Na+ pump inhibition and non-selective cation channel activation by cyanide and anoxia in guinea-pig chromaffin cells

Hypoxia-regulated catecholamine secretion in chromaffin cells

Adrenal catecholamine (CAT) secretion is a general physiological response of animals to environmental stressors such as hypoxia. This represents an important adaptive mechanism to maintain homeostasis and protect vital organs such as the brain. In adult mammals, CAT secretory responses are triggered by activation of the sympathetic nervous system that supplies cholinergic innervation of adrenomedullary chromaffin cells (AMC) via the splanchnic nerve. In the neonate, the splanchnic innervation of AMC is immature or absent, yet hypoxia stimulates a non-neurogenic CAT secretion that is critical for adaptation to extra-uterine life. This non-neurogenic, hypoxia-sensing mechanism in AMC is gradually lost or suppressed postnatally along a time course that parallels the development of splanchnic innervation. Moreover, denervation of adult AMC results in a gradual return of the direct hypoxia-sensing mechanism. The signaling pathways by which neonatal AMC sense acute hypoxia leading to non-neurogenic CAT secretion and the mechanisms that underlie the re-acquisition of hypoxia-sensing properties by denervated adult AMC, are beginning to be understood. This review will focus on current views concerning the mechanisms responsible for direct acute hypoxia sensing and CAT secretion in perinatal AMC and how they are regulated by innervation during postnatal development. It will also briefly discuss plasticity mechanisms likely to contribute to CAT secretion during exposures to chronic and intermittent hypoxia.

Catecholamine secretion by chemical hypoxia in guinea-pig, but not rat, adrenal medullary cells: differences in mitochondria

The effects of mitochondrial inhibitors (CN(-), a complex IV inhibitor and CCCP, protonophore) on catecholamine (CA) secretion and mitochondrial function were explored functionally and biochemically in rat and guinea-pig adrenal chromaffin cells. Guinea-pig chromaffin cells conspicuously secreted CA in response to CN(-) or CCCP, but rat cells showed a little, if any, secretory response to either of them. The resting metabolic rates in rat adrenal medullae did not differ from those in guinea-pig adrenal medullae. On the other hand, the time course of depolarization of the mitochondrial membrane potential (ΔΨm) in guinea-pig chromaffin cells in response to CN(-) was slower than that in rat chromaffin cells, and this difference was abolished by oligomycin, an F1F0-ATPase inhibitor. The extent of CCCP-induced decrease in cellular ATP in guinea-pig chromaffin cells, which was indirectly measured using a Mg(2+) indicator, was smaller than that in rat chromaffin cells. Relative expression levels of F1F0-ATPase inhibitor factor in guinea-pig adrenal medullae were smaller than in rat adrenal medullae, and the opposite was true for F1F0-ATPase α subunit. The present results indicate that guinea-pig chromaffin cells secrete more CA in response to a mitochondrial inhibitor than rat chromaffin cells and this higher susceptibility in the former is accounted for by a larger extent of reversed operation of F1F0-ATPase with the consequent decrease in ATP under conditions where ΔΨm is depolarized.

Ontogeny of O2 and CO2//H+ chemosensitivity in adrenal chromaffin cells: role of innervation

The adrenal medulla plays a key role in the physiological responses of developing and mature mammals by releasing catecholamines (CAT) during stress. In rodents and humans, the innervation of CAT-producing, adrenomedullary chromaffin cells (AMCs) is immature or absent during early postnatal life, when these cells possess 'direct' hypoxia- and CO2/H(+)-chemosensing mechanisms. During asphyxial stressors at birth, these mechanisms contribute to a CAT surge that is critical for adaptation to extra-uterine life. These direct chemosensing mechanisms regress postnatally, in parallel with maturation of splanchnic innervation. Here, we review the evidence that neurotransmitters released from the splanchnic nerve during innervation activate signaling cascades that ultimately cause regression of direct AMC chemosensitivity to hypoxia and hypercapnia. In particular, we consider the roles of cholinergic and opioid receptor signaling, given that splanchnic nerves release acetylcholine and opiate peptides onto their respective postsynaptic nicotinic and opioid receptors on AMCs. Recent in vivo and in vitro studies in the rat suggest that interactions involving α7 nicotinic acetylcholine receptors (nAChRs), the hypoxia inducible factor (HIF)-2α signaling pathway, protein kinases and ATP-sensitive K(+) (KATP) channels contribute to the selective suppression of hypoxic chemosensitivity. In contrast, interactions involving μ- and/or δ-opiod receptor signaling pathways contribute to the suppression of both hypoxic and hypercapnic chemosensitivity, via regulation of the expression of KATP channels and carbonic anhydrase (CA I and II), respectively. These data suggest that the ontogeny of O2 and CO2/H(+) chemosensitivity in chromaffin cells can be regulated by the tonic release of presynaptic neurotransmitters.

Hydrogen peroxide removes TRPM4 current desensitization conferring increased vulnerability to necrotic cell death

Necrosis is associated with an increase in plasma membrane permeability, cell swelling, and loss of membrane integrity with subsequent release of cytoplasmic constituents. Severe redox imbalance by overproduction of reactive oxygen species is one of the main causes of necrosis. Here we demonstrate that H(2)O(2) induces a sustained activity of TRPM4, a Ca(2+)-activated, Ca(2+)-impermeant nonselective cation channel resulting in an increased vulnerability to cell death. In HEK 293 cells overexpressing TRPM4, H(2)O(2) was found to eliminate in a dose-dependent manner TRPM4 desensitization. Site-directed mutagenesis experiments revealed that the Cys(1093) residue is crucial for the H(2)O(2)-mediated loss of desensitization. In HeLa cells, which endogenously express TRPM4, H(2)O(2) elicited necrosis as well as apoptosis. H(2)O(2)-mediated necrosis but not apoptosis was abolished by replacement of external Na(+) ions with sucrose or the non-permeant cation N-methyl-d-glucamine and by knocking down TRPM4 with a shRNA directed against TRPM4. Conversely, transient overexpression of TRPM4 in HeLa cells in which TRPM4 was previously silenced re-established vulnerability to H(2)O(2)-induced necrotic cell death. In addition, HeLa cells exposed to H(2)O(2) displayed an irreversible loss of membrane potential, which was prevented by TRPM4 knockdown.

Catecholamine secretion from rat foetal adrenal chromaffin cells and hypoxia sensitivity

The adrenal medulla chromaffin cells (AMCs) secrete catecholamines in response to various types of stress. We examined the hypoxia-sensitivity of catecholamine secretion by rat foetal chromaffin cells in which the innervation by the splanchnic nerve is not established. The experiments were performed in primary cultured cells from two different ages of foetuses (F15 and F19). Membrane potential of AMCs was monitored with the patch clamp technique, and the catecholamine secretion was detected by amperometry. We found that: (1) AMCs from F19 foetuses showed hypoxia-induced catecholamine release. (2) This hypoxia-induced secretion is produced by membrane depolarization generated by an inhibition of Ca(2+)-activated K(+) current [I (K(Ca))] current. (3) Chromaffin precursor cells from F15 foetuses secrete catecholamine. The quantal release is calcium-dependent, but the size of the quantum is reduced. (4) In the precursor cells, a hypoxia-induced membrane hyperpolarization is originated by an ATP-sensitive K(+) current [I (K(ATP))] activation. (5) During the prenatal period, at F15, the percentage of the total outward current for I (K(ATP)) and I (K(Ca)) was 50 and 29.5%, respectively, whereas at F19, I (K(ATP)) is reduced to 14%, and I (K(Ca)) became 64% of the total current. We conclude that before birth, the age-dependent hypoxia response of chromaffin cells is modulated by the functional activity of K(ATP) and K(Ca) channels.

CCCP enhances catecholamine release from the perfused rat adrenal medulla

The present study was designed to investigate the effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP), a mitochondrial uncoupler, on secretion of catecholamines from the isolated perfused model of the rat adrenal gland and to establish the mechanism of its adrenomedullary secretion. The perfusion of CCCP (3x10(-5) M) into an adrenal vein of for 90 min caused a great increase in catecholamine secretion. Tachyphylaxis to catecholamine-releasing effect of CCCP was not observed by repeated perfusion of it. The net catecholamine-releasing effects of CCCP were depressed by pretreament with pirenzepine (a selective muscarinic M(1)-receptor antagonist), chlorisondamine (a selective neuronal nicotinic receptor antagonist), nicardipine (an L-type Ca2+-channel antagonist), TMB-8 (an intracellular Ca2+-antagonist), and the perfusion of EGTA plus Ca2+-free medium, respectively. In the presence of CCCP (3x10(-5) M), catecholamine secretory responses induced by ACh (5.32x10(-3) M), high K+ (5.6x10(-2) M, a direct membrane depolarizer), DMPP (10(-4) M, (a selective neuronal nicotinic receptor agonist), and McN-A-343 (10(-4) M, (a selective muscarinic M1-receptor agonist) were significantly enhanced. CCCP also significantly enhanced the catecholamine secretory responses evoked by Bay-K-8644 (10(-5) M), L-type Ca2+ channel activator, and cyclopiazonic acid (10(-5) M), an inhibitor of Ca2+-ATPase. Furthermore, the perfusion of FCCP (3x10(-5) M), a similar mitochondrial uncoupler, into an adrenal vein of for 90 min also caused a great increase in catecholamine secretion in a similar pattern with CCCP. Taken together, the results demonstrate that CCCP causes the catecholamine secretion from the perfused rat adrenal medulla in a calcium-dependent fashion. It is thought that this catecholamine secretory enhancement of CCCP may be mediated by both cholinergic receptor stimulation and membrane depolarization, which are relevant to the cytoplasmic Ca2+ increase by stimulation of the Ca2+ influx as well as by the inhibition of Ca2+ uptake into the cytoplasmic Ca2+ stores (both endoplasmic reticulum and mitochondria in chromaffin cells). It also seems that protonophores, such as CCCP, suppress mitochondrial Ca2+ uptake and increase the stimulated secretion of catecholamine by the secretagogues. These results indicate that mitochondria modulate catecholamine secretion by regulating the Ca2+ mobilization for exocytosis.

Pivotal role of reduced glutathione in oxygen-induced regulation of the Na(+)/K(+) pump in mouse erythrocyte membranes

This study addresses the mechanisms of oxygen-induced regulation of ion transport pathways in mouse erythrocyte, specifically focusing on the role of cellular redox state and ATP levels. Mouse erythrocytes possess Na(+)/K(+) pump, K(+)-Cl(-) and Na(+)-K(+)-2Cl(-) cotransporters that have been shown to be potential targets of oxygen. The activity of neither cotransporter changed in response to hypoxia-reoxygenation. In contrast, the Na(+)/K(+) pump responded to hypoxic treatment with reversible inhibition. Hypoxia-induced inhibition was abolished in Na(+)-loaded cells, revealing no effect of O(2) on the maximal operation rate of the pump. Notably, the inhibitory effect of hypoxia was not followed by changes in cellular ATP levels. Hypoxic exposure did, however, lead to a rapid increase in cellular glutathione (GSH) levels. Decreasing GSH to normoxic levels under hypoxic conditions abolished hypoxia-induced inhibition of the pump. Furthermore, GSH added to the incubation medium was able to mimic hypoxia-induced inhibition. Taken together these data suggest a pivotal role of intracellular GSH in oxygen-induced modulation of the Na(+)/K(+) pump activity.

The effect of mitochondrial inhibitors on membrane currents in isolated neonatal rat carotid body type I cells

Inhibitors of mitochondrial energy metabolism have long been known to be potent stimulants of the carotid body, yet their mechanism of action remains obscure. We have therefore investigated the effects of rotenone, myxothiazol, antimycin A, cyanide (CN(-)) and oligomycin on isolated carotid body type I cells. All five compounds caused a rapid rise in intracellular Ca(2+), which was inhibited on removal of extracellular Ca(2+). Under current clamp conditions rotenone and CN(-) caused a rapid membrane depolarization and elevation of [Ca(2+)](i). Voltage clamping cells to -70 mV substantially attenuated this rise in [Ca(2+)](i). Rotenone, cyanide, myxothiazol and oligomycin significantly inhibited resting background K(+) currents. Thus rotenone, myxothiazol, cyanide and oligomycin mimic the effects of hypoxia in that they all inhibit background K(+) current leading to membrane depolarization and voltage-gated calcium entry. Hypoxia, however, failed to have any additional effect upon membrane currents in the presence of CN(-) or rotenone or the mitochondrial uncoupler p-trifluoromethoxyphenyl hydrazone (FCCP). Thus not only do mitochondrial inhibitors mimic the effects of hypoxia, but they also abolish oxygen sensitivity. These observations suggest that there is a close link between oxygen sensing and mitochondrial function in type I cells. Mechanisms that could account for this link and the actions of mitochondrial inhibitors are discussed.

Mechanisms for Hypoxia Detection in O2-Sensitive Cells

Since O(2) is the bare necessity for multicellular organisms, they develop multiple protective mechanisms against hypoxia. Mammals will adapt to hypoxia in short and long terms. The short-term responses include enhancement of the respiratory and cardiac functions, adrenaline secretion from adrenal medullary cells, and pulmonary vasoconstriction, whereas the long-term response is the increase in erythropoietin production with the consequent increase in red blood cells. Although much work has been done to elucidate molecular mechanisms for O(2)-sensing for the last ten years, the majority of the mechanisms remain unclear. We will review mechanisms proposed for hypoxia detection in carotid body type I cells, pulmonary artery smooth muscle, adrenal medullary cells, and liver cells, with the special focus on adrenal medullary cells.

Role of ATP decrease in secretion induced by mitochondrial dysfunction in guinea-pig adrenal chromaffin cells

The mechanism related to mitochondrial dysfunction-induced catecholamine (CA) secretion in dispersed guinea-pig adrenal chromaffin cells was investigated using amperometry and confocal laser microscopy. Application of CCCP, which does not stimulate generation of reactive oxygen species (ROS), reversibly induced CA secretion, whereas application of either cyanide or oligomycin (OL), a stimulator for ROS, enhanced CA secretion to a smaller extent. The CCCP-induced secretion was abolished by removal of external Ca2+ ions and was markedly diminished by D600. The mitochondrial membrane potential, measured using rhodamine 123, was rapidly lost in response to CCCP, but did not change noticeably during a 3 min exposure to OL. Prior exposure to OL markedly facilitated depolarization of the mitochondrial membrane potential in response to cyanide. The mitochondrial inhibitors rapidly produced an increase in Magnesium Green (MgG) fluorescence in the absence of external Ca2+ and Mg2+ ions, an increase that was larger in the cytoplasm than in the nucleus. The rank order of potency in increasing MgG fluorescence among the inhibitors was similar to that in increasing secretion. Thus, mitochondrial inhibition rapidly decreases [ATP] and the mitochondrial dysfunction-induced secretion is not due to ROS generation or to mitochondrial depolarization, but is possibly mediated by a decrease in ATP.

Necrotic volume increase and the early physiology of necrosis

Whether a lethally injured mammalian cell undergoes necrosis or apoptosis may be determined by the early activation of specific ion channels at the cell surface. Apoptosis requires K+ and Cl- efflux, which leads to cell shrinking, an active phenomenon termed apoptotic volume decrease (AVD). In contrast, necrosis has been shown to require Na+ influx through membrane carriers and more recently through stress-activated non-selective cation channels (NSCCs). These ubiquitous channels are kept dormant in viable cells but become activated upon exposure to free-radicals. The ensuing Na+ influx leads to cell swelling, an active response that may be termed necrotic volume increase (NVI). This review focuses on how AVD and NVI become conflicting forces at the beginning of cell injury, on the events that determine irreversibility and in particular, on the ion fluxes that decide whether a cell is to die by necrosis or by apoptosis.

Effects of cyanide and deoxyglucose on Ca2+ signalling in macrovascular endothelial cells

1. We have studied the effects of the metabolic inhibitors cyanide (CN) and deoxyglucose (DG) on the intracellular Ca2+ concentration ([Ca2+]i) in macrovascular endothelial cells derived from human umbilical vein (EA cells). 2. CN- and DG increased [Ca2-]i in non-voltage clamped cells. This effect was dependent on extracellular Ca2+ concentration and membrane potential, indicating that CN- induced a Ca2+ entry. 3. During expose to CN- and/or DG, EA cells depolarise. This depolarisation is sometimes preceded by a small, but transient hyperpolarisation due to activation of a big - conductance K+ channels, BKCa, present in EA cells. However, in approximately 90% of the cells tested, the CN- and/or DG induced elevation of [Ca2+]i was insufficient to activate BKCa. 4. CN- and/or DG enhanced BKCa currents preactivated by an elevation of [Ca2+]i via cell dialysis with 0.5 and 1 microM, respectively. Thus, metabolic inhibition sensitises BKCa. 5. The CN- induced depolarisation of EA cells occurs by activating a current that reversed at positive membrane potentials. Substituting extracellular cations abolished the inward component of this current by NMDG, indicating that CN- activated a non-selective cation channel, NSC. This current was reduced by extracellular Ca2+ and Mg2'+ but is partially carried by Ca2+. 6. It is concluded that CN elevates [Ca2+]i by activating Ca2+ permeable NSC channels. The properties of these channels are similar to those of the recently described trp3 channels expressed in endothelium.

Pituitary adenylate cyclase-activating polypeptide may function as a neuromodulator in guinea-pig adrenal medulla

The role of pituitary adenylate cyclase-activating polypeptide (PACAP) in catecholamine secretion from dissociated adrenal chromaffin cells of the guinea-pig was investigated using amperometry, the patch clamp technique and immunochemistry. Pretreatment of adrenal chromaffin cells with 0.3-10 nM PACAP for 2 min resulted in enhancement of nicotine- and muscarine-induced secretions in either the presence of external Ca2+ ions or nominally Ca2+-free solution, with no change in basal secretion or the holding current at -60 mV in most of the cells tested. Pretreatment with PACAP augmented the muscarine-induced non-selective cation current, but did not affect the muscarine-induced outward current or nicotine-induced current. PACAP-induced enhancement of nicotine- and muscarine-induced secretions was suppressed by the simultaneous application of PACAP and the protein kinase inhibitors 100 microM HA1004 or 2 microM H89. Application of forskolin enhanced both muscarine- and nicotine-induced secretions, whereas application of a phorbol ester augmented the nicotine-induced secretion, but suppressed the muscarine-induced secretion in a reversible manner. Immunohistochemical analysis of adrenal medullae revealed that PACAP-like immunoreactivity was present in nerve fibres surrounding putative chromaffin cells. PAC1R-like immunoreactivity was distributed diffusely in the plasma membrane, whereas nicotinic ACh receptor-like immunoreactivity was concentrated at the plasma membrane near the nucleus, where the synapses were mainly localized. These observations suggest that PACAP in the guinea-pig adrenal medulla functions as a neuromodulator to facilitate ACh-induced secretion through a cAMP-protein kinase A-dependent pathway.

Mitochondrial inhibitors evoke catecholamine release from pheochromocytoma cells

Quantal catecholamine secretion evoked from individual pheochromocytoma (PC12) cells by exposure to mitochondrial inhibitors and uncouplers was monitored in real time using amperometry. Cyanide (0.05-5 mM) caused a concentration-dependent increase in the frequency of amperometric events. This secretory response was abolished by removal of extracellular Ca(2+) and by the application of Cd(2+) (200 microM), a nonselective blocker of voltage-gated Ca(2+) channels. Secretion was also inhibited by ca. 75% following pretreatment of cells with omega-conotoxin GVIA to inhibit N-type Ca(2+) channels selectively. Secretion was also detected when cells were exposed to rotenone (10 microM), dinitrophenol (250 microM) and p-trifluoromethoxyphenyl hydrazone (1 microM) and, as for cyanide, these secretory responses were abolished by removal of extracellular Ca(2+) or application of 200 microM Cd(2+). These results indicate that, like hypoxia, mitochondrial inhibitors and uncouplers evoke catecholamine secretion from PC12 cells which is wholly dependent on Ca(2+) influx through voltage-gated Ca(2+) channels.

Retardation of cation channel deactivation by mitochondrial dysfunction in adrenal medullary cells

The mechanism for cyanide (CN) activation of a nonselective cation (NS) channel coupled with a muscarinic receptor in a guinea pig chromaffin cell was studied with the perforated-patch method. Bath application of a protein kinase inhibitor resulted in a dose-dependent inhibition of muscarine-induced current (I(M)) but had no apparent effect on the CN-induced current (I(CN)). On the other hand, production of I(CN) occluded muscarine activation of NS channels in an amplitude-dependent manner. Deactivation of I(M) after washout was retarded while I(CN) was also active, and the extent of the retardation increased with an increase in the relative production of I(CN) on muscarinic stimulation. Restoration of Na(+) pump activity from CN suppression was conspicuously retarded below 19-20 degrees C, and the apparent diminution of I(M) and I(CN) after washout was retarded in parallel with a decrease in temperature. The results suggest that CN activation of NS channels is due to suppression of deactivation of the channel.