Intracellular pH and catecholamine secretion from bovine adrenal chromaffin 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.

PKC mediates GnRH activation of a Na+/H+ exchanger in goldfish somatotropes

Previous results suggest that gonadotropin-releasing hormone (GnRH) stimulation of somatotropin secretion in goldfish involves activation of Na(+)/H(+) exchange (NHE). We tested the hypothesis that GnRH alkalinizes intracellular pH (pH(i)) via protein kinase C (PKC) activation of NHE. Two types of alkalinization responses were observed in identified goldfish somatotropes preloaded with the pH-sensitive dye BCECF; the rate of pH(i) changes went from a neutral or slightly negative slope to either a positive or a less negative slope relative to control. Two GnRHs, the PKC-activating TPA, and dioctanoyl glycerol each caused an alkalinization in 70-90% of somatotropes. The PKC inhibitors, Bis II and Gö6976, the NHE inhibitor amiloride, or Na(+)-free solution attenuated TPA and GnRHs actions, suggesting that PKC mediates GnRH activation of NHE. Since amiloride and Na(+)-free solution caused acidification in somatotropes at rest, regulation of basal pH(i) in these cells likely involves Na(+) flux through amiloride-sensitive NHE.

Relative contribution of the Na(+)/Ca(2+) exchanger, mitochondria and endoplasmic reticulum in the regulation of cytosolic Ca(2+) and catecholamine secretion of bovine adrenal chromaffin cells

The relative importance of mitochondria, the Na(+)/Ca(2+) exchanger (NCX) and the endoplasmic reticulum (ER) in the regulation of the cytosolic Ca(2+) concentration ([Ca(2+)](i)) were examined in bovine chromaffin cells using fura-2 for average [Ca(2+)](i) and amperometry for secretory activity, which reflects the local Ca(2+) concentration near the exocytotic sites. Chromaffin cells were stimulated by a high concentration of K(+) when the three Ca(2+) removal mechanisms were individually or simultaneously inhibited. When the mitochondrial Ca(2+) uptake was inhibited, the [Ca(2+)](i) decayed at a significantly slower rate and the secretory activity was higher than the control cells. The NCX appears to function only in the initial phase of [Ca(2+)](i) decay and when the ER Ca(2+) pump is blocked. Similarly, the ER had a significant effect on the [Ca(2+)](i) decay and on the secretion only when the NCX was blocked. Inhibition of all three mechanisms leads to a substantial delay in [Ca(2+)](i) recovery and an increase in the secretion. The results suggest that the three mechanisms work together in the regulation of the Ca(2+) near the Ca(2+) channels and exocytotic sites and therefore modulate the secretory activity. When Ca(2+) diffuses away from the exocytotic sites, the mitochondrial Ca(2+) uptake becomes the dominant mechanism.

The adrenergic stress response in fish: control of catecholamine storage and release

In fish, the catecholamine hormones adrenaline and noradrenaline are released into the circulation, from chromaffin cells, during numerous 'stressful' situations. The physiological and biochemical actions of these hormones (the efferent adrenergic response) have been the focus of numerous investigations over the past several decades. However, until recently, few studies have examined aspects involved in controlling/modulating catecholamine storage and release in fish. This review provides a detailed account of the afferent limb of the adrenergic response in fish, from the biosynthesis of catecholamines to the exocytotic release of these hormones from the chromaffin cells. The emphasis is on three particular topics: (1) catecholamine biosynthesis and storage within the chromaffin cells including the different types of chromaffin cells and their varying arrangement amongst species; (2) situations eliciting the secretion of catecholamines (e.g. hypoxia, hypercapnia, chasing); (3) cholinergic and non-cholinergic (i.e. serotonin, adrenocorticotropic hormone, angiotensin, adenosine) control of catecholamine secretion. As such, this review will demonstrate that the control of catecholamine storage and release in fish chromaffin cells is a complex processes involving regulation via numerous hormones, neurotransmitters and second messenger systems.

CO interact with intracellular [H+] with and without CO2-HCO3- in the cat carotid chemosensory discharge

To test the hypothesis whether CO2-HCO3- buffer is essential for the expression of chemoreception and to distinguish between pHi and pHo interaction with pCO in the carotid chemosensory response, we superfused-perfused in vitro cat carotid bodies using HEPES-Tyrode's solution with and without CO2-HCO3-, and compared the responses at the same pHo in the absence and presence of light. In the absence of light, pCO (> 138 Torr) stimulated the carotid body chemoreceptors in CO2-HCO3- buffer at pHo of 7.40, whereas pCO (69-550 Torr) did not stimulate the neural discharge in HEPES buffer at the pHo of 7.4-7.1 but did so below pHo 7.1. In the presence of light, all the responses were diminished proportionately.

Purine and pyrimidine nucleotides activate distinct signalling pathways in PC12 cells

The role of extracellular nucleotides in intracellular signalling and neurosecretion was assessed in PC12 cells. Activation of phospholipase C and increased [Ca2+]i were mediated by purinoceptors with an agonist potency profile, ATP approximately UTP > 2-methylthioadenosine triphosphate (2-MeSATP), typical of P2U. ATP also evoked a rapid acidification followed by a more gradual alkalinization (measured with 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF)), while UTP induced only a gradual alkalinization. The amiloride analogue 5-(N-ethyl-N-isopropyl)amiloride (EIPA) attenuated the alkalinization phase suggesting activation of the Na+/H+ exchanger by ATP and UTP. Using bisoxonol and [3H]tetraphenylphosphonium ([3H]TPP+) as potential-sensitive probes, we showed that while ATP rapidly depolarized PC12 cells in an Na(+)-dependent manner, UTP evoked a much reduced and delayed response. The potency profile (ATP approximately 2-MeSATP approximately adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) >> UTP, alpha, beta-methyleneATP) suggested involvement of a receptor subtype distinct from P2U. Secretion of endogenous dopamine was also assessed. Those nucleotides that induced depolarization (ATP, 2-MeSATP, ATP gamma S) were also the most potent secretagogues. UTP was ineffective. Our results suggest that ATP stimulates distinct purinoceptor subtypes and induces neurosecretion through the activation of multiple signalling pathways.

Characterization of extracellular pH drop due to the activation of the secretory process by acetylcholine in the bovine adrenal medulla

A progressive and reversible decrease of external pH accompanied the catecholamine release elicited by acetylcholine in decorticated bovine adrenal glands perfused with buffer-free Locke solution adjusted to an initial pH of 7.4. Both the secretory response as well as the extracellular acid shift promoted by the cholinergic agonist were antagonized by hexamethonium plus atropine, Mg2+ and verapamil. Experiments performed to assess the effects of the reduction of external pH on acetylcholine-induced release of catecholamines revealed that increasing the extracellular concentration of H+ significantly and reversibly reduced this secretory response. These findings are consistent with the idea that adrenomedullary activation of secretion by acetylcholine could be associated with a transient acidification of the extracellular fluid. This release of protons, arising mainly from the chromaffin granules, may be involved in a local automodulatory mechanism of the regulated secretory process.

Atrial natriuretic factor modifies noradrenaline release in a sodium-free medium

1. The effects of atrial natriuretic factor (ANF) on 3H-noradrenaline (3H-NA) release evoked by a sodium-free medium (SFM) were studied. The experiments were carried out in rat hypothalamic slices incubated in vitro. 2. ANF (1, 10 and 100 nM) decreased NA release evoked by the omission of sodium in a concentration-dependent way. When calcium was omitted from a SFM, NA output was partially diminished. However, if ANF was added to the SFM/calcium free medium NA secretion showed no modifications. 3. Present results suggest that, in rat hypothalamus, NA release evoked by Na+ omission is divided into two fractions: one independent of and the other dependent on extracellular calcium. In addition, ANF modifies NA release evoked by SFM dependent on extracellular calcium.

A role for ions in barrier recovery after acute perturbation

The epidermal cutaneous permeability barrier can be disrupted by treatment with topical solvents. Recent studies have shown that barrier recovery, measured by the recovery of transepidermal water loss towards normal, is inhibited by high extracellular Ca++ and K+, and accelerated by low extracellular concentrations of these ions. To examine the effects of Ca++ or K+ fluxes on barrier recovery, we tested the effects on transepidermal water loss recovery of agents that modify these fluxes. K+ channel agonists or blockers modified the inhibitory effects on barrier recovery induced by raised extracellular Ca++ and K+. In addition, Na+/K+ adenosine 5' triphosphatase inhibitors reversed the inhibitory effects of high extracellular Ca++ and K+. Our results suggest that barrier recovery requires both Ca++ and K+ fluxes and are consistent with the hypothesis that both verapamil or dihydropyridine-sensitive Ca++-permeable channels and Ca++-sensitive K+ channels participate in epidermal permeability barrier homeostasis.

Characterization of low pH-induced catecholamine secretion in the rat adrenal medulla

Catecholamine (CA) secretion was evoked when the isolated rat adrenal gland was perfused with HEPES-buffered Krebs solution acidified by the addition of HCl or by gassing with 95% O2/5% CO2. The secretion was detectable at pH 7.0 and increased with decreasing pH until at approximately 6.4. The low pH-induced CA secretion consisted of two phases, an initial transient response followed by a sustained phase. An intracellular Ca2+ antagonist, 3,4,5-trimethoxybenzoic acid 8-(N,N-diethylamino)octyl ester, selectively inhibited the initial phase of secretion. Both of the responses were resistant to nifedipine, a blocker of voltage-gated Ca2+ channel, but were completely inhibited in Ca(2+)-free (1 mM EGTA containing) solution. Adrenaline was an exclusive component in CAs released by low pH. The time course and extent of intracellular acidification caused either by low pH in the external medium or by the offset of a transitory NH4Cl application had no correlation with those of the secretory responses in the corresponding period. These results suggest that extracellular acidification preferentially activates adrenaline secretive cells to evoke CA secretion and that this low pH-induced CA secretion may be mediated by dihydropyridine-insensitive Ca2+ influx. Furthermore, the initial transient phase of the low pH-induced CA secretion might be caused by a Ca2+ release from intracellular stores, which is also induced by the Ca2+ influx.

Modulation of the rat adrenal medulla norepinephrine secretion in a sodium-free medium by atrial natriuretic factor

The effects of atrial natriuretic factor (ANF) on [3H]norepinephrine ([3H]NE) release evoked by a sodium-free medium (SFM) were studied. Experiments were performed in rat adrenal medulla slices incubated in vitro. Results showed that [3H]NE release evoked by the omission of Na+ was decreased by 10 nM ANF. In addition, when the Ca2+ was omitted from the SFM, NE output was partially diminished. Nevertheless, if ANF was added to SFM/Ca(2+)-free medium (CFM), NE secretion showed no modifications compared with SFM/CFM. Present results raise the hypothesis that two mechanisms could be involved in NE output evoked by a SFM in the rat adrenal medulla: one independent of and the other dependent on the extracellular calcium. Moreover, ANF only diminished NE secretion evoked by SFM dependent on extracellular calcium and did not modify calcium-independent NE release.

Decrease of extracellular pH associated with the secretion of catecholamines induced by barium in perfused bovine adrenal medulla

1. The possible modifications of extracellular pH produced during buffer-free 5 mM Ba2+ stimulation was studied in decorticated perfused bovine adrenal glands. 2. A significant and reversible drop of pH accompanied the release of catecholamines each time the tissue was exposed to a buffer-free 5 mM Ba2+ solution. 3. A progressive declination of the magnitude of this acidification associated with a gradual attenuation of the secretory response was observed consecutive to successive periods of Ba(2+)-stimulation. 4. D-600 (methoxyverapamil), in a concentration of 3 x 10(-4) M, markedly antagonized both Ba(2+)-induced secretory response and extracellular pH drop. 5. Perfusion of adrenal medulla for 4 min period with Locke solution buffered at pH 6.9, significantly and reversibly reduced the secretory response to 5 mM Ba2+ (pH 6.9) compared to a first control response obtained 35 min before at pH 7.4. 6. These results are compatible with the view that Ba(2+)-induced secretory activity is accompanied by the release of protons which could be involved in a local negative automodulatory mechanism of adrenomedullary secretion.

Scinderin and chromaffin cell actin network dynamics during neurotransmitter release

It has been demonstrated that filamentous actin (F-A) is mainly localized in the cortical surface of the chromaffin cell. This F-A network acts as a barrier to the chromaffin granules impeding their contact with the plasma membrane. Stimulation of chromaffin cells with either nicotine or a depolarizing concentration of K+ induces the disassembly of cortical F-A in focal areas underneath the plasma membrane. Sites of exocytosis are localised to these areas with low concentration of F-A. The cortical surface of the chromaffin cell also contains scinderin, a Ca(2+)-dependent actin filament-severing protein recently isolated in our laboratory. Nicotine and high K+ stimulation also induce redistribution of cortical scinderin. Both nicotine and high K(+)-induced scinderin redistribution and F-A disassembly are Ca(2+)-dependent events which seem to precede neurotransmitter secretion. A possible target for protein kinase C in the modulation of secretion is the cortical F-A network. Treatment of chromaffin cells with phorbol esters prior to secretion induced scinderin redistribution, F-A disassembly and enhanced the initial rate of subsequent nicotine-evoked catecholamine release. The present results strongly indicate the involvement of the cortical cytoskeleton in the regulation of neurotransmitter release.

Ca2+ and pH determine the interaction of chromaffin cell scinderin with phosphatidylserine and phosphatidylinositol 4,5,-biphosphate and its cellular distribution during nicotinic-receptor stimulation and protein kinase C activation

Nicotinic stimulation and high K(+)-depolarization of chromaffin cells cause disassembly of cortical filamentous actin networks and redistribution of scinderin, a Ca(2+)-dependent actin filament-severing protein. These events which are Ca(2+)-dependent precede exocytosis. Activation of scinderin by Ca2+ may cause disassembly of actin filaments leaving cortical areas of low cytoplasmic viscosity which are the sites of exocytosis (Vitale, M. L., A. Rodríguez Del Castillo, L. Tchakarov, and J.-M. Trifaró. 1991. J. Cell. Biol. 113:1057-1067). It has been suggested that protein kinase C (PKC) regulates secretion. Therefore, the possibility that PKC activation might modulate scinderin redistribution was investigated. Here we report that PMA, a PKC activator, caused scinderin redistribution, although with a slower onset than that induced by nicotine. PMA effects were independent of either extra or intracellular Ca2+ as indicated by measurements of Ca2+ transients, and they were likely to be mediated through direct activation of PKC because inhibitors of the enzyme completely blocked the response to PMA. Scinderin was not phosphorylated by the kinase and further experiments using the Na+/H+ antiport inhibitors and intracellular pH determinations, demonstrated that PKC-mediated scinderin redistribution was a consequence of an increase in intracellular pH. Moreover, it was shown that scinderin binds to phosphatidylserine and phosphatidylinositol 4,5-biphosphate liposomes in a Ca(2+)-dependent manner, an effect which was modulated by the pH. The results suggest that under resting conditions, cortical scinderin is bound to plasma membrane phospholipids. The results also show that during nicotinic receptor stimulation both a rise in intracellular Ca2+ and pH are observed. The rise in intracellular pH might be the result of the translocation and activation of PKC produced by Ca2+ entry. This also would explain why scinderin redistribution induced by nicotine is partially (26-40%) inhibited by inhibitors of either PKC or the Na+/H+ antiport. In view of these findings, a model which can explain how scinderin redistribution and activity may be regulated by pH and Ca2+ in resting and stimulated conditions is proposed.