Direct autocrine inhibition and cAMP-dependent potentiation of single L-type Ca2+ channels in bovine chromaffin cells

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.

Low pHo boosts burst firing and catecholamine release by blocking TASK-1 and BK channels while preserving Cav1 channels in mouse chromaffin cells

KEY POINTS

Mouse chromaffin cells (MCCs) generate spontaneous burst-firing that causes large increases of Ca²⁺ -dependent catecholamine release, and is thus a key mechanism for regulating the functions of MCCs. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pH_o ) from 7.4 to 6.6 induces cell depolarizations of 10-15 mV that generate bursts of ∼330 ms at 1-2 Hz and a 7.4-fold increase of cumulative catecholamine-release. Burst-firing originates from the inhibition of the pH-sensitive TASK-1-channels and a 60% reduction of BK-channel conductance at pH_o 6.6. Blockers of the two channels (A1899 and paxilline) mimic the effects of pH_o 6.6, and this is reverted by the Cav1 channel blocker nifedipine. MCCs act as pH-sensors. At low pH_o , they depolarize, undergo burst-firing and increase catecholamine-secretion, generating an effective physiological response that may compensate for the acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.

ABSTRACT

Mouse chromaffin cells (MCCs) generate action potential (AP) firing that regulates the Ca²⁺ -dependent release of catecholamines (CAs). Recent findings indicate that MCCs possess a variety of spontaneous firing modes that span from the common 'tonic-irregular' to the less frequent 'burst' firing. This latter is evident in a small fraction of MCCs but occurs regularly when Nav1.3/1.7 channels are made less available or when the Slo1β2-subunit responsible for BK channel inactivation is deleted. Burst firing causes large increases of Ca²⁺ -entry and potentiates CA release by ∼3.5-fold and thus may be a key mechanism for regulating MCC function. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pH_o ) from 7.4 to 7.0 and 6.6 induces cell depolarizations of 10-15 mV that generate repeated bursts. Bursts at pH_o 6.6 lasted ∼330 ms, occurred at 1-2 Hz and caused an ∼7-fold increase of CA cumulative release. Burst firing originates from the inhibition of the pH-sensitive TASK-1/TASK-3 channels and from a 40% BK channel conductance reduction at pH_o 7.0. The same pH_o had little or no effect on Nav, Cav, Kv and SK channels that support AP firing in MCCs. Burst firing of pH_o 6.6 could be mimicked by mixtures of the TASK-1 blocker A1899 (300 nm) and BK blocker paxilline (300 nm) and could be prevented by blocking L-type channels by adding 3 μm nifedipine. Mixtures of the two blockers raised cumulative CA-secretion even more than low pH_o (∼12-fold), showing that the action of protons on vesicle release is mainly a result of the ionic conductance changes that increase Ca²⁺ -entry during bursts. Our data provide direct evidence suggesting that MCCs respond to low pH_o with sustained depolarization, burst firing and enhanced CA-secretion, thus mimicking the physiological response of CCs to acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.

Dopamine Modulates the Activity of Sensory Hair Cells

The senses of hearing and balance are subject to modulation by efferent signaling, including the release of dopamine (DA). How DA influences the activity of the auditory and vestibular systems and its site of action are not well understood. Here we show that dopaminergic efferent fibers innervate the acousticolateralis epithelium of the zebrafish during development but do not directly form synapses with hair cells. However, a member of the D1-like receptor family, D1b, tightly localizes to ribbon synapses in inner ear and lateral-line hair cells. To assess modulation of hair-cell activity, we reversibly activated or inhibited D1-like receptors (D1Rs) in lateral-line hair cells. In extracellular recordings from hair cells, we observed that D1R agonist SKF-38393 increased microphonic potentials, whereas D1R antagonist SCH-23390 decreased microphonic potentials. Using ratiometric calcium imaging, we found that increased D1R activity resulted in larger calcium transients in hair cells. The increase of intracellular calcium requires Cav1.3a channels, as a Cav1 calcium channel antagonist, isradipine, blocked the increase in calcium transients elicited by the agonist SKF-38393. Collectively, our results suggest that DA is released in a paracrine fashion and acts at ribbon synapses, likely enhancing the activity of presynaptic Cav1.3a channels and thereby increasing neurotransmission.

SIGNIFICANCE STATEMENT

The neurotransmitter dopamine acts in a paracrine fashion (diffusion over a short distance) in several tissues and bodily organs, influencing and regulating their activity. The cellular target and mechanism of the action of dopamine in mechanosensory organs, such as the inner ear and lateral-line organ, is not clearly understood. Here we demonstrate that dopamine receptors are present in sensory hair cells at synaptic sites that are required for signaling to the brain. When nearby neurons release dopamine, activation of the dopamine receptors increases the activity of these mechanosensitive cells. The mechanism of dopamine activation requires voltage-gated calcium channels that are also present at hair-cell synapses.

Elementary properties of Ca(2+) channels and their influence on multivesicular release and phase-locking at auditory hair cell ribbon synapses

Voltage-gated calcium (Ca_v1.3) channels in mammalian inner hair cells (IHCs) open in response to sound and the resulting Ca²⁺ entry triggers the release of the neurotransmitter glutamate onto afferent terminals. At low to mid sound frequencies cell depolarization follows the sound sinusoid and pulses of transmitter release from the hair cell generate excitatory postsynaptic currents (EPSCs) in the afferent fiber that translate into a phase-locked pattern of action potential activity. The present article summarizes our current understanding on the elementary properties of single IHC Ca²⁺ channels, and how these could have functional implications for certain, poorly understood, features of synaptic transmission at auditory hair cell ribbon synapses.

Reduced availability of voltage-gated sodium channels by depolarization or blockade by tetrodotoxin boosts burst firing and catecholamine release in mouse chromaffin cells

KEY POINTS

Mouse chromaffin cells (MCCs) of the adrenal medulla possess fast-inactivating Nav channels whose availability alters spontaneous action potential firing patterns and the Ca(2+)-dependent secretion of catecholamines. Here, we report MCCs expressing large densities of neuronal fast-inactivating Nav1.3 and Nav1.7 channels that carry little or no subthreshold pacemaker currents and can be slowly inactivated by 50% upon slight membrane depolarization. Reducing Nav1.3/Nav1.7 availability by tetrodotoxin or by sustained depolarization near rest leads to a switch from tonic to burst-firing patterns that give rise to elevated Ca(2+)-influx and increased catecholamine release. Spontaneous burst firing is also evident in a small percentage of control MCCs. Our results establish that burst firing comprises an intrinsic firing mode of MCCs that boosts their output. This occurs particularly when Nav channel availability is reduced by sustained splanchnic nerve stimulation or prolonged cell depolarizations induced by acidosis, hyperkalaemia and increased muscarine levels.

ABSTRACT

Action potential (AP) firing in mouse chromaffin cells (MCCs) is mainly sustained by Cav1.3 L-type channels that drive BK and SK currents and regulate the pacemaking cycle. As secretory units, CCs optimally recruit Ca(2+) channels when stimulated, a process potentially dependent on the modulation of the AP waveform. Our previous work has shown that a critical determinant of AP shape is voltage-gated sodium channel (Nav) channel availability. Here, we studied the contribution of Nav channels to firing patterns and AP shapes at rest (-50 mV) and upon stimulation (-40 mV). Using quantitative RT-PCR and immunoblotting, we show that MCCs mainly express tetrodotoxin (TTX)-sensitive, fast-inactivating Nav1.3 and Nav1.7 channels that carry little or no Na(+) current during slow ramp depolarizations. Time constants and the percentage of recovery from fast inactivation and slow entry into closed-state inactivation are similar to that of brain Nav1.3 and Nav1.7 channels. The fraction of available Nav channels is reduced by half after 10 mV depolarization from -50 to -40 mV. This leads to low amplitude spikes and a reduction in repolarizing K(+) currents inverting the net current from outward to inward during the after-hyperpolarization. When Nav channel availability is reduced by up to 20% of total, either by TTX block or steady depolarization, a switch from tonic to burst firing is observed. The spontaneous occurrence of high frequency bursts is rare under control conditions (14% of cells) but leads to major Ca(2+)-entry and increased catecholamine release. Thus, Nav1.3/Nav1.7 channel availability sets the AP shape, burst-firing initiation and regulates catecholamine secretion in MCCs. Nav channel inactivation becomes important during periods of high activity, mimicking stress responses.

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.

Fine Tuning of CaV1.3 Ca2+ channel properties in adult inner hair cells positioned in the most sensitive region of the Gerbil Cochlea

Hearing relies on faithful signal transmission by cochlear inner hair cells (IHCs) onto auditory fibres over a wide frequency and intensity range. Exocytosis at IHC ribbon synapses is triggered by Ca²⁺ inflow through Ca_V1.3 (L-type) Ca²⁺ channels. We investigated the macroscopic (whole-cell) and elementary (cell-attached) properties of Ca²⁺ currents in IHCs positioned at the middle turn (frequency ∼2 kHz) of the adult gerbil cochlea, which is their most sensitive hearing region. Using near physiological recordings conditions (body temperature and a Na⁺ based extracellular solution), we found that the macroscopic Ca²⁺ current activates and deactivates very rapidly (time constant below 1 ms) and inactivates slowly and only partially. Single-channel recordings showed an elementary conductance of 15 pS, a sub-ms latency to first opening, and a very low steady-state open probability (P_o: 0.024 in response to 500-ms depolarizing steps at ∼−18 mV). The value of P_o was significantly larger (0.06) in the first 40 ms of membrane depolarization, which corresponds to the time when most Ca²⁺ channel openings occurred clustered in bursts (mean burst duration: 19 ms). Both the P_o and the mean burst duration were smaller than those previously reported in high-frequency basal IHCs. Finally, we found that middle turn IHCs are likely to express about 4 times more Ca²⁺ channels per ribbon than basal cells. We propose that middle-turn IHCs finely-tune Ca_V1.3 Ca²⁺ channel gating in order to provide reliable information upon timing and intensity of lower-frequency sounds.

Pannexin 1 channels: new actors in the regulation of catecholamine release from adrenal chromaffin cells

Chromaffin cells of the adrenal gland medulla synthesize and store hormones and peptides, which are released into the blood circulation in response to stress. Among them, adrenaline is critical for the fight-or-flight response. This neurosecretory process is highly regulated and depends on cytosolic [Ca²⁺]. By forming channels at the plasma membrane, pannexin-1 (Panx1) is a protein involved in many physiological and pathological processes amplifying ATP release and/or Ca²⁺ signals. Here, we show that Panx1 is expressed in the adrenal gland where it plays a role by regulating the release of catecholamines. In fact, inhibitors of Panx1 channels, such as carbenoxolone (Cbx) and probenecid, reduced the secretory activity induced with the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP, 50 μM) in whole adrenal glands. A similar inhibitory effect was observed in single chromaffin cells using Cbx or ¹⁰Panx1 peptide, another Panx1 channel inhibitors. Given that the secretory response depends on cytosolic [Ca²⁺] and Panx1 channels are permeable to Ca²⁺, we studied the possible implication of Panx1 channels in the Ca²⁺ signaling occurring during the secretory process. In support of this possibility, Panx1 channel inhibitors significantly reduced the Ca²⁺ signals evoked by DMPP in single chromaffin cells. However, the Ca²⁺ signals induced by caffeine in the absence of extracellular Ca²⁺ was not affected by Panx1 channel inhibitors, suggesting that this mechanism does not involve Ca²⁺ release from the endoplasmic reticulum. Conversely, Panx1 inhibitors significantly blocked the DMPP-induce dye uptake, supporting the idea that Panx1 forms functional channels at the plasma membrane. These findings indicate that Panx1 channels participate in the control the Ca²⁺ signal that triggers the secretory response of adrenal chromaffin cells. This mechanism could have physiological implications during the response to stress.

Burst activity and ultrafast activation kinetics of CaV1.3 Ca²⁺ channels support presynaptic activity in adult gerbil hair cell ribbon synapses

Auditory information transfer to afferent neurons relies on precise triggering of neurotransmitter release at the inner hair cell (IHC) ribbon synapses by Ca²⁺ entry through Ca_V1.3 Ca²⁺ channels. Despite the crucial role of Ca_V1.3 Ca²⁺ channels in governing synaptic vesicle fusion, their elementary properties in adult mammals remain unknown. Using near-physiological recording conditions we investigated Ca²⁺ channel activity in adult gerbil IHCs. We found that Ca²⁺ channels are partially active at the IHC resting membrane potential (−60 mV). At −20 mV, the large majority (>70%) of Ca²⁺ channel first openings occurred with an estimated delay of about 50 μs in physiological conditions, with a mean open time of 0.5 ms. Similar to other ribbon synapses, Ca²⁺ channels in IHCs showed a low mean open probability (0.21 at −20 mV), but this increased significantly (up to 0.91) when Ca²⁺ channel activity switched to a bursting modality. We propose that IHC Ca²⁺ channels are sufficiently rapid to transmit fast signals of sound onset and support phase-locking. Short-latency Ca²⁺ channel opening coupled to multivesicular release would ensure precise and reliable signal transmission at the IHC ribbon synapse.

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.

Equal sensitivity of Cav1.2 and Cav1.3 channels to the opposing modulations of PKA and PKG in mouse chromaffin cells

Mouse chromaffin cells (MCCs) express high densities of L-type Ca2+ channels (LTCCs), which control pacemaking activity and catecholamine secretion proportionally to their density of expression. In vivo phosphorylation of LTCCs by cAMP-PKA and cGMP–PKG, regulate LTCC gating in two opposing ways: the cAMP-PKA pathway potentiates while the cGMP–PKG cascade inhibits LTCCs. Despite this, no attempts have been made to answer three key questions related to the two Cav1 isoforms expressed in MCCs (Cav1.2 and Cav1.3): (i) how much are the two Cav1 channels basally modulated by PKA and PKG?, (ii) to what extent can Cav1.2 and Cav1.3 be further regulated by PKA or PKG activation?, and (iii) are the effects of both kinases cumulative when simultaneously active? Here, by comparing the size of L-type currents of wild-type (WT; Cav1.2+Cav1.3) and Cav1.3−/− KO (Cav1.2) MCCs, we provide new evidence that both PKA and PKG pathways affect Cav1.2 and Cav1.3 to the same extent either under basal conditions or induced stimulation. Inhibition of PKA by H89 (5 μM) reduced the L-type current in WT and KO MCCs by∼60%,while inhibition of PKG by KT 5823 (1 μM) increased by∼40% the same current in both cell types. Given that Cav1.2 and Cav1.3 carry the same quantity of Ca2+ currents, this suggests equal sensitivity of Cav1.2 and Cav1.3 to the two basal modulatory pathways. Maximal stimulation of cAMP–PKA by forskolin (100 μM) and activation of cGMP–PKG by pCPT-cGMP (1mM) uncovered a∼25% increase of L-type currents in the first case and∼65% inhibition in the second case in both WT and KO MCCs, suggesting equal sensitivity of Cav1.2 and Cav1.3 during maximal PKA or PKG stimulation. The effects of PKA and PKG were cumulative and most evident when one pathway was activated and the other was inhibited. The two extreme combinations(PKA activation–PKG inhibition vs. PKG activation-PKA inhibition) varied the size of L-type currents by one order of magnitude (from 180% to 18% of control size). Taken together our data suggest that: (i) Cav1.2 and Cav1.3 are equally sensitive to PKA and PKG action under both basal conditions and maximal stimulation, and (ii) PKA and PKG act independently on both Cav1.2 and Cav1.3, producing cumulative effects when opposingly activated. These extreme Cav1 channel modulations may occur either during high-frequency sympathetic stimulation to sustain prolonged catecholamine release (maximal L-type current) or following activation of the NO–cGMP–PKG signalling pathway (minimal L-type current) to limit the steady release of catecholamines.

Functional chromaffin cell plasticity in response to stress: focus on nicotinic, gap junction, and voltage-gated Ca2+ channels

An increase in circulating catecholamines constitutes one of the mechanisms whereby human body responds to stress. In response to chronic stressful situations, the adrenal medullary tissue exhibits crucial morphological and functional changes that are consistent with an improvement of chromaffin cell stimulus-secretion coupling efficiency. Stimulus-secretion coupling encompasses multiple intracellular (chromaffin cell excitability, Ca(2+) signaling, exocytosis, endocytosis) and intercellular pathways (splanchnic nerve-mediated synaptic transmission, paracrine and endocrine communication, gap junctional coupling), each of them being potentially subjected to functional remodeling upon stress. This review focuses on three chromaffin cell incontrovertible actors, the cholinergic nicotinic receptors and the voltage-dependent T-type Ca(2+) channels that are directly involved in Ca(2+)-dependent events controlling catecholamine secretion and electrical activity, and the gap junctional communication involved in the modulation of catecholamine secretion. We show here that these three actors react differently to various stressors, sometimes independently, sometimes in concert or in opposition.

Are Ca(v)1.3 pacemaker channels in chromaffin cells? Possible bias from resting cell conditions and DHP blockers usage

Mouse and rat chromaffin cells (MCCs, RCCs) fire spontaneously at rest and their activity is mainly supported by the two L-type Ca(2+) channels expressed in these cells (Ca(v)1.2 and Ca(v)1.3). Using Ca(v)1.3(-/-) KO MCCs we have shown that Ca(v)1.3 possess all the prerequisites for carrying subthreshold currents that sustain low frequency cell firing near resting (0.5 to 2 Hz at -50 mV): low-threshold and steep voltage dependence of activation, slow and incomplete inactivation during pulses of several hundreds of milliseconds. Ca(v)1.2 contributes also to pacemaking MCCs and possibly even Na(+) channels may participate in the firing of a small percentage of cells. We now show that at potentials near resting (-50 mV), Ca(v)1.3 carries equal amounts of Ca(2+) current to Ca(v)1.2 but activates at 9 mV more negative potentials. MCCs express only TTX-sensitive Na(v)1 channels that activate at 24 mV more positive potentials than Ca(v)1.3 and are fully inactivating. Their blockade prevents the firing only in a small percentage of cells (13%). This suggests that the order of importance with regard to pacemaking MCCs is: Ca(v)1.3, Ca(v)1.2 and Na(v)1. The above conclusions, however, rely on the proper use of DHPs, whose blocking potency is strongly holding potential dependent. We also show that small increases of KCl concentration steadily depolarize the MCCs causing abnormally increased firing frequencies, lowered and broadened AP waveforms and an increased facility of switching "non-firing" into "firing" cells that may lead to erroneous conclusions about the role of Ca(v)1.3 and Ca(v)1.2 as pacemaker channels in MCCs.

Extracellular K+ elevates outward currents through Kir2.1 channels by increasing single-channel conductance

Outward currents through inward rectifier K+ channels (Kir) play a pivotal role in determining resting membrane potential and in controlling excitability in many cell types. Thus, the regulation of outward Kir current (IK1) is important for appropriate physiological functions. It is known that outward IK1 increases with increasing extracellular K+ concentration ([K+]o), but the underlying mechanism is not fully understood. A "K+-activation of K+-channel" hypothesis and a "blocking-particle" model have been proposed to explain the [K+]o-dependence of outward IK1. Yet, these mechanisms have not been examined at the single-channel level. In the present study, we explored the mechanisms that determine the amplitudes of outward IK1 at constant driving forces [membrane potential (Vm) minus reversal potential (EK)]. We found that increases in [K+]o elevated the single-channel current to the same extent as macroscopic IK1 but did not affect the channel open probability at a constant driving force. In addition, spermine-binding kinetics remained unchanged when [K+]o ranged from 1 to 150 mM at a constant driving force. We suggest the regulation of K+ permeation by [K+]o as a new mechanism for the [K+]o-dependence of outward IK1.

Modulation by endogenously released ATP and opioids of chromaffin cell calcium channels in mouse adrenal slices

Modulation of high-threshold voltage-dependent calcium channels by neurotransmitters has been the subject of numerous studies in cultures of neurons and chromaffin cells. However, no studies on such modulation exist in chromaffin cells in their natural environment, the intact adrenal medullary tissue. Here we performed such a study in voltage-clamped chromaffin cells of freshly prepared mouse adrenal slices under the whole cell configuration of the patch-clamp technique. The subcomponents of the whole cell inward Ca(2+) current (I(Ca)) accounted for 49% for L-, 28% for N-, and 36% for P/Q-type channels. T-type Ca(2+) channels or residual R-type Ca(2+) currents were not seen. However, under the perforated-patch configuration, 20% of I(Ca) accounted for a toxin-resistant R-type Ca(2+) current. Exogenously applied ATP and methionine-enkephalin (Met-enk) inhibited I(Ca) by 33%. Stop-flow and Ca(2+) replacement by Ba(2+), which favored the release of endogenous ATP and opioids, also inhibited I(Ca), with no changes in activation or inactivation kinetics. This inhibition was partially voltage independent and insensitive to prepulse facilitation. Furthermore, in about half of the cells, suramin and naloxone augmented I(Ca) in the absence of exogenous application of ATP/Met-enk. No additional modulation of I(Ca) was obtained after bath application of exogenous ATP and opioids to these already inhibited cells. Augmentation of I(Ca) was also seen upon intracellular dialysis of guanosine 5'-[β-thio]diphosphate (GDPβS), indicating the existence in the intact slice of a tonic inhibition of I(Ca) in resting conditions. These results suggest that in the intact adrenal tissue a tonic inhibition of I(Ca) exists, mediated by purinergic and opiate receptors.

Different roles attributed to Cav1 channel subtypes in spontaneous action potential firing and fine tuning of exocytosis in mouse chromaffin cells

J. Neurochem. (2011) 116, 105–121.

Abstract

This study examines the Cav1 isoforms expressed in mouse chromaffin cells and compares their biophysical properties and roles played in cell excitability and exocytosis. Using immunocytochemical and electrophysiological techniques in mice lacking the Cav1.3α1 subunit (Cav1.3^−/−) or the high sensitivity of Cav1.2α1 subunits to dihydropyridines, Cav1.2 and Cav1.3 channels were identified as the only Cav1 channel subtypes expressed in mouse chromaffin cells. Cav1.3 channels were activated at more negative membrane potentials and inactivated more slowly than Cav1.2 channels. Cav1 channels, mainly Cav1.2, control cell excitability by functional coupling to BK channels, revealed by nifedipine blockade of BK channels in wild type (WT) and Cav1.3^−/− cells (53% and 35%, respectively), and by the identical change in the shape of the spontaneous action potentials elicited by the dihydropyridine in both strains of mice. Cav1.2 channels also play a major role in spontaneous action potential firing, supported by the following evidence: (i) a similar percentage of WT and Cav1.3^−/− cells fired spontaneous action potentials; (ii) firing frequency did not vary between WT and Cav1.3^−/− cells; (iii) mostly Cav1.2 channels contributed to the inward current preceding the action potential threshold; and (iv) in the presence of tetrodotoxin, WT or Cav1.3^−/− cells exhibited spontaneous oscillatory activity, which was fully abolished by nifedipine perfusion. Finally, Cav1.2 and Cav1.3 channels were essential for controlling the exocytotic process at potentials above and below −10 mV, respectively. Our data reveal the key yet differential roles of Cav1.2 and Cav1.3 channels in mediating action potential firing and exocytotic events in the neuroendocrine chromaffin cell.

Down-modulation of Ca2+ channels by endogenously released ATP and opioids: from the isolated chromaffin cell to the slice of adrenal medullae

Modifications in Ca(2+) influx may lead to profound changes in the cell activity associated with Ca(2+)-dependent processes, from muscle contraction and neurotransmitter release to calcium-mediated cell death. Therefore, calcium entry into the cell requires fine regulation. In this context, understanding of the modulation of voltage-dependent Ca(2+) channels seems to be critical. The modulatory process results in the enhancement or decrement of calcium influx that may regulate the local and global cytosolic Ca(2+) concentrations. Here, we summarize the well-established data on this matter described in isolated chromaffin cells by our laboratory and others, and the new results we have obtained in a more physiological preparation: freshly isolated slices of mouse adrenal medullae.

The extracellular K+ concentration dependence of outward currents through Kir2.1 channels is regulated by extracellular Na+ and Ca2+

It has been known for more than three decades that outward Kir currents (I(K1)) increase with increasing extracellular K(+) concentration ([K(+)](o)). Although this increase in I(K1) can have significant impacts under pathophysiological cardiac conditions, where [K(+)](o) can be as high as 18 mm and thus predispose the heart to re-entrant ventricular arrhythmias, the underlying mechanism has remained unclear. Here, we show that the steep [K(+)](o) dependence of Kir2.1-mediated outward I(K1) was due to [K(+)](o)-dependent inhibition of outward I(K1) by extracellular Na(+) and Ca(2+). This could be accounted for by Na(+)/Ca(2+) inhibition of I(K1) through screening of local negative surface charges. Consistent with this, extracellular Na(+) and Ca(2+) reduced the outward single-channel current and did not increase open-state noise or decrease the mean open time. In addition, neutralizing negative surface charges with a carboxylate esterifying agent inhibited outward I(K1) in a similar [K(+)](o)-dependent manner as Na(+)/Ca(2+). Site-directed mutagenesis studies identified Asp(114) and Glu(153) as the source of surface charges. Reducing K(+) activation and surface electrostatic effects in an R148Y mutant mimicked the action of extracellular Na(+) and Ca(2+), suggesting that in addition to exerting a surface electrostatic effect, Na(+) and Ca(2+) might inhibit outward I(K1) by inhibiting K(+) activation. This study identified interactions of K(+) with Na(+) and Ca(2+) that are important for the [K(+)](o) dependence of Kir2.1-mediated outward I(K1).

The influences of g proteins, ca, and k channels on electrical field stimulation in cat esophageal smooth muscle

NO released by myenteric neurons controls the off contraction induced by electrical field stimulation (EFS) in distal esophageal smooth muscle, but in the presence of nitric oxide synthase (NOS) inhibitor, L-NAME, contraction by EFS occurs at the same time. The authors investigated the intracellular signaling pathways related with G protein and ionic channel EFS-induced contraction using cat esophageal muscles. EFS-induced contractions were significantly suppressed by tetrodotoxin (1 microM) and atropine (1 microM). Furthermore, nimodipine inhibited both on and off contractions by EFS in a concentration dependent meaner. The characteristics of 'on' and 'off' contraction and the effects of G-proteins, phospholipase, and K(+) channel on EFS-induced contraction in smooth muscle were also investigated. Pertussis toxin (PTX, a G(i) inactivator) attenuated both EFS-induced contractions. Cholera toxin (CTX, G(s) inactivator) also decreased the amplitudes of EFS-induced off and on contractions. However, phospholipase inhibitors did not affect these contractions. Pinacidil (a K(+) channel opener) decreased these contractions, and tetraethylammonium (TEA, K(+) (Ca) channel blocker) increased them. These results suggest that EFS-induced on and off contractions can be mediated by the activations Gi or Gs proteins, and that L-type Ca(2+) channel may be activated by G-protein alpha subunits. Furthermore, K(+) (Ca)-channel involve in the depolarization of esophageal smooth muscle. Further studies are required to characterize the physiological regulation of Ca(2+) channel and to investigate the effects of other K(+) channels on EFS-induced on and off contractions.

L-type channel inhibition by CB1 cannabinoid receptors is mediated by PTX-sensitive G proteins and cAMP/PKA in GT1-7 hypothalamic neurons

Using immortalized hypothalamic GT1-7 neurons, which express the CB1 cannabinoid receptor (CB1R) and three Ca2+ channel types (T, R and L), we found that the CB1R agonist WIN 55,212-2 inhibited the voltage-gated Ca2+ currents by about 35%. The inhibition by WIN 55,212-2 (10 microM) was reversible and prevented by nifedipine (3 microM), suggesting a selective action on L-type Ca2+ channels (LTCCs). WIN 55,212-2 action exhibited all the features of voltage-independent Ca2+ channel modulation: (1) no changes of the activation kinetics, (2) equal depressive action at all potentials and (3) no facilitation following strong prepulses. At variance with WIN 55,212-2, the CB1R inverse agonist AM-251 (10 microM) caused 20% increase of Ca2+ currents. The inhibition of LTCCs by WIN 55,212-2 was prevented by overnight PTX-incubation and by intracellular perfusion with GDP-beta-S. The latter caused also a 20% Ca2+ current up-regulation. WIN 55,212-2 action was also prevented by application of the PKA-blocker H89 or by loading the neurons with 8-CPT-cAMP. Our results suggest that LTCCs in GT1-7 neurons are partially inhibited at rest due to a constitutive CB1R activity removed by AM-251 and GDP-beta-S. Activation of CB1R via PTX-sensitive G proteins and cAMP/PKA pathway selectively depresses LTCCs that critically control the synchronized spontaneous firing and pulsatile release of gonadotropin-releasing hormone in GT1-7 neurons.

Calcium channels in chromaffin cells: focus on L and T types

Voltage-gated Ca2+ channels (Cav) are highly expressed in the adrenal chromaffin cells of mammalian species. Besides shaping action potential waveforms, they are directly involved in the excitation-secretion coupling underlying catecholamine release and, possibly, control other Ca2+-dependent events that originate near the membrane. These functions are shared by a number of Cav channel types (L, N, P/Q, R and T) which have different structure-function characteristics and whose degree of expression changes remarkably among mammalian species. Understanding precisely the functioning of each voltage-gated Ca2+ channels is a crucial task that helps clarifying the Ca2+-dependent mechanisms controlling exocytosis during physiological and pathological conditions. In this paper, we focus on classical and new roles that L- and T-type channels play in the control of chromaffin cell excitability and neurotransmitter release. Interestingly, L-type channels are shown to be implicated in the spontaneous autorhythmicity of chromaffin cells, while T-type channels, which are absent in adult chromaffin cells, are coupled with secretion and can be recruited following long-term beta-adrenergic stimulation or chronic hypoxia. This suggests that like other cells, adrenal chromaffin cells undergo effective remodelling of membrane ion channels and cell functioning during prolonged stress conditions.

Lysophospholipids regulate excitability and exocytosis in cultured bovine chromaffin cells

Bioactive lysophospholipids (LPLs) are released by blood cells and can modulate many cellular activities such as angiogenesis and cell survival. In this study, the effects of sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) on excitability and exocytosis in bovine chromaffin cells were investigated using the whole-cell configuration of the patch-clamp. Voltage-gated Ca(2+) current was inhibited by S1P and LPA pre-treatment in a concentration-dependent manner with IC(50)s of 0.46 and 0.79 mumol/L, respectively. Inhibition was mostly reversible upon washout and prevented by suramin, an inhibitor of G-protein signaling. Na(+) current was inhibited by S1P, but not by LPA. However, recovery of Na(+) channels from inactivation was slowed by both LPLs. The outward K(+) current was also significantly reduced by both LPLs. Chromaffin cells fired repetitive action potentials in response to minimal injections of depolarizing current. Repetitive activity was dramatically reduced by LPLs. Consistent with the reduction in Ca(2+) current, exocytosis elicited by a train of depolarizations and the ensuing endocytosis were both inhibited by LPL pre-treatments. These data demonstrate the interaction between immune and endocrine systems mediated by the inhibitory effects of LPLs on the excitability of adrenal chromaffin cells.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Single-channel L-type Ca2+ currents in chicken embryo semicircular canal type I and type II hair cells

Few data are available concerning single Ca channel properties in inner ear hair cells and particularly none in vestibular type I hair cells. By using the cell-attached configuration of the patch-clamp technique in combination with the semicircular canal crista slice preparation, we determined the elementary properties of voltage-dependent Ca channels in chicken embryo type I and type II hair cells. The pipette solutions included Bay K 8644. With 70 mM Ba(2+) in the patch pipette, Ca channel activity appeared as very brief openings at -60 mV. Ca channel properties were found to be similar in type I and type II hair cells; therefore data were pooled. The mean inward current amplitude was -1.3 +/- 0.1 (SD) pA at - 30 mV (n = 16). The average slope conductance was 21 pS (n = 20). With 5 mM Ba(2+) in the patch pipette, very brief openings were already detectable at -80 mV. The mean inward current amplitude was -0.7 +/- 0.2 pA at -40 mV (n = 9). The average slope conductance was 11 pS (n = 9). The mean open time and the open probability increased significantly with depolarization. Ca channel activity was still present and unaffected when omega-agatoxin IVA (2 microM) and omega-conotoxin GVIA (3.2 microM) were added to the pipette solution. Our results show that types I and II hair cells express L-type Ca channels with similar properties. Moreover, they suggest that in vivo Ca(2+) influx might occur at membrane voltages more negative than -60 mV.

Low-threshold exocytosis induced by cAMP-recruited CaV3.2 (alpha1H) channels in rat chromaffin cells

We have studied the functional role of CaV3 channels in triggering fast exocytosis in rat chromaffin cells (RCCs). CaV3 T-type channels were selectively recruited by chronic exposures to cAMP (3 days) via an exchange protein directly activated by cAMP (Epac)-mediated pathway. Here we show that cAMP-treated cells had increased secretory responses, which could be evoked even at very low depolarizations (-50, -40 mV). Potentiation of exocytosis in cAMP-treated cells did not occur in the presence of 50 microM Ni2+, which selectively blocks T-type currents in RCCs. This suggests that the "low-threshold exocytosis" induced by cAMP is due to increased Ca2+ influx through cAMP-recruited T-type channels, rather than to an enhanced secretion downstream of Ca2+ entry, as previously reported for short-term cAMP treatments (20 min). Newly recruited T-type channels increase the fast secretory response at low voltages without altering the size of the immediately releasable pool. They also preserve the Ca2+ dependence of exocytosis, the initial speed of vesicle depletion, and the mean quantal size of single secretory events. All this indicates that cAMP-recruited CaV3 channels enhance the secretory activity of RCCs at low voltages by coupling to the secretory apparatus with a Ca2+ efficacy similar to that of already existing high-threshold Ca2+ channels. Finally, using RT-PCRs we found that the fast inactivating low-threshold Ca2+ current component recruited by cAMP is selectively associated to the alpha1H (CaV3.2) channel isoform.

Memory in astrocytes: a hypothesis

Background

Recent work has indicated an increasingly complex role for astrocytes in the central nervous system. Astrocytes are now known to exchange information with neurons at synaptic junctions and to alter the information processing capabilities of the neurons. As an extension of this trend a hypothesis was proposed that astrocytes function to store information. To explore this idea the ion channels in biological membranes were compared to models known as cellular automata. These comparisons were made to test the hypothesis that ion channels in the membranes of astrocytes form a dynamic information storage device.

Results

Two dimensional cellular automata were found to behave similarly to ion channels in a membrane when they function at the boundary between order and chaos. The length of time information is stored in this class of cellular automata is exponentially related to the number of units. Therefore the length of time biological ion channels store information was plotted versus the estimated number of ion channels in the tissue. This analysis indicates that there is an exponential relationship between memory and the number of ion channels. Extrapolation of this relationship to the estimated number of ion channels in the astrocytes of a human brain indicates that memory can be stored in this system for an entire life span. Interestingly, this information is not affixed to any physical structure, but is stored as an organization of the activity of the ion channels. Further analysis of two dimensional cellular automata also demonstrates that these systems have both associative and temporal memory capabilities.

Conclusion

It is concluded that astrocytes may serve as a dynamic information sink for neurons. The memory in the astrocytes is stored by organizing the activity of ion channels and is not associated with a physical location such as a synapse. In order for this form of memory to be of significant duration it is necessary that the ion channels in the astrocyte syncytium be electrically in contact with each other. This function may be served by astrocyte gap junctions and suggests that agents that selectively block these gap junctions should disrupt memory.

Caveolin-1 expression and membrane cholesterol content modulate N-type calcium channel activity in NG108-15 cells

Caveolins are the main structural proteins of glycolipid/cholesterol-rich plasmalemmal invaginations, termed caveolae. In addition, caveolin-1 isoform takes part in membrane remodelling as it binds and transports newly synthesized cholesterol from endoplasmic reticulum to the plasma membrane. Caveolin-1 is expressed in many cell types, including hippocampal neurons, where an abundant SNAP25-caveolin-1 complex is detected after induction of persistent synaptic potentiation. To ascertain whether caveolin-1 influences neuronal voltage-gated Ca2+ channel basal activity, we stably expressed caveolin-1 into transfected neuroblastoma x glioma NG108-15 hybrid cells [cav1(+) clone] that lack endogenous caveolins but express N-type Ca2+ channels upon cAMP-induced neuronal differentiation. Whole-cell patch-clamp recordings of cav1(+) cells demonstrated that N-type current density was reduced in size by approximately 70% without any significant change in the time course of activation and inactivation and voltage dependence. Moreover, the cav1(+) clone exhibited a significantly increased proportion of membrane cholesterol compared to wild-type NG108-15 cells. To gain insight into the mechanism underlying caveolin-1 lowering of N-current density, and more precisely to test whether this was indirectly caused by caveolin-1-induced enhancement of membrane cholesterol, we compared single N-type channel activities in cav1(+) clone and wild-type NG108-15 cells enriched with cholesterol after exposure to a methyl-beta-cyclodextrin-cholesterol complex. A lower Ca2+ channel activity was recorded from cell-attached patches of both cell types, thus supporting the view that the increased proportion of membrane cholesterol is ultimately responsible for the effect. This is due to a reduction in the probability of channel opening caused by a significant decrease of channel mean open time and by an increase of the frequency of null sweeps.

Pertussis toxin-sensitive pathway inhibits glucose-stimulated Ca2+ signals of rat islet β-cells by affecting L-type Ca2+ channels and voltage-dependent K+ channels

A role of pertussis toxin (PTX)-sensitive pathway in regulation of glucose-stimulated Ca2+ signaling in rat islet beta-cells was investigated by using clonidine as a selective agonist to alpha2-adrenoceptors which link to the pathway. An elevation of extracellular glucose concentration from 5.5 to 22.2 mM (glucose stimulation) increased the levels of [Ca2+]i of beta-cells, and clonidine reversibly reduced the elevated levels of [Ca2+]i. This clonidine effect was antagonized by yohimbine, and abolished in beta-cells pre-treated with PTX. Clonidine showed little effect on membrane currents including those through ATP-sensitive K+ channels induced by voltage ramps from -90 to -50 mV. Clonidine showed little effect on the magnitude of whole-cell currents through L-type Ca2+ channels (ICa(L)), but increased the inactivation process of the currents. Clonidine increased the magnitude of the voltage-dependent K+ currents (IVK). These clonidine effects on ICa(L) and IVK were abolished in beta-cells treated with PTX or GDP-betaS. These results suggest that the PTX-sensitive pathway increases IVK activity and decreases ICa(L) activity of islet beta-cells, resulting in a decrease in the levels of [Ca2+]i elevated by depolarization-induced Ca2+ entry. This mechanism seems responsible at least in part for well-known inhibitory action of PTX-sensitive pathway on glucose-stimulated insulin secretion from islet beta-cells.

L-type calcium channel gating is modulated by bradykinin with a PKC-dependent mechanism in NG108-15 cells

Bradykinin (BK) excites dorsal root ganglion cells, leading to the sensation of pain. The actions of BK are thought to be mediated by heterotrimeric G protein-regulated pathways. Indeed there is strong evidence that in different cell types BK is involved in phosphoinositide breakdown following activation of G(q/11). In the present study we show that the Ca(2+) current flowing through L-type voltage-gated Ca(2+) channels in NG108-15 cells (differentiated in vitro to acquire a neuronal phenotype), measured using the whole-cell patch clamp configuration, is reversibly inhibited by BK in a voltage-independent fashion, suggesting a cascade process where a second messenger system is involved. This inhibitory action of BK is mimicked by the application of 1,2-oleoyl-acetyl glycerol (OAG), an analog of diacylglycerol that activates PKC. Interestingly, OAG occluded the effects of BK and both effects were blocked by selective PKC inhibitors. The down modulation of single L-type Ca(2+) channels by BK and OAG was also investigated in cell-attached patches. Our results indicate that the inhibitory action of BK involves activation of PKC and mainly shows up in a significant reduction of the probability of channel opening, caused by an increase and clustering of null sweeps in response to BK.

Neurotransmitter modulation of neuronal calcium channels

There are many different calcium channels expressed in the mammalian nervous system, but N-type and P/Q-type calcium channels appear to dominate the presynaptic terminals of central and peripheral neurons. The neurotransmitter-induced modulation of these channels can result in alteration of synaptic transmission. This review highlights the mechanisms by which neurotransmitters affect the activity of N-type and P/Q-type calcium channels. The inhibition of these channels by voltage-dependent and voltage-independent mechanisms is emphasized because of the wealth of information available on the intracellular mediators and on the effect of these pathways on the single-channel gating.

Direct and remote modulation of L-channels in chromaffin cells: distinct actions on alpha1C and alpha1D subunits?

Understanding precisely the functioning of voltage-gated Ca2+ channels and their modulation by signaling molecules will help clarifying the Ca(2+)-dependent mechanisms controlling exocytosis in chromaffin cells. In recent years, we have learned more about the various pathways through which Ca2+ channels can be up- or down-modulated by hormones and neurotransmitters and how these changes may condition chromaffin cell activity and catecolamine release. Recently, the attention has been focused on the modulation of L-channels (CaV 1), which represent the major Ca2+ current component in rat and human chromaffin cells. L-channels are effectively inhibited by the released content of secretory granules or by applying mixtures of exogenous ATP, opioids, and adrenaline through the activation of receptor-coupled G proteins. This unusual inhibition persists in a wide range of potentials and results from a direct (membrane-delimited) interaction of G protein subunits with the L-channels co-localized in membrane microareas. Inhibition of L-channels can be reversed when the cAMP/PKA pathway is activated by membrane permeable cAMP analog or when cells are exposed to isoprenaline (remote action), suggesting the existence of parallel and opposite effects on L-channel gating by distinctly activated membrane autoreceptors. Here, the authors review the molecular components underlying these two opposing signaling pathways and present new evidence supporting the presence of two L-channel types in rat chromaffin cells (alpha1C and alpha1D), which open new interesting issues concerning Ca(2+)-channel modulation. In light of recent findings on the regulation of exocytosis by Ca(2+)-channel modulation, the authors explore the possible role of L-channels in the autocontrol of catecholamine release.

Exposure to cAMP and beta-adrenergic stimulation recruits Ca(V)3 T-type channels in rat chromaffin cells through Epac cAMP-receptor proteins

T-type channels are expressed weakly or not at all in adult rat chromaffin cells (RCCs) and there is contrasting evidence as to whether they play a functional role in catecholamine secretion. Here we show that 3-5 days after application of pCPT-cAMP, most RCCs grown in serum-free medium expressed a high density of low-voltage-activated T-type channels without altering the expression and characteristics of high-voltage-activated channels. The density of cAMP-recruited T-type channels increased with time and displayed the typical biophysical and pharmacological properties of low-voltage-activated Ca(2+) channels: (1) steep voltage-dependent activation from -50 mV in 10 mm Ca(2+), (2) slow deactivation but fast and complete inactivation, (3) full inactivation following short conditioning prepulses to -30 mV, (4) effective block of Ca(2+) influx with 50 microM Ni(2+), (5) comparable permeability to Ca(2+) and Ba(2+), and (6) insensitivity to common Ca(2+) channel antagonists. The action of exogenous pCPT-cAMP (200 microM) was prevented by the protein synthesis inhibitor anisomycin and mimicked in most cells by exposure to forskolin and 1-methyl-3-isobutylxanthine (IBMX) or isoprenaline. The protein kinase A (PKA) inhibitor H89 (0.3 microM) and the competitive antagonist of cAMP binding to PKA, Rp-cAMPS, had weak or no effect on the action of pCPT-cAMP. In line with this, the selective Epac agonist 8CPT-2Me-cAMP nicely mimicked the action of pCPT-cAMP and isoprenaline, suggesting the existence of a dominant Epac-dependent recruitment of T-type channels in RCCs that may originate from the activation of beta-adrenoceptors. Stimulation of beta-adrenoceptors occurs autocrinally in RCCs and thus, the neosynthesis of low-voltage-activated channels may represent a new form of 'chromaffin cell plasticity', which contributes, by lowering the threshold of action potential firing, to increasing cell excitability and secretory activity during sustained sympathetic stimulation and/or increased catecholamine circulation.

Distinct potentiation of L-type currents and secretion by cAMP in rat chromaffin cells

We have investigated the potentiating action of cAMP on L-currents of rat chromaffin cells and the corresponding increase of Ca(2+)-evoked secretory responses with the aim of separating the action of cAMP on Ca(2+) entry through L-channels and the downstream effects of cAMP/protein kinase A (PKA) on exocytosis. In omega-toxin-treated rat chromaffin cells, exposure to the permeable cAMP analog 8-(4-chlorophenylthio)-adenosine 3',5'-monophosphate (pCPT-cAMP; 1 mM, 30 min) caused a moderate increase of Ca(2+) charge carried through L-channels (19% in 10 mM Ca(2+) at +10 mV) and a drastic potentiation of secretion ( approximately 100%), measured as membrane capacitance increments (deltaC). The apparent Ca(2+) dependency of exocytosis increased with pCPT-cAMP and was accompanied by 83% enhancement of the readily releasable pool of vesicles with no significant change of the probability of release, as evaluated with paired-pulse stimulation protocols. pCPT-cAMP effects could be mimicked by stimulation of beta(1)-adrenoreceptors and reversed by the PKA inhibitor H89, suggesting strict PKA dependence. For short pulses to +10 mV (100 ms), potentiation of exocytosis by pCPT-cAMP was proportional to the quantity of charge entering the cell and occurred independently of whether L, N, or P/Q channels were blocked, suggesting that cAMP acts as a constant amplification factor for secretion regardless of the channel type carrying Ca(2+). Analysis of statistical variations among depolarization-induced capacitance increments indicates that pCPT-cAMP acts downstream of Ca(2+) entry by almost doubling the mean size of unitary exocytic events, most likely as a consequence of an increased granule-to-granule rather than a granule-to-membrane fusion.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Opposite action of beta1- and beta2-adrenergic receptors on Ca(V)1 L-channel current in rat adrenal chromaffin cells

Voltage-gated Ca(2+) channels of chromaffin cells are modulated by locally released neurotransmitters through autoreceptor-activated G-proteins. Clear evidence exists in favor of a Ca(2+) channel gating inhibition mediated by purinergic, opioidergic, and alpha-adrenergic autoreceptors. Few and contradictory data suggest also a role of beta-adrenergic autoreceptors (beta-ARs), the action of which, however, remains obscure. Here, using patch-perforated recordings, we show that rat chromaffin cells respond to the beta-AR agonist isoprenaline (ISO) by either upmodulating or downmodulating the amplitude of Ca(2+) currents through two distinct modulatory pathways. ISO (1 microm) could cause either fast inhibition (approximately 25%) or slow potentiation (approximately 25%), or a combination of the two actions. Both effects were completely prevented by propranolol. Slow potentiation was more evident in cells pretreated with pertussis toxin (PTX) or when beta(1)-ARs were selectively stimulated with ISO + ICI118,551. Potentiation was absent when the beta(2)-AR-selective agonist zinterol (1 microm), the protein kinase A (PKA) inhibitor H89, or nifedipine was applied, suggesting that potentiation is associated with a PKA-mediated phosphorylation of L-channels (approximately 40% L-current increase) through beta(1)-ARs. The ISO-induced inhibition was fast and reversible, preserved in cell treated with H89, and mimicked by zinterol. The action of zinterol was mostly on L-channels (38% inhibition). Zinterol action preserved the channel activation kinetics, the voltage-dependence of the I-V characteristic, and was removed by PTX, suggesting that beta(2)AR-mediated channel inhibition was mainly voltage independent and coupled to G(i)/G(o)-proteins. Sequential application of zinterol and ISO mimicked the dual action (inhibition/potentiation) of ISO alone. The two kinetically and pharmacologically distinct beta-ARs signaling uncover alternative pathways, which may serve the autocrine control of Ca(2+)-dependent exocytosis and other related functions of rat chromaffin cells.

Opposite action of beta1- and beta2-adrenergic receptors on Ca(V)1 L-channel current in rat adrenal chromaffin cells

Voltage-gated Ca(2+) channels of chromaffin cells are modulated by locally released neurotransmitters through autoreceptor-activated G-proteins. Clear evidence exists in favor of a Ca(2+) channel gating inhibition mediated by purinergic, opioidergic, and alpha-adrenergic autoreceptors. Few and contradictory data suggest also a role of beta-adrenergic autoreceptors (beta-ARs), the action of which, however, remains obscure. Here, using patch-perforated recordings, we show that rat chromaffin cells respond to the beta-AR agonist isoprenaline (ISO) by either upmodulating or downmodulating the amplitude of Ca(2+) currents through two distinct modulatory pathways. ISO (1 microm) could cause either fast inhibition (approximately 25%) or slow potentiation (approximately 25%), or a combination of the two actions. Both effects were completely prevented by propranolol. Slow potentiation was more evident in cells pretreated with pertussis toxin (PTX) or when beta(1)-ARs were selectively stimulated with ISO + ICI118,551. Potentiation was absent when the beta(2)-AR-selective agonist zinterol (1 microm), the protein kinase A (PKA) inhibitor H89, or nifedipine was applied, suggesting that potentiation is associated with a PKA-mediated phosphorylation of L-channels (approximately 40% L-current increase) through beta(1)-ARs. The ISO-induced inhibition was fast and reversible, preserved in cell treated with H89, and mimicked by zinterol. The action of zinterol was mostly on L-channels (38% inhibition). Zinterol action preserved the channel activation kinetics, the voltage-dependence of the I-V characteristic, and was removed by PTX, suggesting that beta(2)AR-mediated channel inhibition was mainly voltage independent and coupled to G(i)/G(o)-proteins. Sequential application of zinterol and ISO mimicked the dual action (inhibition/potentiation) of ISO alone. The two kinetically and pharmacologically distinct beta-ARs signaling uncover alternative pathways, which may serve the autocrine control of Ca(2+)-dependent exocytosis and other related functions of rat chromaffin cells.

Nitric oxide inhibits neuroendocrine Ca(V)1 L-channel gating via cGMP-dependent protein kinase in cell-attached patches of bovine chromaffin cells

Nitric oxide (NO) regulates the release of catecholamines from the adrenal medulla but the molecular targets of its action are not yet well identified. Here we show that the NO donor sodium nitroprusside (SNP, 200 microM) causes a marked depression of the single Ca(V)1 L-channel activity in cell-attached patches of bovine chromaffin cells. SNP action was complete within 3-5 min of cell superfusion. In multichannel patches the open probability (NP(o)) decreased by approximately 60 % between 0 and +20 mV. Averaged currents over a number of traces were proportionally reduced and showed no drastic changes to their time course. In single-channel patches the open probability (P(o)) at +10 mV decreased by the same amount as that of multichannel patches (approximately 61 %). Such a reduction was mainly associated with an increased probability of null sweeps and a prolongation of mean shut times, while first latency, mean open time and single-channel conductance were not significantly affected. Addition of the NO scavenger carboxy-PTIO or cell treatment with the guanylate cyclase inhibitor ODQ prevented the SNP-induced inhibition. 8-Bromo-cyclicGMP (8-Br-cGMP; 400 microM) mimicked the action of the NO donor and the protein kinase G blocker KT-5823 prevented this effect. The depressive action of SNP was preserved after blocking the cAMP-dependent up-regulatory pathway with the protein kinase A inhibitor H89. Similarly, the inhibitory action of 8-Br-cGMP proceeded regardless of the elevation of cAMP levels, suggesting that cGMP/PKG and cAMP/PKA act independently on L-channel gating. The inhibitory action of 8-Br-cGMP was also independent of the G protein-induced inhibition of L-channels mediated by purinergic and opiodergic autoreceptors. Since Ca(2+) channels contribute critically to both the local production of NO and catecholamine release, the NO/PKG-mediated inhibition of neuroendocrine L-channels described here may represent an important autocrine signalling mechanism for controlling the rate of neurotransmitter release from adrenal glands.