Two components of calcium-activated potassium current in rat adrenal chromaffin cells

Lead exerts a depression of neurotransmitter release through a blockade of voltage dependent calcium channels in chromaffin cells

The present work, using chromaffin cells of bovine adrenal medullae (BCCs), aims to describe what type of ionic current alterations induced by lead (Pb2+) underlies its effects reported on synaptic transmission. We observed that the acute application of Pb2+ lead to a drastic depression of neurotransmitters release in a concentration-dependent manner when the cells were stimulated with both K+ or acetylcholine, with an IC50 of 119,57 μM and of 5,19 μM, respectively. This effect was fully recovered after washout. Pb2+ also blocked calcium channels of BCCs in a time- and concentration-dependent manner with an IC50 of 6,87 μM. This blockade was partially reversed upon washout. This compound inhibited the calcium current at all test potentials and shows a shift of the I-V curve to more negative values of about 8 mV. The sodium current was not blocked by acute application of high Pb2+ concentrations. Voltage-dependent potassium current was also shortly affected by high Pb2+. Nevertheless, the calcium- and voltage-dependent potassium current was drastically depressed in a dose-dependent manner, with an IC50 of 24,49 μM. This blockade was related to the prevention of Ca2+ influx through voltage-dependent calcium channels coupled to Ca2+-activated K+-channels (BK) instead a direct linking to these channels. Under current-clamp conditions, BCCs exhibit a resting potential of -52.7 mV, firing spontaneous APs (1-2 spikes/s) generated by the opening of Na+ and Ca2+-channels, and terminated by the activation of K+ channels. In spite of the effect on ionic channels exerted by Pb2+, we found that Pb2+ didn't alter cellular excitability, no modification of the membrane potential, and no effect on action potential firing. Taken together, these results point to a neurotoxic action evoked by Pb2+ that is associated with changes in neurotransmitter release by blocking the ionic currents responsible for the calcium influx.

Two firing modes and well-resolved Na+, K+, and Ca2+ currents at the cell-microelectrode junction of spontaneously active rat chromaffin cell on MEAs

We recorded spontaneous extracellular action potentials (eAPs) from rat chromaffin cells (CCs) at 37 °C using microelectrode arrays (MEAs) and compared them with intracellularly recorded APs (iAPs) through conventional patch clamp recordings at 22 °C. We show the existence of two distinct firing modes on MEAs: a ~ 4 Hz irregular continuous firing and a frequent intermittent firing mode where periods of high-intraburst frequency (~ 8 Hz) of ~ 7 s duration are interrupted by silent periods of ~ 12 s. eAPs occurred either as negative- or positive-going signals depending on the contact between cell and microelectrode: either predominantly controlled by junction-membrane ion channels (negative-going) or capacitive/ohmic coupling (positive-going). Negative-going eAPs were found to represent the trajectory of the Na⁺, Ca²⁺, and K⁺ currents passing through the cell area in tight contact with the microelectrode during an AP (point-contact junction). The inward Nav component of eAPs was blocked by TTX in a dose-dependent manner (IC₅₀ ~ 10 nM) while the outward component was strongly attenuated by the BK channel blocker paxilline (200 nM) or TEA (5 mM). The SK channel blocker apamin (200 nM) had no effect on eAPs. Inward Nav and Cav currents were well-resolved after block of Kv and BK channels or in cells showing no evident outward K⁺ currents. Unexpectedly, on the same type of cells, we could also resolve inward L-type currents after adding nifedipine (3 μM). In conclusion, MEAs provide a direct way to record different firing modes of rat CCs and to estimate the Na⁺, Ca²⁺, and K⁺ currents that sustain cell firing and spontaneous catecholamines secretion.

Fast inactivation of Nav current in rat adrenal chromaffin cells involves two independent inactivation pathways

Voltage-dependent sodium (Nav) current in adrenal chromaffin cells (CCs) is rapidly inactivating and tetrodotoxin (TTX)–sensitive. The fractional availability of CC Nav current has been implicated in regulation of action potential (AP) frequency and the occurrence of slow-wave burst firing. Here, through recordings of Nav current in rat CCs, primarily in adrenal medullary slices, we describe unique inactivation properties of CC Nav inactivation that help define AP firing rates in CCs. The key feature of CC Nav current is that recovery from inactivation, even following brief (5 ms) inactivation steps, exhibits two exponential components of similar amplitude. Various paired pulse protocols show that entry into the fast and slower recovery processes result from largely independent competing inactivation pathways, each of which occurs with similar onset times at depolarizing potentials. Over voltages from −120 to −80 mV, faster recovery varies from ∼3 to 30 ms, while slower recovery varies from ∼50 to 400 ms. With strong depolarization (above −10 mV), the relative entry into slow or fast recovery pathways is similar and independent of voltage. Trains of short depolarizations favor recovery from fast recovery pathways and result in cumulative increases in the slow recovery fraction. Dual-pathway fast inactivation, by promoting use-dependent accumulation in slow recovery pathways, dynamically regulates Nav availability. Consistent with this finding, repetitive AP clamp waveforms at 1–10 Hz frequencies reduce Nav availability 80–90%, depending on holding potential. These results indicate that there are two distinct pathways of fast inactivation, one leading to conventional fast recovery and the other to slower recovery, which together are well-suited to mediate use-dependent changes in Nav availability.

Small and Intermediate Calcium Activated Potassium Channels in the Heart: Role and Strategies in the Treatment of Cardiovascular Diseases

Calcium-activated potassium channels are a heterogeneous family of channels that, despite their different biophysical characteristics, structures, and pharmacological signatures, play a role of transducer between the ubiquitous intracellular calcium signaling and the electric variations of the membrane. Although this family of channels was extensively described in various excitable and non-excitable tissues, an increasing amount of evidences shows their functional role in the heart. This review aims to focus on the physiological role and the contribution of the small and intermediate calcium-activated potassium channels in cardiac pathologies.

Upregulation of Beta4 subunit of BKCa channels in the anterior cingulate cortex contributes to mechanical allodynia associated anxiety-like behaviors

The anterior cingulate cortex (ACC) serves as a critical hub for the anxiety and pain perception. The large-conductance Ca²⁺-activated potassium channels, or BK_Ca channels, are ubiquitously expressed throughout the central nervous system including the cingulate cortex. However, what changes of cortical BK_Ca channels undergo in the ACC remains unknown in pain-related anxiety. In the present study, a significant upregulation of synaptic and non-synaptic BK_Ca channel accessory β4 subunits in the ACC was accompanied with pain-associated anxiety-like behaviors in the chronic compression of multiple dorsal root ganglia (mCCD) of the rat. NS1619, an opener of BK_Ca channels, significantly rescued the alteration of fAHP and AP duration of ACC pyramidal neurons in mCCD rats. The mRNA expression of BK_Ca β4 subunits was extremely upregulated in the ACC after mCCD with the increased amount of both synaptic and non-synaptic BK_Ca β4 subunit protein. Meanwhile, NS1619 reversed the enhanced AMPA receptor-mediated spontaneous excitatory postsynaptic current (sEPSC) frequency and the attenuated PPR of ACC neurons in mCCD rats. Local activation of BK_Ca channels in the ACC reversed mechanical allodynia and anxiety-like behaviors. These results suggest that the upregulation of postsynaptic and presynaptic BK_Ca β4 subunit may contribute to neuronal hyperexcitability and the enhanced synaptic transmission in the ACC in neuropathic pain state, and then may result in anxiety-like behavior induced by neuropathic pain.

Old and emerging concepts on adrenal chromaffin cell stimulus-secretion coupling

The chromaffin cells (CCs) of the adrenal medulla play a key role in the control of circulating catecholamines to adapt our body function to stressful conditions. A huge research effort over the last 35 years has converted these cells into the Escherichia coli of neurobiology. CCs have been the testing bench for the development of patch-clamp and amperometric recording techniques and helped clarify most of the known molecular mechanisms that regulate cell excitability, Ca²⁺ signals associated with secretion, and the molecular apparatus that regulates vesicle fusion. This special issue provides a state-of-the-art on the many well-known and unsolved questions related to the molecular processes at the basis of CC function. The issue is also the occasion to highlight the seminal work of Antonio G. García (Emeritus Professor at UAM, Madrid) who greatly contributed to the advancement of our present knowledge on CC physiology and pharmacology. All the contributors of the present issue are distinguished scientists who are either staff members, external collaborators, or friends of Prof. García.

Roles of Na+, Ca2+, and K+ channels in the generation of repetitive firing and rhythmic bursting in adrenal chromaffin cells

Adrenal chromaffin cells (CCs) are the main source of circulating catecholamines (CAs) that regulate the body response to stress. Release of CAs is controlled neurogenically by the activity of preganglionic sympathetic neurons through trains of action potentials (APs). APs in CCs are generated by robust depolarization following the activation of nicotinic and muscarinic receptors that are highly expressed in CCs. Bovine, rat, mouse, and human CCs also express a composite array of Na⁺, K⁺, and Ca²⁺ channels that regulate the resting potential, shape the APs, and set the frequency of AP trains. AP trains of increasing frequency induce enhanced release of CAs. If the primary role of CCs is simply to relay preganglionic nerve commands to CA secretion, why should they express such a diverse set of ion channels? An answer to this comes from recent observations that, like in neurons, CCs undergo complex firing patterns of APs suggesting the existence of an intrinsic CC excitability (non-neurogenically controlled). Recent work has shown that CCs undergo occasional or persistent burst firing elicited by altered physiological conditions or deletion of pore-regulating auxiliary subunits. In this review, we aim to give a rationale to the role of the many ion channel types regulating CC excitability. We will first describe their functional properties and then analyze how they contribute to pacemaking, AP shape, and burst waveforms. We will also furnish clear indications on missing ion conductances that may be involved in pacemaking and highlight the contribution of the crucial channels involved in burst firing.

Nanosecond electric pulses differentially affect inward and outward currents in patch clamped adrenal chromaffin cells

This study examined the effect of 5 ns electric pulses on macroscopic ionic currents in whole-cell voltage-clamped adrenal chromaffin cells. Current-voltage (I-V) relationships first established that the early peak inward current was primarily composed of a fast voltage-dependent Na+ current (INa), whereas the late outward current was composed of at least three ionic currents: a voltage-gated Ca2+ current (ICa), a Ca2+-activated K+ current (IK(Ca)), and a sustained voltage-dependent delayed rectifier K+ current (IKV). A constant-voltage step protocol was next used to monitor peak inward and late outward currents before and after cell exposure to a 5 ns pulse. A single pulse applied at an electric (E)-field amplitude of 5 MV/m resulted in an instantaneous decrease of ~4% in peak INa that then declined exponentially to a level that was ~85% of the initial level after 10 min. Increasing the E-field amplitude to 8 or 10 MV/m caused a twofold greater inhibitory effect on peak INa. The decrease in INa was not due to a change in either the steady-state inactivation or activation of the Na+ channel but instead was associated with a decrease in maximal Na+ conductance. Late outward current was not affected by a pulse applied at 5 MV/m. However, for a pulse applied at the higher E-field amplitudes of 8 and 10 MV/m, late outward current in some cells underwent a progressive ~22% decline over the course of the first 20 s following pulse exposure, with no further decline. The effect was most likely concentrated on ICa and IK(Ca) as IKV was not affected. The results of this study indicate that in whole-cell patch clamped adrenal chromaffin cells, a 5 ns pulse differentially inhibits specific voltage-gated ionic currents in a manner that can be manipulated by tuning E-field amplitude.

Arrestin-biased AT1R agonism induces acute catecholamine secretion through TRPC3 coupling

Acute hormone secretion triggered by G protein-coupled receptor (GPCR) activation underlies many fundamental physiological processes. GPCR signalling is negatively regulated by β-arrestins, adaptor molecules that also activate different intracellular signalling pathways. Here we reveal that TRV120027, a β-arrestin-1-biased agonist of the angiotensin II receptor type 1 (AT1R), stimulates acute catecholamine secretion through coupling with the transient receptor potential cation channel subfamily C 3 (TRPC3). We show that TRV120027 promotes the recruitment of TRPC3 or phosphoinositide-specific phospholipase C (PLCγ) to the AT1R-β-arrestin-1 signalling complex. Replacing the C-terminal region of β-arrestin-1 with its counterpart on β-arrestin-2 or using a specific TAT-P1 peptide to block the interaction between β-arrestin-1 and PLCγ abolishes TRV120027-induced TRPC3 activation. Taken together, our results show that the GPCR-arrestin complex initiates non-desensitized signalling at the plasma membrane by coupling with ion channels. This fast communication pathway might be a common mechanism of several cellular processes.

Single Channel Recordings Reveal Differential β2 Subunit Modulations Between Mammalian and Drosophila BKCa(β2) Channels

Large-conductance Ca²⁺- and voltage-activated potassium (BK) channels are widely expressed in tissues. As a voltage and calcium sensor, BK channels play significant roles in regulating the action potential frequency, neurotransmitter release, and smooth muscle contraction. After associating with the auxiliary β2 subunit, mammalian BK(β2) channels (mouse or human Slo1/β2) exhibit enhanced activation and complete inactivation. However, how the β2 subunit modulates the Drosophila Slo1 channel remains elusive. In this study, by comparing the different functional effects on heterogeneous BK(β2) channel, we found that Drosophila Slo1/β2 channel exhibits “paralyzed”-like and incomplete inactivation as well as slow activation. Further, we determined three different modulations between mammalian and Drosophila BK(β2) channels: 1) dSlo1/β2 doesn’t have complete inactivation. 2) β2(K33,R34,K35) delays the dSlo1/Δ3-β2 channel activation. 3) dSlo1/β2 channel has enhanced pre-inactivation than mSlo1/β2 channel. The results in our study provide insights into the different modulations of β2 subunit between mammalian and Drosophila Slo1/β2 channels and structural basis underlie the activation and pre-inactivation of other BK(β) complexes.

Age-related changes of inactivating BK channels in rat dorsal root ganglion neurons

The large-conductance, voltage- and Ca(2+)-activated K(+) channels (termed BK) are associated with age-related dysfunctions or diseases. Previously, with our colleagues, we reported that the rβ2-associated inactivating BK (BKi) channels play an essential role in rat dorsal root ganglion (DRG) neurons. However, the age-dependent changes in BKi channels are still elusive. Here, we identify three types of BK channels in small DRG neurons, the single exponential BKi, the double exponential BKi and the non-inactivating BK. Interestingly, compared to the increased occurrence of the non-inactivating BK, the presence of BKi channels declined with age. Furthermore, the peak amplitude of the single exponential BKi current increased from infancy to youth, but decreased from youth to old age. The inactivation time constant, however, did not change with age. The double exponential BKi also displayed age-related change in current amplitude with an intricate kinetics. Physiologically, the decay speed of the action potential was significantly increased in Youth, which correlated with the change of current amplitude of BKi channels. Collectively, these results reveal an age-related change pattern of BKi channels in small DRG neurons, providing potential mechanistic clues for different susceptibility to sensation in different ages.

Knockout of the BK β2 subunit abolishes inactivation of BK currents in mouse adrenal chromaffin cells and results in slow-wave burst activity

Chromaffin cells from mice lacking the BK β2 subunit show decreased action potential firing during current injection but an increase in spontaneous burst firing.

Rat and mouse adrenal medullary chromaffin cells (CCs) express an inactivating BK current. This inactivation is thought to arise from the assembly of up to four β2 auxiliary subunits (encoded by the kcnmb2 gene) with a tetramer of pore-forming Slo1 α subunits. Although the physiological consequences of inactivation remain unclear, differences in depolarization-evoked firing among CCs have been proposed to arise from the ability of β2 subunits to shift the range of BK channel activation. To investigate the role of BK channels containing β2 subunits, we generated mice in which the gene encoding β2 was deleted (β2 knockout [KO]). Comparison of proteins from wild-type (WT) and β2 KO mice allowed unambiguous demonstration of the presence of β2 subunit in various tissues and its coassembly with the Slo1 α subunit. We compared current properties and cell firing properties of WT and β2 KO CCs in slices and found that β2 KO abolished inactivation, slowed action potential (AP) repolarization, and, during constant current injection, decreased AP firing. These results support the idea that the β2-mediated shift of the BK channel activation range affects repetitive firing and AP properties. Unexpectedly, CCs from β2 KO mice show an increased tendency toward spontaneous burst firing, suggesting that the particular properties of BK channels in the absence of β2 subunits may predispose to burst firing.

Cav1.3 Channels as Key Regulators of Neuron-Like Firings and Catecholamine Release in Chromaffin Cells

Neuronal and neuroendocrine L-type calcium channels (Cav1.2, Cav1.3) open readily at relatively low membrane potentials and allow Ca(2+) to enter the cells near resting potentials. In this way, Cav1.2 and Cav1.3 shape the action potential waveform, contribute to gene expression, synaptic plasticity, neuronal differentiation, hormone secretion and pacemaker activity. In the chromaffin cells (CCs) of the adrenal medulla, Cav1.3 is highly expressed and is shown to support most of the pacemaking current that sustains action potential (AP) firings and part of the catecholamine secretion. Cav1.3 forms Ca(2+)-nanodomains with the fast inactivating BK channels and drives the resting SK currents. These latter set the inter-spike interval duration between consecutive spikes during spontaneous firing and the rate of spike adaptation during sustained depolarizations. Cav1.3 plays also a primary role in the switch from "tonic" to "burst" firing that occurs in mouse CCs when either the availability of voltage-gated Na channels (Nav) is reduced or the β2 subunit featuring the fast inactivating BK channels is deleted. Here, we discuss the functional role of these "neuron-like" firing modes in CCs and how Cav1.3 contributes to them. The open issue is to understand how these novel firing patterns are adapted to regulate the quantity of circulating catecholamines during resting condition or in response to acute and chronic stress.

Decreased calcium-activated potassium channels by hypoxia causes abnormal firing in the spontaneous firing medial vestibular nuclei neurons

Vertebrobasilar insufficiency (VBI) presents complex varied clinical symptoms, including vertigo and hearing loss. Little is known, however, about how Ca(2+)-activated K(+) channel attributes to the medial vestibular nucleus (MVN) neural activity in VBI. To address this issue, we performed whole-cell patch clamp and quantitative polymerase chain reaction (qPCR) to examine the effects of hypoxia on neural activity and the changes of the large conductance Ca(2+) activated K(+) channels (BKCa channels) in the MVN neurons in brain slices of male C57BL/6 mice. Brief hypoxic stimuli of the brain slices containing MVN were administrated by switching the normoxic artificial cerebrospinal fluid (ACSF) equilibrated with 21% O2/5% CO2 to hypoxic ACSF equilibrated with 5% O2/5% CO2 (balance N2). 3-min hypoxia caused a depolarization in the resting membrane potential (RM) in 8/11 non-spontaneous firing MVN neurons. 60/72 spontaneous firing MVN neurons showed a dramatic increase in firing frequency and a depolarization in the RM following brief hypoxia. The amplitude of the afterhyperpolarization (AHPA) was significantly decreased in both type A and type B spontaneous firing MVN neurons. Hypoxia-induced firing response was alleviated by pretreatment with NS1619, a selective BKCa activator. Furthermore, brief hypoxia caused a decrease in the amplitude of iberiotoxin-sensitive outward currents and mRNA level of BKCa in MVN neurons. These results suggest that BKCa channels protect against abnormal MVN neuronal activity induced by hypoxia, and might be a key target for treatment of vertigo and hearing loss in VBI.

The role of large-conductance, calcium-activated potassium channels in a rat model of trigeminal neuropathic pain

BACKGROUND

Trigeminal neuralgia is a disorder of paroxysmal and severely disabling facial pain and continues to be a real therapeutic challenge. At present there are few effective drugs. Here the aim of this study was to investigate the role of BKCa channels in trigeminal neuropathic pain.

METHODS

Rats were divided into two groups: a sham and a chronic constriction injury of infraorbital branch of trigeminal nerve (ION-CCI) group. Nociceptive behavior testing, immunohistochemistry, RT-PCR, Western blotting and whole-cell patch clamp recording were used.

RESULTS

Relative to the sham group, rats with ION-CCI consistently displayed lower mechanical pain thresholds in the vibrissal pad region from day 6 to 42 after ION-CCI operation. ION-CCI induced a significant down-regulation of BKCa channels both in mRNA and protein levels in the ipsilateral trigeminal ganglion (TG), a lower threshold intensity of action potential, and decreased total BKCa currents in cultured TG neurons. TG target injection of NS1619 (20-100 µg), an opener of BKCa channels, dose-dependently increased the mechanical pain threshold, which was blocked by the BKCa channel inhibitor iberiotoxin (IbTX, 20 µg). NS1619 (10 µM) significantly increased the mean threshold intensities of action potentials in ION-CCI rats, while failing to affect those in the sham rats. The levels of phosphorylated extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinases (JNK) in TG were significantly increased after ION-CCI operation. The ERK1/2 antagonist U0126, p38 antagonist SB203580 and JNK antagonist SP600125 significantly reversed the facial mechanical allodynia in ION-CCI rats. However, the ERK1/2 antagonist U0126, p38 antagonist SB203580 but not JNK antagonist SP600125 significantly increased BKCa currents in ION-CCI TG neurons.

CONCLUSIONS

Our results indicate the important involvement of mainly ERK and p38 MAPK pathways in modulating BKCa channels in ION-CCI TG neurons. BKCa channels represent a new therapeutic target for the clinical treatment of trigeminal neuropathic pain.

Iberiotoxin-sensitive and -insensitive BK currents in Purkinje neuron somata

Purkinje cells have specialized intrinsic ionic conductances that generate high-frequency action potentials. Disruptions of their Ca or Ca-activated K (KCa) currents correlate with altered firing patterns in vitro and impaired motor behavior in vivo. To examine the properties of somatic KCa currents, we recorded voltage-clamped KCa currents in Purkinje cell bodies isolated from postnatal day 17-21 mouse cerebellum. Currents were evoked by endogenous Ca influx with approximately physiological Ca buffering. Purkinje somata expressed voltage-activated, Cd-sensitive KCa currents with iberiotoxin (IBTX)-sensitive (>100 nS) and IBTX-insensitive (>75 nS) components. IBTX-sensitive currents activated and partially inactivated within milliseconds. Rapid, incomplete macroscopic inactivation was also evident during 50- or 100-Hz trains of 1-ms depolarizations. In contrast, IBTX-insensitive currents activated more slowly and did not inactivate. These currents were insensitive to the small- and intermediate-conductance KCa channel blockers apamin, scyllatoxin, UCL1684, bicuculline methiodide, and TRAM-34, but were largely blocked by 1 mM tetraethylammonium. The underlying channels had single-channel conductances of ∼150 pS, suggesting that the currents are carried by IBTX-resistant (β4-containing) large-conductance KCa (BK) channels. IBTX-insensitive currents were nevertheless increased by small-conductance KCa channel agonists EBIO, chlorzoxazone, and CyPPA. During trains of brief depolarizations, IBTX-insensitive currents flowed during interstep intervals, and the accumulation of interstep outward current was enhanced by EBIO. In current clamp, EBIO slowed spiking, especially during depolarizing current injections. The two components of BK current in Purkinje somata likely contribute differently to spike repolarization and firing rate. Moreover, augmentation of BK current may partially underlie the action of EBIO and chlorzoxazone to alleviate disrupted Purkinje cell firing associated with genetic ataxias.

Ca(V)1.3-driven SK channel activation regulates pacemaking and spike frequency adaptation in mouse chromaffin cells

Mouse chromaffin cells (MCCs) fire spontaneous action potentials (APs) at rest. Ca(v)1.3 L-type calcium channels sustain the pacemaker current, and their loss results in depolarized resting potentials (V(rest)), spike broadening, and remarkable switches into depolarization block after BayK 8644 application. A functional coupling between Ca(v)1.3 and BK channels has been reported but cannot fully account for the aforementioned observations. Here, using Ca(v)1.3(-/-) mice, we investigated the role of Ca(v)1.3 on SK channel activation and how this functional coupling affects the firing patterns induced by sustained current injections. MCCs express SK1-3 channels whose tonic currents are responsible for the slow irregular firing observed at rest. Percentage of frequency increase induced by apamin was found inversely correlated to basal firing frequency. Upon stimulation, MCCs build-up Ca(v)1.3-dependent SK currents during the interspike intervals that lead to a notable degree of spike frequency adaptation (SFA). The major contribution of Ca(v)1.3 to the subthreshold Ca(2+) charge during an AP-train rather than a specific molecular coupling to SK channels accounts for the reduced SFA of Ca(v)1.3(-/-) MCCs. Low adaptation ratios due to reduced SK activation associated with Ca(v)1.3 deficiency prevent the efficient recovery of Na(V) channels from inactivation. This promotes a rapid decline of AP amplitudes and facilitates early onset of depolarization block following prolonged stimulation. Thus, besides serving as pacemaker, Ca(v)1.3 slows down MCC firing by activating SK channels that maintain Na(V) channel availability high enough to preserve stable AP waveforms, even upon high-frequency stimulation of chromaffin cells during stress responses.

Toll-like receptor 4 activation reduces adrenal chromaffin cell excitability through a nuclear factor-κB-dependent pathway

The adrenal medulla contains fenestrated capillaries that allow catecholamines and neuropeptides secreted by adrenal chromaffin cells (ACCs) to readily access the circulation. These capillaries may also allow bacterial products to enter the adrenal medulla and interact with ACCs during infection. One potential mediator of this interaction is toll-like receptor 4 (TLR-4), a pattern-recognition receptor that detects lipopolysaccharide (LPS) from Gram-negative bacteria. Evidence suggests that excitable cells can express TLR-4 and that LPS can modulate important neuronal and endocrine functions. The present study was therefore performed to test the hypothesis that TLR-4 activation by LPS affects ACC excitability and secretory output. RT-PCR revealed that TLR-4, cluster of differentiation 14, myeloid differentiation protein-2, and myeloid-derived factor 88 are expressed within mouse adrenal medullae. TLR-4 immunoreactivity was observed within all tyrosine hydroxylase immunoreactive ACCs. Incubation of isolated ACCs in LPS dose dependently hyperpolarized the resting membrane potential and enhanced large conductance (BK) Ca(2+)-activated K(+) currents. LPS (10 μg/ml) also increased rheobase, decreased the number of action potentials fired at rheobase, and reduced the percentage of ACCs exhibiting spontaneous and anodal break action potentials. Although catecholamine release was unaltered, LPS significantly reduced high-K(+)-stimulated neuropeptide Y release from isolated ACCs. LPS did not alter the excitability of ACCs from TLR-4(-/-) mice. Inhibition of nuclear factor-κB signaling with SC-514 (20 μm) abolished the effects of LPS on ACC excitability. Our findings suggest that LPS acts at TLR-4 to reduce ACC excitability and neuropeptide Y release through an nuclear factor-κB-dependent pathway.

Cav1.3 and Cav1.2 channels of adrenal chromaffin cells: emerging views on cAMP/cGMP-mediated phosphorylation and role in pacemaking

Voltage-gated Ca²⁺ channels (VGCCs) are voltage sensors that convert membrane depolarizations into Ca²⁺ signals. In the chromaffin cells of the adrenal medulla, the Ca²⁺ signals driven by VGCCs regulate catecholamine secretion, vesicle retrievals, action potential shape and firing frequency. Among the VGCC-types expressed in these cells (N-, L-, P/Q-, R- and T-types), the two L-type isoforms, Ca(v)1.2 and Ca(v)1.3, control key activities due to their particular activation-inactivation gating and high-density of expression in rodents and humans. The two isoforms are also effectively modulated by G protein-coupled receptor pathways delimited in membrane micro-domains and by the cAMP/PKA and NO/cGMP/PKG phosphorylation pathways which induce prominent Ca²⁺ current changes if opposingly regulated. The two L-type isoforms shape the action potential and directly participate to vesicle exocytosis and endocytosis. The low-threshold of activation and slow rate of inactivation of Ca(v)1.3 confer to this channel the unique property of carrying sufficient inward current at subthreshold potentials able to activate BK and SK channels which set the resting potential, the action potential shape, the cell firing mode and the degree of spike frequency adaptation during spontaneous firing or sustained depolarizations. These properties help chromaffin cells to optimally adapt when switching from normal to stress-mimicking conditions. Here, we will review past and recent findings on cAMP- and cGMP-mediated modulations of Ca(v)1.2 and Ca(v)1.3 and the role that these channels play in the control of chromaffin cell firing. This article is part of a Special Issue entitled: Calcium channels.

Nerve injury increases brain-derived neurotrophic factor levels to suppress BK channel activity in primary sensory neurons

Abnormal hyperexcitability of primary sensory neurons contributes to neuropathic pain development after nerve injury. Nerve injury profoundly reduces the expression of big conductance Ca(2+) -activated K(+) (BK) channels in the dorsal root ganglion (DRG). However, little is known about how nerve injury affects BK channel activity in DRG neurons. In this study, we determined the changes in BK channel activity in DRG neurons in a rat model of neuropathic pain and the contribution of brain-derived neurotrophic factor (BDNF) to reduced BK channel activity. The BK channel activity was present predominantly in small and medium DRG neurons, and ligation of L5 and L6 spinal nerves profoundly decreased the BK current density in these neurons. Blocking BK channels significantly increased neuronal excitability in sham control, but not in nerve-injured, rats. The BDNF concentration in the DRG was significantly greater in nerve-injured rats than in control rats. BDNF treatment largely reduced BK currents in DRG neurons in control rats, which was blocked by either anti-BDNF antibody or K252a, a Trk receptor inhibitor. Furthermore, either anti-BDNF antibody or K252a reversed reduction in BK currents in injured DRG neurons. BDNF treatment reduced the mRNA levels of BKα1 subunit in DRG neurons, and anti-BDNF antibody attenuated the reduction in the BKα1 mRNA level in injured DRG neurons. These findings suggest that nerve injury primarily diminishes the BK channel activity in small and medium DRG neurons. Increased BDNF levels contribute to reduced BK channel activity in DRG neurons through epigenetic and transcriptional mechanisms in neuropathic pain.

Loss of Cav1.3 channels reveals the critical role of L-type and BK channel coupling in pacemaking mouse adrenal chromaffin cells

We studied wild-type (WT) and Cav1.3(-/-) mouse chromaffin cells (MCCs) with the aim to determine the isoform of L-type Ca(2+) channel (LTCC) and BK channels that underlie the pacemaker current controlling spontaneous firing. Most WT-MCCs (80%) were spontaneously active (1.5 Hz) and highly sensitive to nifedipine and BayK-8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridinecarboxylic acid, methyl ester). Nifedipine blocked the firing, whereas BayK-8644 increased threefold the firing rate. The two dihydropyridines and the BK channel blocker paxilline altered the shape of action potentials (APs), suggesting close coupling of LTCCs to BK channels. WT-MCCs expressed equal fractions of functionally active Cav1.2 and Cav1.3 channels. Cav1.3 channel deficiency decreased the number of normally firing MCCs (30%; 2.0 Hz), suggesting a critical role of these channels on firing, which derived from their slow inactivation rate, sizeable activation at subthreshold potentials, and close coupling to fast inactivating BK channels as determined by using EGTA and BAPTA Ca(2+) buffering. By means of the action potential clamp, in TTX-treated WT-MCCs, we found that the interpulse pacemaker current was always net inward and dominated by LTCCs. Fast inactivating and non-inactivating BK currents sustained mainly the afterhyperpolarization of the short APs (2-3 ms) and only partially the pacemaker current during the long interspike (300-500 ms). Deletion of Cav1.3 channels reduced drastically the inward Ca(2+) current and the corresponding Ca(2+)-activated BK current during spikes. Our data highlight the role of Cav1.3, and to a minor degree of Cav1.2, as subthreshold pacemaker channels in MCCs and open new interesting features about their role in the control of firing and catecholamine secretion at rest and during sustained stimulations matching acute stress.

Differential regulation of action potentials by inactivating and noninactivating BK channels in rat adrenal chromaffin cells

Large-conductance Ca(2+)-activated K(+) (BK) channels can regulate cellular excitability in complex ways because they are able to respond independently to two distinct cellular signals, cytosolic Ca(2+) and membrane potential. In rat chromaffin cells (RCC), inactivating BK(i) and noninactivating (BK(s)) channels differentially contribute to RCC action potential (AP) firing behavior. However, the basis for these differential effects has not been fully established. Here, we have simulated RCC action potential behavior, using Markovian models of BK(i) and BK(s) current and other RCC currents. The analysis shows that BK current influences both fast hyperpolarization and afterhyperpolarization of single APs and that, consistent with experimental observations, BK(i) current facilitates repetitive firing of APs, whereas BK(s) current does not. However, the key functional difference between BK(i) and BK(s) current that accounts for the differential firing is not inactivation but the more negatively shifted activation range for BK(i) current at a given [Ca(2+)].

Inhibition of N and PQ calcium channels by calcium entry through L channels in chromaffin cells

Why adrenal chromaffin cells express various subtypes of voltage-dependent Ca(2+) channels and whether a given channel is specialized to perform a specific function are puzzling and unanswered questions. In this study, we have used the L Ca(2+) channel activator FPL64176 (FPL) to test the hypothesis that enhanced Ca(2+) entry through this channel favors the inhibition of N and PQ channels in voltage-clamped bovine adrenal chromaffin cells. Using 2 mM Ca(2+) as charge carrier and under the perforated-patch configuration (PPC) of the patch-clamp technique, FPL caused a paradoxical inhibition of the whole-cell inward Ca(2+) current (I (Ca)). Such inhibition turned on into an augmentation upon cell loading with EGTA-AM. Also, under the whole-cell configuration (WCC) of the patch-clamp technique, FPL decreased I (Ca) in the absence of EGTA from the pipette solution and increased the current in its presence. Using 2 mM Ba(2+) as charge carrier, FPL augmented the Ba(2+) current under both recording conditions, WCC and PPC. FPL augmented the residual current remaining after blockade of N and PQ channels with omega-conotoxin MVIIC or by holding the membrane potential at -50 mV. The data support the view that Ca(2+) entering the cell through the lesser inactivating L channels serves to modulate the more inactivating N and PQ channels. They also suggest a close colocalization of L and N/PQ Ca(2+) channels. This kind of L channel specialization may be relevant to cell excitability, exocytosis, and cell survival mechanisms.

Molecular mechanisms of BK channel activation

Large conductance, Ca(2+)-activated potassium (BK) channels are widely expressed throughout the animal kingdom and play important roles in many physiological processes, such as muscle contraction, neural transmission and hearing. These physiological roles derive from the ability of BK channels to be synergistically activated by membrane voltage, intracellular Ca(2+) and other ligands. Similar to voltage-gated K(+) channels, BK channels possess a pore-gate domain (S5-S6 transmembrane segments) and a voltage-sensor domain (S1-S4). In addition, BK channels contain a large cytoplasmic C-terminal domain that serves as the primary ligand sensor. The voltage sensor and the ligand sensor allosterically control K(+) flux through the pore-gate domain in response to various stimuli, thereby linking cellular metabolism and membrane excitability. This review summarizes the current understanding of these structural domains and their mutual interactions in voltage-, Ca(2+)- and Mg(2+)-dependent activation of the channel.

Chronic hypoxia up-regulates alpha1H T-type channels and low-threshold catecholamine secretion in rat chromaffin cells

alpha(1H) T-type channels recruited by beta(1)-adrenergic stimulation in rat chromaffin cells (RCCs) are coupled to fast exocytosis with the same Ca(2+) dependence of high-threshold Ca(2+) channels. Here we show that RCCs exposed to chronic hypoxia (CH) for 12-18 h in 3% O(2) express comparable densities of functional T-type channels that depolarize the resting cells and contribute to low-voltage exocytosis. Following chronic hypoxia, most RCCs exhibited T-type Ca(2+) channels already available at -50 mV with the same gating, pharmacological and molecular features as the alpha(1H) isoform. Chronic hypoxia had no effects on cell size and high-threshold Ca(2+) current density and was mimicked by overnight incubation with the iron-chelating agent desferrioxamine (DFX), suggesting the involvement of hypoxia-inducible factors (HIFs). T-type channel recruitment occurred independently of PKA activation and the presence of extracellular Ca(2+). Hypoxia-recruited T-type channels were partially open at rest (T-type 'window-current') and contributed to raising the resting potential to more positive values. Their block by 50 microm Ni(2+) caused a 5-8 mV hyperpolarization. The secretory response associated with T-type channels could be detected following mild cell depolarizations, either by capacitance increases induced by step depolarizations or by amperometric current spikes induced by increased [KCl]. In the latter case, exocytotic bursts could be evoked even with 2-4 mm KCl and spike frequency was drastically reduced by 50 microm Ni(2+). Chronic hypoxia did not alter the shape of spikes, suggesting that hypoxia-recruited T-type channels increase the number of secreted vesicles at low voltages, without altering the mechanism of catecholamine release and the quantal content of released molecules.

Characterization of voltage-and Ca2+-activated K+ channels in rat dorsal root ganglion neurons

Auxiliary beta-subunits associated with pore-forming Slo1 alpha-subunits play an essential role in regulating functional properties of large-conductance, voltage- and Ca(2+)-activated K(+) channels commonly termed BK channels. Even though both noninactivating and inactivating BK channels are thought to be regulated by beta-subunits (beta1, beta2, beta3, or beta4), the molecular determinants underlying inactivating BK channels in native cells have not been extensively demonstrated. In this study, rbeta2 (but not rbeta3-subunit) was identified as a molecular component in rat lumbar L4-6 dorsal root ganglia (DRG) by RT-PCR responsible for inactivating large-conductance Ca(2+)-dependent K(+) currents (BK(i) currents) in small sensory neurons. The properties of native BK(i) currents obtained from both whole-cell and inside-out patches are very similar to inactivating BK channels produced by co-expressing mSlo1 alpha- and hbeta2-subunits in Xenopus oocytes. Intracellular application of 0.5 mg/ml trypsin removes inactivation of BK(i) channels, and the specific blockers of BK channels, charybdotoxin (ChTX) and iberiotoxin (IbTX), inhibit these BK(i) currents. Single BK(i) channel currents derived from inside-out patches revealed that one BK(i) channel contained three rbeta2-subunits (on average), with a single-channel conductance about 217 pS under 160 K(+) symmetrical recording conditions. Blockade of BK(i) channels by 100 nM IbTX augmented firing frequency, broadened action potential waveform and reduced after-hyperpolarization. We propose that the BK(i) channels in small diameter DRG sensory neurons might play an important role in regulating nociceptive input to the central nervous system (CNS).

Depolarization preconditioning produces cytoprotection against veratridine-induced chromaffin cell death

The hypothesis that K(+) channels and cell depolarization are involved in neuronal death and neuroprotection was tested in bovine chromaffin cells subjected to two treatment periods: the first period (preconditioning period) lasted 6 to 48 h and consisted of treatment with high K(+) solutions or with tetraethylammonium (TEA), a K(+) channel blocker; the second period consisted of incubation with veratridine for 24 h, to cause cell damage. Preconditioning with high K(+) (20-80 mM) or TEA (10-30 mM) for 24 h caused 20-60% cytoprotection against veratridine-induced cell death in bovine chromaffin cells. The absence of Ca(2+) ions during the first 9 h of an 18-h preconditioning period abolished the cytoprotection. Preconditioning with K(+) or TEA increased by 2.5-fold the expression of brain-derived neurotrophic factor and by nearly 2-fold the expression of the antiapoptotic protein Bcl-2. However, preconditioning did not modify the veratridine-evoked Ca(2+) signal. High K(+) shifted the Em by about 10 mV and TEA evoked a transient burst of action potentials superimposed on a sustained depolarization. We conclude that preconditioning may protect chromaffin cells from death by blocking K(+) channels that depolarize the cell and cause a cytosolic Ca(2+) signal, leading to enhanced expression of BDNF and Bcl-2.

BmP09, a “Long Chain” Scorpion Peptide Blocker of BK Channels

A novel "long chain" toxin BmP09 has been purified and characterized from the venom of the Chinese scorpion Buthus martensi Karsch. The toxin BmP09 is composed of 66 amino acid residues, including eight cysteines, with a mass of 7721.0 Da. Compared with the B. martensi Karsch AS-1 as a Na(+) channel blocker (7704.8 Da), the BmP09 has an exclusive difference in sequence by an oxidative modification at the C terminus. The sulfoxide Met-66 at the C terminus brought the peptide a dramatic switch from a Na(+) channel blocker toaK(+) channel blocker. Upon probing the targets of the toxin BmP09 on the isolated mouse adrenal medulla chromaffin cells, where a variety of ion channels coexists, we found that the toxin BmP09 specifically blocked large conductance Ca(2+)- and voltage-dependent K(+) channels (BK) but not Na(+) channels at a range of 100 nm concentration. This was further confirmed by blocking directly the BK channels encoded with mSlo1 alpha-subunits in Xenopus oocytes. The half-maximum concentration EC(50) of BmP09 was 27 nm, and the Hill coefficient was 1.8. In outside-out patches, the 100 nm BmP09 reduced approximately 70% currents of BK channels without affecting the single-channel conductance. In comparison with the "short chain" scorpion peptide toxins such as Charybdotoxin, the toxin BmP09 behaves much better in specificity and reversibility, and thus it will be a more efficient tool for studying BK channels. A three-dimensional simulation between a BmP09 toxin and an mSlo channel shows that the Lys-41 in BmP09 lies at the center of the interface and plugs into the entrance of the channel pore. The stable binding between the toxin BmP09 and the BK channel is favored by aromatic pi -pi interactions around the center.

Differential effects of beta 1 and beta 2 subunits on BK channel activity

High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β) subunits. β1 and β2 subunits increase apparent channel calcium sensitivity. The β1 subunit also decreases the voltage sensitivity of the channel and the β2 subunit produces an N-type inactivation of BK currents. We further characterized the effects of the β1 and β2 subunits on the calcium and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan and Aldrich, 2002). In this study, we used a β2 subunit without its N-type inactivation domain (β2IR). The results indicate that the β2IR subunit, like the β1 subunit, has a small effect on the calcium binding affinity of the channel. Unlike the β1 subunit, the β2IR subunit also has no effect on the voltage sensitivity of the channel. The limiting voltage dependence for steady-state channel activation, unrelated to voltage sensor movements, is unaffected by any of the studied β subunits. The same is observed for the limiting voltage dependence of the deactivation time constant. Thus, the β1 subunit must affect the voltage sensitivity by altering the function of the voltage sensors of the channel. Both β subunits reduce the intrinsic equilibrium constant for channel opening (L₀). In the allosteric activation model, the reduction of the voltage dependence for the activation of the voltage sensors accounts for most of the macroscopic steady-state effects of the β1 subunit, including the increase of the apparent calcium sensitivity of the BK channel. All allosteric coupling factors need to be increased in order to explain the observed effects when the α subunit is coexpressed with the β2IR subunit.

A novel scorpion toxin blocking small conductance Ca2+ activated K+ channel

Small conductance calcium activated potassium channels (SK) are crucial in the regulation of cell firing frequency in the nervous system and other tissues. In the present work, a novel SK channel blocker, designated BmSKTx1, was purified from the scorpion Buthus martensi Karsh venom. The sequence of the N-terminal 22 amino acid residues was determined by Edman degradation. Using this sequence information, the full-length cDNA and genomic gene of BmSKTx1 were cloned and sequenced. By these analyses, BmSKTx1 was found to be a peptide composed of 31 amino acid residues with three disulfide bonds. It shared little sequence homology with other known scorpion alpha-KTxs but showed close relationship with SK channel blockers in the phylogenetic tree. According to the previous nomenclature, BmSKTx1 was classified as alpha-KTx14.1. We examined the effects of BmSKTx1 on different ion channels of rat adrenal chromaffin cells (RACC) and locust dorsal unpaired median (DUM) neurons. BmSKTx1 selectively inhibited apamin-sensitive SK currents in RACC with Kd of 0.72 microM and Hill coefficient of 2.2. And it had no effect on Na+, Ca2+, Kv, and BK currents in DUM neuron, indicating that BmSKTx1 was a selective SK toxin.

Control of secretion by temporal patterns of action potentials in adrenal chromaffin cells

Action potentials (APs) are the principal physiological stimuli for neurotransmitter secretion in neurons. Most studies on stimulus-secretion coupling have been performed under voltage clamp using artificial electrical stimuli. To investigate the modulatory effects of AP codes on neural secretion, we introduce a capacitance method to study AP-induced secretion in single cells. The action potential pattern was defined by a four-parameter "code function:" F(n, m, f, d). With this method, cell secretion evoked by stimulation with an AP code was quantified in real time by membrane capacitance (Cm) in adrenal chromaffin cells. We found, in addition to AP frequency (f), for a given number of APs, another parameter of the AP code, the number of AP bursts (m) in which the set of APs occurs, can effectively modulate cell secretion. Possible mechanisms of the m effect are depletion of the readily releasable pool and inactivation of Ca2+ channels during a burst of APs. The physiological m effect may play a key role in AP-mediated neural information transfer within a single neuron and among the elements of a neural network.

Control of secretion by temporal patterns of action potentials in adrenal chromaffin cells

Action potentials (APs) are the principal physiological stimuli for neurotransmitter secretion in neurons. Most studies on stimulus-secretion coupling have been performed under voltage clamp using artificial electrical stimuli. To investigate the modulatory effects of AP codes on neural secretion, we introduce a capacitance method to study AP-induced secretion in single cells. The action potential pattern was defined by a four-parameter "code function:" F(n, m, f, d). With this method, cell secretion evoked by stimulation with an AP code was quantified in real time by membrane capacitance (Cm) in adrenal chromaffin cells. We found, in addition to AP frequency (f), for a given number of APs, another parameter of the AP code, the number of AP bursts (m) in which the set of APs occurs, can effectively modulate cell secretion. Possible mechanisms of the m effect are depletion of the readily releasable pool and inactivation of Ca2+ channels during a burst of APs. The physiological m effect may play a key role in AP-mediated neural information transfer within a single neuron and among the elements of a neural network.

A novel conotoxin from Conus betulinus, kappa-BtX, unique in cysteine pattern and in function as a specific BK channel modulator

A novel conotoxin, kappa-conotoxin (kappa-BtX), has been purified and characterized from the venom of a worm-hunting cone snail, Conus betulinus. The toxin, with four disulfide bonds, shares no sequence homology with any other conotoxins. Based on a partial amino acid sequence, its cDNA was cloned and sequenced. The deduced sequence consists of a 26-residue putative signal peptide, a 31-residue mature toxin, and a 13-residue extra peptide at the C terminus. The extra peptide is cleaved off by proteinase post-processing. All three Glu residues are gamma-carboxylated, one of the two Pro residues is hydroxylated at position 27, and its C-terminal residue is Pro-amidated. The monoisotopic mass of the toxin is 3569.0 Da. Electrophysiological experiments show that: 1) among voltage-gated channels, kappa-BtX is a specific modulator of K(+) channels; 2) among the K channels, kappa-BtX specifically up-modulates the Ca(2+)- and voltage-sensitive BK channels (252 +/- 47%); 3) its EC(50) is 0.7 nm with a single binding site (Hill = 0.88); 4) the time constant of wash-out is 8.3 s; and 5) kappa-BtX has no effect on single channel conductance, but increases the open probability of BK channels. It is concluded that kappa-BtX is a novel specific biotoxin against BK channels.

Na+ channel inactivation: a comparative study between pancreatic islet beta-cells and adrenal chromaffin cells in rat

A comparative study was carried out on the inactivation of Na+ channels in two types of endocrine cells in rats, beta-cells and adrenal chromaffin cells (ACCs), using patch-clamp techniques. The beta-cells were very sensitive to hyperpolarization; the Na+ currents increased ninefold when the holding potential was shifted from -70 mV to -120 mV. ACCs were not sensitive to hyperpolarization. The half-inactivation voltages were -90 mV (rat beta-cells) and -62 mV (ACCs). The time constant for recovery from inactivation at -70 mV was 10.5 times slower in beta-cells (60 ms) than in ACCs (5.7 ms). The rate of Na+-channel inactivation at physiological resting potential was more than three times slower in beta-cells than in ACCs. Na+ influx through Na+ channels had no effect on the secretory machinery in rat beta-cells. However, these 'silent Na+ channels' could contribute to the generation of action potentials in some conditions, such as when the cell is hyperpolarized. It is concluded that the fractional availability of Na+ channels in beta-cells at a holding potential of -70 mV is about 15 % of that in ACCs. This value in rat beta-cells is larger than that observed in mouse (0 %), but is smaller than those observed in human or dog (90 %).

Martentoxin, a novel K+-channel-blocking peptide: purification, cDNA and genomic cloning, and electrophysiological and pharmacological characterization

Martentoxin, a novel K+-channel-specific peptide has been purified and characterized from the venom of the East-Asian scorpion (Buthus martensi Karsch). The whole cDNA precursor sequence suggested that martentoxin was composed of 37 residues with a unique sequence compared with other scorpion neurotoxins. The genomic DNA of martentoxin showed an additional intron situated unexpectedly in the 5' UTR region, besides one located close to the C-terminal of the signal peptide. The patch-clamp recording found that martentoxin at the applied dose of 100 nm could strongly block large-conductance Ca2+-activated K+ (BKCa) currents in adrenal medulla chromaffin cells, and BKCa currents blocked by martentoxin could be fully recovered within 30 seconds after washing, which is at least 10 times faster than recovery after charybdotoxin. Meanwhile, a biosensor binding assay showed a fast association rate and a slow dissociation rate of martentoxin binding on rat brain synaptosomes. The binding of martentoxin on rat brain synaptosomes could be inhibited regularly by charybdotoxin, and gradually by toosendanin in a concentration-dependent manner, but not by either apamin or P03 from Buthus martensi. The results thus indicate that martentoxin is a new member in the family of K+-channel-blocking ligands.

Steady-state and closed-state inactivation properties of inactivating BK channels

Calcium-dependent potassium (BK-type) Ca2+ and voltage-dependent K+ channels in chromaffin cells exhibit an inactivation that probably arises from coassembly of Slo1 alpha subunits with auxiliary beta subunits. One goal of this work was to determine whether the Ca2+ dependence of inactivation arises from any mechanism other than coupling of inactivation to the Ca2+ dependence of activation. Steady-state inactivation and the onset of inactivation were studied in inside-out patches and whole-cell recordings from rat adrenal chromaffin cells with parallel experiments on inactivating BK channels resulting from cloned alpha + beta2 subunits. In both cases, steady-state inactivation was shifted to more negative potentials by increases in submembrane [Ca2+] from 1 to 60 microM. At 10 and 60 microM Ca2+, the maximal channel availability at negative potentials was similar despite a shift in the voltage of half availability, suggesting there is no strictly Ca2+-dependent inactivation. In contrast, in the absence of Ca2+, depolarization to potentials positive to +20 mV induces channel inactivation. Thus, voltage-dependent, but not solely Ca2+-dependent, kinetic steps are required for inactivation to occur. Finally, under some conditions, BK channels are shown to inactivate as readily from closed states as from open states, indicative that a key conformational change required for inactivation precedes channel opening.

Oxygen sensitivity in the sheep adrenal medulla: role of SK channels

The hypoxia-evoked secretion of catecholamines from the noninnervated fetal adrenal gland is essential for surviving intrauterine hypoxemia. The ion channels responsible for the initial depolarization that leads to catecholamine secretion have not been identified. Patch-clamp studies of adrenal chromaffin cells isolated from fetal and adult sheep revealed the presence of a Ca(2+)-dependent K(+) current that was reduced by hypoxia. Apamin, a blocker of small-conductance K(+) (SK) channels, reduced the Ca(2+)-dependent K(+) current, and the sensitivity of the channels to apamin indicated that the channels involved were of the SK2 subtype. In the presence of apamin, the hypoxia-evoked change in K(+) currents was largely eliminated. Both hypoxia and apamin blocked a K(+) current responsible for maintaining the resting potential of the cell, and the depolarization resulting from both led to an influx of Ca(2+). Simultaneous application of hypoxia and apamin did not potentiate the increase in cytosolic Ca(2+) concentration beyond that seen with either agent alone. Similar results were seen with curare, another blocker of SK channels. These results indicate that closure of SK2 channels would be the initiating event in the hypoxia-evoked catecholamine secretion in the adrenal medulla.

Ca(2+)-dependent K(+) current and exocytosis in responses to caffeine and muscarine in voltage-clamped guinea-pig adrenal chromaffin cells

We characterized changes in membrane currents and the cytosolic Ca(2+) concentration, [Ca(2+)](i), in response to caffeine, and compared them with those in response to muscarine using the perforated patch-clamp technique and fura-2 microfluorimetry in guinea-pig adrenal chromaffin cells. Catecholamine release from single voltage-clamped cells was monitored with amperometry using carbon microelectrodes. Caffeine produced a transient outward current (I(out)) at holding potentials over - 60 mV, increasing in amplitude with increasing the potentials. It also evoked a rapid increase of [Ca(2+)](i) at all potentials examined. The current-voltage relation revealed that the activation of K(+) channels was responsible for the I(out) evoked by caffeine. Both current and [Ca(2+)](i) responses were reversibly abolished by cyclopiazonic acid, an inhibitor of Ca(2+)-pump ATPase. At - 30 mV, the caffeine-induced I(out), but not [Ca(2+)](i), was partly inhibited by either charybdotoxin or apamin. In the majority of cells tested, caffeine induced a larger I(out) but a smaller [Ca(2+)](i) increase than muscarine. Caffeine and muscarine increased catecholamine release from voltage-clamped single cells concomitant with the transient increase of [Ca(2+)](i), and there was a positive correlation between them. These results indicate that caffeine activates Ca(2+)-dependent K(+) channels and catecholamine secretion due to the release of Ca(2+) from internal stores in voltage-clamped adrenal chromaffin cells of the guinea-pig. There seems to be a spatial difference between [Ca(2+)](i) increased by Ca(2+) release from caffeine-sensitive stores and that released from muscarine (inositol 1,4,5-trisphosphate)-sensitive ones.

Pituitary control of BK potassium channel function and intrinsic firing properties of adrenal chromaffin cells

The discovery that the hypothalamic-pituitary-adrenocortical (HPA) endocrine stress axis controls an alternative splicing decision in chromaffin Slo-encoded BK (big potassium) channels raised the possibility that activation of the HPA could serve as a mechanism to tune the intrinsic electrical properties of epinephrine-secreting adrenal chromaffin cells. To test this, we compared BK functional properties and cell excitability in chromaffin cells from normal and hypophysectomized (pituitary-ablated) rats. Hypophysectomy was found to alter the voltage dependence and kinetics of BK gating, making channels less accessible for activation from rest. Perforated-patch recordings revealed changes in action potential waveform and repetitive firing properties. The maximum number of spikes that could be elicited with a 2 sec depolarizing current pulse was reduced by approximately 50% by hypophysectomy. The results indicate that pituitary hormones can adapt the mechanics of adrenal catecholamine release by tailoring BK channel function.

Influence of quinine on catecholamine release evoked by cholinergic stimulation and membrane depolarization from the rat adrenal gland

The present study was attempted to investigate the effect of quinine on secretion of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from the isolated perfused rat adrenal gland. The perfusion of quinine (15-150 microM) into an adrenal vein for 60 min produced dose- and time-dependent inhibition in CA secretion evoked by ACh (5.32 x 10(-3) M), high K+ (5.6 x 10(-2) M), DMPP (10(-4) M for 2 min), McN-A-343 (10(-4) M for 2 min), cyclopiazonic acid (10(-5) M for 4 min) and Bay-K-8644 (10(-5) M for 4 min). Also, under the presence of pinacidil (10(-4) M), which is also known to be a selective potassium channel activator, CA secretory responses evoked by ACh, high potassium, DMPPF McN-A-343, Bay-K-8644 and cyclopiazonic acid were also greatly reduced. When preloaded along with quinine (5 x 10(-5) M) and glibenclamide (10(-6) M), a specific blocker of ATP-regulated potassium channels, CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were recovered as compared to those of quinine-treatment only. Taken together, these results demonstrate that quinine inhibits CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors as well as by membrane depolarization through inhibiting influx of extracellular calcium and release in intracellular calcium in the rat adrenomedullary chromaffin cells. These findings suggest that activation of potassium channels may be involved at least in inhibitory action of quinine on CA secretion from the rat adrenal gland.

Pituitary control of BK potassium channel function and intrinsic firing properties of adrenal chromaffin cells

The discovery that the hypothalamic-pituitary-adrenocortical (HPA) endocrine stress axis controls an alternative splicing decision in chromaffin Slo-encoded BK (big potassium) channels raised the possibility that activation of the HPA could serve as a mechanism to tune the intrinsic electrical properties of epinephrine-secreting adrenal chromaffin cells. To test this, we compared BK functional properties and cell excitability in chromaffin cells from normal and hypophysectomized (pituitary-ablated) rats. Hypophysectomy was found to alter the voltage dependence and kinetics of BK gating, making channels less accessible for activation from rest. Perforated-patch recordings revealed changes in action potential waveform and repetitive firing properties. The maximum number of spikes that could be elicited with a 2 sec depolarizing current pulse was reduced by approximately 50% by hypophysectomy. The results indicate that pituitary hormones can adapt the mechanics of adrenal catecholamine release by tailoring BK channel function.

Activation of BK channels in rat chromaffin cells requires summation of Ca(2+) influx from multiple Ca(2+) channels

Large-conductance Ca(2+) and voltage-dependent K(+) channels (BK channels) in many tissues require high Ca(2+) concentrations for activation and therefore might be expected to be tightly coupled to Ca(2+) channels. However, in most cases, little is known about the relative organization of the BK channels and the Ca(2+) channels involved in their activation. We probed the nature of the organization of BK and Ca(2+) channels in rat chromaffin cells by manipulating Ca(2+) influx through Ca(2+) channels and by altering cellular Ca(2+) buffering using EGTA and bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA). The results were analyzed to determine the distance between Ca(2+) and BK channels that would be most consistent with the experimental data. Most BK channels are close enough to Ca(2+) channels to be resistant to the buffering action of millimolar of EGTA, but are far enough to be inhibited by BAPTA. Analysis of the EGTA/BAPTA results suggests that BK channels are at a distance of 50 to 160 nm from Ca(2+) channels. A model that assumes random distribution of Ca(2+) and BK channels fails to account for the observed [Ca(2+)](i) detected by BK channels, suggesting that a specific mechanism may exist to mediate the functional coupling between these channels. Importantly, the effects of EGTA and BAPTA cannot be explained by assuming a one-to-one coupling between Ca(2+) and BK channels. Rather, Ca(2+) influx through a number of Ca(2+) channels appears to act in concert to regulate the behavior of any individual BK channel. Thus differences in BK channel open probabilities may be explained by differences in the extent of Ca(2+) domain overlap at the sites of individual BK channels.

Bovine versus rat adrenal chromaffin cells: big differences in BK potassium channel properties

Both bovine and rat adrenal chromaffin cells have served as pioneering model systems in cellular neurophysiology, including in the study of large conductance calcium- and voltage-dependent K(+) (BK) channels. We now report that while BK channels dominate the outward current profile of both species, specific gating properties vary widely across cell populations, and the distributions of these properties differ dramatically between species. Although BK channels were first described in bovine chromaffin cells, rapidly inactivating ones were discovered in rat chromaffin cells. We report that bovine cells can also exhibit inactivating BK channels with varying properties similar to those in rat cells. However, a much smaller proportion of bovine cells exhibit inactivating BK current, the proportion of the total current that inactivates is usually smaller, and the rate of inactivation is often much slower. Other gating features differ as well; the voltage dependence of channel activation is much more positive for bovine cells, and their rates of activation and deactivation are faster and slower, respectively. Modeling studies suggest that channel heterogeneity is consistent with varying tetrameric combinations of inactivation-competent versus -incompetent subunits. The results suggest that chromaffin BK channel functional nuances represent an important level for evolutionary tailoring of autonomic stress responses.

Properties and expression of Ca2+-activated K+ channels in H9c2 cells derived from rat ventricle

H9c2 is a clonal myogenic cell line derived from embryonic rat ventricle that can serve as a surrogate for cardiac or skeletal muscle in vitro. Using whole cell clamp with H9c2 myotubes, we observed that depolarizing pulses activated slow outward K+ currents and then slow tail currents. The K+ currents were abolished in a Ca2+-free external solution, indicating that they were Ca2+-activated K+ currents. They were blocked by apamin, a small-conductance Ca2+-activated K+ (SK) channel antagonist (IC50 = 6.2 nM), and by d-tubocurarine (IC50 = 49.4 microM). Activation of SK channels exhibited a bell-shaped voltage dependence that paralleled the current-voltage relation for L-type Ca2+ currents (ICa,L). ICa,L exhibited a slow time course similar to skeletal ICa, L, were unaffected by apamin, and were only slightly depressed by d-tubocurarine. RT-PCR analysis of the mRNAs revealed that rSK3, but not rSK1 or rSK2, was expressed in H9c2 myotubes but not in myoblasts. These results suggest that rSK3 channels are expressed in H9c2 myotubes and are primarily activated by ICa,L directly or indirectly via Ca2+-induced Ca2+ release from sarcoplasmic reticulum.