Modulation by L-type Ca2+ channels and apamin-sensitive K+ channels of muscarinic responses in cat chromaffin cells

Muscarinic excitation-secretion coupling in chromaffin cells

Excitation-secretion coupling in adrenomedullary chromaffin cells physiologically commences when acetylcholine molecules released from splanchnic nerve terminals bind to cholinergic receptors located at the cell's plasma membrane. While nicotinic acetylcholine receptors ensure a rapid and efficacious transmission of preganglionic impulses, muscarinic acetylcholine receptors are considered to play a subsidiary role mostly by facilitating the nicotinic responses. Nevertheless, the variety of effects brought about by muscarinic stimulation in chromaffin cells (release of intracellular Ca2+, activation of Ca2+ entry through non-selective cation channels and voltage-dependent Ca2+ channels, impairment and/or enhancement of action potential firing, etc.) and the long-lasting nature of many of them suggests that muscarinic receptors might contribute to the fine tuning of the catecholamine secretory response upon graded preganglionic stimulation and prolonged periods of time. Such a variety of effects probably reflects not only the diversity of muscarinic receptors expressed in chromaffin cells but also the existence of differences among the animal species employed in the reported investigations. Accordingly, we first review on an animal species-based approach the most relevant features of the muscarinic response in chromaffin cells from a set of mammals, and finally present a unified picture of the mechanisms of muscarinic excitation-secretion coupling in chromaffin cells.

Key role of the nicotinic receptor in neurotransmitter exocytosis in human chromaffin cells

The whole-cell secretory response evoked by acetylcholine (ACh) in human chromaffin cells was examined using a new protocol based on quickly switching from the voltage-clamp to the current-clamp (CC) configuration of the patch-clamp technique. Our experiments revealed that Ca(2+) entry through the nicotinic receptor at hyperpolarized membrane potentials contributed as much to the exocytosis (100.4 +/- 27.3 fF) evoked by 200 ms pulses of ACh, as Ca(2+) flux through voltage-dependent Ca(2+) channels at depolarized membrane potentials. The nicotinic current triggered a depolarization event with a peak at +49.3 mV and a 'plateau' phase that ended at -23.9 mV, which was blocked by 10 mumol/L mecamylamine. When a long ACh stimulus (15 s) was applied, the nicotinic current at the end of the pulse reached a value of 15.45 +/- 3.6 pA, but the membrane potential depolarization still remained at the 'plateau' stage until withdrawal of the agonist. Perfusion with 200 mumol/L Cd(2+) during the 15 s ACh pulse completely abolished the plasma membrane depolarization at the end of the pulse, indicating that Ca(2+) entry through Ca(2+) channels contributed to the membrane potential depolarization provoked by prolonged ACh pulses. These findings also reflect that voltage-dependent Ca(2+) channels were recruited by the small current flowing through the desensitized nicotinic receptor to maintain the depolarization. Finally, muscarinic receptor activation triggered a delayed exocytotic process after prolonged ACh stimulation, dependent on Ca(2+) mobilization from the endoplasmic reticulum. In summary, we show here that nicotinic and muscarinic receptors contribute to the exocytosis of neurotransmitters in human chromaffin cells, and that the nicotinic receptor plays a key role in several stages of the stimulus-secretion coupling process in these cells.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

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

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.

Interaction of SK(Ca) channels and L-type Ca(2+) channels in catecholamine secretion in the rat adrenal gland

We elucidated the interaction of small-conductance Ca(2+)-activated K(+) (SK(Ca)) channels and L-type Ca(2+) channels in muscarinic receptor-mediated control of catecholamine secretion in the isolated perfused rat adrenal gland. The muscarinic agonist methacholine (10-300 microM) produced concentration-dependent increases in adrenal output of epinephrine and norepinephrine. The SK(Ca) channel blocker apamin (1 microM) enhanced the methacholine-induced catecholamine responses. The facilitatory effect of apamin on the methacholine-induced catecholamine responses was not observed during treatment with the L-type Ca(2+) channel blocker nifedipine (3 microM) or Ca(2+)-free solution. Nifedipine did not affect the methacholine-induced catecholamine responses, but it inhibited the responses during treatment with apamin. The L-type Ca(2+) channel activator Bay k 8644 (1 microM) enhanced the methacholine-induced catecholamine responses, whereas the enhancement of the methacholine-induced epinephrine and norepinephrine responses were prevented and attenuated by apamin, respectively. These results suggest that SK(Ca) channels are activated by muscarinic receptor stimulation, which inhibits the opening of L-type Ca(2+) channels and thereby attenuates adrenal catecholamine secretion.

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

We elucidated the functional contribution of K(+) channels to cholinergic control of catecholamine secretion in the perfused rat adrenal gland. The small-conductance Ca(2+)-activated K(+) (SK(Ca))-channel blocker apamin (10-100 nM) enhanced the transmural electrical stimulation (ES; 1-10 Hz)- and 1, 1-dimethyl-4-phenyl-piperazinium (DMPP; 5-40 microM)-induced increases in norepinephrine (NE) output, whereas it did not affect the epinephrine (Epi) responses. Apamin enhanced the catecholamine responses induced by acetylcholine (6-200 microM) and methacholine (10-300 microM). The putative large-conductance Ca(2+)-activated K(+) channel blocker charybdotoxin (10-100 nM) enhanced the catecholamine responses induced by ES, but not the responses induced by cholinergic agonists. Neither the K(A) channel blocker mast cell degranulating peptide (100-1000 nM) nor the K(V) channel blocker margatoxin (10-100 nM) affected the catecholamine responses. These results suggest that SK(Ca) channels play an inhibitory role in adrenal catecholamine secretion mediated by muscarinic receptors and also in the nicotinic receptor-mediated secretion of NE, but not of Epi. Charybdotoxin-sensitive Ca(2+)-activated K(+) channels may control the secretion at the presynaptic site.

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.

Effects of adrenomedullin and PAMP on adrenal catecholamine release in dogs

We examined the effects of proadrenomedullin-derived peptides on the release of adrenal catecholamines in response to cholinergic stimuli in pentobarbital sodium-anesthetized dogs. Drugs were administered into the adrenal gland through the phrenicoabdominal artery. Splanchnic nerve stimulation (1, 2, and 3 Hz) and ACh injection (0.75, 1.5, and 3 microgram) produced frequency- or dose-dependent increases in adrenal catecholamine output. These responses were unaffected by infusion of adrenomedullin (1, 3, and 10 ng. kg-1. min-1) or its selective antagonist adrenomedullin-(22-52) (5, 15, and 50 ng. kg-1. min-1). Proadrenomedullin NH2-terminal 20 peptide (PAMP; 5, 15, and 50 ng. kg-1. min-1) suppressed both the splanchnic nerve stimulation- and ACh-induced increases in catecholamine output in a dose-dependent manner. PAMP also suppressed the catecholamine release responses to the nicotinic agonist 1, 1-dimethyl-4-phenylpiperazinium (0.5, 1, and 2 microgram) and to muscarine (0.5, 1, and 2 microgram), although the muscarine-induced response was relatively resistant to PAMP. These results suggest that PAMP, but not adrenomedullin, can act as an inhibitory regulator of adrenal catecholamine release in vivo.

Effect of cilnidipine, a novel dihydropyridine Ca2+ channel blocker, on adrenal catecholamine secretion in anesthetized dogs

We investigated the effect of cilnidipine, a novel dihydropyridine Ca2+ channel blocker possessing blocking actions on N-type and L-type voltage-dependent Ca2+ channels (VDCCs), in comparison with the L-type VDCC blocker nifedipine, on adrenal catecholamine secretion in response to splanchnic nerve stimulation (SNS), acetylcholine (ACh), the nicotinic receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium (DMPP), and muscarine in anesthetized dogs. Ca2+ channel blockers and cholinergic agonists were infused and injected, respectively, into the adrenal gland through the phrenicoabdominal artery. Cilnidipine (0.3-3 microg/min) inhibited increases in both epinephrine (EPI) and norepinephrine (NE) output induced by SNS (2 Hz), ACh (1.5 microg), and DMPP (0.2 microg). However, cilnidipine inhibited increase in NE output induced by muscarine (1 microg) without affecting increase in EPI output. Nifedipine (0.3-3 microg/min) inhibited the ACh- and DMPP-induced increases in EPI and NE output without affecting the SNS- and muscarine-induced increases in EPI and NE output. From these results, it seems likely that the inhibition by cilnidipine of the SNS-induced EPI and NE secretion and of the muscarine-induced NE secretion is related to its blocking action on N-type VDCCs.

Role of K+ channels in adrenal catecholamine secretion in anesthetized dogs

We examined the role of K+ channels in the secretion of adrenal catecholamine (CA) in response to splanchnic nerve stimulation (SNS), acetylcholine (ACh), 1,1-dimethyl-4-phenyl-piperazinium (DMPP), and muscarine in anesthetized dogs. K+ channel blockers and the cholinergic agonists were infused and injected, respectively, into the adrenal gland. The voltage-dependent K+ channel (KA type) blocker mast cell degranulating (MCD) peptide infusion (10-100 ng/min) enhanced increases in CA output induced by SNS (1-3 Hz), but it did not affect increases in CA output induced by ACh (0.75-3 micrograms), DMPP (0.1-0.4 microgram), or muscarine (0.5-2 micrograms). The small-conductance Ca(2+)-activated K+ (SKCa) channel blocker scyllatoxin infusion (10-100 ng/min) enhanced the ACh-, DMPP-, and muscarine-induced increases in CA output, but it did not affect the SNS-induced increases in CA output. These results suggest that KA channels may play an inhibitory role in the regulation of adrenal CA secretion in response to SNS and that SKCa channels may play the same role in the secretion in response to exogenously applied cholinergic agonists.

Apamin-sensitive SK(Ca) channels modulate adrenal catecholamine release in anesthetized dogs

We investigated the role of high conductance (BK(Ca)) and small conductance Ca2(+)-activated K+ (SK(Ca)) channels in adrenal catecholamine release in response to splanchnic nerve stimulation, acetylcholine, the nicotinic receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium (DMPP), and muscarine in anesthetized dogs. The selective SK(Ca) channel blocker apamin and the selective BK(Ca) channel blocker charybdotoxin were infused into the adrenal gland through the phrenicoabdominal artery, and the cholinergic agonists were injected into the same artery. Splanchnic nerve stimulation (1, 2, 3 and 10 Hz), acetylcholine (0.75, 1.5 and 3 microg), DMPP (0.1, 0.2 and 0.4 microg) and muscarine (0.5, 1 and 2 microg) produced frequency- or dose-dependent increases in catecholamine output as measured in adrenal venous blood. Apamin infusion (1, 3 and 10 ng/min) enhanced the acetylcholine-, DMPP- and muscarine-induced increases in catecholamine output in a dose-dependent manner, but it did not affect the splanchnic nerve stimulation-induced catecholamine response. Charybdotoxin infusion (10, 30 and 100 ng/min) did not affect the increases in catecholamine output induced by the agonists and splanchnic nerve stimulation. Neither apamin nor charybdotoxin affected basal catecholamine output. These results suggest that apamin-sensitive SK(Ca) channels located in adrenal medullary cells may play an inhibitory role in the regulation of adrenal catecholamine release mediated by extrasynaptic nicotinic and muscarinic receptors.

Characteristics of cytosolic Ca2+ elevation induced by muscarinic receptor activation in single adrenal chromaffin cells of the guinea pig

In Fura-2 loaded-single guinea pig adrenal chromaffin cells, muscarine, nicotine and KCl all caused an early peak rise in intracellular Ca concentration ([Ca2+]i) followed by a sustained rise. In Ca(2+)-free solution, muscarine, but neither nicotine nor KCl, caused a transient increase in [Ca2+]i, which was partially reduced by preceding application of caffeine or by treatment with ryanodine plus caffeine. In voltage-clamped cells at a holding potential of -60 mV, the muscarine-induced [Ca2+]i rise, especially its sustained phase, decreased in magnitude. Intracellular application of inositol 1,4,5-trisphosphate caused a transient increase in [Ca2+]i and inhibited the following [Ca2+]i response to muscarine without affecting responses to nicotine and a depolarizing pulse. Muscarine evoked membrane depolarization following brief hyperpolarization in most cells tested. There was a significant positive correlation between the amplitude of the depolarization and the magnitude of the sustained rise in [Ca2+]i. Muscarine-induced sustained [Ca2+]i rise was much greater in the current-clamp mode than that in the voltage-clamp mode. The sustained phase of [Ca2+]i rise and Mn2+ influx in response to muscarine were suppressed by a voltage-dependent Ca2+ channel blocker, methoxyverapamil. These results suggest that stimulation of muscarinic receptors causes not only extracellular Ca2+ entry, but also Ca2+ mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Voltage-dependent Ca(2+)-channels may function as one of the Ca2+ entry pathways activated by muscarinic receptor in guinea pig adrenal chromaffin cells.

Contribution of SK and BK channels in the control of catecholamine release by electrical stimulation of the cat adrenal gland

1. Transmural electrical stimulation (10 Hz, 1 ms, 40 V for 10 s) of cat adrenal glands perfused at room temperature with Krebs-Hepes solution produced catecholamine secretory responses which were reproducible when stimulations were applied at 5 min intervals. Such responses were inhibited about 20% by atropine (1 microM) and 80% by hexamethonium (30 microM). Apamin (100 nM) increased the secretory response 2.5-fold in the presence of atropine and 8-fold in the presence of hexamethonium. 2. Potentiation by apamin of secretory responses evoked by 100-pulse trains was similar at 5, 10 and 20 Hz (about 2-fold). When glands were continuously stimulated at 3 Hz, apamin increased 4-fold the initial secretion plateau. Continuous stimulation at a higher frequency (20 Hz) produced a sharp secretory peak followed by a small, sustained plateau; apamin did not alter this plateau. Apamin also enhanced the secretory responses obtained with sustained stimulation with acetylcholine (10 or 200 microM). 3. Secretion peaks induced by brief acetylcholine pulses (10 microM for 10 s) applied to isolated and superfused cat adrenal chromaffin cells were enhanced more than 3-fold by 100 nM apamin. Charybdotoxin (10 nM) did not enhance these secretory peaks. 4. In perfused cat adrenal glands, charybdotoxin (10 nM) affected neither the secretion evoked by trains of electrical stimulation applied at different frequencies nor the secretion evoked by acetylcholine pulses. 5. In 0.5 mM [Ca2+]o, apamin enhanced 3-fold the secretion evoked by electrical stimulation trains of 100 pulses (10 Hz, 10 s) and almost 6-fold the acetylcholine (10 microM for 10 s)-induced secretion. In 5 mM Ca2+, apamin enhanced the secretory responses to electrical stimulation and acetylcholine 2- and 10-fold, respectively. Charybdotoxin enhanced 2.5-fold the secretory response to electrical stimulation in 0.5 mM Ca2+, although this effect was not statistically significant. A synergistic interaction between the two toxins on catecholamine release induced by electrical stimulation was observed at low but not at high [Ca2+]o. 6. Simultaneous release of acetylcholine and catecholamines upon electrical stimulation was achieved in glands in which the endogenous acetylcholine stores in the splanchnic nerve terminals had been prelabelled by perfusion with [3H]choline. While apamin enhanced more than 2-fold the postsynaptic release of catecholamines, the presynaptic release of acetylcholine remained unaffected. 7. The results are compatible with the hypothesis that, under physiological conditions, Ca(2+)-activated SK channels present in chromaffin cells control the firing patterns of action potentials induced by the acetylcholine released from splanchnic nerves during stress.(ABSTRACT TRUNCATED AT 400 WORDS)