A dihydropyridine-resistant component in the rat adrenal secretory response to splanchnic nerve stimulation

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.

Distinguishing splanchnic nerve and chromaffin cell stimulation in mouse adrenal slices with fast-scan cyclic voltammetry

Electrical stimulation is an indispensible tool in studying electrically excitable tissues in neurobiology and neuroendocrinology. In this work, the consequences of high-intensity electrical stimulation on the release of catecholamines from adrenal gland slices were examined with fast-scan cyclic voltammetry at carbon fiber microelectrodes. A biphasic signal, consisting of a fast and slow phase, was observed when electrical stimulations typically used in tissue slices (10 Hz, 350 μA biphasic, 2.0 ms/phase pulse width) were applied to bipolar tungsten-stimulating electrodes. This signal was found to be stimulation dependent, and the slow phase of the signal was abolished when smaller (≤250 μA) and shorter (1 ms/phase) stimulations were used. The slow phase of the biphasic signal was found to be tetrodotoxin and hexamethonium independent, while the fast phase was greatly reduced using these pharmacological agents. Two different types of calcium responses were observed, where the fast phase was abolished by perfusion with a low-calcium buffer while both the fast and slow phases could be modulated when Ca²(+) was completely excluded from the solution using EGTA. Perfusion with nifedipine resulted in the reduction of the slow catecholamine release to 29% of the original signal, while the fast phase was only decreased to 74% of predrug values. From these results, it was determined that high-intensity stimulations of the adrenal medulla result in depolarizing not only the splanchnic nerves, but also the chromaffin cells themselves resulting in a biphasic catecholamine release.

L-type Ca2+ channels in the enteric nervous system mediate oscillatory Cl- secretion in guinea pig colon

The enteric nervous system regulates epithelial ion and fluid secretion. Our previous study has shown that the low (0.2-1 mM) concentrations of Ba2+, a K+ channel inhibitor, evoke Ca2+-dependent oscillatory Cl- secretion via activation of submucosal cholinergic neurons in guinea pig distal colon. However, it is still unclear which types of Ca2+ channels are involved in the oscillation at the neuroepithelial junction. We investigated the inhibitory effects of organic and inorganic Ca2+ channel antagonists on the short circuit current (I(sc)) of colonic epithelia (mucosa-submucosa sheets) mounted in Ussing chambers. The amplitude (412 +/- 37 microA cm(-2)) and frequency (2.6 +/- 0.1 cycles min(-1)) of the Ba2+-induced I(sc) in normal (1.8 mM) Ca2+ solution (n = 26) significantly decreased by 37.6% and 38.5%, respectively, in the low (0.1 mM) Ca2+ solution (n = 14). The I(sc) amplitude was reversibly inhibited by either verapamil (an L-type Ca2+ channel antagonist) or divalent cations (Cd2+, Mn2+, Ni2+) in a concentration-dependent manner. The concentration of verapamil for half-maximum inhibition (IC50) was 4 and 2 microM in normal and low Ca2+ solution, respectively. The relative blocking potencies of metal ions were Cd2+ > Mn2+, Ni2+ in normal Ca2+ solution. In contrast, the frequency of I(sc) was unchanged over the range of concentrations of the Ca2+ channel antagonists used. Our results show that the oscillatory I(sc) evoked by Ba2+ involves L-type voltage-gated Ca2+ channels. We conclude that L-type Ca2+ channels play a key role in the oscillation at the neuroepithelial junctions of guinea pig colon.

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.

Role of calcium channels in catecholamine secretion in the rat adrenal gland

1. We elucidated the contribution of voltage-dependent Ca2+ channels to cholinergic control of catecholamine secretion in the isolated perfused rat adrenal gland. 2. Nifedipine (0.3-3 microM) inhibited increases in noradrenaline output induced by transmural electrical stimulation (1-10 Hz) and acetylcholine (6-200 microM), whereas it only slightly inhibited the adrenaline output responses. Nifedipine also inhibited the catecholamine output response induced by 1, 1-dimethyl-4-phenyl-piperazinium (DMPP; 5-40 microM) but not by methacholine (10-300 microM). 3. omega-Conotoxin MVIIC (10-1000 nM) inhibited the catecholamine output responses induced by electrical stimulation but not by acetylcholine, DMPP and methacholine. 4. omega-Conotoxin GVIA (50-500 nM) had no inhibitory effect on the catecholamine output responses. 5. These results suggest that L-type Ca2+ channels are responsible for adrenal catecholamine secretion mediated by nicotinic receptors but not by muscarinic receptors, and that their contribution to noradrenaline secretion may be greater than that to adrenaline secretion. P/Q-type Ca2+ channels may control the secretion at a presynaptic site.

Voltage-independent autocrine modulation of L-type channels mediated by ATP, opioids and catecholamines in rat chromaffin cells

The inhibition of L-type channels induced by either bath application of ATP, opioids and catecholamines or by endogenously released neurotransmitters was investigated in rat chromaffin cells with whole-cell recordings (5 mM Ba2+). In both cases, the L-type current, isolated pharmacologically using omega-toxin peptides and potentiated by Bay K 8644, was inhibited by approximately 50% with nearly no changes to the activation-inactivation kinetics. Inhibition was voltage independent at a wide range of potentials (-20 to +50 mV) and insensitive to depolarizing prepulses (+100 mV, 50 ms). Onset and offset of the inhibition were fast (time constants: tau(on) approximately 0.9 s, tau(off) approximately 3.6 s), indicating a rapid mechanism of channel modulation. Whether induced exogenously or from the released granules content in conditions of stopped cell superfusion, the neurotransmitter action was reversible and largely prevented by either intracellular GDP-beta-S, cell treatment with pertussis toxin or simultaneous application of P2y,2x delta/mu-opioidergic and alpha/beta-adrenergic antagonists. This suggests the existence of converging modulatory pathways by which autoreceptors-activated G-proteins reduce the activity of L-type channels through fast interactions. The autocrine inhibition of L-type currents, which was absent in superfused isolated cells, was effective on cell clusters, suggesting that L-type channels may be potently inhibited by cell exocytosis under physiological conditions resembling the intact adrenal glands.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

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.

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.

Multiple calcium channel subtypes in isolated rat chromaffin cells

By using the whole-cell configuration of the patch-clamp technique we have investigated the pharmacological properties of Ca2+ channels in short-term cultured rat chromaffin cells. In cells held at a membrane potential of --80 mV, using 10 mM Ba2+ as the charge carrier, only high-voltage-activated (HVA) Ca2+ channels were found. Ba2+ currents (IBa) showed variable sensitivity to dihydropyridine (DHP) Ca2+ channel agonists and antagonists. Furnidipine, a novel DHP antagonist, reversibly blocked the current amplitude by 22% and 48%, at 1 microM and 10 microM respectively, during short (15-50 ms) depolarizing pulses to 0 mV. The L-type Ca2+ channel agonist Bay K 8644 (1 microM) caused a variable potentiation of HVA currents that could be better appreciated at low rather than at high depolarizing steps. Increase of IBa was accompanied by a 20-mV shift in the activation curves for Ca2+ channels towards more hyperpolarizing potentials. Application of the conus toxin omega-conotoxin GVIA (GVIA; 1 microM) blocked 31% of IBa; blockade was irreversible upon removal of the toxin from the extracellular medium. omega-Agatoxin IVA (IVA; 100 nM) produced a 15% blockade of IBa. omega-Conotoxin MVIIC (MVIIC; 5 microM) produced a 36% blockade of IBa; such blockade seems to be related to both GVIA-sensitive (N-type) and GVIA-resistant Ca2+ channels. The sequential addition of supramaximal concentrations of furnidipine (10 microM), GVIA (1 microM), IVA (100 nM) and MVIIC (3 microM) produced partial inhibition of IBa, which were additive. Our data suggest that the whole cell IBa in rat chromaffin cells exhibits at least four components.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of the voltage-gated calcium channels mediating dopamine and acetylcholine release from the isolated rat retina

We examined the properties of voltage-gated calcium channels mediating endogenous dopamine (DA) and acetylcholine (ACh) release in the isolated rat retina. Application of 30 mM KCl elicited the release of DA and ACh, and these releases were abolished in Ca(2+)-free medium. The high K(+)-evoked DA release was largely blocked by both of omega-agatoxin IVA and omega-conotoxin MVIIC, P- and Q-type calcium channel antagonists, and partly blocked by isradipine, and L-type calcium channel antagonist, and omega-conotoxin GVIA, an N-type calcium channel antagonist. omega-Agatoxin IVA at a small dose, sufficient to block P-type channels alone, was however without effect. On the other hand, the high K(+)-evoked ACh release was partly blocked by omega-agatoxin IVA and omega-conotoxin MVIIC, but was resistant to isradipine and omega-conotoxin GVIA. Flunarizine, a non-selective T-type calcium channel antagonist, did not inhibit the release of DA and ACh. Cd2+ markedly blocked the release of both DA and ACh, Co2+ and Ni2+ slightly blocked the release of DA, and the release of ACh was not blocked by these two divalent cations. These results suggest that the high K(+)-evoked release of retinal DA is largely mediated by omega-agatoxin IVA and omega-conotoxin MVIIC sensitive calcium channels (probably Q-type channels), while the release of retinal ACh is largely mediated by as yet uncharacterized Cd2+ sensitive calcium channels. The properties of voltage-gated calcium channels involved in the release of ACh in the rat retina differ from those of DA.

Contribution of L- and N-type calcium currents to exocytosis in rat adrenal medullary chromaffin cells

The excitation-secretion coupling process requires Ca2+ influx through voltage-dependent Ca2+ channels (VDCC) but the contribution of L-type or N-type VDCC during the secretion from adrenal chromaffin cell is still on debate. In this study we explored the contribution of each VDCC to exocytosis in single rat adrenal chromaffin cells. Chromaffin cells were voltage-clamped clamped in the whole-cell recording mode. Ca2+ inward current (ICa) was elicited by depolarization from -70 mV to +10 mV and the change in cell membrane capacitance (Cm) was monitored as an indicator of the resultant exocytosis. The increase in Cm had positive correlation with the amount of ICa and replacing the internal Ca2+ buffer to high EGTA (5 mM) decreased the sensitivity of Cm increase to Ca2+ influx. After blockage of ICa with 100 microM Cd2+, there was no increase in Cm following membrane potential depolarization while INa was intact. To clarify the contribution of each type of VDCC to induce exocytosis during membrane potential depolarization, L- and N-type ICa were blocked selectively by Ca2+ channel antagonists. After blockage of L-type ICa with nicardipine (1 microM), ICa was blocked to 35 +/- 6.2% (mean +/- standard error) of control and the resultant change in Cm was reduced to 38 +/- 4.6% of control. Bay K-8644 (1 microM) enhanced ICa and the similar proportion of Cm was increased by this L-type VDCC agonist. On the other hand omega-conotoxin GVIA (1 microM), an N-type VDCC antagonist, blocked ICa to 60 +/- 4.3% of control and reduced the change in Cm to 58 +/- 3.9% of control.(ABSTRACT TRUNCATED AT 250 WORDS)

Different calcium channels mediate transmitter release evoked by transient or sustained depolarization at mammalian sympathetic ganglia

We have compared the effect of calcium channel blockers on the potassium-evoked release of tritium-labeled acetylcholine and on preganglionic spike-evoked synaptic transmission in the rat superior cervical ganglion. Transmitter release at the nerve terminals is mediated by the influx of calcium through voltage-gated calcium channels. While four types of voltage-gated calcium channels (T, L, N and P) have been identified in neurons, it is not clear which may actually be involved in excitation-secretion coupling. Release of tritiated acetylcholine evoked by sustained depolarization in high (40 mM) extracellular potassium decreased markedly in the absence of calcium or the presence of cadmium. High potassium-evoked release was substantially inhibited by the P-type channel blockers, purified from funnel-web spider toxin, and omega-agatoxin-IVA, and by the N-type channel blocker omega-conotoxin-GVIA, but was unaffected by the L-type channel blocker nitrendipine. In contrast, postganglionic compound action potentials synaptically triggered by preganglionic stimulation were strongly blocked by funnel-web spider toxin and slightly blocked by a high concentration of omega-agatoxin-IVA, but were unaffected by either omega-conotoxin-GVIA, nitrendipine or a low concentration of omega-agatoxin-IVA. Thus, at the superior cervical ganglion, funnel-web spider toxin-sensitive calcium channels play a dominant role in transmitter release evoked by transient, spike-mediated depolarization, but other types of voltage-gated calcium channels in addition to the funnel-web spider toxin-sensitive channel mediate the transmitter release that is evoked by sustained high potassium depolarization.

Calcium channel subtypes in cat chromaffin cells

1. Using the patch-clamp technique we have investigated the kinetic and pharmacological properties of high-voltage-activated (HVA) Ca2+ channels in short-term-cultured cat chromaffin cells. 2. In 10 mM Ba2+, HVA currents activated around -40 mV, reached maximal amplitude at 0 mV and reversed at about +60 mV. At 0 mV, HVA current activation was fast (mean tau act, 2.45 ms), and followed by either an incomplete inactivation or by a second slow phase of activation (mean tau slow, 36.8 ms) that was lost when Ba2+ was replaced by Ca2+. HVA Ba2+ currents deactivate quickly on repolarization to -50 mV (mean tau deact, 0.36 ms). 3. In most cells, HVA currents were sensitive to common dihydropyridine (DHP) derivatives. Nisoldipine blocked the currents maximally at low membrane potentials (mean block 76% at -30 mV, 3 microM) and gradually less at higher voltages. Nisoldipine block was clearly time dependent (33 and 56% after 30 and 600 ms, respectively, to 0 mV). 4. Bay K 8644 (3 microM) action was variable and caused (1) a 2- to 4-fold increase of Ba2+ currents at -40 to -20 mV, (2) a -15 mV shift of the current-voltage relationship and (3) a 10- to 20-fold prolongation of HVA channel deactivation at -50 mV. 5. Nisoldipine block and Bay K 8644 potentiation of HVA currents increased markedly in omega-conotoxin GVIA (omega-CgTX)-pretreated cells, suggesting an increased fraction of DHP-sensitive currents in these cells. Nisoldipine block of residual omega-CgTX-resistant currents was almost complete (mean block, 82%) during pulses of 1 s to 0 mV. 6. The degree of inhibition produced by omega-CgTX (2 microM for 1 min) varied from cell to cell (mean block, 46%) and was partly reversible. Residual omega-CgTX-resistant currents exhibited faster activation-deactivation kinetics than control currents. 7. The slow phase of HVA current activation was abolished if a conditioning prepulse of 40 ms to +70 mV preceded a test pulse to 0 mV. Double-pulse protocols caused an average current increase (facilitation) of 37% that was voltage dependent and which correlated with the slow phase of Ca2+ channel activation. Facilitation was lost in most omega-CgTX-treated cells and was little affected by nisoldipine (3 microM) and Bay K 8644 (1 microM). Facilitation was potentiated in cells dialysed with 100 microM guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and fully prevented by 1 mM guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S).(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic mechanisms involved in the secretory effects of histamine in the rat adrenal medulla

Histamine activation of H1 receptors elicited the release of adrenaline from in vitro perfused rat adrenal with an EC50 of 3 microM. Neither Na+ deprivation nor complete membrane depolarization abolished the histamine-mediated secretory response but it was partially dependent on extracellular Ca2+. Nitrendipine and BAY-K-8644 affected the release induced by histamine concentrations at over 3 microM. Delayed application of histamine pulses, after external Ca2+ removal, led to a decline in to a plateau at 50% of the initial release. Pretreatment with ionomycin abolished this Ca2+ deprivation-resistant component. These data suggest that secretion evoked by low concentrations of histamine occurs by mobilization of Ca2+ from internal stores whereas higher concentrations use Ca2+ from both intracellular and extracellular sources.