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

Molecular mechanism for muscarinic M1 receptor-mediated endocytosis of TWIK-related acid-sensitive K+ 1 channels in rat adrenal medullary cells

KEY POINTS

The muscarinic acetylcholine receptor (mAChR)-mediated increase in excitability in rat adrenal medullary cells is at least in part due to inhibition of TWIK (tandem of P domains in a weak inwardly rectifying K⁺ channel)-related acid-sensitive K⁺ (TASK)1 channels. In this study we focused on the molecular mechanism of mAChR-mediated inhibition of TASK1 channels. Exposure to muscarine resulted in a clathrin-dependent endocytosis of TASK1 channels following activation of the muscarinic M₁ receptor (M₁ R). This muscarinic signal for the endocytosis was mediated in sequence by phospholipase C (PLC), protein kinase C (PKC), and then the non-receptor tyrosine kinase Src with the consequent tyrosine phosphorylation of TASK1. The present results establish that TASK1 channels are tyrosine phosphorylated and internalized in a clathrin-dependent manner in response to M₁ R stimulation and this translocation is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

ABSTRACT

Activation of muscarinic receptor (mAChR) in rat adrenal medullary (AM) cells induces depolarization through the inhibition of TWIK-related acid-sensitive K⁺ (TASK)1 channels. Here, pharmacological and immunological approaches were used to elucidate the molecular mechanism for this mAChR-mediated inhibition. TASK1-like immunoreactive (IR) material was mainly located at the cell periphery in dissociated rat AM cells, and its majority was internalized in response to muscarine. The muscarine-induced inward current and translocation of TASK1 were suppressed by dynasore, a dynamin inhibitor. The muscarinic translocation was suppressed by MT7, a specific M₁ antagonist, and the dose-response curves for muscarinic agonist-induced translocation were similar to those for the muscarinic inhibition of TASK1 currents. The muscarine-induced inward current and/or translocation of TASK1 were suppressed by inhibitors for phospholipase C (PLC), protein kinase C (PKC), and/or Src. TASK1 channels in AM cells and PC12 cells were transiently associated with Src and were tyrosine phosphorylated in response to muscarinic stimulation. After internalization, TASK1 channels were quickly dephosphorylated even while they remained in the cytoplasm. The cytoplasmic TASK1-like IR material quickly recycled back to the cell periphery after muscarine stimulation for 0.5 min, but not 10 min. We conclude that M₁ R stimulation results in internalization of TASK1 channels through the PLC-PKC-Src pathway with the consequent phosphorylation of tyrosine and that this M₁ R-mediated internalization is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

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

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

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

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

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.

Effect of nicotine on neuromuscular transmission in mouse motor synapses

Nicotine (10 nM) inhibits rhythmic activity of the neuromuscular synapse in mice. This effect was prevented by alpha-cobratoxin and apamin. Hence, the effects of nicotine are realized via presynaptic neuronal nicotinic cholinoceptors and Ca(2+)-activated potassium channels.

Blockade of Ca2+-activated K+ channels by galantamine can also contribute to the potentiation of catecholamine secretion from chromaffin cells

Galantamine is a drug in clinical use for the treatment of Alzheimer's disease, but its mechanism(s) of action remains controversial. Here we addressed the question whether galantamine could potentiate neurotransmitter release by inhibiting small conductance Ca2+ -activated K+ channels (KCa2). Galantamine potentiated catecholamine secretory responses induced by 10 s pulses of acetylcholine and high [K+]o applied to fast-superfused bovine adrenal chromaffin cell populations. Catecholamine release was significantly enhanced by galantamine although we did not find concentration dependence in the range 0.1-1 microM. The KCa2 channel blocker apamin (0.3 microM) occluded the potentiating effects of galantamine on acetylcholine-evoked secretion. Like apamin, galantamine also modified the firing of action potentials, but to a lesser extent. In addition, 1 microM galantamine reduced by 41% the KCa2 current without modifying the voltage-dependent Ca2+ currents. These results constitute the first direct evidence that galantamine can potentiate neurotransmitter release by blocking KCa2 channels, in addition to its already demonstrated capacity to mildly block acetylcholinesterase or potentiate allosterically nicotinic receptors.

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 choline-evoked [Ca2+]c signal causes catecholamine release and hyperpolarization of chromaffin cells

In bovine chromaffin cells fast-superfused with Krebs-HEPES solution containing 1-2 mM Ca2+, 5 s pulses of choline (1-10 mM), elicited catecholamine secretory responses that were only approximately 10% of those evoked by ACh (0.01-0.1 mM). However, in high-Ca2+ solutions (10-20 mM) the size of the choline secretory responses approached those of ACh. The choline responses (10 mM choline in 20 mM Ca2+, 10Cho/20Ca2+) tended to decline upon repetitive pulsing, whereas those of ACh were well maintained. The confocal [Ca2+]c increases evoked by 10Cho/20Ca2+ were similar to those of ACh. Whereas 10Cho/20Ca2+ caused mostly hyperpolarization of chromaffin cells, 0.1ACh/20 Ca2+ caused first depolarization and then hyperpolarization; in regular solutions (2 mM Ca2+), the hyperpolarizing responses did not show up. In Xenopus oocytes injected with mRNA for bovine alpha7 nicotinic receptors (nAChRs), 10Cho/20 Ca2+ fully activated an inward current; in oocytes expressing alpha3beta4, however, the inward current elicited by choline amounted to only 4% of the size of alpha7 current. Our results suggest that choline activates the entry of Ca2+ through alpha7 nAChRs; this leads to a cytosolic concentration of calcium ([Ca2+]c) rise that causes the activation of nearby Ca2+-dependent K+ channels and the hyperpolarization of the chromaffin cell. This response, which could be unmasked provided that cells were stimulated with high-Ca2+ solutions, may be the underlying mechanism through which choline exerts a modulatory effect on the electrical activity of the chromaffin cell and on neurotransmitter release at cholinergic synapses.

Opioid receptor stimulation suppresses the adrenal medulla hypoxic response in sheep by actions on Ca(2+) and K(+) channels

Before the preganglionic regulation of the adrenal medulla is established, hypoxia acts directly on the chromaffin cells to evoke the secretion of catecholamines. This direct action of hypoxia is suppressed by the gradual development of the preganglionic innervation and we have proposed that opioid peptides released from the adrenal splanchnic nerves may be responsible for this suppression. The effects of the specific opioid agonists DPDPE (delta-agonist), U-62066 (kappa-agonist) and DALDA (mu-agonist) on the hypoxia-evoked response were investigated in both a whole-gland preparation and in isolated adrenal chromaffin cells using amperometry, whole-cell patch clamping and measurement of cytosolic [Ca(2+)]. The combined application of mu- and kappa-type agonists abolished the hypoxia-evoked catecholamine secretion from whole perfused adrenal gland. In isolated chromaffin cells, mu- and kappa-opioid agonists reduced the rise in [Ca(2+)](i) that results from exposure to hypoxia. Both agonists decreased the voltage-dependent Ca(2+) current in these cells. The mu-agonist increased the conductance through SK-type K(+) channels and this action offset the decrease in K(+) conductance produced by exposure to hypoxia. The kappa-type agonist decreased the conductance through an action on BK-type K(+) channels, a class of channels that are not involved in initiating the direct response to hypoxia. These data suggest that opioids, through their action on SK channels and voltage-dependent Ca(2+) channels, may be responsible for the nerve-induced suppression of the hypoxic response of adrenal chromaffin cells and that these effects of endogenous opioids are mediated via mu- and kappa-type receptors.

ERG K+ channel blockade enhances firing and epinephrine secretion in rat chromaffin cells: the missing link to LQT2-related sudden death?

The ether-a-go-go-related genes (erg) are expressed in tissues other than heart and brain, in which human erg (HERG) K+ channels are known to regulate the repolarization of heart action potentials and neuronal spike-frequency accommodation. We provide evidence that erg1 transcripts and ERG proteins are present in rat chromaffin cells in which we could isolate a K+ current that was biophysically and pharmacologically similar to the ERG current. Firing frequency and catecholamine release were analyzed at the single-cell level by means of perforated patch-clamp and carbon fiber electrochemical detection. It was found that the blocking of ERG, KATP, and KCa channels led to hyperexcitability and an increase in catecholamine release. Combined immunocytochemical experiments with antibodies directed against phenylethanolamine N-methyltransferase and ERG channels suggested expression of these channels in epinephrine- but not in norepinephrine-containing cells. It is concluded that, in addition to being crucial in regulating the QT period in the heart, ERG channels play a role in modulating epinephrine, a fundamental neurotransmitter shaping cardiac function. This finding suggests that the sudden death phenotype associated with LQT2 syndrome mutations may be the result of an emotionally triggered increase in epinephrine in a long-QT running heart.

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.

Apamin improves reference memory but not procedural memory in rats by blocking small conductance Ca(2+)-activated K(+) channels in an olfactory discrimination task

Apamin blocks SK channels responsible for long-lasting hyperpolarization following the action potential. Using an olfactory associative task, the effect of an intracerebroventricular 0.3 ng apamin injection was tested on learning and memory. Apamin did not modify the learning of the procedure side of the task or the learning of the odor-reward association. To test reference memory specifically, the rats were trained on a new odor-association problem using the same procedure (acquisition session), and they were tested for retention 24 h later. Apamin injected before or after the acquisition session improved retention of the valence of a new odor pair. Apamin injected before the retention session did not affect the retrieval of the new valence. Thus, the results indicate that the blockage of apamin-sensitive SK channels facilitate consolidation on new-odor-reward association.

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.

UCL 1684: a potent blocker of Ca2+ -activated K+ channels in rat adrenal chromaffin cells in culture

The novel K+ channel blocker 6,10-diaza-3(1,3)8,(1,4)-dibenzena-1,5(1,4)-diquinolinacy clodecaphane (UCL 1684) has been tested for its ability to inhibit Ca2+ -activated K+ currents in cultured rat chromaffin cells. Low nanomolar concentrations of UCL 1684 produced a rapid and reversible inhibition of the slow, apamin-sensitive, tail current activated by a depolarizing voltage command. This compound also inhibited the muscarine activated outward current with an IC50 of 6 nM. These results confirm UCL 1684 to be the most potent non-peptidic blocker of the apamin-sensitive Ca2+ -activated K+ channel so far described.

The role of BK(Ca) channels in the nitric oxide-mediated regulation of adrenal catecholamine secretion

We examined whether high conductance Ca2+-activated K+ (BK(Ca)) channels are involved in the modulatory action of nitric oxide (NO) on the secretion of adrenal catecholamines in response to splanchnic nerve stimulation and acetylcholine in anesthetized dogs. The NO donor 3-(2-hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-1-propanamin e (NOC 7), the BK(Ca) channel blocker charybdotoxin and acetylcholine were administered intraarterially (i.a.) into the adrenal gland. NOC 7 infusion (2 microg min(-1)) inhibited increases in catecholamine output induced by splanchnic nerve stimulation (1-3 Hz) and acetylcholine (0.75-3 microg). Charybdotoxin infusion (100 ng min(-1)) did not affect increases in catecholamine output induced by splanchnic nerve stimulation and acetylcholine. Charybdotoxin blocked the NOC 7-induced inhibition of increases in catecholamine output induced by splanchnic nerve stimulation but not by acetylcholine. These results suggest that NO may inhibit the secretion of adrenal catecholamines induced by splanchnic nerve stimulation through activation of BK(Ca) channels.

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.

Structural diversity among subtypes of small-conductance Ca2+-activated potassium channels

125I-Apamin and photolabile derivatives of the toxin have been used to investigate the binding properties and subunit composition of small conductance Ca2+-activated potassium channels (SK(Ca) channels) expressed on plasma membranes from rat brain, rabbit liver, or rat pheochromocytoma (PC12) cells. On all preparations, 125I-apamin recognized single classes of acceptor binding sites with similar high affinity (Kd approximately 3-6 pM). Gallamine, however, was found to readily discriminate between 125I-apamin acceptors present in these preparations, showing a maximal approx nine-fold difference in affinity for acceptors expressed by rabbit liver or PC12 cells. Affinity-labeling patterns revealed the expression of different hetero-oligomeric combinations of high (86 or 59 kDa) and low (33 or 30 kDa) molecular mass 125I-apamin-binding polypeptides, consistent with pharmacological differences. Alternative expression of either 86- or 59-kDa polypeptides appeared to be the most important factor influencing gallamine's affinity for SK(Ca) channel subtypes. Both high- and low-molecular-mass polypeptides are integral membrane proteins, the latter being glycosylated in a tissue-specific manner.

A novel small conductance Ca2+-activated K+ channel blocker from Oxyuranus scutellatus taipan venom. Re-evaluation of taicatoxin as a selective Ca2+ channel probe

Taicatoxin, isolated from the venom of the Australian taipan snake Oxyuranus scutellatus, has been previously regarded as a specific blocker of high threshold Ca2+ channels in heart. Here we show that taicatoxin (in contrast to a range of other Ca2+ channel blockers) interacts with apamin-sensitive, small conductance, Ca2+-activated potassium channels on both chromaffin cells and in the brain. Taicatoxin displays high affinity recognition of 125I-apamin acceptor-binding sites, present on rat synaptosomal membranes (Ki = 1.45 +/- 0.22 nM) and also specifically blocks affinity-labeling of a 33-kDa 125I-apamin-binding polypeptide on rat brain membranes. Taicatoxin (50 nM) completely blocks apamin-sensitive after-hyperpolarizing slow tail K+ currents generated in rat chromaffin cells (mean block 97 +/- 3%, n = 12) while only partially reducing total voltage-dependent Ca2+ currents (mean block 12 +/- 4%, n = 6). In view of these findings, the use of taicatoxin as a specific ligand for Ca2+ channels should now be reconsidered.

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.