omega-Agatoxin-IVA-sensitive calcium channels in bovine chromaffin cells

Modulating Ca2+ influx into adrenal chromaffin cells with short-duration nanosecond electric pulses

Isolated bovine adrenal chromaffin cells exposed to single 2-, 4-, or 5-ns pulses undergo a rapid, transient rise in intracellular Ca2+ mediated by Ca2+ entry via voltage-gated Ca2+ channels (VGCCs), mimicking the activation of these cells in vivo by acetylcholine. However, pulse durations 150 ns or longer elicit larger amplitude and longer-lived Ca2+ responses due to Ca2+ influx via both VGCCs and a yet to be identified plasma membrane pathway(s). To further our understanding of the differential effects of ultrashort versus longer pulse durations on Ca2+ influx, chromaffin cells were loaded with calcium green-1 and exposed to single 3-, 5-, 11-, 25-, or 50-ns pulses applied at their respective Ca2+ activation threshold electric fields. Increasing pulse duration from 3 or 5 ns to only 11 ns was sufficient to elicit increased amplitude and longer-lived Ca2+ responses in the majority of cells, a trend that continued as pulse duration increased to 50 ns. The amplification of Ca2+ responses was not the result of Ca2+ release from intracellular stores and was accompanied by a decreased effectiveness of VGCC inhibitors to block the responses and a reduced reliance on extracellular Na+ and membrane depolarization to evoke the responses. Inhibitors of pannexin channels, P2X receptors, or non-selective cation channels failed to attenuate 50-ns-elicited Ca2+ responses, ruling out these Ca2+-permeable channels as secondary Ca2+ entry pathways. Analytical calculations and numerical modeling suggest that the parameter that best determines the response of chromaffin cells to increasing pulse durations is the time the membrane charges to its peak voltage. These results highlight the pronounced sensitivity of a neuroendocrine cell to pulse durations differing by only tens of nanoseconds, which has important implications for the future development of nanosecond pulse technologies enabling electrostimulation applications for spatially focused and graded in vivo neuromodulation.

Regulation by L channels of Ca(2+)-evoked secretory responses in ouabain-treated chromaffin cells

It is known that the sustained depolarisation of adrenal medullary bovine chromaffin cells (BCCs) with high K(+) concentrations produces an initial sharp catecholamine release that subsequently fades off in spite depolarisation persists. Here, we have recreated a sustained depolarisation condition of BCCs by treating them with the Na(+)/K(+) ATPase blocker ouabain; in doing so, we searched experimental conditions that permitted the development of a sustained long-term catecholamine release response that could be relevant during prolonged stress. BCCs were perifused with nominal 0Ca(2+) solution, and secretion responses were elicited by intermittent application of short 2Ca(2+) pulses (Krebs-HEPES containing 2 mM Ca(2+)). These pulses elicited a biphasic secretory pattern with an initial 30-min period with secretory responses of increasing amplitude and a second 30-min period with steady-state, non-inactivating responses. The initial phase was not due to gradual depolarisation neither to gradual increases of the cytosolic calcium transients ([Ca(2+)]c) elicited by 2Ca(2+) pulses in BBCs exposed to ouabain; both parameters increased soon after ouabain addition. Νifedipine blocked these responses, and FPL64176 potentiated them, suggesting that they were triggered by Ca(2+) entry through non-inactivating L-type calcium channels. This was corroborated by nifedipine-evoked blockade of the L-type Ca(2+) channel current and the [Ca(2+)]c transients elicited by 2Ca(2+) pulses. Furthermore, the plasmalemmal Na(+)/Ca(2+) exchanger (NCX) blocker SEA0400 caused a mild inhibition followed by a large rebound increase of the steady-state secretory responses. We conclude that these two phases of secretion are mostly contributed by Ca(2+) entry through L calcium channels, with a minor contribution of Ca(2+) entry through the reverse mode of the NCX.

T-type channel-mediated neurotransmitter release

Besides controlling a wide variety of cell functions, T-type channels have been shown to regulate neurotransmitter release in peripheral and central synapses and neuroendocrine cells. Growing evidence over the last 10 years suggests a key role of Cav3.2 and Cav3.1 channels in controlling basal neurosecretion near resting conditions and sustained release during mild stimulations. In some cases, the contribution of low-voltage-activated (LVA) channels is not directly evident but requires either the activation of coupled presynaptic receptors, block of ion channels, or chelation of metal ions. Concerning the coupling to the secretory machinery, T-type channels appear loosely coupled to neurotransmitter and hormone release. In neurons, Cav3.2 and Cav3.1 channels mainly control the asynchronous appearance of "minis" [miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory postsynaptic currents (mEPSCs)]. The same loose coupling is evident from membrane capacity and amperometric recordings in chromaffin cells and melanotropes where the low-threshold-driven exocytosis possesses the same linear Ca(2+) dependence of the other voltage-gated Ca(2+) channels (Cav1 and Cav2) that is strongly attenuated by slow calcium buffers. The intriguing issue is that, despite not expressing a consensus "synprint" site, Cav3.2 channels do interact with syntaxin 1A and SNAP-25 and, thus, may form nanodomains with secretory vesicles that can be regulated at low voltages. In this review, we discuss all the past and recent issues related to T-type channel-secretion coupling in neurons and neuroendocrine cells.

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.

Calcium entry through slow-inactivating L-type calcium channels preferentially triggers endocytosis rather than exocytosis in bovine chromaffin cells

Calcium (Ca(2+))-dependent endocytosis has been linked to preferential Ca(2+) entry through the L-type (α(1D), Ca(V)1.3) of voltage-dependent Ca(2+) channels (VDCCs). Considering that the Ca(2+)-dependent exocytotic release of neurotransmitters is mostly triggered by Ca(2+) entry through N-(α(1B), Ca(V)2.2) or PQ-VDCCs (α(1A), Ca(V)2.1) and that exocytosis and endocytosis are coupled, the supposition that the different channel subtypes are specialized to control different cell functions is attractive. Here we have explored this hypothesis in primary cultures of bovine adrenal chromaffin cells where PQ channels account for 50% of Ca(2+) current (I(Ca)), 30% for N channels, and 20% for L channels. We used patch-clamp and fluorescence techniques to measure the exo-endocytotic responses triggered by long depolarizing stimuli, in 1, 2, or 10 mM concentrations of extracellular Ca(2+) ([Ca(2+)](e)). Exo-endocytotic responses were little affected by ω-conotoxin GVIA (N channel blocker), whereas ω-agatoxin IVA (PQ channel blocker) caused 80% blockade of exocytosis as well as endocytosis. In contrast, nifedipine (L channel blocker) only caused 20% inhibition of exocytosis but as much as 90% inhibition of endocytosis. Conversely, FPL67146 (an activator of L VDCCs) notably augmented endocytosis. Photoreleased caged Ca(2+) caused substantially smaller endocytotic responses compared with those produced by K(+) depolarization. Using fluorescence antibodies, no colocalization between L, N, or PQ channels with clathrin was found; a 20-30% colocalization was found between dynamin and all three channel antibodies. This is incompatible with the view that L channels are coupled to the endocytotic machine. Data rather support a mechanism implying the different inactivation rates of L (slow-inactivating) and N/PQ channels (fast-inactivating). Thus a slow but more sustained Ca(2+) entry through L channels could be a requirement to trigger endocytosis efficiently, at least in bovine chromaffin cells.

Regulation of glutamatergic and GABAergic neurotransmission in the chick nucleus laminaris: role of N-type calcium channels

Neurons in the chicken nucleus laminaris (NL), the third order auditory nucleus involved in azimuth sound localization, receive bilaterally segregated (ipsilateral vs contralateral) glutamatergic excitation from the cochlear nucleus magnocellularis and GABAergic inhibition from the ipsilateral superior olivary nucleus (SON). Here, I investigate the voltage-gated calcium channels (VGCCs) that trigger the excitatory and the inhibitory transmission in the NL. Whole-cell recordings were performed in acute brainstem slices. The excitatory transmission was predominantly mediated by N-type VGCCs, as the specific N-type blocker omega-Conotoxin-GVIA (omega-CTx-GVIA, 1-2.5 microM) inhibited excitatory postsynaptic currents (EPSCs) by approximately 90%. Blockers for P/Q- and L-type VGCCs produced no inhibition, and blockade of R-type VGCCs produced a small inhibition. In individual cells, the effect of each VGCC blocker on the EPSC elicited by activation of the ipsilateral input was the same as that on the EPSC elicited by activation of the contralateral input, and the two EPSCs had similar kinetics, suggesting physiological symmetry between the two glutamatergic inputs to single NL neurons. The inhibitory transmission in NL neurons was almost exclusively mediated by N-type VGCCs, as omega-CTx-GVIA (1 microM) produced a approximately 90% reduction of inhibitory postsynaptic currents, whereas blockers for other VGCCs produced no inhibition. In conclusion, N-type VGCCs play a dominant role in triggering both the excitatory and the inhibitory transmission in the NL, and the presynaptic VGCCs that mediate the two bilaterally segregated glutamatergic inputs to individual NL neurons are identical. These features may play a role in optimizing coincidence detection in NL neurons.

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.

Pharmacological and biophysical properties of Ca2+ channels and subtype distributions in human adrenal chromaffin cells

In this study, we explored the pharmacological and biophysical properties of voltage-activated Ca2+ channels in human chromaffin cells using the perforated-patch configuration of the patch-clamp technique. According to their pharmacological sensitivity to Ca2+ channel blockers, cells could be sorted into two groups of similar size showing the predominance of either N- or P/Q-type Ca2+ channels. R-type Ca2+ channels, blocked by 77% with 20 muM Cd2+ and not affected by 50 muM Ni2+, were detected for the first time in human chromaffin cells. Immunocytochemical experiments revealed an even distribution of alpha (1E) Ca2+ channels in these cells. With regard to their biophysical properties, L- and R-type channels were activated at membrane potentials that were 15-20 mV more negative than P/Q- and N-type channels. Activation time constants showed no variation with voltage for the L-type channels, decreased with increasing potentials for the R- and P/Q-type channels, and displayed a bell shape with a maximum at 0 mV for the N-type channels. R-type channels were also the most inactivated channels. We thus show here that human chromaffin cells possess all the Ca2+ channel types described in neurons, L, N, P/Q, and R channels, but the relative contributions of N and P/Q channels differ among cells. Given that N- and P/Q-type Ca2+ channel types can be differentially modulated, these findings suggest the possibility of cell-specific regulation in human chromaffin cells.

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.

Calcium channel subtypes differentially regulate fusion pore stability and expansion

Various studies have focused in the relative contribution of different voltage-activated Ca(2+) channels (VACC) to total transmitter release. However, how Ca(2+) entry through a given VACC subtype defines the pattern of individual exocytotic events remains unknown. To address this question, we have used amperometry in bovine chromaffin cells. L, N, and P/Q channels were individually or jointly blocked with furnidipine, omega-conotoxin GVIA, omega-agatoxin IVA, or omega-conotoxin MVIIC. The three channel types contributed similarly to cytosolic Ca(2+) signals induced by 70 mmol/L K(+). However, they exhibited different contributions to the frequency of exocytotic events and they were shown to differently regulate the final steps of the exocytosis. When compared with the other VACC subtypes, Ca(2+) entry through P/Q channels effectively induced exocytosis, it decreased fusion pore stability and accelerated its expansion. Conversely, Ca(2+) entry through N channels was less efficient in inducing exocytotic events, also slowing fusion pore expansion. Finally, Ca(2+) entry through L channels inefficiently induced exocytosis, and the individual blockade of this channel significantly modified fusion pore dynamics. The distance between a given VACC subtype and the release sites could account for the differential effects of the distinct VACC on the fusion pore dynamics.

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.

Glycerotoxin stimulates neurotransmitter release from N-type Ca2+ channel expressing neurons

Glycerotoxin (GLTx) is capable of stimulating neurotransmitter release at the frog neuromuscular junction by directly interacting with N-type Ca2+ (Cav2.2) channels. Here we have utilized GLTx as a tool to investigate the functionality of Cav2.2 channels in various mammalian neuronal preparations. We first adapted a fluorescent-based high-throughput assay to monitor glutamate release from rat cortical synaptosomes. GLTx potently stimulates glutamate secretion and Ca2+ influx in synaptosomes with an EC50 of 50 pm. Both these effects were prevented using selective Cav2.2 channel blockers suggesting the functional involvement of Cav2.2 channels in mediating glutamate release in this system. We further show that both Cav2.1 (P/Q-type) and Cav2.2 channels contribute equally to depolarization-induced glutamate release. We then investigated the functionality of Cav2.2 channels at the neonatal rat neuromuscular junction. GLTx enhances both spontaneous and evoked neurotransmitter release causing a significant increase in the frequency of postsynaptic action potentials. These effects were blocked by specific Cav2.2 channel blockers demonstrating that either GLTx or its derivatives could be used to selectively enhance the neurotransmitter release from Cav2.2-expressing mammalian neurons.

Bovine CACNA1A gene and comparative analysis of the CAG repeats associated to human spinocerebellar ataxia type-6

A small expansion of a CAG repeat domain in exon 47 of the human CACNA1A gene, which codes for the pore-forming alpha1A subunit of P/Q-type Ca2+ channels, causes spinocerebellar ataxia type-6. Only the human alpha1A protein has been demonstrated to contain the poly(Q) tract, although this locus has also recently been detected in ape genomes. To our knowledge, no further information has been published on other mammal species. Here, we have cloned the full-length alpha1A subunit in a non-primate species, the cow. The results have made it possible to explore the exon organization of the bovine CACNA1A gene as well as the splice alpha1A isoforms expressed by bovine chromaffin cells. We found a splice variant of the protein that, as in humans, also contains a polymorphic poly(Q) tract. Based on this result and using data from different Genome Databases, we performed an interspecies comparison of exon 47 and discovered that the poly(Q) tract is present in all the species studied, with the exception of primitive fish and rodents. Our results provide insight into the evolution of the CAG repeat tract at the C-terminus coding region of the CACNA1A gene.

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.

Preclinical profile of PF9404C, a nitric oxide donor with beta receptor blocking properties

PF9404C ((2'S),(2S)-3-isopropylamine, 1-[4-(2,3-dinitroxy)propoxymethyl]-phenoxy-2'-propranol) is the S-S diesteroisomer of a novel blocker of beta-adrenergic receptors with vasorelaxing properties. It causes a concentration-dependent relaxation of rat aorta helical strips precontracted with 10(-6) M norepinephrine (NE; IC50 33 nM). It is equipotent to nitroglycerin (NTG; IC50 49 nM), but much more potent than isosorbide dinitrate (ISD; IC50 15,000 nM). In rat aorta smooth muscle cells, at 10 microM, PF9404C increased the formation of cGMP from 3 pmol/mg protein in basal conditions to 53 pmol/mg protein, suggesting that the mechanism of its vasorelaxing effects involves the slow generation of NO. This is supported by the facts that (i) ODQ (a blocker of guanylate cyclase) inhibited the vasodilatory effects of PF9404C; and (ii) PF9404C generates NO, as indirectly measured by the Griess reaction. In the electrically driven guinea pig left atrium, PF9404C blocks the inotropic effects of isoproterenol in a concentration-dependent manner. Its IC50 (30 nM) was similar to that of S-propranolol (22.4 nM) and lower than that of metoprolol (120 nM) or atenolol (192 nM). The beta adrenergic ligand (-)-[3H]-CGP12177 (4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one hydrochloride) (0.2 nM) is displaced from its binding sites in rat brain membranes with a K(i) of 7, 17, 170, and 1200 nM for PF9404C, S-(-)propranolol, metoprolol, and atenolol, respectively. PF9404C blocks 45Ca2+ entry into bovine adrenal chromaffin cells induced by direct depolarization with 70 mM K+ or by the nicotinic agonist dimethylphenylpiperazinium (DMPP). PF9404C exhibits about 3-fold higher potency than NTG to relax the majority of the vessels studied, especially when they were contracted with K+, and shows a certain selectivity of action for the renal artery. It produces auto-tolerance that is ca. 20-fold less pronounced than that observed with NTG. Cross-tolerance in preparations pre-exposed to PF9404C and later relaxed with NTG, was much greater than auto-tolerance. This makes PF9404C a useful pharmacological tool for the development of novel NO-donor compounds with a lesser degree of vascular tolerance than those currently available.

Linopirdine Modulates Calcium Signaling and Stimulus-Secretion Coupling in Adrenal Chromaffin Cells by Targeting M-Type K+Channels and Nicotinic Acetylcholine Receptors

Adrenal chromaffin cells synthesize and release catecholamines and several other transmitters that play important physiological roles in the coordinated response to stress or danger. The main trigger for secretion is acetylcholine (ACh) released from splanchnic nerve terminals that activates nicotinic ACh receptors (nAChRs) on the chromaffin cells, causing membrane depolarization and Ca2+ entry primarily through voltage-gated Ca2+ channels (Ca-channels). G protein-coupled receptors (GPCRs) can also trigger secretion, and it has been suggested that closure of M-type K+ channels might contribute to this process. However, GPCRs have multiple effects on calcium signaling and secretion, including release of intracellular Ca2+ stores, activation of second messenger pathways and kinases, and Ca2+ entry through store/receptor-operated channels. Hence, the effects of M-channel closure on [Ca2+]i signaling and transmitter release remain unclear. We have investigated the effects of linopirdine, a relatively selective blocker of M-channels, on stimulus-secretion coupling in chromaffin cells. Linopirdine produced a small increase in [Ca2+]i in approximately 63% of cells because of influx through Ca-channels. However, this was not sufficient to promote catecholamine release. We also show that linopirdine reduced cholinergic-stimulated increases in [Ca2+]i and secretion, primarily through potent block of nAChRs and a subtle effect on Ca2+ entry via Ca-channels. Hence, our data support the hypothesis that M-channels help control the excitability of chromaffin cells, but additional pathways need to be recruited by GPCRs to trigger catecholamine release. Furthermore, linopirdine potently targets nAChRs to modulate stimulus-secretion coupling in adrenal chromaffin cells.

Dynamic association of the Ca2+channel α1Asubunit and SNAP-25 in round or neurite-emitting chromaffin cells

Although the specific interaction between synaptic protein SNAP-25 and the alpha1A subunit of the Cav2.1 channels, which conduct P/Q-type Ca2+ currents, has been confirmed in in vitro-translated proteins and brain membrane studies, the question of how native proteins can establish this association in situ in developing neurons remains to be elucidated. Here we report data regarding this interaction in bovine chromaffin cells natively expressing both proteins. The two carboxyl-terminal splice variants of the alpha1A subunit identified in these cells share a synaptic protein interaction ('synprint') site within the II/III loop segment and are immunodetected by a specific antibody against bovine alpha1A protein. Moreover, both alpha1A isoforms form part of the P/Q-channels-SNARE complexes in situ because they are coimmunoprecipitated from solubilized chromaffin cell membranes by a monoclonal SNAP-25 antibody. The distribution of alpha1A and SNAP-25 was studied in round or transdifferentiated chromaffin cells using confocal microscopy and specific antibodies: the two proteins are colocalized at the cell body membrane in both natural cell types. However, during the first stages of the cell transdifferentiation process, SNAP-25 migrates alone out to the developing growth cone and what will become the nerve endings and varicosities of the mature neurites; alpha1A follows and colocalizes to SNAP-25 in the now mature processes. These observations lead us to propose that the association between SNAP-25 and alpha1A during neuritogenesis might promote not only the efficient coupling of the exocytotic machinery but also the correct insertion of P/Q-type channels at specialized active zones in presynaptic neuronal terminals.

Voltage-activated Ca(2+) channels and their role in the endocrine function of the pituitary gland in newborn and adult mice

We have prepared fresh pituitary gland slices from adult and, for the first time, from newborn mice to assess modulation of secretory activity via voltage-activated Ca(2+) channels (VACCs). Currents through VACCs and membrane capacitance have been measured with the whole-cell patch-clamp technique. Melanotrophs in newborns were significantly larger than in adults. In both newborn and adult melanotrophs activation of VACCs triggered exocytosis. All pharmacologically isolated VACC types contributed equally to the secretory activity. However, the relative proportion of VACCs differed between newborns and adults. In newborn cells L-type channels dominated and, in addition, an exclusive expression of a toxin-resistant R-type-like current was found. The expression of L-type VACCs was up-regulated by the increased oestrogen levels observed in females, and was even more emphasized in the cells of pregnant females and oestrogen-treated adult male mice. We suggest a general mechanism modulating endocrine secretion in the presence of oestrogen and particularly higher sensitivity to treatments with L-type channel blockers during high oestrogen physiological states.

Adrenal medulla calcium channel population is not conserved in bovine chromaffin cells in culture

During the stress response adrenal medullary chromaffin cells release catecholamines to the bloodstream. Voltage-activated calcium channels present in the cell membrane play a crucial role in this process. Although the electrophysiological and pharmacological properties of chromaffin cell calcium channels have been studied in detail, the molecular composition of these channels has not been defined yet. Another aspect that needs to be explored is the extent to which chromaffin cells in culture reflect the adrenal medulla calcium channel characteristics. In this sense, it has been described that catecholamine release in the intact adrenal gland recruits different calcium channels than those recruited during secretion from cultured chromaffin cells. Additionally, recent electrophysiological studies show that chromaffin cells in culture differ from those located in the intact adrenal medulla in the contribution of several calcium channel types to the whole cell current. However there is not yet any study that compares the population of calcium channels in chromaffin cells with that one present in the adrenal medulla. In order to gain some insight into the roles that calcium channels might play in the adrenal medullary cells we have analyzed the alpha1 subunit mRNA expression profile. We demonstrate that the expression pattern of voltage-dependent calcium channels in cultured bovine chromaffin cells markedly differs from that found in the native adrenal medulla and that glucocorticoids are only partially involved in those differences. Additionally, we show, for the first time, that the cardiac isoform of L-type calcium channel is present in both bovine adrenal medulla and cultured chromaffin cells and that its levels of expression do not vary during culture.

Depolarization-induced ERK phosphorylation depends on the cytosolic Ca2+ level rather than on the Ca2+ channel subtype of chromaffin cells

The contribution of Ca2+ entry through different voltage-activated Ca2+ channel (VACC) subtypes to the phosphorylation of extracellular signal regulated kinase (ERK) was examined in bovine adrenal-medullary chromaffin cells. High K+ depolarization (40 mM, 3 min) induced ERK phosphorylation, an effect that was inhibited by specific mitogen-activated protein kinase kinase inhibitors. By using selective inhibitors, we observed that depolarization-induced ERK phosphorylation completely depended on protein kinase C-alpha (PKC-alpha), but not on Ca2+/calmodulin-dependent protein kinase nor cyclic AMP-dependent protein kinase. Blockade of L-type Ca2+ channels by 3 microm furnidipine, or blockade of N channels by 1 micromomega-conotoxin GVIA reduced ERK phosphorylation by 70%, while the inhibition of P/Q channels by 1 micromomega-agatoxin IVA only caused a 40% reduction. The simultaneous blockade of L and N, or P/Q and N channels completely abolished this response, yet 23% ERK phosphorylation remained when L and P/Q channels were simultaneously blocked. Confocal imaging of cytosolic Ca2+ elevations elicited by 40 mm K+, showed that Ca2+ levels increased throughout the entire cytosol, both in the presence and the absence of Ca2+ channel blockers. Fifty-eight percent of the fluorescence rise depended on Ca2+ entering through N channels. Thus, ERK phosphorylation seems to depend on a critical level of Ca2+ in the cytosol rather than on activation of a given Ca2+ channel subtype.

Calcium entry, calcium redistribution, and exocytosis

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca(2+) concentration, [Ca(2+)](c), depend on at least three efficient regulatory mechanisms: (1) the plasmalemmal Ca(2+) channels; (2) the endoplasmic reticulum (ER); and (3) the mitochondria. High-voltage activated Ca(2+) channels of the L, N, P/Q, and R subtypes are expressed with different densities in various mammalian species; they are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca(2+)](c). Targeted aequorin and confocal microscopy show that Ca(2+) entry through Ca(2+) channels can refill the ER to near millimolar concentrations and causes the release of ER Ca(2+) (CICR). We have also seen that, depending on its degree of filling, the ER may act as a sink or source of Ca(2+) that modulates the release of catecholamine. Targeted aequorins with different Ca(2+) affinities show that mitochondria undergo surprisingly rapid millimolar Ca(2+) transients ([Ca(2+)](M)) upon stimulation of chromaffin cells with ACh, high K(+), or caffeine. Physiological stimuli generate [Ca(2+)](c) microdomains at these functional complexes in which the local subplasmalemmal [Ca(2+)](c) rises abruptly from 0.1 micro M to about 50 micro M. This triggers CICR, mitochondrial Ca(2+) uptake, and exocytosis in nearby secretory active sites. That this is true is shown by the observation that protonophores abolish mitochondrial Ca(2+) uptake and drastically increase catecholamine release by 3- to 5-fold. This increase is likely due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; such transport might be controlled by Ca(2+) redistribution to the cytoskeleton, through CICR and/or mitochondrial Ca(2+) release.

A perforated patch-clamp study of calcium currents and exocytosis in chromaffin cells of wild-type and alpha(1A) knockout mice

Simultaneous recordings of inward whole-cell Ca(2+) channel currents (I(Ca) ) and increments of capacitance as an indication of exocytosis (Delta(Cm)), were performed in voltage-clamped single adrenal chromaffin cells from wild-type and alpha(1A) subunit deficient mice, using the perforated-patch configuration of the patch-clamp technique. Using protocol #1 (one single Ca(2+) channel blocker per cell), to dissect the components of I(Ca), L channels contributed 43%, N channels 35% and P/Q channels 30% to the total I(Ca) of wild-type cells. Using protocol #2 (cumulative sequential addition of 3 microm nifedipine, 1 microm omega-conotoxin GVIA, and 1 microm omega-agatoxin IVA), L, N and P/Q channels contributed 40%, 34% and 14%, respectively, to I(Ca); an R component of around 11% remained. In wild-type mice the changes of Delta(Cm) paralleled those of I(Ca). In alpha(1A) deficient mice the L component of I(Ca) rose to 53% while the P/Q disappeared; the N and R components were similar. In these mice, Delta(Cm) associated to N and R channels did not vary; however, the P/Q component was abolished while the L component increased by 20%. In conclusion, exocytosis was proportional to the relative density of each Ca(2+) channel subtype, L, N, P/Q, R. Ablation of the alpha(1A) gene led to a loss of P/Q channel current and to a compensatory increase of L channel-associated secretion; however, this compensation was not sufficient to maintain the overall exocytotic response, that was diminished by 35% in alpha(1A) -deficient mice. This may be due to altered Ca(2+) homeostasis in these mice, as compared to wild mouse chromaffin cells.

Voltage-independent inhibition of P/Q-type Ca2+ channels in adrenal chromaffin cells via a neuronal Ca2+ sensor-1-dependent pathway involves Src family tyrosine kinase

In common with many neurons, adrenal chromaffin cells possess distinct voltage-dependent and voltage-independent pathways for Ca(2+) channel regulation. In this study, the voltage-independent pathway was revealed by addition of naloxone and suramin to remove tonic blockade of Ca(2+) currents via opioid and purinergic receptors due to autocrine feedback inhibition. This pathway requires the Ca(2+)-binding protein neuronal calcium sensor-1 (NCS-1). The voltage-dependent pathway was pertussis toxin-sensitive, whereas the voltage-independent pathway was largely pertussis toxin-insensitive. Characterization of the voltage-independent inhibition of Ca(2+) currents revealed that it did not involve protein kinase C-dependent signaling pathways but did require the activity of a Src family tyrosine kinase. Two structurally distinct Src kinase inhibitors, 4-amino-5-(4-methylphenyl)7-(t-butyl)pyrazolo[3,4-d] pyrimidine (PP1) and a Src inhibitory peptide, increased the Ca(2+) currents, and no further increase in Ca(2+) currents was elicited by addition of naloxone and suramin. In addition, the Src-like kinase appeared to act in the same pathway as NCS-1. In contrast, addition of PP1 did not prevent a voltage-dependent facilitation elicited by a strong pre-pulse depolarization indicating that this pathway was independent of Src kinase activity. PPI no longer increased Ca(2+) currents after addition of the P/Q-type channel blocker omega-agatoxin TK. The alpha(1A) subunit of P/Q-type Ca(2+) channels was immunoprecipitated from chromaffin cell extracts and found to be phosphorylated in a PP1-sensitive manner by endogenous kinases in the immunoprecipitate. A high molecular mass (around 220 kDa) form of the alpha(1A) subunit was detected by anti-phosphotyrosine, suggesting a possible target for Src family kinase action. These data demonstrate a voltage-independent mechanism for autocrine inhibition of P/Q-type Ca(2+) channel currents in chromaffin cells that requires Src family kinase activity and suggests that this may be a widely distributed pathway for Ca(2+) channel regulation.

Differential expression of calcium channel subtypes in the bovine adrenal medulla

This study aimed at determining the distribution and expression levels of different subtypes of Ca(2+) channels in the bovine adrenal medulla, and whether individual subtypes were more abundant in chromaffin cells exhibiting an adrenergic or a noradrenergic phenotype. In situ hybridization using riboprobes specific for the pore-forming Ca(2+) channel alpha(1D) (L-type channel), alpha(1B) (N-type channel), and alpha(1A) (P/Q-type channel) subunits of bovine chromaffin cells showed a broad distribution of the three transcripts in adrenal medulla tissue. However, a tissue-specific expression pattern of individual subunits was found; whereas alpha(1B) mRNA was homogeneously distributed throughout the medulla, alpha(1D) and alpha(1A) transcripts were present at higher densities in the internal medullary area, far away from the adrenal cortex. These results were corroborated by comparative analysis of the alpha(1B), alpha(1D), and alpha(1A) products amplified by RT-PCR from total RNA extracted from small pieces of tissue dissected out from external or internal medullary areas. Interestingly, immunohistochemical experiments performed in adrenal gland sections, using antidopamine-beta-hydroxylase and anti-phenylethanolamine-N-methyltransferase antibodies, indicated a higher density of noradrenergic over adrenergic chromaffin cells in the internal medullary region. These results provide direct evidence in favor of a heterogeneous distribution of Ca(2+) channel subtypes in the adrenal medulla, in agreement with previous functional data showing that blockade of the high K+ -elicited responses by dihydropyridines was greater in noradrenergic than in adrenergic chromaffin cells. These differences may be relevant for the differential release regulation of each catecholamine under physiological and pathophysiological conditions.

R-Type Ca2+ channels are coupled to the rapid component of secretion in mouse adrenal slice chromaffin cells

Patch-clamp measurements of Ca(2+) currents and membrane capacitance were performed on slices of mouse adrenal glands, using the perforated-patch configuration of the patch-clamp technique. These recording conditions are much closer to the in vivo situation than those used so far in most electrophysiological studies in adrenal chromaffin cells (isolated cells maintained in culture and whole-cell configuration). We observed profound discrepancies in the quantities of Ca(2+) channel subtypes (P-, Q-, N-, and L-type Ca(2+) channels) described for isolated mouse chromaffin cells maintained in culture. Differences with respect to previous studies may be attributable not only to culture conditions, but also to the patch-clamp configuration used. Our experiments revealed the presence of a Ca(2+) channel subtype never before described in chromaffin cells, a toxin and dihydropyridine-resistant Ca(2+) channel with fast inactivation kinetics, similar to the R-type Ca(2+) channel described in neurons. This channel contributes 22% to the total Ca(2+) current and controls 55% of the rapid secretory response evoked by short depolarizing pulses. Our results indicate that R-type Ca(2+) channels are in close proximity with the exocytotic machinery to rapidly regulate the secretory process.

Neuronal Ca2+ sensor-1/frequenin functions in an autocrine pathway regulating Ca2+ channels in bovine adrenal chromaffin cells

NCS-1/frequenin belongs to a family of EF-hand-containing Ca(2+) sensors expressed mainly in neurons. Overexpression of NCS-1/frequenin has been shown to stimulate neurotransmitter release but little else is known of its cellular roles. We have constructed an EF-hand mutant, NCS-1(E120Q), as a likely dominant inhibitor of cellular NCS-1 function. Recombinant NCS-1(E120Q) showed an impaired Ca(2+)-dependent conformational change but could still bind to cellular proteins. Transient expression of this mutant, but not NCS-1, in bovine adrenal chromaffin cells increased non-L-type Ca(2+) channel currents. Cells expressing NCS-1(E120Q) no longer responded effectively to the removal of autocrine purinergic/opioid inhibition of Ca(2+) currents but still showed voltage-dependent facilitation. These data are consistent with the existence of both voltage-dependent and voltage-independent pathways for Ca(2+) channel inhibition in chromaffin cells. Our results suggest a novel function for NCS-1 specific for the voltage-independent autocrine pathway that negatively regulates non-L-type Ca(2+) channels in chromaffin cells.

Greater diversity than previously thought of chromaffin cell Ca2+ channels, derived from mRNA identification studies

Using reverse transcription followed by PCR amplification (RT-PCR), we have identified multiple messenger RNAs encoding for the neuronal pore-forming Ca(2+) channel subunits alpha(1A) (P/Q channel), alpha(1B) (N channel), alpha(1D) (neuronal/endocrine L channel), alpha(1E) (R channel), alpha(1G-H) (T channel) and alpha(1S) (skeletal muscle L channel) in bovine chromaffin cells. mRNAs for the auxiliary beta(2), beta(3), beta(4), alpha(2)/delta and gamma(2) subunits were also identified. In agreement with these molecular data, perforated patch-clamp recordings of whole-cell Ca(2+) currents reveal the existence of functional R-type Ca(2+) channels in these cells that were previously undetected with other techniques. Our results provide a molecular frame for a much wider functional diversity of Ca(2+) channels in chromaffin cells than that previously established using pharmacological and electrophysiological approaches.

Mechanism for increase in expression of cerebral diazepam binding inhibitor mRNA by nicotine: involvement of L-type voltage-dependent calcium channels

We investigated the mechanisms underlying the increase in diazepam binding inhibitor (DBI) and its mRNA expression induced by nicotine (0.1 microM) exposure for 24 h using mouse cerebral cortical neurons in primary culture. Nicotine-induced (0.1 microM) increases in DBI mRNA expression were abolished by hexamethonium, a nicotinic acetylcholine (nACh) receptor antagonist. Agents that stabilize the neuronal membrane, including tetrodotoxin (TTX), procainamide (a Na(+) channel inhibitor), and local anesthetics (dibucaine and lidocaine), dose-dependently inhibited the increased expression of DBI mRNA by nicotine. The nicotine-induced increase in DBI mRNA expression was inhibited by L-type voltage-dependent Ca(2+) channel (VDCC) inhibitors such as verapamil, calmodulin antagonist (W-7), and Ca(2+)/calmodulin-dependent protein kinase II (CAM II kinase) inhibitor (KN-62), whereas P/Q- and N-type VDCC inhibitors showed no effects. In addition, nicotine exposure for 24 h induced [3H]nicotine binding to the particulate fractions of the neurons with an increased B(max) value and no changes in K(d). Under these conditions, the 30 mM KCl- and nicotine-induced 45Ca(2+) influx into the nicotine-treated neurons was significantly higher than those into non-treated neurons. These results suggest that the nicotine-stimulated increase in DBI mRNA expression is mediated by CAM II kinase activation resulting from the increase in intracellular Ca(2+) through L-type VDCCs subsequent to the neuronal membrane depolarization associated with nACh receptor activation.

Ba(2+)-induced chromaffin cell death: cytoprotection by Ca(2+) channel antagonists

Exposure of bovine adrenal medullary chromaffin cells to Ba(2+) ions (in the absence of Ca(2+) ions) caused their death, measured as lactate dehydrogenase (LDH) release. The concentration of Ba(2+) required to damage the cells by about 65% ranged between 1 and 10 mM (no Ca(2+) added); the required exposure time was rather brief (15 min-4 h). The simultaneous presence of Ca(2+), Mg(2+) or Zn(2+) together with Ba(2+) (2 mM, 4 h) afforded cyprotection (60-80%). Individual selective blockers of Ca(2+) channel subtypes afforded no protection. However, combined nifedipine (3 microM) plus omega-conotoxin MVIIC (3 microM) offered full protection. Substantial protection was also seen with the "wide-spectrum" Ca(2+) channel blockers penfluridol (0.3 microM), lubeluzole (3 microM), dotarizine (3 microM), flunarizine (3 microM), and mibefradil (3 microM). This protection was due to blockade of Ba(2+) entry through Ca(2+) channels because dotarizine (10 microM) inhibited the increase in cytosolic [Ba(2+)] seen in fura-2-loaded chromaffin cells. Once Ba(2+) accumulated in the cytosol, it was not extruded by the Na(+)/Ca(2+) exchanger, as shown by the prolonged and sustained elevation of the fura-2 signal. This contrasts with the fast dissipation of the fura-2 signal generated by [Ca(2+)](i) elevation. Thus, Ba(2+) overload can cause cell death by mechanisms similar to those reported for Ca(2+) overload and might be used as a novel and convenient tool to search for new cytoprotective compounds.

Coexpression of cloned alpha(1B), beta(2a), and alpha(2)/delta subunits produces non-inactivating calcium currents similar to those found in bovine chromaffin cells

Chromaffin cells express N-type calcium channels identified on the basis of their high sensitivity to block by omega-conotoxin GVIA (omega-CgTx GVIA). In contrast to neuronal N-type calcium currents that inactivate during long depolarizations and that require negative holding potentials to remove inactivation, many chromaffin cells exhibit N-type calcium channel currents that show little inactivation during maintained depolarizations and that exhibit no decrease in channel availability at depolarized holding potentials. N-type calcium channels are thought to be produced by combination of the pore-forming alpha(1B) subunit and accessory beta and alpha(2)/delta subunits. To examine the molecular composition of the non-inactivating N-type calcium channel, we cloned the alpha(1B) and accessory beta (beta(1b), beta(1c,) beta(2a), beta(2b), and beta(3a)) subunits found in bovine chromaffin cells. Expression of the subunits in either Xenopus oocytes or human embryonic kidney 293 cells produced high-threshold calcium currents that were blocked by omega-CgTx GVIA. Coexpression of bovine alpha(1B) with beta(1b), beta(1c), beta(2b), or beta(3a) produced currents that were holding potential dependent. In contrast, coexpression of bovine alpha(1B) with beta(2a) produced holding potential-independent calcium currents that closely mimicked native non-inactivating currents, suggesting that non-inactivating N-type channels consist of bovine alpha(1B), alpha(2)/delta, and beta(2a).

Regulation of Ca2+ influx by a protein kinase C activator in chromaffin cells: differential role of P/Q- and L-type Ca2+ channels

Phorbol esters reduce depolarization-evoked Ca2+ influx in adrenal chromaffin cells, suggesting that voltage-sensitive Ca2+ channels (VSCCs) are inhibited by protein kinase C-mediated phosphorylation. We now address the possibility that L- and P/Q-type Ca2+ channel subtypes might be differentially involved in phorbol ester action. In bovine chromaffin cells, short-term (10 min) incubations with phorbol 12-myristate 13-acetate (PMA) inhibited early high K+-evoked rises in cytosolic free Ca2+ concentration ([Ca2+]i) and the early component of the depolarization-evoked Mn2+ quenching of fura-2 fluorescence in a dose-dependent manner (IC50: 18 and 7 nM; maximal inhibitions: 45 and 48%, respectively). The protein kinase C inhibitor staurosporine (100 nM) reverted the inhibitory action of PMA. PMA (0.1-1 microM) inhibited the early and late phases of the ionomycin (2 microM)-evoked [Ca2+]i transients by 14-23%. Omega-agatoxin IVA, a blocker of P/Q-type Ca2+ channels, inhibited high K+-evoked [Ca2+]i rises in a dose-dependent fashion (IC50 = 50 nM). In contrast, 0.1 microM omega-conotoxin GVIA, a blocker of N-type channels, was without effect. A sizeable (< 45%) component of early Ca2+ influx persisted in the combined presence of omega-agatoxin IVA (100 nM) and nitrendipine (1 microM). Simultaneous exposure to omega-agatoxin IVA and PMA inhibited both the early [Ca2+]i transients and Mn2+ quenching to a much greater extent than each drug separately. Inhibition of the [Ca2+]i transients by nitrendipine and PMA did not significantly exceed that produced by PMA alone. It is concluded that phorbol ester-mediated activation of protein kinase C inhibits preferentially L-type VSCCs over P/Q type channels in adrenal chromaffin cells. However, the possibility cannot be ruled out that dihydropyridine-resistant, non-P/Q type channels might also be negatively regulated by protein kinase C. This may represent an important pathway for the specific control of VSCCs by protein kinase C-linked receptors, not only in paraneurones but presumably also in neurones and other excitable cells.

Effects of the neuroprotectant lubeluzole on the cytotoxic actions of veratridine, barium, ouabain and 6-hydroxydopamine in chromaffin cells

1. Incubation of bovine adrenal chromaffin cells with veratridine (10-100 microM) during 24 h, caused a concentration-dependent release of the cytosolic lactate dehydrogenase (LDH) into the bathing medium, an indicator of cell death. Lubeluzole or its R(-) enantiomer, R91154, did not enhance LDH release. Both lubeluzole and R91154 (0.3-10 microM) decreased the veratridine-induced LDH release. 2. Penfluridol did not increase LDH release at concentrations 0.003-1 microM; 3-10 microM increased LDH release to 50-60%, after 24 h exposure. Penfluridol (0.03-0.3 microM) did not protect against the cytotoxic effects of veratridine; at 1 microM, 15% protection was produced. Higher concentrations (3-10 microM) enhanced the cytotoxic effects of veratridine. 3. Ba2+ ions caused a concentration-dependent increase of LDH release. This cytotoxic effect was partially prevented by 3 microM lubeluzole and fully counteracted by 1 microM penfluridol. R91154 was less potent than lubeluzole and only protected against the lesion induced by 0.5 mM Ba2+. 4. Ouabain (10 microM during 24 h) increased LDH release to about 30%. Both lubeluzole (0.3-10 microM) and the lower concentrations of penfluridol (0.003-0.3 microM) prevented the ouabain cytotoxic effects. At higher concentrations (3 microM), penfluridol increased drastically the ouabain cytotoxic effects. 5. 6-Hydroxydopamine (6-OHDA) caused significant cytotoxic effects at 30 and 100 microM. Lubeluzole (3-10 microM) or penfluridol (0.03-0.3 microM) had no cytoprotective effects against 6-OHDA. 6. Lubeluzole (3 microM), R91154 (3 microM) and penfluridol (1 microM) blocked the current through Na+ channels in voltage-clamped chromaffin cells (I(Na)) by around 20-30%. Ca2+ current through Ca2+ channels (I(Ca)) was inhibited 57% by lubeluzole and R91154 and 50% by penfluridol. The effects of penfluridol were not washed out, but those of lubeluzole and R91154 were readily reversible. 7. Lubeluzole (3 microM) induced reversible blockade of the oscillations of the cytosolic Ca2+, [Ca2+]i, in fura-2-loaded cells exposed to 30 or 100 microM veratridine. Penfluridol (1 microM) inhibited those oscillations in an irreversible manner. 8. The results suggest that lubeluzole and its R-isomer caused cytoprotection against veratridine cell damage, by blocking the veratridine stimulated Na+ and Ca2+ entry, as well as the [Ca2+]i oscillations. The Ba2+ and ouabain cytotoxic effects were prevented more efficiently by penfluridol, likely by blocking the plasmalemmal Na+/Ca2+ exchanger. It remains dubious whether these findings are relevant to the reported neuroprotective action of lubeluzole in stroke; the doubt rests in the stereoselective protecting effects of lubeluzole in in vivo stroke models, as opposed to its lack of stereoselectivity in the in vitro model reported here.

Antagonists-resistant calcium currents in rat embryo motoneurons

Ca2+ channels diversity of cultured rat embryo motoneurons was investigated with whole-cell current recordings. In 5-20 mM Ba2+, the whole-cell currents were separated in low- (LVA) and high-voltage-activated (HVA) current. The LVA current was evident since the first day in culture, while the HVA component was small and increased with time. Recordings after 4 days revealed approximately 20% L-, approximately 45% N- and approximately 35% P- and R-type currents. P-type currents were revealed only in 40% of motoneurons, in which 20-200 nM omega-Aga-IVA caused 20% irreversible block of total current. The remaining 60% of cells were insensitive even to higher doses of the toxin (500 nM in 5 mM Ba2+), suggesting weak expression and heterogeneous distribution of P-type channels compensated by high densities of HVA Ca2+ channels resistant to all the antagonists (R-type). A significant residual current could also be resolved after prolonged applications of 5 microM omega-CTx-MVIIC, which allowed separation of N- and P-type currents by the distinct onset of toxin block. The antagonists-resistant current reveals biophysical characteristics typical of HVA channels, but distinct from the alphaE channel. The current activates around -20 mV in 20 mM Ba2+; inactivates slowly and independently of Ca2+; is blocked by low [Cd2+] and high [Ni2+]; and is larger with Ba2+ than Ca2+. The uncovered R-type calcium current can account for part of the presynaptic Ca2+ current controlling neurotransmitter release at the mammalian neuromuscular junction whose activity is resistant to DHP-and omega-CTx-GVIA, and displays anomalous sensitivity to omega-Aga-IVA and omega-CTx-MVIIC.

Differential effects of the neuroprotectant lubeluzole on bovine and mouse chromaffin cell calcium channel subtypes

1. The effects of lubeluzole (a neuroprotective benzothiazole derivative) and its (-) enantiomer R91154 on whole-cell currents through Ca2+ channels, with 10 mM Ba2+ as charge carrier (IBa), have been studied in bovine and mouse voltage-clamped adrenal chromaffin cells. Currents generated by applying 50 ms depolarizing test pulses to 0 mV, from a holding potential of -80 mV, at 10 s intervals had an average magnitude of 1 nA. 2. Lubeluzole and R91154 blocked the peak IBa of bovine chromaffin cells in a time and concentration-dependent manner; their IC50s were 1.94 microM for lubeluzole and 2.54 microM for R91154. In a current-voltage protocol, lubeluzole (3 microM) inhibited peak IBa at all test potentials. However, no obvious shifts of the I-V curve were detected. 3. After 10 min exposure to 3 microM lubeluzole, the late current (measured at the end of the pulse) was inhibited more than the peak current. Upon wash out of the drug, the inactivation reversed first and then the peak current recovered. 4. Blockade of peak current was greater at more depolarizing holding potentials (i.e. 35% at -110 mV and 87% at -50 mV, after 10 min superfusion with lubeluzole). Inactivation of the current was pronounced at -110 mV, decreased at -80 mV and did not occur at -50 mV. 5. Intracellular dialysis of bovine voltage-clamped chromaffin cells with 3 microM lubeluzole caused neither blockade nor inactivation of IBa. The external application of 3 microM lubeluzole to those dialysed cells produced inhibition as well as inactivation of IBa. 6. The effects of lubeluzole (3 microM) on IBa in mouse chromaffin cells were similar to those in bovine chromaffin cells. At -80 mV holding potential, a pronounced inactivation of the current led to greater blockade of the late IBa (66%) as compared with peak IBa (46% after 10 min superfusion with lubeluzole). 7. In mouse chromaffin cells approximately half of the whole-cell IBa was sensitive to 3 microM nifedipine (L-type Ca2+ channels) and the other half to 3 microM omega-conotoxin MVIIC (non-L-type Ca2+ channels). In omega-conotoxin MVIIC-treated cells, 3 microM lubeluzole caused little blockade and inactivation of IBa. However in nifedipine-treated cells, lubeluzole caused a pronounced blockade and inactivation of IBa that reversed upon wash out of the compound. 8. The results are compatible with the idea that lubeluzole preferentially blocks non-L-types of voltage-dependent Ca2+ channels expressed by bovine and mouse chromaffin cells. The higher concentrations of the compound also block L-type Ca2+ channels. The mechanism of inhibition involves the access of lubeluzole to the open channel from the outside of the cell and promotion of its inactivation. The differential blockade of Ca2+ channel subtypes might contribute to the neuroprotective actions of lubeluzole (which exhibit stereoselectivity). However, in view of the lack of stereoselectivity in blocking Ca2+ channels, this effect cannot be the only explanation for the protective activity of lubeluzole in stroke.

Effects of N- and L-type calcium channel antagonists and (+/-)-Bay K8644 on nerve-induced catecholamine secretion from bovine perfused adrenal glands

1. The effects of N- and L-type calcium channel antagonists and (+/-)-Bay K8644 on catecholamine release from chromaffin cells and acetylcholine release from splanchnic nerve terminals was investigated in bovine perfused adrenal glands. 2. Adrenal glands were perfused retrogradely and preloaded with [3H]-choline. Subsequent efflux of 3H-labelled compounds was taken as an index of acetylcholine release from the splanchnic nerve terminals. Noradrenaline and adrenaline release from the glands was measured by h.p.l.c. with electrochemical detection. 3. A maximally effective frequency of field stimulation of the adrenal nerves, 10 Hz, induced release of catecholamines and 3H-labelled compounds. Tetrodotoxin (1 microM) abolished release of both catecholamines and 3H-labelled compounds. A combination of mecamylamine (5 microM) and atropine (1 microM) inhibited nerve-induced catecholamine release by about 75% but did not inhibit release of 3H-labelled compounds. Reducing the concentration of extracellular calcium 5 fold to 0.5 mM inhibited nerve-induced catecholamine release by 80% and release of 3H-labelled compounds by 50%. 4. (+/-)-Bay K8644 (1 microM), nitrendipine (1 microM), omega-conotoxin-GVIA (10 nM) and the combination of nitrendipine and omega-conotoxin-GVIA each had no effect on nerve-induced release of 3H-labelled compounds. 5. (+/-)-Bay K8644 (1 microM) potentiated nerve-induced catecholamine release by 75%. Nitrendipine (1 microM) reduced release by 20% but this did not reach statistical significance, omega-Conotoxin-GVIA (10 nM) reduced nerve-induced catecholamine release by 75%, while the combination of omega-conotoxin-GVIA and nitrendipine reduced release to the same extent as omega-conotoxin-GVIA alone. 6. Exogenous acetylcholine perfusion through the glands produced a concentration-dependent increase in catecholamine release. The maximally effective concentration of acetylcholine for catecholamine release was > or = 300 microM, while 30 microM acetylcholine gave comparable catecholamine release to that obtained with 10 Hz field stimulation. 7. (+/-)-Bay K8644 (1 microM), nitrendipine (1 microM) and omega-conotoxin-GVIA (10 nM) each had no significant effect on catecholamine release evoked by perfusion of the gland with either a near maximally effective concentration of acetylcholine, 100 microM, or with the lower concentration of 30 microM. 8. The results show that the omega-conotoxin-GVIA-sensitive N-type voltage-sensitive calcium channels located on the chromaffin cells are largely responsible for catecholamine release induced by nerve stimulation in bovine adrenal glands. In contrast, N-type calcium channels are not involved in catecholamine release induced by exogenous acetylcholine. L-type voltage sensitive calcium channels do not play a major role in nerve-induced or exogenously applied acetylcholine-induced catecholamine release. However, the L-type calcium channels do have the potential to augment powerfully nerve-induced catecholamine release. N- and L-type calcium channels do not play a major role in the presynaptic release of acetylcholine.

Distinct effects of omega-toxins and various groups of Ca(2+)-entry inhibitors on nicotinic acetylcholine receptor and Ca2+ channels of chromaffin cells

The effects of omega-toxins and various Ca2+ antagonist subtypes on the 45Ca2+ entry into bovine adrenal medullary chromaffin cells stimulated via nicotinic acetylcholine receptors or via direct depolarization with K+, have been compared. The conditions selected to stimulate the 45Ca2+ entry consisted of a 60-s period of exposure of cells to 100 microM of the nicotinic acetylcholine receptor agonist dimethylphenylpiperazinium or to 70 mM K+. The N-type voltage-dependent Ca2+ channel blockers omega-conotoxin GVIA and MVIIA (1 microM) inhibited 45Ca2+ entry stimulated by dimethylphenylpiperazinium or K+ by around 25-30%. The P-type Ca2+ channel blocker omega-agatoxin IVA (10 nM) did not affect the dimethylphenylpiperazinium nor the K+ responses; 1 microM (Q-channel blockade) inhibited both responses by around 50%. The N/P/Q-type Ca2+ channel blocker omega-contoxin MVIIC (1 microM) inhibited the K+ evoked 45Ca2+ entry by 70%, while dimethylphenylpiperazinium was blocked by 50% (P < 0.001). The L-type Ca2+ channel blockers nifedipine, furnidipine, diltiazem or verapamil (3 microM each) inhibited much more the dimethylphenylpiperazinium than the K+ response. The dimethylphenylpiperazinium signal was blocked 71, 88, 89, and 53%, respectively, by nifedipine, furnidipine, diltiazem and verapamil, and the K+ response by 38, 29, 22, and 10%. Combined omega-conotoxin MVIIC (1 microM) and furnidipine (3 microM) blocked 100% of the K+ evoked 45Ca2+ entry. However, combined omega-conotoxin GVIA (1 microM), and furnidipine left unblocked 50% of the K+ response. The "wide spectrum' Ca2+ channel antagonists flunarizine or dotarizine (3 microM each) blocked the dimethylphenylpiperazinium and the K+ responses to a similar extent (50%); cinnarizine (3 microM) inhibited more the dimethylphenylpiperazinium (82%) than the K+ response (21%). At 3 microM, the highly lipophilic beta-adrenoceptor antagonist (+/-)-propranolol, reduced by 68% the dimethylphenylpiperazinium signal and by 23% the K+ signal. Other high lipophilic beta-adrenoceptor antagonists such as metoprolol and labetalol, reduced little the dimethylphenylpiperazinium and the K+ responses. The highly lipophilic agent penfluridol blocked the dimethylpiperazinium response by 30% and the K+ response by 50%. One of the least lipophilic compounds tested, (+)-lubeluzole, blocked by 40% the dimethylphenylpiperazinium and the K+ responses. These data are compatible with the idea that the various omega-toxin peptides used to separate pharmacologically the different voltage-dependent Ca2+ channels expressed by neurones, do not block the neuronal nicotinic acetylcholine receptor ion channel. In contrast the L-type Ca2+ channel blockers do block the nicotinic acetylcholine receptor ionophore. Lipophilicity of the compounds is not a requirement for Ca2+ channel or nicotinic acetylcholine receptor blockade.

Re-evaluation of the P/Q Ca2+ channel components of Ba2+ currents in bovine chromaffin cells superfused with solutions containing low and high Ba2+ concentrations

This study was undertaken to reassess the set of voltage-dependent Ca2+ channel subtypes expressed by bovine adrenal chromaffin cells maintained in primary cultures. Previous views on the pharmacology of such channels had to be revised in the light of the novel data which arose from the use in this study of low and high micromolar concentrations of omega-agatoxin IVA, and low (2 mM) and high (10 mM) concentrations of the charge carrier Ba2+. Whole-cell Ba2+ currents (IBa) through Ca2+ channels were elicited in voltage-clamped chromaffin cells, with a holding potential of -80 mV and depolarising pulses to 0 mV. Mean peak IBa was 425 pA in 2 mM Ba2+ (59 cells) and 787 pA in 10 mM Ba2+ (42 cells). In 2 mM Ba2+, omega-conotoxin MVIIC (3 microM) inhibited IBa by 79%; in 10 mM Ba2+, the blockade developed much more slowly and reached only 44%. A low concentration of omega-agatoxin IVA (20 nM) inhibited IBa by 9%; 2 microM inhibited IBa by 60%. This blockade was similar in low and high Ba2+ concentrations. After giving furnidipine (3 microM) and omega-conotoxin GVIA (1 microM), 2 microM omega-agatoxin IVA inhibited the remaining current (about 40-45%); this blockade was independent of the Ba2+ concentration. The current could be fully blocked by the cocktail furnidipine/omega-conotoxin GVIA/high omega-agatoxin IVA, both in low and high Ba2+ concentrations. The large Q-type channel component of IBa is blocked by micromolar concentrations of omega-agatoxin IVA and omega-conotoxin MVIIC. While solutions with a high Ba2+ concentration strongly delayed the development of blockade by omega-conotoxin MVIIC, the blockade by high concentrations of omega-agatoxin IVA was equally effective in solutions with a low or a high Ba2+ concentration. Hence, the use of appropriate Ba2+ and toxin concentrations in this study reveals that P-type Ca2+ channels are poorly expressed in bovine chromaffin cells; in contrast, a robust component of the current depends on Q-type Ca2+ channels. An R-type residual current is not present in these cells.

The mechanism of calcium channel facilitation in bovine chromaffin cells

1. This study was planned to clarify the mechanism of Ca2+ channel facilitation by depolarizing prepulses given to voltage-clamped bovine chromaffin cells. The hypothesis for an autocrine modulation of such channels was tested by studying the effects of a soluble vesicle lysate (SVL) on whole-cell Ba2+ currents (IBa). 2. SVL was prepared from a bovine adrenal medullary homogenate. The ATP content in this concentrated SVL amounted to 3.18 +/- 0.12 mM (n = 4). The concentration of noradrenaline and adrenaline present in the SVL was 11.2 +/- 0.97 and 15.2 +/- 2 mM, respectively (n = 5). A 1:1000 dilution of SVL in the external solution halved the magnitude of IBa and produced a 7-fold slowing of its activation kinetics. The blocking effects of SVL were concentration dependent and quickly reversed upon washout. 3. Inhibition and slowing of the kinetics of IBa by SVL could be partially reversed by strong depolarizing prepulses (+90 mV, 45 ms). This reversal of inhibition, called Ca2+ channel facilitation, persisted in the presence of 3 microM nifedipine. 4. Intracellular dialysis of GDP-beta-S (0.5 mM) or pretreatment of the cells with pertussis toxin (100 ng ml-1 for 18-24 h) prevented the reduction in peak current caused by a 1:100 dilution of SVL; no prepulse facilitation could be observed under these conditions. 5. The receptor blockers naloxone (10 microM) or suramin (100 microM) and PPADS (100 microM) largely antagonized the effects of SVL. Treatment of SVL with alkaline phosphatase or dialysis against a saline buffer to remove low molecular mass materials (< 10 kDa) considerably reduced the activity of SVL. 6. Stopping the flow of the external solution (10 mM Ba2+) gradually reduced the size, and slowed down the activation phase, of the current. Prepulse facilitation of IBa was absent or weak in a superfused cell, but was massive upon flow-stop conditions in the presence or absence of 3 microM nifedipine. 7. Our experiments suggest that facilitation by prepulses of whole-cell current through Ca2+ channels is due to the suppression of an autoinhibitory autocrine loop present in bovine chromaffin cells. By acting at least on purinergic and opiate receptors, the exocytotic release of ATP and opiates will cause a tonic inhibition of the current through a G-protein-mediated mechanism. Such a mechanism will be removed by strong depolarizing prepulses, and will involve preferentially non-L-type channels. In the light of these and other recent results, previously held views on the selective recruitment by prepulses of dihydropyridine-sensitive Ca2+ channels are not tenable.

Inhibition of nicotinic receptor-mediated responses in bovine chromaffin cells by diltiazem

1. The effects of diltiazem on various functional parameters were studied in bovine cultured adrenal chromaffin cells stimulated with the nicotinic receptor agonist dimethylphenylpiperazinium (DMPP) or with depolarizing Krebs-HEPES solutions containing high K+ concentrations. 2. The release of [3H]-noradrenaline induced by DMPP (100 microM for 5 min) was gradually and fully inhibited by increasing concentrations of diltiazem (IC50 = 1.3 microM). In contrast, the highest concentration of diltiazem used (10 microM) inhibited the response to high K+ (59 mM for 5 min) by only 25%. 3. 45Ca2+ uptake into cells stimulated with DMPP (100 microM for 1 min) was also blocked by diltiazem in a concentration-dependent manner (IC50 = 0.4 microM). Again, diltiazem blocked the K(+)-evoked 45Ca2+ uptake (70 mM K+ for 1 min) only by 20%. In contrast, the N-P-Q-type Ca2+ channel blocker omega-conotoxin MVIIC depressed the K+ signal by 70%. In the presence of this toxin, diltiazem exhibited an additional small inhibitory effect, indicating that the compound was acting on L-type Ca2+ channels. 4. Whole-cell Ba2+ currents through Ca2+ channels in voltage-clamped chromaffin cells were inhibited by 3-10 microM diltiazem by 20-25%. The inhibition was readily reversed upon washout of the drug. 5. The whole-cell currents elicited by 100 microM DMPP (IDMPP) were inhibited in a concentration-dependent and reversible manner by diltiazem. Maximal effects were found at 10 microM, which reduced the peak IDMPP by 70%. The area of each curve represented by total current (QDMPP) was reduced more than the peak current. At 10 microM, the inhibition amounted to 80%; the IC50 for QDMPP inhibition was 0.73 microM, a figure close to the IC50 for 45Ca2+ uptake (0.4 microM) and [3H]-noradrenaline release (1.3 microM). The blocking effects of diltiazem developed very quickly and did not exhibit use-dependence; thus the drug blocked the channel in its closed state. The blocking effects of 1 microM diltiazem on IDMPP were similar at different holding potentials (inhibition by around 30% at -100, -80 or -50 mV). Diltiazem did not affect the current flow through voltage-dependent Na+ channels. 6. These data are compatible with the idea that diltiazem has little effect on Ca2+ entry through voltage-dependent Ca2+ channels in bovine chromaffin cells. Neither, does diltiazem affect INa. Rather, diltiazem acts directly on the neuronal nicotinic receptor ion channel and blocks ion fluxes, cell depolarization and the subsequent Ca2+ entry and catecholamine release. This novel effect of diltiazem might have clinical relevance since it might reduce the sympathoadrenal drive to the heart and blood vessels, thus contributing to the well established antihypertensive and cardioprotective effects of the drug.

Effects of omega-conotoxin MVIIC on veratridine-induced cytotoxicity and cytosolic Ca(2+) oscillations

External Ca(2+) entry through various Ca(2+)-channel subtypes is responsible for the large oscillations of the cytosolic Ca(2+) concentrations, [Ca(2+)](i), and cell death induced by veratridine in primary cultures of bovine chromaffin cells. Blockade by omega-conotoxin GVIA (GVIA) of N-type Ca(2+) channels, by omega-agatoxin IVA (IVA) of P-type Ca(2+) channels, or by furnidipine of L-type Ca(2+) channels did not afford cytoprotection. However, (omega-conotoxin MVIIC (MVIIC), a wide-spectrum blocker of N-, P- and Q-type Ca(2+) channels greatly protected the cells against the cytotoxic effects of veratridine. Furnidipine further enhanced the cytoprotecting effects of MVIIC. MVIIC but not furuidipine, markedly reduced the oscillations of [Ca(2+)](i) induced by veratridine in single fura-2-loaded chromaffin cells. The results suggest that Ca(2+) entry through any of the different Ca(2+) channel subtypes present in bovine chromaffin cells might be cytotoxic. They also support two ideas: (i) that wide-spectrum neuronal Ca(2+) channel blockers (i.e. MVIIC) might be better cytoprotecting agents than more specific neuronal Ca(2+) channel blockers (i.e., GVIA, IVA, furnidipine); and (ii) that combined Ca(2+) channel blockers may provide greater cytoprotection than single compounds.

Blocking effects of otilonium on Ca2+ channels and secretion in rat chromaffin cells

We describe here the effects of otilonium bromide (an anticholinergic agent widely used as an intestinal spasmolytic) on whole-cell currents through Ca2+ channels (IBa) and catecholamine secretion in rat adrenal glands and isolated rat chromaffin cells. Otilonium blocked the peak IBa current in voltage-clamped chromaffin cells in a concentration-dependent manner; the IC50 to block IBa was 4.7 microM. Blockade was not accompanied by a significant shift in the I-V relationship for IBa, suggesting that such blockade was not affecting a specific subtype of Ca2+ channel. When given intracellularly through the patch pipette, otilonium (10 microM) did not block IBa. However, its external application to the same cell (10 microM) reversibly reduced IBa by 70%. Otilonium caused a concentration-dependent blockade of catecholamine release from perfused rat adrenal glands intermittently stimulated with methacholine, high K+ or histamine. The IC50 to block secretion after a 5 min incubation with otilonium was 0.02, 0.7 and 3 microM, respectively, for methacholine, K+ and histamine. The blocking effects of otilonium were fully reversible at concentrations below 10 microM. The Ca2+ channel agonist Bay K 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine-5- carboxylate) partially antagonized the effects of otilonium on K(+)-evoked secretion and accelerated the time course of recovery from inhibition. The results are compatible with the idea that otilonium blocks Ca2+ entry into chromaffin cells by blocking voltage-dependent Ca2+ channels. This would lead to a limitation in the rise in cytosolic Ca2+ at secretory sites and to inhibition of catecholamine release in response to stimulation of chromaffin cells.

Effects of tyramine and calcium on the kinetics of secretion in intact and electroporated chromaffin cells superfused at high speed

Fast superfusion of electroporated bovine adrenal chromaffin cells with a K+ glutamate-based solution containing 50 nM free Ca2+ and 2 mM adenosine 5'-triphosphate, dipotassium salt (K2ATP), produced a steady-state low catecholamine secretion, measured on-line with an electrochemical detector (about 20 nA). Rapid switching to electroporation solutions containing increasing Ca2+ concentrations ([Ca2+]) produced a rapid increase in the rate and peak secretion, followed by a decline. At intermediate [Ca2+] (3-100 microM), a fast peak and a slow secretory plateau were distinguished. The fast secretory peak identifies a readily releasable catecholamine pool consisting of about 200-400 vesicles per cell. Pretreatment of cells with tyramine (10 microM for 4 min before electroporation) supressed the initial fast secretory peak, leaving intact the slower phase of secretion. With [Ca2+] in the range of 0.1-3 microM, the activation rate of secretion increased from 2.3 to 35.3 nA.s-1, reached a plateau between 3-30 microM and rose again from 100 to 1000 microM [Ca2+] to a maximum of 91.9 nA.s-1. In contrast, total secretion first increased (0.1-1 microM Ca2+), then plateaud (1-100 microM Ca2+) and subsequently decreased (100-1000 microM Ca2+). At 30 and 1000 microM extracellular [Ca2+] or [Ca2+]o, the activation rates of secretion from intact cells depolarised with 70 mM K+ were close to those obtained in electroporated cells. However, secretion peaks were much lower in intact (93 nA at 30 microM Ca2+) than in electroporated cells (385 nA). On the other hand, inactivation of secretion was much faster in intact than in electroporated cells; as a consequence, total secretion in a 5-min period was considerably smaller in intact (10.6 microA.s at 1000 microM Ca2+) than in electroporated cells (42.4 microA.s at 1 microM Ca2+). Separation of the time-courses of changes in intracellular [Ca2+] or [Ca2+]i and secretion in intact chromaffin cells depolarised with 70 mM K+ was demonstrated at different [Ca2+]o. The increase in the rate of catecholamine release was substantially higher than the increase of the average [Ca2+]i. In contrast, the decline of secretion was faster than the decline of the peak [Ca2+]i. The results are compatible with the idea that the peak and the amount of catecholamine released from depolarised intact cells is determined essentially by plasmalemmal factors, rather than by vesicle supply from reserve pools. These plasmalemmal factors limit the supply of Ca2+ by the rates of opening and closing of voltage-dependent Ca2+ channels of the L- and Q-subtypes, which control the local [Ca2+]i near to exocytotic sites.

Catecholamine secretion, calcium levels and calcium influx in response to membrane depolarization in bovine chromaffin cells

In this paper, we show that exposure of chromaffin cells to high K+ (75 mM) for 5 min releases about 15% of total norepinephrine and 8% of total epinephrine contained in chromaffin cells. The measured resting membrane potential of these cells was -55 mV. Long (10 s) depolarizing electrical pulses applied from a holding potential of -55 mV to 5 mV, that would produce a depolarization similar to exposure to high K+ (75 mM), induced an inward Ca2+ current that inactivated with a time constant of about 0.8 s and promoted the influx of about 1 fmol of Ca2+ into the cell. Both high K+ and electrically-induced depolarization increased intracellular Ca2+ levels to a similar value (about 350 nM). Extrapolation would indicate that total Ca2+ influx in high K+ (75 mM)-stimulated 10(6) chromaffin cells would amount to 1 nmol which would promote the secretion of about 4.9 nmol of norepinephrine and 3.5 nmol of epinephrine from 10(6) chromaffin cells. The results indicate that Ca2+ influx in response to depolarization is short-lived, likely due to Ca(2+)-dependent inactivation of voltage-dependent Ca2+ channels. However, intracellular Ca2+ levels remain high as long as depolarization is present and long after Ca2+ influx has ceased. This would suggest that some processes related to either Ca2+ buffering or extrusion from the cell may be voltage dependent.

Pharmacological protection against the cytotoxicity induced by 6-hydroxydopamine and H2O2 in chromaffin cells

We present in this report the characteristics of the damage induced by 6-hydroxydopamine and H2O2 on bovine chromaffin cells in primary culture. Cytotoxicity was quantified using catecholamine cell contents, lactate dehydrogenase (LDH) release, trypan blue exclusion and morphological appearance. An excellent correlation between these four parameters was found. The cytotoxic effects of 6-hydroxydopamine were Ca(2+)-independent. In spite of this, the Ca2+ channel antagonists R56865 (N-[1-(4-(fluorophenoxy)butyl)]-4-piperidinyl-N-methyl-2-benzo-thiazo lamine) lidoflazine exhibited marked cytoprotective effects against both 6-hydroxydopamine and H2O2. The selective dopamine uptake blocker, bupropion, increased the viability of 6-hydroxydopamine and H2O2-treated cells from 20% to around 80%. Catalase drastically protected against the cytotoxic effects of 6-hydroxydopamine and H2O2. In contrast, desferrioxamine gave better protection against H2O2 cytotoxicity; glutathione and N-acetylcysteine only afforded substantial protection against 6-hydroxydopamine. Three main conclusions emerge from this study. (1st) 6-Hydroxydopamine causes chromaffin cell damage via a mechanism probably related to the production of free radicals, but unrelated to Ca2+ ions. Cytoprotection afforded by R56865 and lidoflazine must be unrelated to their Ca2+ antagonist properties. This suggests a novel component in the cytoprotective mechanism of action of these drugs. (2nd) The strong cytoprotective effects of bupropion seem to be unrelated to its ability to block the plasmalemmal dopamine carrier. (3rd) Bovine adrenal chromaffin cells in primary cultures are a suitable model for adult neurons to study the basic mechanism of cell damage, and to screen new drugs with putative neuroprotective properties.

omega-Conotoxin GVIA blocks nicotine-induced catecholamine secretion by blocking the nicotinic receptor-activated inward currents in bovine chromaffin cells

We have studied the contribution of N-type voltage-dependent Ca2+ channels to both norepinephrine and epinephrine secretion from bovine chromaffin cells induced by high K+ or nicotine using omega-conotoxin GVIA, a selective blocker of N-type voltage-dependent Ca2+ channels. We found that high K+ (75 mM) induced catecholamine secretion was not affected by exposure of bovine chromaffin cells to omega-conotoxin GVIA (1 microM). However, nicotine-induced both norepinephrine and epinephrine secretion were similarly blocked (about 25%) by omega-conotoxin GVIA (1 microM). This effect could be explained by a potent (about 80%) and reversible blockade of the inward current induced by nicotine receptor activation in bovine chromaffin cells. The results indicate that besides the blockade of N-type voltage-dependent channels, omega-conotoxin GVIA is a potent and reversible blocker of the nicotinic receptor-induced currents in chromaffin cells.