Ethanol blocks cytosolic Ca2+ responses triggered by activation of GABA(A) receptor/Cl- channels in cultured proliferating rat neuroepithelial cells.

GABA(A) receptor/Cl- channels and voltage-gated Ca2+ channels are believed to be important sites of ethanol action in the CNS. Acute exposure of ethanol potentiates GABA(A) receptor/Cl- channel activity and inhibits voltage-gated Ca2+ channels in a number of preparations, mostly post-mitotic neurons. The effects of ethanol on these channels in primary cultures of undifferentiated neural precursor cells remain unknown. To address this issue, we examined the effects of ethanol on GABA(A) agonist-activated elevation of cytosolic Ca2+ in an in vitro model of the cortical neuroepithelium derived from rat basic fibroblast growth factor-expanded neural precursor cells. We found a potent inhibition of GABA(A)-activated elevation of cytosolic Ca2+ by ethanol in actively proliferating cells. Since we had recently demonstrated that GABA(A) receptor activation depolarizes these cells and elevates their cytosolic Ca2+, we tested whether the effects of ethanol involved both GABA(A) receptors and voltage-gated Ca2+ channels. Both extracellular K+- and muscimol-induced cytosolic Ca2+ elevations were abolished by nitrendipine, indicating that both depolarizing stimuli triggered Ca2+ influx through L-type voltage-gated Ca2+ channels. Exposure of proliferating cells to different concentrations of ethanol revealed that the drug was more potent in blocking muscimol-induced compared to K+-evoked cytosolic Ca2+ elevations.These results raise the possibility that ethanol blocks GABAergic stimulation of cytosolic Ca2+ levels in proliferating precursors primarily by interacting with GABA(A) receptor/Cl- channels and secondarily with voltage-gated Ca2+ channels.     

Both external and internal calcium reduce the sensitivity of the olfactory cyclic-nucleotide-gated channel to CAMP.

In vertebrate olfaction, odorous stimuli are first transduced into an electrical signal in the cilia of olfactory receptor neurons. Many odorants cause an increase in ciliary cAMP, which gates cationic channels in the ciliary membrane. The resulting influx of Ca2+ and Na+ produces a depolarizing receptor current. Modulation of the cyclic-nucleotide-gated (CNG) channels is one mechanism of adjusting olfactory sensitivity. Modulation of these channels by divalent cations was studied by patch-clamp recording from single cilia of frog olfactory receptor neurons. In accord with previous reports, it was found that cytoplasmic Ca2+ above 1 microM made the channels less sensitive to cAMP. The effect of cytoplasmic Ca2+ was eliminated by holding the cilium in a divalent-free cytoplasmic solution and was restored by adding calmodulin (CaM). An unexpected result was that external Ca2+ could also greatly reduce the sensitivity of the channels to cAMP. This reduction was seen when external Ca2+ exceeded 30 microM and was not affected by the divalent-free solution, by CaM, or by Ca2+ buffering. The effects of cytoplasmic and external Ca2+ were additive. Thus the effects of cytoplasmic and external Ca2+ are apparently mediated by different mechanisms. There was no effect of CaM on a Ca2+-activated Cl- current that also contributes to the receptor current. Increases in Ca2+ concentration on either side of the ciliary membrane may influence olfactory adaptation.     

Temporal development of cyclic nucleotide-gated and Ca2+ -activated Cl- currents in isolated mouse olfactory sensory neurons

A Ca(2+)-activated Cl(-) current constitutes a large part of the transduction current in olfactory sensory neurons. The binding of odorants to olfactory receptors in the cilia produces an increase in cAMP concentration; Ca(2+) enters into the cilia through CNG channels and activates a Cl(-) current. In intact mouse olfactory sensory neurons little is known about the kinetics of the Ca(2+)-activated Cl(-) current. Here, we directly activated CNG channels by flash photolysis of caged cAMP or 8-Br-cAMP and measured the current response with the whole cell voltage-clamp technique in mouse neurons. We measured multiphasic currents in the rising phase of the response at -50 mV. The current rising phase became monophasic in the absence of extracellular Ca(2+), at +50 mV, or when most of the intracellular Cl(-) was replaced by gluconate to shift the equilibrium potential for Cl(-) to -50 mV. These results show that the second phase of the current in mouse intact neurons is attributed to a Cl(-) current activated by Ca(2+), similarly to previous results on isolated frog cilia. The percentage of the total saturating current carried by Cl(-) was estimated in two ways: 1) by measuring the maximum secondary current and 2) by blocking the Cl(-) channel with niflumic acid. We estimated that in the presence of 1 mM extracellular Ca(2+) and in symmetrical Cl(-) concentrations the Cl(-) component can constitute up to 90% of the total current response. These data show how to unravel the CNG and Ca(2+)-activated Cl(-) component of the current rising phase.     

Calcium-activated chloride channels in cultured embryonic Xenopus spinal neurons.

1. Single-channel currents were recorded from Xenopus spinal neurons developing in vitro using the patch-clamp technique, to identify the channels underlying the large and small macroscopic Ca(2+)-activated Cl- currents (ICl(Ca)) present in these cells. 2. Channels of large (maxi-channels; 310 pS) and smaller conductance (mini-channels; 50-60 pS) are activated by elevation of cytoplasmic Ca2+ concentration. Channel activity is not altered by subsequent removal of Ca2+ from the bath, arguing against a direct ligand-type Ca2+ dependence. The much higher incidence of channel activation in cell-attached patches from cells permeabilized with the Ca2+ ionophore A23187 than in excised patches also suggests the involvement of some unidentified intracellular factor. 3. The reversal potential of maxi-Cl- channels is not altered by changes in Na+ concentration, but is shifted in the negative direction by the substitution of Cl- by methanesulfonate on the intracellular side of the patch, indicating their anionic selectivity. 4. Maxi-Cl- channels exhibited the presence of multiple probable subconductance states and showed marked voltage-dependent inactivation above and below +/- 20 mV. 5. Examination of maxi-Cl- channels at early times in culture (6-9 h) and 24 h later did not reveal any developmental change in the characteristics described above. However, the mean open duration of the channel was found to increase twofold during this period of time. 6. The simultaneous presence of maxi- and mini-Cl- channels prevented detailed characterization of the latter. The anionic selectivity of mini-Cl- channels is suggested by their reversal potential that lies close to the Cl- equilibrium potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of large conductance calcium-activated potassium channels from rat hippocampal neurons by glutathione

We have investigated the modulation of neuronal large conductance Ca2+-activated K+ channels by glutathione. Single channel recordings were made from cultured neonatal rat hippocampal neurons by using excised inside-out patch clamp method. Glutathione, a physiological sulfhydryl specific reducing reagent, increased channel activities in concentration dependent manner with half activation concentration of 710 microM. Conversely, oxidized form of glutathione inhibited channel activities with half inhibition concentration of 520 microM. Our results provide direct evidence that when neuronal large conductance Ca2+-activated K+ channels are exposed to reducing or oxidizing environments, channel activities are increased or decreased in opposite directions due to the redox modification. This may constitute an important regulatory mechanism of neuronal Ca2+-activated K+ channel activities.     

Anoctamin 2/TMEM16B: a calcium-activated chloride channel in olfactory transduction

In vertebrate olfactory transduction, a Ca(2+)-dependent Cl(-) efflux greatly amplifies the odorant response. The binding of odorants to receptors in the cilia of olfactory sensory neurons activates a transduction cascade that involves the opening of cyclic nucleotide-gated channels and the entry of Ca(2+) into the cilia. The Ca(2+) activates a Cl(-) current that, in the presence of a maintained elevated intracellular Cl(-) concentration, produces an efflux of Cl(-) ions and amplifies the depolarization. In this review, we summarize evidence supporting the hypothesis that anoctamin 2/TMEM16B is the main, or perhaps the only, constituent of the Ca(2+)-activated Cl(-) channels involved in olfactory transduction. Indeed, studies from several laboratories have shown that anoctamin 2/TMEM16B is expressed in the ciliary layer of the olfactory epithelium, that there are remarkable functional similarities between currents in olfactory sensory neurons and in HEK 293 cells transfected with anoctamin 2/TMEM16B, and that knockout mice for anoctamin 2/TMEM16B did not show any detectable Ca(2+)-activated Cl(-) current. Finally, we discuss the involvement of Ca(2+)-activated Cl(-) channels in the transduction process of vomeronasal sensory neurons and the physiological role of these channels in olfaction.     

Characterization of an apamin-sensitive potassium current in suprachiasmatic nucleus neurons

In neurons of the suprachiasmatic nucleus, spike frequency adaptation and membrane afterhyperpolarization occur during a train of action potentials. Extracellular Ca2+ may regulate neuronal excitability by several mechanisms, including activation of small conductance and large conductance Ca(2+)-activated K+ channels. The overall goal of this study was to examine the role of Ca(2+)-activated K+ currents in individual suprachiasmatic nucleus neurons. To this end, we used the nystatin-perforated patch technique to record currents from suprachiasmatic nucleus neurons. Iberiotoxin and tetraethylammonium, antagonists of large conductance Ca(2+)-activated K+ channels, had no effect on the membrane afterhyperpolarization. However, antagonists of small conductance Ca(2+)-activated K+ channels, apamin and d-tubocurarine, reduced the amplitude of the membrane afterhyperpolarization and inhibited the spike frequency adaptation that occurred during a train of action potentials. Although there was no significant difference in membrane AHP between different portions of the circadian day, apamin and d-tubocurarine increased the spontaneous firing frequency of suprachiasmatic nucleus neurons during the daytime. In voltage-clamp mode, membrane depolarization-activated currents were followed by an outward tail current reversing near the K+ equilibrium potential. The tail current decayed with a time constant of 220 ms at +20 mV and 149 ms at -40 mV. Apamin irreversibly and d-tubocurarine reversibly inhibited the tail current. The tail current amplitude was also reduced by the GABAA receptor antagonist, bicuculline methiodide, while picrotoxin (another GABAA receptor antagonist) was without effect. Removal of extracellular Ca2+ or the addition of Cd2+ reversibly inhibited the tail current. These results indicate that apamin- and d-tubocurarine-sensitive small conductance Ca(2+)-activated K+ channels have a modulatory function on the action potential firing frequency as well as the membrane afterhyperpolarization that follows a train of action potentials in suprachiasmatic nucleus neurons. Importantly, our data also indicate that a portion of the effects of bicuculline methiodide on suprachiasmatic nucleus neurons may be mediated by inhibition of small conductance Ca(2+)-activated K+ channels.     

Ca2+-activated Cl− currents are dispensable for olfaction

Canonical olfactory signal transduction involves the activation of cyclic AMP-activated cation channels that depolarize the cilia of receptor neurons and raise intracellular calcium. Calcium then activates Cl(-) currents that may be up to tenfold larger than cation currents and are believed to powerfully amplify the response. We identified Anoctamin2 (Ano2, also known as TMEM16B) as the ciliary Ca(2+)-activated Cl(-) channel of olfactory receptor neurons. Ano2 is expressed in the main olfactory epithelium (MOE) and in the vomeronasal organ (VNO), which also expresses the related Ano1 channel. Disruption of Ano2 in mice virtually abolished Ca(2+)-activated Cl(-) currents in the MOE and VNO. Ano2 disruption reduced fluid-phase electro-olfactogram responses by only ～40%, did not change air-phase electro-olfactograms and did not reduce performance in olfactory behavioral tasks. In contrast with the current view, cyclic nucleotide-gated cation channels do not need a boost by Cl(-) channels to achieve near-physiological levels of olfaction.     

The Drosophila tweety family: molecular candidates for large-conductance Ca2+-activated Cl- channels

Calcium-activated chloride currents (I(Cl(Ca))) can be recorded in almost all cells, but the molecular identity of the channels underlying this Cl- conductance is still incompletely understood. Here, I report that tweety, a gene located in Drosophila flightless, possesses five or six transmembrane segments, and that a human homologue of tweety (hTTYH3) is a novel large-conductance Ca2+-activated Cl- channel, while the related gene, hTTYH1, is a swelling-activated Cl- current. hTTYH3 is expressed in excitable tissues, including the heart, brain and skeletal muscle, whereas hTTYH1 is expressed mainly in the brain. Expression of hTTYH3 in CHO cells generated a unique Cl- current activated by an increase in the intracellular Ca2+ concentration. The hTTYH3-induced Cl- current had a linear current-voltage (I-V) relationship, a large single-channel conductance (260 pS) and the anion permeability sequence I- > Br- > Cl-. Like native Ca2+-activated Cl- channels, the hTTYH3 channel showed complex gating kinetics and voltage-dependent inactivation, and was dependent on micromolar intracellular Ca2+ concentration. Expression in CHO cells of an hTTYH1 splice variant that lacks the C-terminal glutamate-rich domain of hTTYH1 (hTTYH1sv) generated a swelling-activated Cl- current. I conclude that investigation of the tweety family will provide important information about large-conductance Cl- channel molecules.     

Calcium channel gating and modulation by transmitters depend on cellular compartmentalization

Voltage-gated Ca2+ channels participate in dendritic integration, yet functional properties of Ca2+ channels and mechanisms of their modulation by neurotransmitters in dendrites are unknown. Here we report how pharmacologically identified Ca2+ channels behave in different neural compartments. Whole-cell and cell-attached patch-clamp recordings were made on both cell bodies and electrically isolated dendrites of sympathetic neurons. We found not only that Ca2+ channel populations differentially contribute to somatic and dendritic currents but also that families of Ca2+ channels display gating properties and neurotransmitter modulation that depend on channel compartmentalization. By comparison with their somatic counterparts, dendritic N-type Ca2+ currents were hypersensitive to neurotransmitters and G proteins. Single-channel analysis showed that dendrites express a unique N-type channel that has enhanced interaction with Gbetagamma. Thus Ca2+ channels in dendrites seem to be specialized elements with unique regulatory mechanisms.     

Cyclic AMP cascade mediates the inhibitory odor response of isolated toad olfactory receptor neurons

Odor stimulation may excite or inhibit olfactory receptor neurons (ORNs). It is well established that the excitatory response involves a cyclic AMP (cAMP) transduction mechanism that activates a nonselective cationic cyclic nucleotide-gated (CNG) conductance, accompanied by the activation of a Ca2+-dependent Cl(-) conductance, both causing a depolarizing receptor potential. In contrast, odor inhibition is attributed to a hyperpolarizing receptor potential. It has been proposed that a Ca2+-dependent K+ (K(Ca)) conductance plays a key role in odor inhibition, both in toad and rat isolated olfactory neurons. The mechanism underlying odor inhibition has remained elusive. We assessed its study using various pharmacological agents and caged compounds for cAMP, Ca2+, and inositol 1,4,5-triphosphate (InsP3) on isolated toad ORNs. The odor-triggered K(Ca) current was reduced on exposing the cell either to the CNG channel blocker LY83583 (20 microM) or to the adenylyl cyclase inhibitor SQ22536 (100 microM). Photorelease of caged Ca2+ activated a Cl- current sensitive to niflumic acid (10 microM) and a K+ current blockable by charybdotoxin (20 nM) and iberiotoxin (20 nM). In contrast, photoreleased Ca2+ had no effect on cells missing their cilia, indicating that these conductances are confined to the cilia. Photorelease of cAMP induced a charybdotoxin-sensitive K+ current in intact ORNs. Photorelease of InsP3 did not increase the membrane conductance of olfactory neurons, arguing against a direct role of InsP3 in chemotransduction. We conclude that a cAMP cascade mediates the activation of the ciliary Ca2+-dependent K+ current and that the Ca2+ ions that activate the inhibitory current enter the cilia through CNG channels.     

Axo-somatic and apical dendritic Kv7/M channels differentially regulate the intrinsic excitability of adult rat CA1 pyramidal cells

Kv7/KCNQ/M channel subunits are widely expressed in peripheral and central neurons, where they give rise to a muscarinic-sensitive, subthreshold, and noninactivating K+ current (M current). Immunohistochemical data suggest that Kv7/M channels are expressed in both axons, somata and dendrites, but their distinctive roles in these compartments are not known. Here we used intracellular microelectrode recordings to monitor the effects of selective Kv7/M channel modulators focally applied to the axo-somatic region and to the apical dendrites of adult rat CA1 pyramidal cells. We show that both compartments express functional Kv7/M channels that synergistically control intrinsic neuronal excitability, albeit in different ways. Axo-somatic Kv7/M channels activate during the spike afterdepolarization (ADP) and counteract the depolarizing drive furnished by conjointly activated persistent Na+ channels. Thereby they limit the size and duration of the spike ADP and prevent its escalation into a somatic spike burst. Apical dendritic Kv7/M channels do not ordinarily regulate the somatic spike ADP and spike output. In hyperexcitable conditions that promote Ca2+ electrogenesis in these dendrites, they elevate the threshold for initiating Ca2+ spikes and associated downstream spike bursts. Thus the concerted activity of Kv7/M channels in both compartments serves to reduce the propensity to generate self-sustained burst responses and fosters a regular, stimulus-graded spike output of the neuron. Given that the activity of Kv7/M channels is regulated by multiple neurotransmitters, they may provide a substrate for neuromodulation of neuronal input/output relations at both the axo-somatic and apical dendritic regions.     

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.     

Reciprocal interactions between calcium and chloride in rod photoreceptors

This study used imaging and electrophysiological techniques in salamander retinal slices to correlate Ca2+ and Cl- levels in rods and thus test the hypothesis of a feedback interaction between Ca2+- and Ca2+-activated Cl- channels whereby Cl- efflux through Cl- channels can inhibit Ca2+ channels. Increasing [K+]o levels produced a concentration-dependent depolarization of rods accompanied by increases in [Ca2+]i measured with Fura-2. The voltage dependence of increases in [Ca2+]i was compared with the voltage dependence of the calcium current (ICa). [Cl-]i was measured with the dye, MEQ. Depolarization with high K+ to membrane potentials below -20 mV reduced [Cl-]i; larger depolarizations increased [Cl-]i. The Na/K/Cl cotransport inhibitor, bumetanide, shifted the apparent Cl- equilibrium potential (ECl) to more negative potentials, suggesting that this cotransporter helps establish a relatively depolarized ECl. MEQ fluorescence changes evoked by high K+ were inhibited by niflumic acid (0.1 mM), NPPB (2 microM), or replacement of Ca2+ with Ba2+, suggesting that depolarization-evoked Cl- changes result partly from stimulation of Ca2+-activated Cl- channels. Replacing >/=12 mM [Cl-]o with CH3SO4- produced a significant reduction in [Cl-]i. [Ca2+]i increases evoked by 20 or 50 mM K+ were also significantly inhibited by replacing >/=12 mM [Cl-]o with CH3SO4-. Thus modest depolarization can evoke increases in [Ca2+]i that lead to reductions in [Cl-]i, and conversely, reductions in [Cl-]i inhibit depolarization-evoked [Ca2+]i increases. These findings support the hypothesis that feedback interactions between Ca2+- and Ca2+-activated Cl- channels may contribute to the regulation of presynaptic Ca2+ currents involved in synaptic transmission from rod photoreceptors.     

Neurobiology of TRPC2: from gene to behavior

The mammalian vomeronasal organ (VNO), a part of the accessory olfactory system, plays an essential role in the sensing of pheromonal signals. The VNO has emerged as an excellent model to investigate the functional role of transient receptor potential (TRP) channels in intact neurons and intact physiological systems. TRPC2, a member of the (canonical) TRPC subfamily, is highly localized to the dendritic tip of vomeronasal sensory neurons. Phenotypic analysis of mice exhibiting a targeted deletion in the TRPC2 gene has established that TRPC2 occupies a fundamental role in the transduction machinery underlying the detection of pheromone signals by the VNO. TRPC2-deficient mice exhibit striking behavioral defects in the regulation of sexual and social behaviors. A previously unknown Ca²⁺-permeable, diacylglycerol (DAG)-activated cation channel found at the dendritic tip of vomeronasal neurons is severely defective in TRPC2 mutants, providing the first clear example for the existence of native DAG-gated cation channels in the mammalian nervous system. The experimental strategy employed in the mouse VNO now serves as a powerful model for examining the native functions of other TRP genes.     

Na+-K+-2Cl- cotransporter in immature cortical neurons: A role in intracellular Cl- regulation

Na+-K+-2Cl- cotransporter has been suggested to contribute to active intracellular Cl- accumulation in neurons at both early developmental and adult stages. In this report, we extensively characterized the Na+-K+-2Cl- cotransporter in primary culture of cortical neurons that were dissected from cerebral cortex of rat fetus at embryonic day 17. The Na+-K+-2Cl- cotransporter was expressed abundantly in soma and dendritic processes of cortical neurons evaluated by immunocytochemical staining. Western blot analysis revealed that an approximately 145-kDa cotransporter protein was present in cerebral cortex at the early postnatal (P0-P9) and adult stages. There was a time-dependent upregulation of the cotransporter activity in cortical neurons during the early postnatal development. A substantial level of bumetanide-sensitive K+ influx was detected in neurons cultured for 4-8 days in vitro (DIV 4-8). The cotransporter activity was increased significantly at DIV 12 and maintained at a steady level throughout DIV 12-14. Bumetanide-sensitive K+ influx was abolished completely in the absence of either extracellular Na+ or Cl-. Opening of gamma-aminobutyric acid (GABA)-activated Cl- channel or depletion of intracellular Cl- significantly stimulated the cotransporter activity. Moreover, the cotransporter activity was elevated significantly by activation of N-methyl-D-aspartate ionotropic glutamate receptor via a Ca2+-dependent mechanism. These results imply that the inwardly directed Na+-K+-2Cl- cotransporter is important in active accumulation of intracellular Cl- and may be responsible for GABA-mediated excitatory effect in immature cortical neurons.     

Intraglomerular inhibition: signaling mechanisms of an olfactory microcircuit

Microcircuits composed of principal neuron and interneuron dendrites have an important role in shaping the representation of sensory information in the olfactory bulb. Here we establish the physiological features governing synaptic signaling in dendrodendritic microcircuits of olfactory bulb glomeruli. We show that dendritic gamma-aminobutyric acid (GABA) release from periglomerular neurons mediates inhibition of principal tufted cells, retrograde inhibition of sensory input and lateral signaling onto neighboring periglomerular cells. We find that L-type dendritic Ca(2+) spikes in periglomerular cells underlie dendrodendritic transmission by depolarizing periglomerular dendrites and activating P/Q type channels that trigger GABA release. Ca(2+) spikes in periglomerular cells are evoked by powerful excitatory inputs from a single principal cell, and glutamate release from the dendrites of single principal neurons activates a large ensemble of periglomerular cells.     

Functional roles of ion channels in lymphocytes.

The application of patch-clamp and video-imaging techniques has enabled responses of lymphocytes to be examined at the level of individual cells. Eight distinct types of ion channel activity have been revealed in T lymphocytes. A variety of external stimuli shifts the pattern of channel activity from the resting state, which is dominated by voltage-gated K+ channels. Channel regulation is achieved both by acute modulation and by altered expression in the membrane. During mitogen stimulation, Ca2+ channels and Ca(2+)-activated K+ channels become active and appear to underlie Ca2+ oscillations. These acute changes are followed by increased expression of voltage-gated K+ channels. In response to osmotic challenge in hypotonic media, cell swelling initiates activation of Cl- channels, which may, in turn, indirectly activate K+ channels and trigger a regulatory decrease in cell volume.     

Calcium sensitivity of a sodium-activated nonselective cation channel in lobster olfactory receptor neurons

We report that a Na+-activated nonselective cation channel described previously in lobster olfactory neurons, in which phosphoinositide signaling mediates olfactory transduction, can also be activated by Ca2+. Ca2+ activates the channel in the presence of Na+, increasing the open probability of the channel with a K1/2 of 490 nM and a Hill coefficient of 1.3. Ca2+ also increases the sensitivity of the channel to Na+. In some cells, the same channel is Ca2+ insensitive in a cell-specific manner. The nonspecific activator of protein phosphatases, protamine, applied to the intracellular face of patches containing the channel irreversibly eliminates the sensitivity to Ca2+. This effect can be blocked by okadaic acid, a nonspecific blocker of protein phosphatases, and restored by the catalytic subunit of protein kinase A in the presence of MgATP. The Ca2+-sensitive form of the channel is predominantly expressed in the transduction zone of the cells in situ. These findings imply that the Ca2+ sensitivity of the channel, and possibly its regulation by phosphorylation, play a role in olfactory transduction and help tie activation of the channel to the canonical phosphoinositide turnover pathway.