS. The small conductance channel was sparse. The large conductance channel is K+-selective and was sensitive to both voltage and internal Ca2+.

Membrane properties of isolated mudpuppy taste cells

The voltage-dependent currents of isolated Necturus lingual cells were studied using the whole-cell configuration of the patch-clamp technique. Nongustatory surface epithelial cells had only passive membrane properties. Small, spherical cells resembling basal cells responded to depolarizing voltage steps with predominantly outward K+ currents. Taste receptor cells generated both outward and inward currents in response to depolarizing voltage steps. Outward K+ currents activated at approximately 0 mV and increased almost linearly with increasing depolarization. The K+ current did not inactivate and was partially Ca++ dependent. One inward current activated at -40 mV, reached a peak at -20 mV, and rapidly inactivated. This transient inward current was blocked by tetrodotoxin (TTX), which indicates that it is an Na+ current. The other inward current activated at 0 mV, peaked at 30 mV, and slowly inactivated. This more sustained inward current had the kinetic and pharmacological properties of a slow Ca++ current. In addition, most taste cells had inwardly rectifying K+ currents. Sour taste stimuli (weak acids) decreased outward K+ currents and slightly reduced inward currents; bitter taste stimuli (quinine) reduced inward currents to a greater extent than outward currents. It is concluded that sour and bitter taste stimuli produce depolarizing receptor potentials, at least in part, by reducing the voltage-dependent K+ conductance.

The prostaglandin response in guinea-pig taenia caeci and trachea smooth muscle of different ages

The concentration-response curves for the carbachol-induced contraction of smooth muscle cells of guinea-pig taenia caeci and trachea were not dependent on tissue age. The prostaglandin E2 and F2 alpha responses increased with age in taenia caeci in contrast with the PGE2 response evoked in trachea. The high-potassium responses evoked in taenia caeci and trachea both increased at higher tissue age. Methoxyverapamil only inhibited the age-dependent responses. The results suggest that it is mainly the voltage-dependent calcium channels that are involved in the age-dependent prostaglandin response.

Both barium and calcium activate neuronal potassium currents

Amphibian spinal neurons in culture possess both rapidly inactivating and sustained calcium-dependent potassium current components, similar to those described for other cells. Divalent cation-dependent whole-cell outward currents were isolated by subtracting the voltage-dependent potassium currents recorded from Xenopus laevis neurons in the presence of impermeant cadmium (100-500 microM) from the currents produced without cadmium but in the presence of permeant divalent cations (50-100 microM). These concentrations of permeant ions were low enough to avoid contamination by macroscopic inward currents through calcium channels. Calcium-dependent potassium currents were reduced by 1 microM tetraethylammonium. These currents can also be activated by barium or strontium. Barium as well as calcium activated outward currents in young neurons (6-8 hr) and in relatively mature neurons (19-26 hr in vitro). However, barium influx appeared to suppress the sustained voltage-dependent potassium current in most cells. Barium also activated at least one class of potassium channels observed in excised membrane patches, while blocking others. The blocking action may have masked and hindered detection of the stimulatory action of barium in other systems.

Single calcium-activated potassium channels recorded from cultured rat sympathetic neurones

1. The properties of single Ca2+-activated K+ channels in cultured rat superior cervical ganglionic neurones were studied in cell-attached and excised patches using the patch-clamp technique. 2. In cell-attached patches using an external K+ concentration ([K+]o) of 150 mM, approximately equal to the internal [K+], the channel slope conductance was approximately 200 pS and independent of membrane voltage between -50 and +50 mV. Using [K+]o of 4.7 mM (providing a near physiological K+ gradient), the I-V relationship was non-linear with a slope conductance of approximately 120 pS at 0 mV. 3. The channel was selective for K+ over Cs+ and Na+ which were impermeant from either side of the membrane. Both Na+ and Cs+ also blocked the movement of K+ through the channel. Cs+ was active on either side of the membrane, whereas Na+ apparently blocked the channel only when applied to the cytoplasmic side. 4. The channel was activated by increasing the Ca2+ concentration on the inside of the membrane ([Ca2+]i). The channel was virtually inactive when [Ca2+]i = 0.01 microM. Depolarizing the patch at a constant [Ca2+]i usually further increased the opening probability. 5. The gating properties of the channel were studied using cell-attached patches. At potentials more negative than the resting membrane potential, the open-time distribution was described by a single exponential. On depolarization, two exponentials were required. The closed-time distribution was fitted by three exponentials. 6. Depolarization of the patch caused the long mean open lifetime to increase whilst the short mean open and closed lifetimes were unaffected. Both the intermediate and long mean closed lifetimes decreased with depolarization from -60 to +60 mV. 7. In cell-attached patches, the long mean open lifetimes were usually smaller than those observed in excised patches at depolarized potentials (greater than 0 mV). 8. A fourth closed state, possibly representing an inactivated form of the channel, was infrequently observed. A 50% substate of the full single-channel current was also observed occasionally. This substate was always associated with openings to the full current state. 9. The channel was blockable by external tetraethylammonium (25 microM-1 mM), Ba2+ (1-10 mM), and quinine (10-200 microM). External d-tubocurarine (25-100 microM) also blocked this IC channel. However it was insensitive to apamin (100-300 nM), muscarine (10 microM) and 4-aminopyridine (1-3 mM). The channel was also blocked by internal tetraethylammonium (5-10 mM) or Ba2+ (0.3-1 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Patch-clamp studies of isolated mouse olfactory receptor neurons

Olfactory receptor neurons isolated from embryonic, neonatal, and adult mice were studied using the patch-clamp technique. Several distinct types of ion channels were characterized in patches of membrane from the neuronal soma and the dendritic knob of receptor neurons, including a 130-pS Ca++-activated K+ channel with voltage-dependent kinetics, an 80-pS Ca++-activated K+ channel with voltage-insensitive kinetics, a 25-pS K+ channel with properties similar to inward rectifiers, and a 40-pS K+ channel that was activated and then inactivated by rapid depolarization. Evidence of large-conductance (greater than 200 pS) Cl- channels, which were Ca++ insensitive and increasingly active at depolarizing membrane potentials, and voltage-activated Ca++ channels (16 pS) was also obtained. From K+ channel activity recorded from cell-attached patches, the intracellular [Ca++] was inferred to be below 0.1 microM, and the membrane potential was inferred to be approximately -50 mV. The receptor neurons had high input resistances, and action potentials could be elicited by picoampere amounts of depolarizing current. The receptor neurons responded to applied odorant molecules and to forskolin with increases in membrane conductance. These results provide a description of the membrane properties of olfactory receptor neurons and a basis for understanding their electrical activity and response to odorants.

Modification of Ca2+-activated K+ channels in cultured medullary thick ascending limb cells by N-bromoacetamide

Ca2+-activated K+ channels were studied in cultured medullary thick ascending limb (MTAL) cells using the patch-clamp technique in the inside-out configuration. The Ca2+ activation site was modified using N-bromoacetamide (NBA). 1 mM NBA in the bath solution, at 2.5 microM Ca2+ reduces the open probability, Po, of the channel to less than 0.01, without an effect on single-channel conductance. NBA-modified channels are still Ca2+-sensitive, requiring 25 mM Ca2+ to raise Po to 0.2. Both before and after NBA modification channel openings display at least two distributions, indicative of more than one open state. High Ca2+ (1 mM) protects the channels from modification. Also presented is a second class of Ca2+-activated K+ channels which are normally present in MTAL cells which open infrequently at 10 microM Ca2+ (Po = 0.01) but have a Po of 0.08 at 1 mM Ca2+. We can conclude (i) that NBA modifies the channel by shifting Ca2+-sensitivity to very high Ca2+, (ii) that NBA acts on a site involved in Ca2+ gating, and (iii) that a low affinity channel is present in the apical cell membrane with characteristics similar to those of normal channels modified with NBA.

Different actions of intracellular free calcium on resting and GABA-gated chloride conductances

The effects of voltage-dependent Ca2+ current (ICa) on resting and gamma-aminobutyric acid (GABA)-gated Cl- conductances were studied in isolated frog sensory neurons. The amplitude of GABA-gated Cl- current (ICl) was greatly suppressed by a preceding ICa. The GABA dose-response curve was shifted to the right without changing the maximum response by increasing [Ca2+]i. An ICa-activated ICl was observed as an inward tail current on the 'off' phase of ICa. This tail current was easily saturated by increasing the amount of Ca2+ influx.