Hypoxia and cyanide induce depolarization and catecholamine release in dispersed guinea-pig chromaffin cells

Expression and function of mitochondrial inhibitor factor-1 and TASK channels in adrenal cells

Adrenal medullary chromaffin (AMC) cells in the perinatal period and carotid body glomus cells after birth respond to hypoxia with catecholamine secretion. The hypoxia detection mechanism in such O2-sensitive cells is still not well defined. One hypothesis is that a decrease in cellular ATP may be involved in the hypoxia detection. This idea is based on ATP dependence of TASK channel activity that regulates the resting membrane potential and is suppressed by hypoxia in glomus cells. Mitochondrial ATPase inhibitor factor-1 (IF1), a physiological regulator of ATP synthase, helps prevent ATP hydrolysis under hypoxic conditions. In cells where IF1 expression is high, exposure to hypoxia is expected to have no effect on TASK channel activity. This possibility was electrophysiologically and immunocytochemically explored. Single channel recordings revealed that 36-pS TASK3-like channels contribute to the resting membrane potential in young rat adrenal cortical (AC) cells. TASK3-like channel activity in a cell-attached patch was not affected by bath application of mitochondrial inhibitors. Consistent with this finding, IF1-like immunoreactive material was well expressed in rat AC cells. In further support of our hypothesis, IF1-like immunoreactive material was well expressed in adult rat AMC cells that are known to be hypoxia-insensitive and minimally expressed in newborn AMC cells that are hypoxia-sensitive. These results provide evidence for the functional relevance of IF1 expression in excitability in O2-sensitive cells in response to mitochondrial inhibition.

Adrenal Medulla Chemo Sensitivity Does Not Compensate the Lack of Hypoxia Driven Carotid Body Chemo Reflex in Guinea Pigs

Guinea pigs (GP), originally from the Andes, have absence of hypoxia-driven carotid body (CB) reflex. Neonatal mammals have an immature CB chemo reflex and respond to hypoxia with metabolic changes arising from direct effects of hypoxia on adrenal medulla (AM). Our working hypothesis is that adult GP would mimic neonatal mammals. Plasma epinephrine (E) has an AM origin, while norepinephrine (NE) is mainly originated in sympathetic endings, implying that specific GP changes in plasma E/NE ratio, and in blood glucose and lactate levels during hypoxia would be observed. Experiments were performed on young adult GP and rats. Hypoxic ventilation (10% O₂) increased E and NE plasma levels similarly in both species but PaO₂ was lower in GP than in rats. Plasma E/NE ratio in GP was higher (≈1.0) than in rats (≈0.5). The hypoxia-evoked increases in blood glucose and lactate were smaller in GP than in the rat. The AM of both species contain comparable E content, but NE was four times lower in GP than in rats. GP superior cervical ganglion also had lower NE content than rats and an unusual high level of dopamine, a negative modulator of sympathetic transmission. Isolated AM from GP released half of E and one tenth of NE than the rat AM, and hypoxia did not alter the time course of CA outflow. These data indicate the absence of direct effects of hypoxia on AM in the GP, and a lower noradrenergic tone in this species. Pathways for hypoxic sympatho-adrenal system activation in GP are discussed.

Oxygen sensing and stem cell activation in the hypoxic carotid body

The carotid body (CB) is the major arterial chemoreceptor responsible for the detection of acute decreases in O₂ tension (hypoxia) in arterial blood that trigger hyperventilation and sympathetic activation. The CB contains O₂-sensitive glomus (chief) cells, which respond to hypoxia with the release of transmitters to activate sensory nerve fibers impinging upon the brain respiratory and autonomic centers. During exposure to sustained hypoxia (for weeks or months), the CB grows several-fold in size, a response associated with acclimatization to high altitude or to medical conditions presenting hypoxemia. Here, I briefly present recent advances on the mechanisms underlying glomus cell sensitivity to hypoxia, in particular the role of mitochondrial complex I in acute oxygen sensing. I also summarize the properties of adult CB stem cells and of glomus cell-stem cell synapses, which contribute to CB hypertrophy in chronic hypoxia. A note on the relationship between hypoxic CB growth and tumorigenesis is included. Finally, the medical implications of CB pathophysiology are discussed.

Gene expression analyses reveal metabolic specifications in acute O2 -sensing chemoreceptor cells

KEY POINTS

Glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM) are essential for reflex cardiorespiratory adaptation to hypoxia. However, the mechanisms whereby these cells detect changes in O₂ tension are poorly understood. The metabolic properties of acute O₂ -sensing cells have been investigated by comparing the transcriptomes of CB and AM cells, which are O₂ -sensitive, with superior cervical ganglion neurons, which are practically O₂ -insensitive. In O₂ -sensitive cells, we found a characteristic prolyl hydroxylase 3 down-regulation and hypoxia inducible factor 2α up-regulation, as well as overexpression of genes coding for three atypical mitochondrial electron transport subunits and pyruvate carboxylase, an enzyme that replenishes tricarboxylic acid cycle intermediates. In agreement with this observation, the inhibition of succinate dehydrogenase impairs CB acute O₂ sensing. The responsiveness of peripheral chemoreceptor cells to acute hypoxia depends on a 'signature metabolic profile'.

ABSTRACT

Acute O₂ sensing is a fundamental property of cells in the peripheral chemoreceptors, e.g. glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM), and is necessary for adaptation to hypoxia. These cells contain O₂ -sensitive ion channels, which mediate membrane depolarization and transmitter release upon exposure to hypoxia. However, the mechanisms underlying the detection of changes in O₂ tension by cells are still poorly understood. Recently, we suggested that CB glomus cells have specific metabolic features that favour the accumulation of reduced quinone and the production of mitochondrial NADH and reactive oxygen species during hypoxia. These signals alter membrane ion channel activity. To investigate the metabolic profile characteristic of acute O₂ -sensing cells, we used adult mice to compare the transcriptomes of three cell types derived from common sympathoadrenal progenitors, but exhibiting variable responsiveness to acute hypoxia: CB and AM cells, which are O₂ -sensitive (glomus cells > chromaffin cells), and superior cervical ganglion neurons, which are practically O₂ -insensitive. In the O₂ -sensitive cells, we found a characteristic mRNA expression pattern of prolyl hydroxylase 3/hypoxia inducible factor 2α and up-regulation of several genes, in particular three atypical mitochondrial electron transport subunits and some ion channels. In addition, we found that pyruvate carboxylase, an enzyme fundamental to tricarboxylic acid cycle anaplerosis, is overexpressed in CB glomus cells. We also observed that the inhibition of succinate dehydrogenase impairs CB acute O₂ sensing. Our data suggest that responsiveness to acute hypoxia depends on a 'signature metabolic profile' in chemoreceptor cells.

Low pHo boosts burst firing and catecholamine release by blocking TASK-1 and BK channels while preserving Cav1 channels in mouse chromaffin cells

KEY POINTS

Mouse chromaffin cells (MCCs) generate spontaneous burst-firing that causes large increases of Ca²⁺ -dependent catecholamine release, and is thus a key mechanism for regulating the functions of MCCs. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pH_o ) from 7.4 to 6.6 induces cell depolarizations of 10-15 mV that generate bursts of ∼330 ms at 1-2 Hz and a 7.4-fold increase of cumulative catecholamine-release. Burst-firing originates from the inhibition of the pH-sensitive TASK-1-channels and a 60% reduction of BK-channel conductance at pH_o 6.6. Blockers of the two channels (A1899 and paxilline) mimic the effects of pH_o 6.6, and this is reverted by the Cav1 channel blocker nifedipine. MCCs act as pH-sensors. At low pH_o , they depolarize, undergo burst-firing and increase catecholamine-secretion, generating an effective physiological response that may compensate for the acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.

ABSTRACT

Mouse chromaffin cells (MCCs) generate action potential (AP) firing that regulates the Ca²⁺ -dependent release of catecholamines (CAs). Recent findings indicate that MCCs possess a variety of spontaneous firing modes that span from the common 'tonic-irregular' to the less frequent 'burst' firing. This latter is evident in a small fraction of MCCs but occurs regularly when Nav1.3/1.7 channels are made less available or when the Slo1β2-subunit responsible for BK channel inactivation is deleted. Burst firing causes large increases of Ca²⁺ -entry and potentiates CA release by ∼3.5-fold and thus may be a key mechanism for regulating MCC function. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pH_o ) from 7.4 to 7.0 and 6.6 induces cell depolarizations of 10-15 mV that generate repeated bursts. Bursts at pH_o 6.6 lasted ∼330 ms, occurred at 1-2 Hz and caused an ∼7-fold increase of CA cumulative release. Burst firing originates from the inhibition of the pH-sensitive TASK-1/TASK-3 channels and from a 40% BK channel conductance reduction at pH_o 7.0. The same pH_o had little or no effect on Nav, Cav, Kv and SK channels that support AP firing in MCCs. Burst firing of pH_o 6.6 could be mimicked by mixtures of the TASK-1 blocker A1899 (300 nm) and BK blocker paxilline (300 nm) and could be prevented by blocking L-type channels by adding 3 μm nifedipine. Mixtures of the two blockers raised cumulative CA-secretion even more than low pH_o (∼12-fold), showing that the action of protons on vesicle release is mainly a result of the ionic conductance changes that increase Ca²⁺ -entry during bursts. Our data provide direct evidence suggesting that MCCs respond to low pH_o with sustained depolarization, burst firing and enhanced CA-secretion, thus mimicking the physiological response of CCs to acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.

Src mediates endocytosis of TWIK-related acid-sensitive K+ 1 channels in PC12 cells in response to nerve growth factor

TWIK-related acid-sensitive K+ (TASK) channels produce background K+ currents. We elucidated that TASK1 channels in rat adrenal medullary cells and PC12 cells are internalized in a clathrin-dependent manner in response to nerve growth factor (NGF). Here, the molecular mechanism for this internalization in PC12 cells was explored. The combination of enzyme inhibitors with tropomyosin receptor kinase A mutants revealed that the internalization was mediated by both phospholipase C and phosphatidylinositol 3-kinase pathways that converge on protein kinase C with the consequent activation of Src, a nonreceptor tyrosine kinase. The NGF-induced endocytosis of TASK1 channels did not occur in the presence of the Src inhibitor or with the expression of a kinase-dead Src mutant. Additionally, NGF induced a transient colocalization of Src with the TASK1 channel, but not the TASK1 mutant, in which tyrosine at 370 was replaced with phenylalanine. This TASK1 mutant showed no increase in tyrosine phosphorylation and markedly diminished internalization in response to NGF. We concluded that NGF induces endocytosis of TASK1 channels via tyrosine phosphorylation in its carboxyl terminus.

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.

Identification of muscarinic receptor subtypes involved in catecholamine secretion in adrenal medullary chromaffin cells by genetic deletion

BACKGROUND AND PURPOSE

Activation of muscarinic receptors results in catecholamine secretion in adrenal chromaffin cells in many mammals, and muscarinic receptors partly mediate synaptic transmission from the splanchnic nerve, at least in guinea pigs. To elucidate the physiological functions of muscarinic receptors in chromaffin cells, it is necessary to identify the muscarinic receptor subtypes involved in excitation.

EXPERIMENTAL APPROACH

To identify muscarinic receptors, pharmacological tools and strains of mice where one or several muscarinic receptor subtypes were genetically deleted were used. Cellular responses to muscarinic stimulation in isolated chromaffin cells were studied with the patch clamp technique and amperometry.

KEY RESULTS

Muscarinic M₁, M₄ and M₅ receptors were immunologically detected in mouse chromaffin cells, and these receptors disappeared after the appropriate gene deletion. Mouse cells secreted catecholamines in response to muscarinic agonists, angiotensin II and a decrease in external pH. Genetic deletion of M₁, but not M₃, M₄ or M₅, receptors in mice abolished secretion in response to muscarine, but not to other stimuli. The muscarine-induced secretion was suppressed by MT7, a snake peptide toxin specific for M₁ receptors. Similarly, muscarine failed to induce an inward current in the presence of MT7 in mouse and rat chromaffin cells. The binding affinity of VU0255035 for the inhibition of muscarine-induced currents agreed with that for the M₁ receptor.

CONCLUSIONS AND IMPLICATIONS

Based upon the effects of genetic deletion of muscarinic receptors and MT7, it is concluded that the M₁ receptor alone is responsible for muscarine-induced catecholamine secretion.

A highly efficient dinuclear Cu(II) chemosensor for colorimetric and fluorescent detection of cyanide in water

A novel dinuclear copper chemosensor selectively binds cyanide over a wide range of inorganic anions, enabling it to detect cyanide in water up to 0.02 ppm which is 10 times lower than the EPA standard for drinking water.

Divergent roles of reactive oxygen species in the responses of perinatal adrenal chromaffin cells to hypoxic challenges

The fetus and neonate experience variable patterns of low P(O)₂(hypoxia) ranging from acute, sustained, and intermittent. Adaptation to hypoxia involves activation of key transcription factors, known as hypoxia-inducible factors (e.g. HIF-1α, HIF-2α), which regulate a number of genes in different cell types. This review focuses on the signaling pathways that mediate proper physiological responses of perinatal adrenomedullary chromaffin cells (AMC) to varying patterns of hypoxic challenges, and particularly on the controversial role of reactive oxygen species (ROS). At birth, acute hypoxia (seconds to minutes) directly stimulates catecholamine release from AMC via K+ channel inhibition, mediated by a decrease in mitochondrial-derived ROS. By contrast, exposure to chronic sustained hypoxia (CSH) induces HIF-2α in a fetal-derived chromaffin cell line independently of changes in ROS. Exposure to chronic intermittent hypoxia (CIH) activates antioxidant responses via the regulator Nrf-2, in association with an increase in ROS and the induction of HIF-1α. We propose that the physiological responses of perinatal AMC to hypoxia and the ensuing directional changes in ROS are dependent on the pattern and duration of the hypoxic exposure.

Neonatal intermittent hypoxia leads to long-lasting facilitation of acute hypoxia-evoked catecholamine secretion from rat chromaffin cells

The objective of the present study was to examine the effects of intermittent hypoxia (IH) and sustained hypoxia (SH) on hypoxia-evoked catecholamine (CA) secretion from chromaffin cells in neonatal rats and assess the underlying mechanism(s). Experiments were performed on rat pups exposed to either IH (15-s hypoxia/5-min normoxia; 8 h/day) or SH (hypobaric hypoxia, 0.4 atm) or normoxia (controls) from P0 to P5. IH treatment facilitated hypoxia-evoked CA secretion and elevations in the intracellular calcium ion concentration ([Ca(2+)](i)) and these responses were attenuated, but not abolished, by treatments designed to eliminate Ca(2+) flux into cells (Ca(2+)-free medium or Cd(2+)), indicating that intracellular Ca(2+) stores were augmented by IH. Norepinephrine (NE) and epinephrine (E) levels of adrenal medullae were elevated in IH-treated pups. IH treatment increased reactive oxygen species (ROS) production in adrenal medullae and antioxidant treatment prevented IH-induced facilitation of CA secretion, elevations in [Ca(2+)](i) by hypoxia, and the up-regulation of NE and E. The effects of neonatal IH treatment on hypoxia-induced CA secretion and elevation in [Ca(2+)](i), CA, and ROS levels persisted in rats reared under normoxia for >30 days. In striking contrast, chromaffin cells from SH-treated animals exhibited attenuated hypoxia-evoked CA secretion. In SH-treated cells hypoxia-evoked elevations in [Ca(2+)](i), NE and E contents, and ROS levels were comparable with controls. These observations demonstrate that: 1) neonatal IH and SH evoke opposite effects on hypoxia-evoked CA secretion from chromaffin cells, 2) ROS signaling mediates the faciltatory effects of IH, and 3) the effects of neonatal IH on chromaffin cells persist into adult life.

Developmental change of T-type Ca2+ channel expression and its role in rat chromaffin cell responsiveness to acute hypoxia

Neonatal chromaffin cells of the adrenal medulla (AM) are intrinsic chemoreceptors that secrete catecholamines in response to hypoxia, thus contributing to fetal adaptation to extrauterine life. In most mammals studied, oxygen sensitivity of AM cells disappears a few days after birth, possibly due to innervation of the adrenal gland by the cholinergic fibres of the splanchnic nerve (approximately postnatal day 7 in the rat). The mechanisms underlying these homeostatic changes in chromaffin cells are unknown. Low voltage-activated, T-type, Ca(2+) channels regulate cell excitability and their expression is up-regulated by hypoxia. Hence, we hypothesized that these channels contribute to the developmental changes in the chemoreceptive properties of AM chromaffin cells. Using electrophysiological, immunocytochemical and molecular biology methodologies we show here that neonatal AM chromaffin cells express T-type Ca(2+) channels (of alpha1H or Ca(v)3.2 sub-type) and that the function of these channels is necessary for catecholamine release in response to acute hypoxia. T-type Ca(2+) channel expression, as well as chromaffin cell responsiveness to hypoxia, decrease with postnatal maturation. Adult chromaffin cell sensitivity to hypoxia reappears after AM denervation in parallel with the recruitment of T-type Ca(2+) channels. These observations indicate that T-type Ca(2+) channels are essential for the acute response of chromaffin cells to hypoxia and help explain the disappearance of O(2) sensitivity in adult AM chromaffin cells. Our results may also be relevant for understanding the pathogenesis of disorders associated with chronic hypoxia or maternal nicotine consumption.

Modulation of TASK-like background potassium channels in rat arterial chemoreceptor cells by intracellular ATP and other nucleotides

The carotid body's physiological role is to sense arterial oxygen, CO(2) and pH. It is however, also powerfully excited by inhibitors of oxidative phosphorylation. This latter observation is the cornerstone of the mitochondrial hypothesis which proposes that oxygen is sensed through changes in energy metabolism. All of these stimuli act in a similar manner, i.e. by inhibiting a background TASK-like potassium channel (K(B)) they induce membrane depolarization and thus neurosecretion. In this study we have evaluated the role of ATP in modulating K(B) channels. We find that K(B) channels are strongly activated by MgATP (but not ATP(4)(-)) within the physiological range (K(1/2) = 2.3 mm). This effect was mimicked by other Mg-nucleotides including GTP, UTP, AMP-PCP and ATP-gamma-S, but not by PP(i) or AMP, suggesting that channel activity is regulated by a Mg-nucleotide sensor. Channel activation by MgATP was not antagonized by either 1 mm AMP or 500 microm ADP. Thus MgATP is probably the principal nucleotide regulating channel activity in the intact cell. We therefore investigated the effects of metabolic inhibition upon both [Mg(2+)](i), as an index of MgATP depletion, and channel activity in cell-attached patches. The extent of increase in [Mg(2+)](i) (and thus MgATP depletion) in response to inhibition of oxidative phosphorylation were consistent with a decline in [MgATP](i) playing a prominent role in mediating inhibition of K(B) channel activity, and the response of arterial chemoreceptors to metabolic compromise.

Developmental changes of chromaffin cell secretory response to hypoxia studied in thin adrenal slices

Adrenomedullary chromaffin (AMC) cells are sensitive to hypoxia in the newborn, but whether this property is lost during postnatal maturation is a matter of controversy. We have developed a rat adrenal slice preparation that allows the study of neonatal and adult AMC cell sensitivity to hypoxia in almost optimal physiological conditions. Responses to secretagogues can be quantitatively and noninvasively monitored in intact cells by amperometry. We have found hypoxia "responsive" (R) and "non-responsive" AMC cells in both neonatal (P0-P8) and juvenile/adult (P12-P60) adrenal glands. However, in the neonate, the proportion of R cells and the magnitude of the response to hypoxia were larger than in the adult. This developmental change of hypoxia responsiveness did not seem to depend on a decrease of the AMC cell's excitability. Spontaneous secretory activity in slices from adult rats was even increased with respect to neonatal animals. The analysis of the secretory events suggests that changes in spike frequency, rather than in vesicle size, account for the increased basal secretion rate in adult AMC cells. Thus, we report a major, but not complete, loss of direct hypoxia sensitivity in adult AMC cells. The adrenal slice appears to be a valuable technique to study acute O(2) sensing and its modifications in pathophysiological states.

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.

Circulating Catecholamines and Cardiorespiratory Responses in Hypoxic Lungfish (Protopterus dolloi): A Comparison of Aquatic and Aerial Hypoxia

Circulating catecholamine levels and a variety of cardiorespiratory variables were monitored in cannulated bimodally breathing African lungfish (Protopterus dolloi) exposed to aquatic or aerial hypoxia. Owing to the purported absence of external branchial chemoreceptors in lungfish and the minor role played by the gill in O2 uptake, it was hypothesized that plasma catecholamine levels would increase only during exposure of fish to aerial hypoxia. The rapid induction of aquatic hypoxia (final PWo2 = 25.9+/-1.6 mmHg) did not affect the levels of adrenaline (A) or noradrenaline (NA) within the plasma. Similarly, none of the measured cardiorespiratory variables--including heart rate (fH), blood pressure, air-breathing frequency (fV), O2 consumption (Mo2), CO2 excretion (Mco2), or blood gases--were influenced by acute aquatic hypoxia. In contrast, however, the rapid induction of aerial hypoxia (inspired Po2=46.6+/-3.3 mmHg) caused a marked increase in the circulating levels of A (from 7.9+/-2.0 to 18.8+/-6.1 nmol L(-1)) and NA (from 7.7+/-2.2 to 19.7+/-6.3 nmol L(-1)) that was accompanied by significant decreases in Mo2, arterial Po2 (Pao2), and arterial O2 concentration (Cao2). Air-breathing frequency was increased (by approximately five breaths per hour) during aerial hypoxia and presumably contributed to the observed doubling of pulmonary Mco2 (from 0.25+/-0.04 to 0.49+/-0.07 mmol kg(-1) h(-1)); fH and blood pressure were unaffected by aerial hypoxia. An in situ perfused heart preparation was used to test the possibility that catecholamine secretion from cardiac chromaffin cells was being activated by a direct localized effect of hypoxia. Catecholamine secretion from the chromaffin cells of the heart, while clearly responsive to a depolarizing concentration of KCl (60 mmol L(-1)), was unaffected by the O2 status of the perfusion fluid. The results of this study demonstrate that P. dolloi is able to mobilize stored catecholamines and increase f(V) during exposure to aerial hypoxia while remaining unresponsive to aquatic hypoxia. Thus, unlike in exclusively water-breathing teleosts, P. dolloi would appear to rely solely on internal/airway O2 chemoreceptors for initiating catecholamine secretion and cardiorespiratory responses.

InsP3 receptor type 2 and oscillatory and monophasic Ca2+ transients in rat adrenal chromaffin cells

Muscarinic receptor stimulation induced oscillatory and monophasic Ca(2+) transients in rat adrenal chromaffin cells in the absence of external Ca(2+). As this Ca(2+) mobilization may be mediated by InsP(3), we first explored types of InsP(3) receptors and their intracellular distribution in chromaffin cells. The InsP(3) receptor type 1 was not immunodetected in precipitates of adrenal medulla homogenates and in dissociated adrenal chromaffin cells, whereas an anti-type 3 mAb recognized a faint band with about 250 kDa, but no significant immunoreaction was visible in chromaffin cells. The anti-type 2 mAb strongly detected a band with about 220 kDa and the immunoreaction was observed perinuclearly and at the cell periphery. These results indicate that InsP(3) receptor type 2 is predominant in chromaffin cells. The oscillatory and monophasic Ca(2+) transients were reproduced in simulation based on a three-state kinetic model (shut, open, and inactivated states). Ca(2+) ions were found experimentally and theoretically to turn over rapidly between stores and the cytosol during stimulation. The results suggest that InsP(3) receptor type 2 is responsible for both oscillatory and monophasic Ca(2+) transients and that change in mode of Ca(2+) responses may be accounted for by the kinetic property of the type 2 receptor.

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.

A possible dual site of action for carbon monoxide-mediated chemoexcitation in the rat carotid body

High tensions of carbon monoxide (CO), relative to oxygen, were used as a tool to investigate the mechanism of chemotransduction. In an in vitro whole organ, rat carotid body preparation, CO increased sinus nerve chemoafferent discharge in the dark, an effect that was significantly reduced (by ca 70 %) by bright white light and by the removal of extracellular Ca(2+) from the superfusate or by the addition of either Ni(2+) (2 mM) or methoxyverapamil (100 microM). Addition of the P(2) purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (50 microM) also significantly reduced the neural response to CO. In perforated patch, whole-cell recordings of isolated rat type I cells, CO induced a depolarisation of ca 11 mV and a decrease in the amplitude of an outward current around and above the resting membrane potential. Membrane conductance between -50 and -60 mV was significantly reduced by ca 40 % by CO. These effects were not photolabile and were present also when a 'blocking solution' containing TEA, 4-AP, Ni(2+) and zero extracellular Ca(2+) was used. In conventional whole-cell recordings, CO only decreased current amplitudes above +10 mV and was without effect around the resting membrane potential. These data demonstrate a direct effect of CO upon type I cell K(+) conductances and strongly suggest an effect upon a background, leak conductance that requires an intracellular mediator. The photolabile effect of CO only upon afferent neural discharge adds further evidence to a dual site of action of CO with a separate action at the afferent nerve terminal that, additionally, requires the permissive action of the neurotransmitter ATP.

Hypoxia induces aortic hypertrophic growth, left ventricular dysfunction, and sympathetic hyperinnervation of peripheral arteries in the chick embryo

BACKGROUND

Low birth weight is associated with an increased incidence of cardiovascular diseases, including hypertension, later in life. This suggests that antenatal insults program for fetal adaptations of the circulatory system. In the present study, we evaluated the effects of mild hypoxia on cardiac function, blood pressure control, and arterial structure and function in near-term chick embryos.

METHODS AND RESULTS

Chick embryos were incubated under normoxic (21% O2) or hypoxic (15% O2) conditions and evaluated at incubation day 19 by use of histological techniques, isolated heart preparations, and in vivo measurements of sympathetic arterial tone and systemic hemodynamics. Chronic hypoxia caused a 33% increase in mortality and an 11% reduction in body weight in surviving embryos. The lumen of the ascending aorta in hypoxic embryos was 23% smaller. Left ventricular systolic pressure was 22% lower, and heart weight/body weight ratio was 14% higher. In resistance arteries of hypoxic embryos, in vivo baseline tone was 23% higher, norepinephrine sensitivity was similar, and norepinephrine release from sympathetic nerves increased 2-fold, indicating sympathetic hyperinnervation. Mean arterial pressure and heart rate were similar under resting conditions, but chronically hypoxic embryos failed to maintain blood pressure during acute stress.

CONCLUSIONS

This study indicates that mild hypoxia during embryonic development induces alterations in cardiac and vascular function and structure and affects hemodynamic regulation. These findings reveal that antenatal insults have profound effects on the control and design of the circulatory system that are already established at birth and may program for hypertension and heart failure at a later age.

Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M-current

The current study has investigated the electrophysiological responses evoked by histamine in bovine adrenal chromaffin cells using perforated-patch techniques. Histamine caused a transient hyperpolarization followed by a sustained depolarization of 7.2 +/- 1.4 mV associated with an increase in spontaneous action potential frequency. The hyperpolarization was abolished after depleting intracellular Ca(2+) stores with thapsigargin (100 nM), and was reduced by 40 % with apamin (100 nM). Membrane resistance increased by about 60 % during the histamine-induced depolarization suggesting inhibition of a K(+) channel. An inward current relaxation, typical of an M-current, was observed in response to negative voltage steps from a holding potential of -30 mV. This current reversed at -81.6 +/- 1.8 mV and was abolished by the M-channel inhibitor linopirdine (100 microM). During application of histamine, the amplitude of M-currents recorded at a time corresponding with the sustained depolarization was reduced by 40 %. No inward current rectification was observed in the range -150 to -70 mV, and glibenclamide (10 microM) had no effect on either resting membrane potential or the response to histamine. The results show that an M-current is present in bovine chromaffin cells and that this current is inhibited during sustained application of histamine, resulting in membrane depolarization and increased discharge of action potentials. These results demonstrate for the first time a possible mechanism coupling histamine receptors to activation of voltage-operated Ca(2+) channels in these 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.

The importance of adrenocortical glucocorticoids for adrenomedullary and physiological response to stress: a study in isolated glucocorticoid deficiency

Glucocorticoids are required for the normal functioning of chromaffin cells and their capacity to produce epinephrine. This was modeled in a unique clinical syndrome of isolated glucocorticoid deficiency due to unresponsiveness to ACTH. The working hypotheses were that in patients with isolated glucocorticoid deficiency, adrenomedullary epinephrine would be suppressed despite replacement therapy; that norepinephrine might show a compensatory response; and that the physiological response to stress would reflect these changes. Toward these hypotheses, patients with ACTH unresponsiveness on glucocorticoid replacement were subjected to three levels of acute stress: assumption of upright posture, cold pressor, and exercise. Their catecholamine and physiological response were monitored. Patients with isolated glucocorticoid deficiency of this study had severe adrenomedullary dysfunction, characterized by a minimal resting production of epinephrine (6 +/- 2 pg/ml compared with 64 +/- 22 pg/ml of the controls) and a minimal response to stress. A slight compensatory increase of norepinephrine was found in response to cold pressor test (754 +/- 200 pg/ml compared with 431 +/- 73 pg/ml of the control). The physiological response is characterized by low systolic blood pressure and high pulse rate in rest and mild stress and in a pressor response to exercise (diastolic 87 +/- 5 mm Hg, compared with 73 +/- 2 mm Hg of the control). It is concluded that intra-adrenal glucocorticoids are essential for epinephrine secretion, that norepinephrine may be compensatory, and that these result in a distinct physiological response. The implications of the pressor response to exercise, the declining pulse pressure, and the increased pulse response insinuate a lower physical fitness in patients with adrenal insufficiency.

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.

Either desipramine or TMB-8 suppresses cyanide-induced norepinephrine efflux from in vivo cardiac sympathetic nerves of cats

To investigate the effect of hypoxia on endogenous norepinephrine (NE) release from cardiac sympathetic nerve ending, we administered sodium cyanide (NaCN) for 30 min into the myocardial interstitial space through a dialysis probe and measured dialysate NE levels. During the NaCN perfusion, a marked and concentration-dependent increase in dialysate NE was observed. This cyanide-induced NE response was suppressed by pretreatment with despiramine (a membraneous NE transport inhibitor). Furthermore, the cyanide-induced NE response was suppressed by pretreatment with TMB-8 (intracellular Ca(2+) antagonist) but unaffected by omega-conotoxin GVIA (NE releasing inhibitor). Our data suggest that two (desipramine or TMB-8 suppressive) mechanisms contributed to the amount of NE efflux induced by cyanide in in vivo cardiac sympathetic nerve.

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.

Mitochondria regulate the Ca(2+)-exocytosis relationship of bovine adrenal chromaffin cells

The present study expands the contemporary view of mitochondria as important participants in cellular Ca(2+) dynamics and provides evidence that mitochondria regulate the supply of release-competent secretory granules. Using pharmacological probes to inhibit mitochondrial Ca(2+) import, the ability of mitochondria to modulate secretory activity in single, patch-clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (DeltaC(m)) and cytosolic Ca(2+) levels ([Ca(2+)](c)). Repetitive step depolarizations or action potential waveforms were found to raise the [Ca(2+)](c) of chromaffin cells into the 1 microM to tens of micromolar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(trifuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revealed that mitochondria are a prominent component for the clearance of Ca(2+) that entered via voltage-activated Ca(2+) channels. Disruption of cellular Ca(2+) homeostasis by poisoning mitochondria enhanced the secretory responsiveness of chromaffin cells by increasing the amplitude of the transient rise and the time course of recovery to baseline of the evoked Delta[Ca(2+)](c). The enhancement of the secretory response was represented by significant deviation of the Ca(2+)-exocytosis relationship from a standard relationship that equates Ca(2+) influx and DeltaC(m). Thus, mitochondria would play a critical role in the control of secretory activity in chromaffin cells that undergo tonic or repetitive depolarizing activity, likely by limiting the Ca(2+)-dependent activation of specific proteins that recruit or prime secretory granules for exocytosis.

Mitochondria regulate the Ca(2+)-exocytosis relationship of bovine adrenal chromaffin cells

The present study expands the contemporary view of mitochondria as important participants in cellular Ca(2+) dynamics and provides evidence that mitochondria regulate the supply of release-competent secretory granules. Using pharmacological probes to inhibit mitochondrial Ca(2+) import, the ability of mitochondria to modulate secretory activity in single, patch-clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (DeltaC(m)) and cytosolic Ca(2+) levels ([Ca(2+)](c)). Repetitive step depolarizations or action potential waveforms were found to raise the [Ca(2+)](c) of chromaffin cells into the 1 microM to tens of micromolar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(trifuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revealed that mitochondria are a prominent component for the clearance of Ca(2+) that entered via voltage-activated Ca(2+) channels. Disruption of cellular Ca(2+) homeostasis by poisoning mitochondria enhanced the secretory responsiveness of chromaffin cells by increasing the amplitude of the transient rise and the time course of recovery to baseline of the evoked Delta[Ca(2+)](c). The enhancement of the secretory response was represented by significant deviation of the Ca(2+)-exocytosis relationship from a standard relationship that equates Ca(2+) influx and DeltaC(m). Thus, mitochondria would play a critical role in the control of secretory activity in chromaffin cells that undergo tonic or repetitive depolarizing activity, likely by limiting the Ca(2+)-dependent activation of specific proteins that recruit or prime secretory granules for exocytosis.

Responses to hypoxia of petrosal ganglia in vitro

NaCN is a classical stimulus used to elicit discharges from carotid body chemoreceptors. The effect is assumed to be mediated by glomus (type I) cells, which release an excitatory transmitter for the excitation of carotid nerve endings. Since the sensory perikarya of the glossopharyngeal nerve (from which the carotid nerve branches) are located in the petrosal ganglion, we tested whether application of this drug to the petrosal ganglion superfused in vitro elicits antidromic discharges in the carotid nerve. NaCN did indeed cause an intense and prolonged burst of nerve impulses in the carotid nerve, while provoking a less intense and much briefer burst of discharges in the glossopharyngeal branch. Carotid nerve responses to NaCN were reduced and shortened by prior or following application of dopamine to the ganglion. Sodium azide applied to the petrosal ganglion evoked a less intense and much briefer burst of impulses in the carotid nerve. Ganglionar application of 2,4-dinitrophenol did not induce discharges in the carotid nerve. Switching the superfusion of the ganglion from a normoxic to a hypoxic solution did not evoke discharges in the carotid nerve. Therefore, the perikarya of carotid nerve neurons are sensitive to NaCN, but are not excited by reducing the pO(2) of the superfusing solution.

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

1. Hypoxia and metabolic inhibition with cyanide (CN) evoke catecholamine secretion in adrenal chromaffin cells through depolarization. We elucidated mechanisms for a CN- or anoxia-induced inward (depolarization) current, using the perforated patch method. 2. Bath application of Ba2+ induced a dose-dependent inhibition of a muscarine-induced current (IMUS) and part of the CN-induced current (ICN) with an IC50 (concentration responsible for 50 % inhibition) of 1.3 mM. The Ba2+-sensitive component was estimated to comprise 58 % of the total ICN. 3. The Ba2+-resistant component of ICN tended to increase with shifts of membrane potential from -40 to 40 mV and was markedly suppressed by exposure to a K+-free solution or 200 microM ouabain, indicating that the majority of the Ba2+-resistant component of ICN is due to suppression of the Na+ pump current (Ipump). 4. The non-Ipump component of ICN diminished progressively in K+-free solution. Substitution of glucose for sucrose in a K+-free CN solution further diminished the CN potency to produce the non-Ipump component. 5. The I-V relationship for the non-Ipump component of ICN had a reversal potential of -3 and -47 mV at 147 and 5.5 mM Na+, respectively, and showed an outward rectification, indicating that the non-Ipump component of ICN is due to activation of non-selective cation channels. 6. Exposure to anoxia induced a current with an amplitude comparable to that of ICN, and the anoxia-induced current apparently occluded development of ICN. The anoxia-induced current diminished by ca 60 % in the absence of K+ and reversed polarity at 5 mV under K+-free conditions. 7. It is concluded that exposure to CN and to anoxia induces suppression of the Na+ pump and activation of non-selective cation channels, probably due to an ATP decrease resulting mainly from consumption by the Na+ pump.

Chronic hypoxia enhances the secretory response of rat phaeochromocytoma cells to acute hypoxia

1. Amperometric recordings were made from individual phaeochromocytoma (PC12) cells using carbon fibre microelectrodes to investigate the effects of chronic hypoxia (10% O2) on the secretory responses evoked by acute hypoxia. 2. Exposure to chronic hypoxia for 21-26 h increased the frequency of exocytotic events evoked in response to acute hypoxia (PO2 ca 10-60 mmHg). 3. Chronic hypoxia increased the value of Q1/3, determined by the integration of amperometric events, indicating an increase in quantal size: this reflects either an increase in vesicular dimensions or vesicular catecholamine concentration. 4. Exocytotic frequency evoked by bath application of tetraethylammonium (1-10 mM) was significantly enhanced following chronic hypoxia. 5. In both control and chronically hypoxic PC12 cells, exocytosis in response to acute hypoxia was completely abolished in Ca2+-free solutions. Cd2+ (200 microM) completely inhibited exocytosis from control cells, but left a significant residual release in chronically hypoxic PC12 cells. 6. The Cd2+-resistant release evoked by acute hypoxia in chronically hypoxic PC12 cells was inhibited by inorganic ions (0.01-10 mM) in a potency order of La3+ > Gd3+ > Zn2+. Ni2+ (10 mM) was without effect. 7. Our results suggest that chronic hypoxia enhances the secretory response of PC12 cells in part by increasing the depolarization mediated by an oxygen-sensitive K+ channel. In addition, acute hypoxia activates a Cd2+-resistant Ca2+ influx pathway in chronically hypoxic PC12 cells.

Effects of mitochondrial uncouplers on intracellular calcium, pH and membrane potential in rat carotid body type I cells

1. Mitochondrial uncouplers are potent stimulants of the carotid body. We have therefore investigated their effects upon isolated type I cells. Both 2,4-dinitrophenol (DNP) and carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) caused an increase in [Ca2+]i which was largely inhibited by removal of extracellular Ca2+ or Na+, or by the addition of 2 mM Ni2+. Methoxyverapamil (D600) also partially inhibited the [Ca2+]i response. 2. In perforated-patch recordings, the rise in [Ca2+]i coincided with membrane depolarization and was greatly reduced by voltage clamping the cell to -70 mV. Uncouplers also inhibited a background K+ current and induced a small inward current. 3. Uncouplers reduced pHi by 0.1 unit. Alkaline media diminished this acidification but had no effect on the [Ca2+]i response. 4. FCCP and DNP also depolarized type I cell mitochondria. The onset of mitochondrial depolarization preceded changes in cell membrane conductance by 3-4 s. 5. We conclude that uncouplers excite the carotid body by inhibiting a background K+ conductance and inducing a small inward current, both of which lead to membrane depolarization and voltage-gated Ca2+ entry. These effects are unlikely to be caused by cell acidification. The inhibition of background K+ current may be related to the uncoupling of oxidative phosphorylation.

Anoxia differentially modulates multiple K+ currents and depolarizes neonatal rat adrenal chromaffin cells

1. Using perforated-patch, whole cell recording, we investigated the membrane mechanisms underlying O2 chemosensitivity in neonatal rat adrenomedullary chromaffin cells (AMC) bathed in extracellular solution containing tetrodotoxin (TTX; 0.5-1 microM), with or without blockers of calcium entry. 2. Under voltage clamp, low PO2 (0-15 mmHg) caused a graded and reversible suppression in macroscopic outward K+ current. The suppression during anoxia (PO2 = 0 mmHg) was approximately 35% (voltage step from -60 to +30 mV) and was due to a combination of several factors: (i) suppression of a cadmium-sensitive, Ca2+-dependent K+ current, IK(CaO2); (ii) suppression of a Ca2+-insensitive, delayed rectifier type K+ current, IK(VO2); (iii) activation of a glibenclamide- (and Ca2+)-sensitive current, IK(ATP). 3. During normoxia (PO2 = 150 mmHg), application of pinacidil (100 microM), an ATP-sensitive potassium channel (KATP) activator, increased outward current density by 45.0 +/- 7.0 pA pF-1 (step from -60 to + 30 mV), whereas the KATP blocker glibenclamide (50 microM) caused only a small suppression by 6.3 +/- 4.0 pA pF-1. In contrast, during anoxia the presence of glibenclamide resulted in a substantial reduction in outward current density by 24.9 +/- 7.9 pA pF-1, which far exceeded that seen in its absence. Thus, activation of IK(ATP) by anoxia appears to reduce the overall K+ current suppression attributable to the combined effects of IK(CaO2) and IK(VO2). 4. Pharmacological tests revealed that IK(CaO2) was carried predominantly by maxi-K+ or BK potassium channels, sensitive to 50-100 nM iberiotoxin; this current also accounted for the major portion (approximately 60%) of the anoxic suppression of outward current. Tetraethylammonium (TEA; 10-20 mM) blocked all of the anoxia-sensitive K+ currents recorded under voltage clamp, i.e. IK(CaO2), IK(VO2) and IK(ATP). 5. Under current clamp, anoxia depolarized neonatal AMC by 10-15 mV from a resting potential of approximately -55 mV. At least part of this depolarization persisted in the presence of either TEA, Cd2+, 4-aminopyridine or charybdotoxin, suggesting the presence of anoxia-sensitive mechanisms additionalto those revealed under voltage clamp. In Na+/Ca2+-free solutions, the membrane hyperpolarized, though at least a portion of the anoxia-induced depolarization persisted. 6. In the presence of glibenclamide, the anoxia-induced depolarization increased significantly to approximately 25 mV, suggesting that activation of KATP channels may function to attenuate the anoxia-induced depolarization or receptor potential.