The effect of ouabain on the secretion of catecholamines and on the intracellular concentration of potassium

Effect of ouabain on calcium signaling in rodent brain: A systematic review of in vitro studies

The Na⁺/K⁺-ATPase is an integral membrane ion pump, essential to maintaining osmotic balance in cells in the presence of cardiotonic steroids; more specifically, ouabain can be an endogenous modulator of the Na⁺/K⁺-ATPase. Here, we conducted a systematic review of the in vitro effects of cardiotonic steroids on Ca²⁺ in the brain of rats and mice. Methods: The review was carried out using the PubMed, Virtual Health Library, and EMBASE databases (between 12 June 2020 and 30 June 2020) and followed the guidelines described in the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA). Results: in total, 829 references were identified in the electronic databases; however, only 20 articles were considered, on the basis of the inclusion criteria. The studies demonstrated the effects of ouabain on Ca²⁺ signaling in synaptosomes, brain slices, and cultures of rat and mouse cells. In addition to the well-known cytotoxic effects of high doses of ouabain, resulting from indirect stimulation of the reverse mode of the Na⁺/Ca²⁺ exchanger and increased intracellular Ca²⁺, other effects have been reported. Ouabain-mediated Ca²⁺ signaling was able to act increasing cholinergic, noradrenergic and glutamatergic neurotransmission. Furthermore, ouabain significantly increased intracellular signaling molecules such as InsPs, IP3 and cAMP. Moreover treatment with low doses of ouabain stimulated myelin basic protein synthesis. Ouabain-induced intracellular Ca²⁺ increase may promote the activation of important cell signaling pathways involved in cellular homeostasis and function. Thus, the study of the application of ouabain in low doses being promising for application in neurological diseases.

Systematic Review Registration:https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42020204498, identifier CRD42020204498.

Ouabain enhances exocytosis through the regulation of calcium handling by the endoplasmic reticulum of chromaffin cells

The augmentation of neurotransmitter and hormone release produced by ouabain inhibition of plasmalemmal Na+/K+-ATPase (NKA) is well established. However, the mechanism underlying this action is still controversial. Here we have shown that in bovine adrenal chromaffin cells ouabain diminished the mobility of chromaffin vesicles, an indication of greater number of docked vesicles at subplasmalemmal exocytotic sites. On the other hand, ouabain augmented the number of vesicles undergoing exocytosis in response to a K+ pulse, rather than the quantal size of single vesicles. Furthermore, ouabain produced a tiny and slow Ca2+ release from the endoplasmic reticulum (ER) and gradually augmented the transient elevations of the cytosolic Ca2+ concentrations ([Ca2+]c) triggered by K+ pulses. These effects were paralleled by gradual increments of the transient catecholamine release responses triggered by sequential K+ pulses applied to chromaffin cell populations treated with ouabain. Both, the increases of K+-elicited [Ca2+]c and secretion in ouabain-treated cells were blocked by thapsigargin (THAPSI), 2-aminoethoxydiphenyl borate (2-APB) and caffeine. These results are compatible with the view that ouabain may enhance the ER Ca2+ load and facilitate the Ca2+-induced-Ca2+ release (CICR) component of the [Ca2+]c signal generated during K+ depolarisation. This could explain the potentiating effects of ouabain on exocytosis.

Ouabain augments and maintains the catecholamine release responses evoked by repetitive pulses of potassium, caffeine or histamine in perifused bovine chromaffin cells

Cardiac glycosides such as ouabain augment the release of neurotransmitters and hormones from various organs, tissues and cell systems. Here we have investigated novel aspects of the ouabain effects on fast-perifused bovine adrenal chromaffin cells subjected to repetitive stimulation during long-time periods with secretagogues that enhance Ca(2+) entry (i.e. 100 mM K(+)solution) or causing the release into the cytosol of the Ca(2+) stored in the endoplasmic reticulum (i.e. 20 mM caffeine or 100 microM histamine). After 1 h of intermittent stimulation, the amperometrically measured catecholamine release responses decayed to 50% with K(+) pulses, and to 10-15% with caffeine or histamine pulses. Ouabain (10 microM) augmented 2-fold the K(+) secretory responses and kept them high, along an hour. When given after the responses had decayed upon repetitive caffeine or histamine pulsing, ouabain gradually restored such responses to their initial control values. On the basis of these results, we raise the hypothesis that ouabain may facilitate the handling by the cell of the endoplasmic reticulum Ca(2+) fluxes that are known to be involved in secretory vesicle transport and the regulation of exocytosis. This may have physiological and pathological interest in the light of an endogenous ouabain steroid found in the adrenal cortex.

Brain soluble fractions which modulate Na+, K+-ATPase activity likewise modify muscarinic receptor

Two brain soluble fractions, named peaks I and II, which respectively stimulate and inhibit neuronal Na+, K+-ATPase activity, have been isolated by gel filtration in Sephadex G-50. Since cholinergic transmission seems related to such enzyme activity, in this study we evaluated the effect of brain peak I, peak II, a more purified fraction II-E and commercial ouabain, on specific binding of the muscarinic antagonist [3H]quinuclidinyl benzilate to membranes from rat cerebellum, hippocampus and cerebral cortex. We found that binding was increased by peak I and decreased by peak II, II-E and ouabain, all effects proving concentration-dependent. Since the changes exerted on the muscarinic receptor followed a pattern similar to the one already described for synaptosomal membrane Na+, K+-ATPase activity, both systems seem to interact at a functional level.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

The actions of ouabain and lithium chloride on cytosolic Ca2+ in single chromaffin cells

The effects of ouabain, Li+ and veratridine on the concentration of cytosolic free Ca2+ ([Ca2+]i) were studied in single fura-2-loaded bovine adrenal chromaffin cells. Superfusion of cells with ouabain (10 microM for 60 min) caused only a delayed mild increase of the Ca2+]i, from around 0.1 microM to 0.2-0.3 microM; this increase was Nao(+)-dependent. Replacement of all NaCl of the Krebs-Hepes solution by LiCl (144 mM) produced a gradual increase of [Ca2+]i, which remained elevated at a stable plateau of 0.4-0.5 microM for 40-50 min. When ouabain (in the presence of normal Nao+) or Li+ (in the absence of Nao+) was given in Krebs-Hepes solution containing no Ca2+, the reintroduction of 2.5 mM Ca2+ produced a fast elevation of the [Ca2+]i. In the case of ouabain-treated cells, the [Ca2+]i curve exhibited an initial phasic component which inactivated to a tonic component. omega-Conotoxin MVIIC (3 microM) and R56865 (10 microM) inhibited the phasic but not the tonic component. Veratridine (30 microM) induced large [Ca2+]i oscillations. Both ouabain or Li+ abolished such oscillations. These results are compatible with ouabain causing elevation of [Ca2+]i in bovine chromaffin cells through a dual mechanism, i.e. cell depolarisation and slowing down of the Na(+)-Ca2+ exchanger of their plasmalemma. Through its binding to the Na+ site on the Na(+)-Ca2+ exchanger, Li+ ions generate powerful Cai2+ signals that might be relevant to its known effects on neurosecretory mechanisms.

Angiotensin II increases catecholamine release from bovine adrenal medulla but does not enhance that evoked by K+ depolarization or by carbachol

The effect of angiotensin II on catecholamine release from bovine adrenal medulla has been investigated. In retrogradely perfused, isolated bovine adrenal glands, angiotensin II increased basal efflux of catecholamines, but the presence of angiotensin II did not increase the release of catecholamines evoked either by bolus injections of the secretagogue carbachol or by depolarization with a perfusing solution containing a raised concentration of K+. In chromaffin cells maintained in primary tissue culture, angiotensin II increased 3H release from cells preloaded with [3H]-noradrenaline but did not enhance the release evoked by carbachol or by depolarization with K+. The increase in 3H release evoked by angiotensin II from chromaffin cells in tissue culture was inhibited by its analogue antagonist Sar1,Ala8-angiotensin II (saralasin) and was entirely dependent on the presence of Ca2+ in the experimental medium. These findings suggest that, in the chromaffin cells of the bovine adrenal medulla, angiotensin II acts on specific receptors to cause a calcium-dependent catecholamine release but triggers no additional response that acts synergistically with depolarizing or nicotinic stimuli to augment catecholamine release.

Studies on secretion of catecholamine evoked by caffeine from the isolated perfused rat adrenal gland

The influence of caffeine on secretion of catecholamines (CA) was examined in the isolated perfused rat adrenal gland. Caffeine (0.3 mM) perfused into an adrenal vein of the gland produced a marked increase in secretion of CA. This secretory effect of CA evoked by perfusion of caffeine for one minute was considerably prolonged, lasting for more than 90 minutes. The tachyphylaxis to releasing effect of CA induced by caffeine was observed by repeated perfusion of this drug. The caffeine-evoked CA secretion was markedly inhibited by pretreatment with ouabain, trifluoperazine, TMB-8 and perfusion with calcium-free Krebs solution containing 5 mM EGTA, but was not affected by perfusion of calcium-free Krebs solution without other addition. CA secretion evoked by caffeine was not reduced significantly by pretreatment with chlorisondamine but after the first collection of perfusate for 3 min was clearly inhibited. Interestingly, the caffeine-evoked CA secretion was considerably potentiated by pretreatment with atropine or pirenzepine, but after the first collection for 3 min it was markedly decreased. These experimental results suggest that caffeine causes a marked increase in secretion of CA from the isolated perfused rat adrenal gland by an extracellular calcium-independent exocytotic mechanism. The secretory effect of caffeine may be mainly due to mobilization of calcium from an intracellular calcium pool in the rat chromaffin cells and partly due to stimulation of both muscarinic and nicotinic receptors.

Brain microdialysis studies on the control of dopamine release and metabolism in vivo

This paper studies the actions of drugs known to release dopamine from brain tissue. Most of the theoretical background to this work has been developed in experiments on slices of brain in vitro but using in vivo microdialysis we have elaborated and extended the ideas from the in vitro experiments and been able to make a direct comparison of the mechanism of action and source of dopamine released by 5 different manipulations. The mode of action of tyramine, amphetamine, veratrine, ouabain and potassium is discussed in the light of the computer model of the nerve terminal published by Justice et al. (1988). The data may lend themselves to such an interpretation, but they could be compatible with several other models.

Influence of ouabain on the cholinergic neurotransmission in the canine trachea

The effect of ouabain on the cholinergic neurotransmission of the trachea was investigated using isolated tracheal strips in dogs. Tracheal strips without epithelium were suspended in organ chambers filled with modified Krebs-Henseleit solution. Ouabain (3 X 10(-7)-10(-5) M) concentration-dependently caused a slow sustained tracheal contraction. The contractile response was significantly inhibited by 10(-6) M atropine and was enhanced by 10(-8) M physostigmine. The ouabain-induced tracheal contraction was unaffected by 10(-7) M tetrodotoxin, but was significantly reduced by 10(-3) M hemicholinium-3. In superfusion experiments, ouabain (10(-5) M) produced an increase in the ACh release. Superfusion with Ca(++)-free solution almost eradicated the ACh release and abolished the tracheal contraction induced by ouabain. omega-Conotoxin (5 X 10(-8) M), but not nicardipine (10(-6) M), inhibited significantly the increase in ACh release induced by ouabain. These results suggest that the ouabain-induced tracheal contraction may be mainly due to acceleration of presynaptic ACh release by enhancing the influx of Ca++, and the Ca(+)+ entry may occur through the N-type Ca(+)+ channels in the canine airway presynaptic site.

Mechanism of calcium transport across the plasma membrane of bovine chromaffin cells

The mechanism of calcium transport across the plasma membrane of chromaffin cells was studied using plasma membrane vesicles prepared from cells of adrenal medulla. Purification of the plasma membrane was about 30-fold, based on the alpha-bungarotoxin binding activity. The isolated membrane vesicles have both Na+/Ca2+ exchange and calcium pump activities. The Na+/Ca2+ exchange activity increased with the free calcium concentration and was not saturated at 1 mM, the highest concentration tried. The K1/2 of the calcium pump for calcium is 0.06 microM. Part of the Na+/Ca2+ exchange activity was inhibited by preincubation of the membrane vesicles with veratridine and the effect of veratridine was reversed by tetrodotoxin. The calcium taken up by the calcium pump was released by 0.005% saponin, but was not affected by oxalate. The calcium taken up by the calcium pump was released by exchanging with the external sodium, which suggests that the two calcium transport systems are located on the same population of membrane vesicles. The above evidence indicates that both calcium transport activities are located on the plasma membrane and not on contaminating organelle membranes. The significance of the two calcium transport systems in regulation of cytosolic calcium concentration of chromaffin cells is discussed.

The key role of sodium in the ouabain-mediated potentiation of potassium-evoked catecholamine release in cat adrenal glands

1. The effect of [Na]o on the catecholamine release evoked by K in ouabain pretreated, isolated adrenal glands of the cat, was investigated. 2. Reduction of [Na]o to 70, 50 and 25 mM, with sucrose as a substitute, did not modify the spontaneous catecholamine release but progressively increased the K (17.7 mM)-evoked secretory response. 3. Ouabain pretreatment (100 microM; 10 min) greatly increased the K (17.7 mM)-evoked catecholamine secretory response in glands perfused with normal Krebs. Such an increase was still seen in glands perfused with 70 mM Na-containing solution but disappeared when [Na]o was reduced to 25 and 10 mM. 4. Preperfusion of non-ouabain treated glands with Li-enriched Krebs, for a 40 min period, caused an increase in the K (17.7 mM)-evoked secretory response which was dependent on [Li]o and essentially similar to that induced by ouabain pretreatment. 5. Ouabain treatment (100 microM; 10 min) of glands perfused with normal Krebs evoked a long lasting catecholamine release, which reached a plateau at about 36 min and amounted to 0.68 +/- 0.25 microgram 2 min-1 (n = 9). Such a secretory response was dramatically increased, and its shape modified, when glands were preperfused with K (17.7 mM)-Krebs: a peak of 3.77 +/- 0.42 micrograms 2 min-1 (n = 7) was reached at 18 min. This response was drastically reduced in the presence of nitrendipine (1 microM). 6. In summary, our results indicate that both [Na]0 reduction or Na accumulation into the chromaffin cell by inhibition of the Na pump with ouabain, greatly enhance the secretory efficacy of small increments of [K]0, and suggest that sodium may play a role in the regulation of catecholamine release mediated by voltage-dependent Ca channels in adrenal glands.

Effect of ouabain applied by intrastriatal microdialysis on the in vivo release of dopamine, acetylcholine, and amino acids in the brain of conscious rats

In the present study we have applied a brain microdialysis technique to investigate the effects of ouabain infusion on the release of dopamine, acetylcholine, and amino acids from striatal neurons in freely moving rats. Ouabain caused an increase in the dialysate levels of dopamine; its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC); and the amino acids glutamate, aspartate, taurine, glycine, alanine, serine, asparagine, and threonine. The ouabain-induced increase in dopamine was dose dependent and explosive (100-fold at an infusion concentration of 1 mmol/L) and contrasted strongly with the small effect of the glycoside on the output of DOPAC. We investigated the nature of ouabain-induced transmitter release by determining its sensitivity to coinfusion with tetrodotoxin or the calcium antagonist Mg2+. In the case of dopamine two mechanisms of ouabain-induced release could be established. At lower infusion concentrations ouabain induced an exocytotic type of release whereas at higher concentrations the release was probably carrier mediated. In the case of amino acids we noticed a calcium-independent release which was nerve impulse flow dependent in the case of glutamate and aspartate and impulse flow independent in the case of alanine, serine, glycine, threonine, and asparagine. Ouabain induced a decrease in the release of acetylcholine and glutamine.

Use of calcium antagonism for the characterization of drug-evoked dopamine release from the brain of conscious rats determined by microdialysis

Dopamine was determined by microdialysis of the striatum of conscious rats. We investigated whether the release of dopamine, induced by nine different pharmacological treatments, was sensitive to calcium antagonism. Calcium antagonism was determined by Mg2+ or Cd2+ infusion. The following conditions were investigated: haloperidol, haloperidol plus GBR 12909, nomifensine, (+)-amphetamine (all administered intraperitoneally), KCl, 1-methyl-4-phenyl-pyridinium ion (MPP+), glutamate, ouabain, and 120 mmol/L magnesium (all applied by infusion through the dialysis membrane). The results on calcium antagonism were combined with data on tetrodotoxin (TTX) sensitivity. With the combined data, three different types of dopamine release were characterized. First, action potential-dependent dopamine release was observed in animals treated with saline, haloperidol, haloperidol plus GBR 12909, nomifensine, and ouabain. Second, action potential-independent release was established in the case of (+)-amphetamine, glutamate, MPP+, and 120 mmol/L Mg2+. Finally, K+-induced dopamine release was classified as TTX independent and calcium dependent. It is concluded that brain dialysis is a powerful method for differentiating between different types of neurotransmitter release.

Ouabain distinguishes between nicotinic and muscarinic receptor-mediated catecholamine secretions in perfused adrenal glands of cat

1. The effect of ouabain on catecholamine (adrenaline and noradrenaline) secretion induced by agents acting on cholinoceptors was studied in perfused cat adrenal glands. Acetylcholine (ACh) (5 x 10(-7) to 10(-3) M), pilocarpine (10(-5) to 10(-3) M) and nicotine (10(-6) to 5 x 10(-5) M) caused dose-dependent increases in catecholamine secretion. Both ACh and nicotine released more noradrenaline than adrenaline and the reverse was the case for pilocarpine. 2. Ouabain (10(-5) M) enhanced catecholamine secretion induced by ACh (10(-5) M), pilocarpine (10(-3) M) and nicotine (3 x 10(-6) M) during perfusion with Locke solution. The ratio of adrenaline to noradrenaline was not affected by ouabain. 3. In the absence of extracellular Ca2+, ACh and pilocarpine, but not nicotine, still caused a small increase in catecholamine secretions, which were enhanced by treatment with ouabain (10(-5) M) plus Ca2+ (2.2 mM) for 25 min. The effect of ouabain was much more significant on noradrenaline secretion than on adrenaline secretion. The enhanced response was blocked by atropine (10(-6) M) but not by hexamethonium (5 x 10(-4) M). 4. Nifedipine (2 x 10(-6) M) inhibited the responses to pilocarpine and nicotine. The treatment with ouabain (10(-5) M) reversed only the response to pilocarpine and resulted in a significant increase in the proportion of noradrenaline released. 5. It is suggested that ouabain enhances evoked catecholamine secretions by facilitating Ca2+ entry through nicotinic receptor-linked Ca2+ channels and by increasing the intracellular Ca2+ pool linked to muscarinic receptors.

Ouabain induces acetylcholine release from pure cholinergic synaptosomes independently of extracellular calcium concentration

We have studied the correlation between [3H]ouabain binding sites, (Na+ + K+)ATPase (EC 3.6.1.3) activity and acetylcholine (ACh) release in different subcellular fractions of Torpedo marmorata electric organ (homogenate, synaptosomes, presynaptic plasma membranes). Presynaptic plasma membranes contained the greater number of [3H]ouabain binding sites, in good agreement with the high (Na+ + K+)ATPase activity found in this fraction. Blockade of this enzymatic activity by ouabain dose-dependently induced ACh release from pure cholinergic synaptosomes, either in the presence or absence of extracellular calcium ions. We suggest that one of the mechanisms involved in the ouabain-induced ACh release in the absence of Ca2+o may be an increase in Na+i that could (a) evoke Ca2+ release from internal stores and (b) inhibit ATP-dependent Ca2+ uptake by synaptic vesicles.

Digitalis glycosides: a discussion of the similarities and differences in actions and existing controversies

The writing of this review was initiated to answer the question of whether differences in the actions of the various digitalis glycosides exist and to discuss current controversies in the research area of the digitalis glycosides. Data obtained in our laboratory indicated that the effect of digoxin on postganglionic cardiac sympathetic neural discharge in the minute prior to the occurrence of arrhythmia differed from that of ouabain. This raised the question of whether data published in other laboratories would support the contention that differences in glycosides do exist. To answer this question, a review of the literature was begun. Our survey of these studies are cited in the tables of this review. These tables summarize the actions of glycosides in vivo and in vitro in different animal models. The reader should bear in mind that the data included within the tables do not represent an inclusive summary of all studies in the literature. For detailed review articles, the reader is referred to the following references: Gillis et al; Gillis and Quest; Roberts et al; Lathers and Roberts; Farah and Alousi; Benthe; Levitt et al; Smith and Haber; Somberg; Lee and Klaus; Mason; Schwartz. Furthermore the summary of the results for each particular study cited in the table may not, in all cases, include each finding of the published data. Nevertheless, the tables do provide a summary of data obtained in various species with different glycosides in several different areas of research, and as such, represent an abridged compendium for the research working in the field of digitalis glycosides. This review has been organized firstly to consider glycoside-induced alterations in the autonomic nervous system and, secondly, to examine their direct actions on the heart.

Effect of the opiate antagonist naloxone on digitalis induced cardiac arrhythmias

To test the hypothesis that the endogenous opioid system is operative in digitalis arrhythmias, guinea pigs anesthetized with pentothal and breathing spontaneously were allocated to a control group or four naloxone groups: 0.1 mg/kg i.v., 1.0 mg/kg i.v., 2 mg/kg i.v. or 4 mg/kg i.v. after the induction of arrhythmias by digoxin 100 micrograms/kg i.v. every 15 min. Naloxone at higher doses resulted in a rapid development of fatal arrhythmias with high degree AV block being more frequent than ventricular tachycardiac or fibrillation. Survival was significantly and inversely related to naloxone dose. The role of the autonomic nervous system was studied using cervical cord transection at the C7 level or bilateral cervical vagotomy, or atropine 1.2 mg/kg pretreatment. Cord transection, but not vagotomy, was associated with a significantly higher digoxin dose to produce arrhythmias. Naloxone 4 mg/kg i.v. shortened survival in cord transected animals, but less than in intact animals. Naloxone did not alter survival in vagotomized animals or animals that were pretreated with atropine. Thus, naloxone accelerates the development of fatal cardiac arrhythmias suggesting that endogenous opioids are involved in digitalis toxic arrhythmias an effect interrelated to the autonomic, mainly parasympathetic, nervous system.

Effects of Bay K 8644 on cat adrenal catecholamine secretory responses to A23187 or ouabain

Calcium ionophore A23187 increases the rate of spontaneous catecholamine release from cat adrenal glands perfused at 37 degrees C with oxygenated Krebs bicarbonate solution, in a time- and Ca-concentration-dependent manner. The secretory profile obtained with the ionophore was not modified in the presence of the Ca channel activator Bay K 8644. Ouabain also enhanced the rate of spontaneous catecholamine outputs in a time- and concentration-dependent manner. The threshold ouabain concentration capable of producing a clear, yet delayed secretory response was 10(-6) M. Increasing ouabain concentrations up to 10(-4) M enhanced catecholamine release and shortened the time to peak release. The dihydropyridine Ca channel activator Bay K 8644 (10(-6) M) markedly potentiated the secretory effects of all ouabain concentrations used (10(-7)-10(-4) M). However, the most impressive potentiations were seen at 10(-5)M ouabain; while at this concentration ouabain alone released 2.6 +/- 0.07 micrograms catecholamines per 30 min, in the presence of Bay K 8644 the release was 73.4 +/- 5.7 micrograms per 30 min. Conversely, at a fixed ouabain concentration (10(-5) M), the potentiation was also dependent on the Bay K 8644 concentration (10(-8)-10(-5) M). Although K deprivation inhibits Na pumping as does ouabain, Bay K 8644 did not modify the rate of catecholamine release evoked by K removal from the perfusion medium. Potassium deletion, nimodipine or high Mg all reversed the fully developed secretory response evoked by ouabain plus Bay K 8644. In glands depolarized by continuous perfusion with high K solutions, once the secretory response was inactivated, the introduction of ouabain caused an enhancement of the catecholamine secretory rate. This increase was dependent on the extracellular Na concentration and was not affected by Bay K 8644. In the presence of 6 mm Na the secretory effects of Bay K 8644 plus ouabain were abolished. 7 These results are compatible with the following conclusions: (i) Bay K 8644 potentiates only those catecholamine secretory responses that are known to be mediated through the activation of voltagesensitive Ca channels; the drug does not seem to affect secretory responses by acting on the membrane Na/Ca exchange system or at some intracellular Ca-dependent component of the secretory machinery of Ca buffering systems. (ii) It is likely that ouabain enhances the rates of adrenal catecholamine release by a dual mechanism: chromaffin cell depolarization and activation of a membrane Na/Ca exchange system.

Facilitation of transmitter action on catecholamine output by cardiac glycoside in perfused adrenal gland of guinea-pig

Effects of K+ deprivation and ouabain on catecholamine secretion evoked by splanchnic nerve stimulation (5 Hz), ACh (10(-5) M) and/or excess K+ (56 mM) were studied in isolated and perfused adrenal glands of guinea-pig. Exposure to K+-free Locke solution initially reduced and later enhanced catecholamine secretion sequentially evoked by splanchnic nerve stimulation and ACh. The enhancement attained a maximum, 185% in magnitude of the corresponding control response at 35 min for splanchnic nerve stimulation and 135% at 65 min for ACh after the start of exposure to K+-free solution. Ouabain (10(-5) M) caused a larger increase in the evoked catecholamine section than K+ deprivation did. The maximum effect was obtained from 40 to 50 min after the start of exposure to ouabain in which the magnitude of responses to splanchnic nerve stimulation, excess K+ and ACh was about 500, 400 and 300% of each control response, respectively. The effect of ouabain on the evoked catecholamine secretion increased as the concentration of extracellular Na+ was increased from 25 to 154 mM, regardless of the kind of stimuli. The ouabain-induced enhancement in the evoked responses was reversibly inhibited by removing Ca2+ from, or by adding Mg2+, Co2+ or Ni2+ to the perfusion medium. The ID50 values for Mg2+ were about 9.4 and 7.3 mM and those for Co2+ were 0.8 and 0.4 mM against ouabain on the responses to ACh and excess K+, respectively. The inhibitory effect of Mg2+ and Co2+ on the ouabain action was counteracted by increasing the concentration of Ca2+ from 2.2 to 8.8 mM in the perfusion medium. These results suggest that ouabain enhances catecholamine secretion evoked by splanchnic nerve stimulation, ACh and excess K+ by increasing the rate of Ca2+ influx through the ACh receptor linked Ca2+ channel and/or voltage-dependent Ca2+ channels on adrenal chromaffin cells of guinea-pig in a Na+-dependent manner.

Regulation of 3-O-methyl-D-glucose uptake in isolated bovine adrenal chromaffin cells

We showed earlier that insulin stimulated sugar transport in adrenal chromaffin cells (Bigornia, L. and Bihler, I. Biochim. Biophys. Acta 885, 335-344). Transport regulation and its Ca2+ -dependence was further investigated in isolated bovine adrenal chromaffin cells, serving as a model of a homogeneous neuronal cell population. Uptake of the nonmetabolizable glucose analogue, 3-O-methyl-D-glucose was stimulated by hyperosmolar medium, and this effect was abolished in the absence of external Ca2+, or depressed in the presence of La3+ or the slow Ca2+ channel blocker methoxyverapamil. Basal transport was also stimulated by factors (acetylcholine, carbamylcholine, low-Na+ medium), which cause Ca2+ -dependent catecholamine release, and these effects were abolished in Ca2+ -free medium. In addition insulin, acetylcholine, hyperosmolar and low-Na+ medium significantly increased 45Ca uptake. Thus, glucose transport in adrenal chromaffin cells was stimulated by insulin and hyperosmolarity in a Ca2+ -dependent manner, as in muscle. Sensitivity to secretory stimuli, a regulatory feature perhaps characteristic of this cell type, was also demonstrated. In contrast to muscle, sugar transport was not affected by Na+ -pump inhibition, metabolic inhibitors or the Na+ ionophore monensin, suggesting that Ca2+ influx by Na+/Ca2+ exchange does not play a significant role in the activation of sugar transport in chromaffin cells.

Inhibition of Na+-pump enhances carbachol-induced influx of 45Ca2+ and secretion of catecholamines by elevation of cellular accumulation of 22Na+ in cultured bovine adrenal medullary cells

In bovine adrenal medullary cells, we reported that 22Na+ influx via nicotinic receptor-associated Na+ channels is involved in 45Ca2+ influx, a requisite for initiating the secretion of catecholamines (Wada et al. 1984, 1985 b). In the present study, we investigated whether the inhibition of Na+-pump modulates carbachol-induced 22Na+ influx, 45Ca2+ influx and catecholamine secretion in cultured bovine adrenal medullary cells. We also measured 86Rb+ uptake by the cells to estimate the activity of Na+,K+-ATPase. Ouabain and extracellular K+ deprivation remarkably potentiated carbachol-induced 22Na+ influx, 45Ca2+ influx and catecholamine secretion; this potentiation of carbachol-induced 45Ca2+ influx and catecholamine secretion was not observed in Na+ free medium. Carbachol increased the uptake of 86Rb+; this increase was inhibited by hexamethonium and d-tubocurarine. In Na+ free medium, carbachol failed to increase 86Rb+ uptake. Ouabain inhibited carbachol-induced 86Rb+ uptake in a concentration-dependent manner, as it increased the accumulation of cellular 22Na+. These results suggest that Na+ influx via nicotinic receptor-associated Na+ channels increases the activity of Na+,K+-ATPase and the inhibition of Na+,K+-ATPase augmented carbachol-induced Ca2+ influx and catecholamine secretion by potentiating cellular accumulation of Na+. It seems that nicotinic receptor-associated Na+ channels and Na+,K+-ATPase, both modulate the influx of Ca2+ and secretion of catecholamines by accommodating cellular concentration of Na+.

Chlorpromazine: cardiac arrhythmogenicity in the cat

To determine the effect of chlorpromazine on ouabain-induced arrhythmia and death, dial-urethane anesthetized cats were pretreated with chlorpromazine (5, 10, 20, 30, 40, or 60 mg/kg, i.v.) and then administered ouabain (2 microgram/kg/min, i.v.). Blood pressure, heart rate and lead II electrocardiogram (ECG) were monitored. The dosages of ouabain necessary to induce premature ventricular contractions, ventricular tachycardia and death were determined. No significant correlation between the dose of chlorpromazine given and the dose of ouabain required to produce arrhythmia or death was found. These doses of chlorpromazine could, therefore, be considered neither arrhythmogenic nor antiarrhythmic in the ouabain model. To determine whether chlorpromazine produced arrhythmia in the dial-urethane anesthetized cat model, the drug was infused at a rate of 1 mg/kg/min, i.v. Chlorpromazine produced arrhythmia at 185 +/- 4.3 minutes and death via cardiovascular collapse at 128 +/- 4.7 minutes. Bilateral adrenal vein ligation, employed to eliminate the influence of adrenal catecholamines, decreased the dosage of chlorpromazine necessary to produce arrhythmia and death to 67.8 +/- 17.7 and 84.7 +/- 15.7 mg/kg, respectively. Thus, adrenal catecholamines did not appear to contribute to chlorpromazine-induced arrhythmia, although the procedure of bilateral adrenal vein ligation appeared to be deleterious in combination with chlorpromazine. In all experiments, chlorpromazine depressed blood pressure without producing the reflex tachycardia normally seen with hypotension. This suggests that the drug may be interfering with the baroreceptor reflex arc. As chlorpromazine modifies the autonomic parameters of blood pressure, heart rate, and cardiac electrophysiology, sudden unexplained death in patients managed with this agent may be due to drug-induced arrhythmia.

The action of excess potassium and calcium on ouabain-evoked [3H]-noradrenaline release from the rabbit pulmonary artery

[3H]-noradrenaline [( 3H]-NA) release from the main pulmonary artery of the rabbit has been measured in the presence of neuronal (cocaine, 3 X 10(-5) M) and extraneuronal (corticosterone, 5 X 10(-5) M) uptake blockers. Removal of K from the external medium increased the [3H]-NA release. In the absence of external K, ouabain (10(-4) M) further enhanced the neurotransmitter release. The 'K-free' stimulated [3H]-NA release was inhibited by an increase of external Ca (7.5 mM), an action antagonized by ouabain. After preperfusion of the preparations for 30 min with either excess K (23.6 mM) or excess Ca (7.5 mM), the ouabain-stimulated [3H]-NA release was inhibited by about 50%; the rates of inhibition did not differ significantly from each other. However, the characteristic initial delay before ouabain-evoked neurotransmitter release was shortened in excess K, and prolonged in excess Ca-containing solution. When both excess K and Ca were applied together 30 min before ouabain perfusion, the action of ouabain in releasing neurotransmitter was also inhibited but the rate of inhibition did not differ significantly from that seen when K or Ca were applied separately. The action of K in shortening the initial delay was partly antagonized by Ca. Excess Ca antagonized the inhibition of ouabain-stimulated [3H]-NA release caused by excess K when Ca and ouabain were applied together after 30 min preperfusion with excess K-containing solution. Again excess Ca failed to inhibit the ouabain-evoked neurotransmitter release if ouabain and excess K were applied together after excess Ca preperfusion (30 min). In both cases the initial delay of ouabain action was greatly shortened. 6 The results suggest a Na-Ca competition at the external activation site of the nerve terminal sodium-pump similar to that of Na-K competition. Furthermore it seems that there is a sort of K-Ca competition as well, suggested by the finding that excess Ca prevented the inhibition caused by excess K of ouabain-evoked noradrenaline release and vice versa.

Potassium ion is indispensable to the catecholamine releasing response of dog adrenals to gamma-aminobutyric acid

The effect of potassium ion on the GABA-evoked catecholamine (CA) release from isolated perfused adrenal glands of the dog was investigated. When omitting the external potassium ion, the basal release of CA was increased. During this period GABA no longer caused the increase in CA release and moreover the increased basal release was diminished reversibly by GABA. 3-Amino-1-propane-sulfonic acid, a GABAA agonist, mimicked the action of GABA in K+-free solution, while baclofen, a GABAB agonist, did not cause CA release in normal solution and did not alter the basal release in K+-free solution. The inhibition by GABA of the basal CA release in K+-free solution was blocked by bicuculline. The potency of the CA releasing action of GABA was dependent on the concentration of external K+ between 1-10 mM. Reintroduction of K+ to glands which had been perfused with K+-free solution immediately reduced the basal release of CA whereas it recovered the CA releasing action of GABA. These results suggest that GABA-evoked CA release is dependent on potassium ion. The possible mechanisms by which GABA evoked CA release are discussed.

Ouabain- and potassium-induced stimulation of amylase release in fragments and acini of rabbit pancreas

Ouabain increases the enzyme secretion from the isolated rabbit pancreas and pancreatic fragments, but not from isolated pancreatic acini. The increase occurs after a delay of 45-60 min and is not accompanied by an increase in lactate dehydrogenase release. The stimulatory effect of ouabain (10(-5) M) is dependent on the presence of extracellular calcium, and is not antagonized by 10(-4) M atropin, 10(-4) M propranolol, 10(-5) M phentolamine, 10(-3) M dibutyryl-cyclic GMP, 10(-6) M tetrodotoxin, 10(-4) M verapamil or 10(-4) M D-600. Elevation of the extracellular potassium concentration to 120 mM in the presence of 10(-4) M atropin also increases the enzyme secretion from rabbit pancreatic fragments. The increase is again dependent on the presence of extracellular calcium and is resistant to adrenergic blockade and to tetrodotoxin, verapamil or D-600. Forskolin also stimulates a Ca2+-dependent release of amylase from pancreatic fragments but not from pancreatic acini. In the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IMX), ouabain (10(-5) M) and K+ (120 mM) cause an immediate increase in the cyclic AMP content of pancreatic fragments which does not occur in the absence of extracellular calcium. In pancreatic acini, the cAMP production is only slightly increased by ouabain. In the absence of IMX, the cAMP levels in fragments or acini are not detectably altered by ouabain or K+. The results suggest that the stimulation of enzyme secretion by ouabain and high K+ is an indirect effect, mediated by the release of an endogenous transmitter from non-cholinergic, non-adrenergic nerves in the intact preparations. The release and/or the effect of the transmitter appears to be mediated primarily by Ca2+ and secondarily by cyclic AMP.

Enhanced oxygen consumption in adrenal medulla on stimulation with acetylcholine

When perfused cortex-free ox adrenal medulla was stimulated to secrete catecholamine by infusion of 0.1 mM acetylcholine for 4 min, the oxygen consumption increased to a value which was 0.15 +/- 0.07 mumole O2/min/g wet weight (+/- S.D., N = 12) above the pre-stimulation value of 0.49 +/- 0.15 (P less than 0.001). 1.4 +/- 0.9 (+/- S.D., N = 12) mole of catecholamine was secreted per mole of enhanced O2 consumption in the 16 min following the start of the stimulation. The rate of ATP hydrolysis by the proton-translocating Mg-ATPase of the chromaffin granule may increase on fusing with the plasma membrane of the chromaffin cell during exocytosis. However, from the amount of catecholamine secreted, this was estimated to account for less than 17% of the oxygen consumption increase. The amount of catecholamine secreted by 4 min 0.1 mM acetylcholine stimulations correlated with the enhancement of oxygen consumption (r = 0.82, P less than 0.001) but, on stimulation with 60 microM veratridine for 4 min, O2 consumption enhancement was anomalously low. This dependence on mode of stimulation suggests that ATP consumption in exocytosis itself is an inadequate explanation. Ouabain-sensitive oxygen consumption rose from undetectable levels to 18 +/- 8% (+/- S.D., N = 4) of the basal respiration during prolonged 0.1 mM acetylcholine stimulation in the absence of Ca, indicating that Na,K-ATPase was not responsible for all of the oxygen consumption enhancement.

Modulation by ouabain and diphenylhydantoin of veratridine-induced 22Na influx and its relation to 45Ca influx and the secretion of catecholamines in cultured bovine adrenal medullary cells

The effects of ouabain and diphenylhydantoin on the secretion of catecholamines induced by veratridine were investigated in cultured bovine adrenal medullary cells with special reference to ion fluxes. Veratridine itself induced an influx of 22Na and 45Ca as well as secretion of catecholamines, which were antagonized by tetrodotoxin, a selective inhibitor of voltage dependent Na channels. The secretion of catecholamines caused by veratridine was not observed either in Na free or Ca free medium. Veratridine-induced influx of 45Ca did not occur in Na free medium, while veratridine-induced influx of 22Na occurred even in Ca free medium. Veratridine-induced influx of 22Na, 45Ca and secretion of catecholamines were all potentiated by ouabain, a potent inhibitor of Na,K-ATPase. Omission of K from the medium, a condition which suppresses the Na,K-ATPase activity, also augmented these cell responses caused by veratridine. On the contrary, diphenylhydantoin, which is known to decrease the intracellular concentration of Na, reduced the veratridine-induced influx of 22Na, 45Ca and secretion of catecholamines. The potentiating effects of ouabain on the veratridine-induced cell responses were all abolished by diphenylhydantoin. These findings imply that veratridine, ouabain and K removal as well as diphenylhydantoin modulate the intracellular accumulation of 22Na which is involved in the influx of 45Ca and the secretion of catecholamines.

Sodium-potassium ATPase inhibition potentiates compound 48/80-induced histamine secretion from mast cells

The effect of ouabain on the histamine secretion induced by compound 48/80 has been studied using rat peritoneal mast cells. Ouabain did not modify histamine release in the presence of millimolar concentrations of extracellular calcium. However, when mast cells were previously washed with a calcium-free buffer, ouabain strongly potentiated histamine release elicited by compound 48/80. The full potentiation of mast cell secretion by ouabain required 30 min preincubation before adding compound 48/80. It was inhibited by lanthanum and EGTA. Potassium deprivation mimicked the effect of ouabain. A 30 min preincubation time without potassium was also required. Potassium concentrations below 2.7 mM increased the effect of ouabain whereas higher potassium concentrations reversed this effect. The potentiation of compound 48/80-induced histamine release by ouabain or potassium deprivation was not immediately reversed by washing away ouabain or by adding potassium, respectively. The data confirm that sodium-potassium ATPase is involved, through a calcium-dependent process, in the regulation of histamine release from mast cells.

Effect of ouabain and furosemide on pepsinogen secretion by gastric glands in vitro

Gastric glands were isolated from rabbit stomach and pepsinogen secretion was measured after stimulation with isoproterenol, forskolin, 8-bromo cyclic adenosine monophosphate (8-bromo cAMP), cholecystokinin octapeptide (CCK-OP), carbachol, and hyperosmolar medium. The responses to these stimuli in medium containing 143 mM Na+ and 5.4 mM K+ (normal medium) were compared with responses to the same stimuli in media containing either 0 Na+ and 5.4 mM K+, or 143 mM Na+ and O K+. In addition, the effects of ouabain and furosemide on secretion elicited by these stimuli were determined. Medium containing 0 Na+ inhibited all stimuli. Medium containing 0 K+ inhibited the action of 8-bromo cAMP and stimuli postulated to be mediated by cAMP. Ouabain inhibited the same stimuli as O K+ medium, and, in addition, inhibited the response to hyperosmolar medium. However, ouabain enhanced the response to CCK-OP. Furosemide inhibited the response to hyperosmolar medium but had no effect on the action of any secretagogue employed. Intraglandular [Na+] increased and [K+] decreased after exposure to K+-free medium or ouabain. cAMP content of the glands was assayed after stimulation with both isoproterenol and hyperosmolar medium. Isoproterenol and hyperosmolar medium significantly increased cAMP levels. The results are discussed in relation to possible involvement of ion transport or intracellular ion concentration in the secretory process.

Inhibition of parathyroid hormone release from dispersed bovine cells by ouabain

Ouabain, at concentrations of 3 X 10(-4)-10(-3) M, inhibited secretion of PTH approximately 50% in a freshly dispersed bovine parathyroid cell preparation. This inhibition was found with both unstimulated and secretagogue-stimulated PTH release. Reductions in PTH secretion were found at all concentrations of Ca++ tested between 0.3 mM and 2.0 mM and in the presence of the divalent cation chelators EDTA and EGTA, indicating that extracellular Ca++ is not an absolute requirement for the inhibition. The ouabain inhibition did not appear to be mediated through changes in either adenylate cyclase activity or total cellular cAMP, implying a locus distal to the generation of this cyclic nucleotide. The data suggest that transmembrane potential and/or distribution of monovalent cations across the plasma membrane is important in the maintenance of PTH secretion. The mechanisms involved in this control do not appear to involve extracellular Ca++ directly.

Ca2+ uptake to purified secretory vesicles from bovine neurohypophyses

Purified secretory vesicles isolated from bovine neurohypophyses were found to take up Ca2+ when incubated at 30 degrees C in media containing 10(-7) to 10(-4) M free Ca2+. At 10(-4) free Ca2+ 19 nmol/mg protein were taken up within 30 min. The initial uptake at this Ca2+ concentration was about 2 nmol/mg protein per min. The uptake of Ca2+ to secretory vesicles was not affected by ATP, oligomycin, ruthenium red, trifluoperazine, Mg2+ or K+, but was inhibited by Na+ and Sr2+. From these characteristics it can be concluded that the uptake system does not utilize directly ATP (as the Ca2+-ATPases known to be present in the cell membrane and the endoplasmic reticulum) and is different from the mitochondrial Ca2+ uptake system driven by respiration and/or ATP hydrolysis. However, Ca2+-Na+ exchange may well operate: In experiments using different concentrations of Na+ we found half-maximal inhibition of Ca2+ uptake with 33.3 mM Na+. An analysis of the data in a Hill plot indicated that at least 2 Na+ would be exchanged for 1 Ca2+. Also, it was found that Ca2+ previously taken up could be released again by external Na+ but not by K+.

Effects of the cardiotonic drug ARL-115 on the release of noradrenaline from the cat atrium, the binding of 3H-ouabain to plasma membranes and the movements of calcium in mitochondria

The cardiotonic pyridine derivative ARL-115 increased the spontaneous and electrically-evoked release of 3H-noradrenaline from the cat right atrium superfused with oxygenated Krebs-bicarbonate solution at 37 degrees C. On the contrary, ouabain inhibited the evoked release while it also enhanced the spontaneous release of the transmitter. Vanadate did not affect either spontaneous or evoked release. Tetraethylammonium chloride (TEA) and 4-aminopyridine (4-AP) greatly potentiated 3H-noradrenaline release induced by electrical stimulation; when applied in addition to each agent, ARL-115 failed to further increase the secretory response. 3H-ouabain specific binding to partially purified bovine adrenal medulla plasma membranes was very efficiently antagonized by cold ouabain, but not by vanadate or ARL-115, even at concentrations as high as 10(-3) mol/l. 45Ca uptake into isolated bovine adrenal medulla mitochondria was prevented by dinitrophenol (DNP) but unchanged in the presence of ARL-115. 45Ca release from preloaded mitochondria was, again, markedly increased by DNP, but not affected by ARL-115. The results suggest that ARL-115 enhances the release of noradrenaline from cardiac sympathetic nerves by a TEA- and 4-AP-like action. In this manner, ARL-115 would inactivate the K+ current in the nerve terminals, thereby prolonging the duration of the action potential, allowing the Ca2+ channels to remain open longer and more Ca2+ to enter the terminal. ARL-115 is not acting like digitalis.

22Na+ uptake and catecholamine secretion by primary cultures of adrenal medulla cells

The uptake of 22Na+ and secretion of catecholamines by primary cultures of adrenal medulla cells under the influence of a variety of agonists and antagonists were determined. Veratridine, batrachotoxin, scorpion venom, and nicotine caused a parallel increase in 22Na+ uptake and Ca2+-dependent catecholamine secretion. Ba2+, depolarizing concentrations of K+, and the Ca2+ ionophore Ionomycin stimulated secretion of catecholamines but did not increase the uptake of 22Na+. Tetrodotoxin inhibited both 22Na+ uptake and catecholamine secretion evoked by veratridine, batrachotoxin, and scorpion venom, but had no effect on 22Na+ uptake and catecholamine secretion caused by nicotine. On the other hand, histrionicotoxin, which blocks the acetylcholine receptor-linked ion conductance channel, blocked nicotine-stimulated 22Na+ uptake and catecholamine secretion, but only partially inhibited veratridine-stimulated catecholamine secretion and had no effect on veratridine-stimulated 22Na+ uptake. The combination of veratridine plus tetrodotoxin, which has been shown to prevent nicotine-stimulated secretion of catecholamines by adrenal medulla cells, also prevented nicotine-stimulated 22Na+ uptake by the primary cultures. These studies demonstrate the presence of tetrodotoxin-sensitive Na+ channels in adrenal medulla cells which are functionally linked to Ca2+-dependent catecholamine secretion. However, These channels are not utilized for Na+ entry upon activation of nicotinic receptors; in this case Na+ entry occurs through the receptor-associated ion conductance channel.

Potentiation of K+-evoked catecholamine release in the cat adrenal gland treated with ouabain

1 A vigorous catecholamine secretory response was evoked by small increments (2-10 mM) of the extracellular concentration of K+ ([K+])o) in cat adrenal glands treated with ouabain (10(-4) M), and perfused with Krebs-bicarbonate solution at room temperature. 2 The secretory response depends on [K+]o; increments of [K+]o as small as 2 mM for 2 min evoked a clear secretory response; at 10-17.7 mM K+, the maximal secretory response was observed. In normal glands, not treated with ouabain, no increase of the rate of catecholamine output was observed by raising [K+]o up to 17.7 mM for 2 min. 3 The K+ secretory response was time-dependent, requiring at least 1 min to be initiated; on continued exposure to 10 mM [K+]o, the enhanced response remained for at least 1 h. 4 In low [Na+]o, the K+-secretory response was unchanged. However, in 0-Ca2+, high-Mg2+ solutions, or in the presence of D600, an organic Ca2+ antagonist, it was abolished. 5 The K+-induced secretory response was not altered in the presence of tetrodoxin or tetraethylammonium. 6 It is concluded that ouabain potentiated the catecholamine secretory response to raised [K+]o by increasing the amount of Ca2+ available to the secretory machinery through (a) mobilization of an enhanced pool of membrane-bound Ca2+, (b) activation of membrane Ca2+ inward current; or (c) decrease of intracellular Ca2+ buffering systems. The activation by ouabain of a membrane Na+-Ca2+ exchange system is not involved in this K+-secretory response. It is suggested that the plasma membrane ATPase enzyme system, by changing the affinity of its Ca2+ binding sites, might control the availability of this cation to the secretory machinery and, therefore, modulate catecholamine secretion in the adrenal gland.

Correlation between catecholamine release and sodium pump inhibition in the perfused adrenal gland of the cat

1 Ca(2+) reintroduction to retrogradely perfused and ouabain (10(-4) M)-treated cat adrenal glands caused a catecholamine secretory response which was greater the longer the time of exposure to the cardiac glycoside. Such a response was proportional to the external Na(+) concentration [Na(+)](o).2 A qualitatively similar, yet smaller response was observed when glands were perfused with Krebs solution lacking K(+) ions; thus, K(+) deprivation mimicked the secretory effects of ouabain. Catecholamine secretion evoked by Ca(2+) reintroduction in K(+)-free solution (0-K(+)) was also proportional to [Na(+)](o) and greater the longer the time of exposure of the gland to 0-K(+) solution.3 The ionophore X537A also mimicked the ouabain effects, since Ca(2+) reintroduction to glands treated with this agent (25 muM) caused a sharp secretory response. When added together with X537A, ouabain (10(-4) M) did not modify the response to the ionophore.4 N-ethylmaleimide (NEM), another Na(+), K(+)-ATPase inhibitor, did not evoke the release of catecholamines; on the contrary, NEM (10(-4) M) inhibited the catecholamine secretory response to high [K(+)](o), acetylcholine, Ca(2+) reintroduction and ouabain.5 Ouabain (10(-4) M) inhibited the uptake of (86)Rb into adreno-medullary tissue by 60%. Maximal inhibition had already occurred 2 min after adding the drug, indicating a lack of temporal correlation between ATPase inhibition and the ouabain secretory response, which took longer (about 30-40 min) to reach its peak. NEM (10(-4) M) blocked (86)Rb uptake in a similar manner.6 The results are further evidence in favour of the presence of a Na(+)-Ca(2+) exchange system in the chromaffin cell membrane, probably involved in the control of [Ca(2+)](i) and in the modulation of catecholamine secretion. This system is activated by increasing [Na(+)](i), either directly (ionophore X537A, increased [Na(+)](o)) or indirectly (Na(+) pump inhibition). However, the simple inhibition of Na(+) pumping does not always lead to a catecholamine secretory response; such is the case for NEM.

Ionophore (A23187)-induced efflux of [3H]norepinephrine and endogenous norepinephrine in the rat vas deferens

The calcium ionophore, A23187, produced a concentration-dependent increase in the release of norepinephrine from nerves in the rat vas deferens. Maximum response to A23187 (10(-6) - 10(-5) M) was delayed in onset, occurring 60-80 min after initiation of continuous superfusion with A23187. In fact. after tissue exposure to A23187 (10(-5) M) for only 5 min with subsequent superfusion in A23187-free buffer, a significant but delayed increase in norepinephrine efflux occurred. The A23187-induced increase in efflux of norepinephrine was not altered when neuronal sodium conductance was blocked with tetrodotoxin (3.1 X 10(-7) M) or when Na+, K+ -stimulated ATPase was blocked with ouabain (10(-4) M). Release of norepinephrine by A23187 was calcium-dependent since A23187-induced efflux of norepinephrine was diminished (approximately 50%), although not abolished, in calcium-free buffer. Thus, one component of A23187 action was calcium independent. A23187 caused an increased efflux of both norepinephrine formed endogenously and [3H]norepinephrine taken up into neuronal stores. However, the effects of A23187, both on rate and maximum amount of release were greater for [3H]norepinephrine than for endogenous norepinephrine. The present studies demonstrate that neurotransmitter efflux can be induced by carboxylic ionophores in a calcium-dependent process, and this approach may prove useful in studies evaluating factors that modulate neurotransmitter release processes.

Distinct regulations by calcium of cyclic GMP levels and catecholamine secretion in isolated bovine adrenal chromaffin cells

The effects of various calcium-dependent secretagogues on cyclic GMP levels and catecholamine (CA) secretion were measured in a preparation of bovine adrenal chromaffin cells. The secretory effect of acetylcholine (ACh; 8--10 fold stimulation) was mimicked by nicotine but not muscarine. Three--five fold stimulations of cyclic GMP levels were also obtained with ACh and muscarine but not nicotine. High concentration of K+, and the ionophore A23187, also elevated cyclic GMP levels. However, secretion produced by veratridine, ouabain, and the ionophore X537A was not accompanied by any rise in cyclic GMP levels. Removal of extracellular calcium significantly decreased both basal levels of CA secretion and of cyclic GMP and completely abolished their stimulation by ACh. The half-maximal effects of calcium on the cholinergic stimulations of cyclic GMP levels and of CA secretion were observed at 0.2 and 2.5 mM, respectively. Substitution of Ca2+ by Sr2+ was more effective in maintaining the cyclic GMP response than the secretory response. The calcium channel blockers Co2+, Mg2+ and Ni2+ inhibited the cholinergic stimulation of cyclic GMP more than that of CA release. On the other hand, the organic calcium channel blockers, verapamil and methoxyverapamil (D--600) were more effective antagonists of the secretory response. These data indicate that the cholinergic stimulations of CA secretion and of cyclic GMP levels in bovine adrenal chromaffin cells are regulated by calcium via two distinct mechanisms.

Possible occurrence of Na+-dependent Ca2+ influx mechanism in isolated bovine chromaffin cells

Chromaffin cells were isolated from bovine adrenals and the effects of experimental manipulations which alter the level of internal Na+ on the release of catecholamines and 45Ca uptake by these cells were investigated. In response to NA+ deprivation both parameters were increased or decreased when internal NA+ was raised or reduced, respectively. The results suggest the existence of Na+-dependent Ca2+ influx mechanism in these cells.

Correlation between catecholamine secretion from bovine isolated chromaffin cells and [3H]-ouabain binding to plasma membranes

1 Secretion of catecholamines (CA) evoked by ouabain, chlormadinone acetate (CMA), phenoxybenzamine (Pbz) and vanadate, four agents known to inhibit Na(+), K(+)-dependent Mg(2+)-activated adenosine triphosphatase (ATPase) activity has been studied in suspensions of bovine isolated adrenal medullary cells.2 Acetylcholine (ACh) evoked a 5 fold increase of the basal CA secretion from isolated cells suspended in oxygenated Krebs-bicarbonate solution kept at 27 degrees C. Secretion was antagonized by Ca(2+)-deprivation or hexamethonium, indicating good functional viability of the cells.3 Ouabain (10(-7) to 10(-4) M) evoked a progressive, dose-dependent release of CA from cell suspensions. Study of the time course of the secretory response for 2 h allowed the separation of two components in the secretory response at all doses studied: a slow initial component (0.011 pg/min CA) and a second faster component (0.032 pg/min CA).4 CMA evoked a clear-cut CA secretory response. The ED(50) for CMA was 10(-4) M, as compared to 3 x 10(-6) M for ouabain. Pbz and vanadate did not induce CA release.5 [(3)H]-ouabain was taken up and bound to intact isolated cells by a non-saturable binding process. However, in semi-purified plasma membranes from bovine adrenal medulla a saturable specific [(3)H]-ouabain binding process was observed with a K(D) of 8.1 nM. Binding to the membranes was ATP-dependent and antagonized by K(+).6 [(3)H]-ouabain specific binding to membranes was antagonized by ouabain and CMA, but not by Pbz or vanadate; the ID(50) for ouabain and CMA were 10(-6) and 10(-5) M respectively.7 Ouabain partially inhibited, in a dose-dependent manner, Na(+), K(+)-Mg(2+) ATPase activity of the semi-purified plasma membranes.8 The results demonstrate a good correlation between the ability of different drugs, known to inhibit ATPase activity, to displace [(3)H]-ouabain binding to adreno-medullary plasma membranes and their capacity to evoke a CA secretory response from isolated chromaffin cells. The data also suggest that the CA secretory effects of ouabain may not be due simply to inhibition of the Na(+) pump and the subsequent ionic redistribution across the plasma membrane; a second mechanism may also be involved.

Noradrenaline output induced by calcium, strontium and barium during exposure of guinea-pig vas deferens to ouabain

The effects of tetrodotoxin (TTX, 5 x 10(-7) g/ml) and excess Mg2+ (20 mM) on the noradrenaline output induced by the addition of 2.5 mM of Ca2+, Sr2+ and Ba2+ during exposure to ouabain were observed using isolated guinea-pig vas deferens. Excess Mg2+ reduced the noradrenaline output induced by Ba2+ or Sr2+, but not that induced by Ca2+, regardless of whether it was added before or after the start of exposure to ouabain. In contrast, when added before the exposure to ouabain, TTX was most effective in retarding and in decreasing the noradrenaline output induced by Ca2+, La3+ (0.25 mM) inhibited, and transmural electrical stimulation potentiated the Ca2+-induced response during exposure to ouabain. These results confirmed that Ca2+-induced response during exposure to ouabain in mainly mediated by a Na+-dependent Ca2+ influx system. In contrast, Sr2+ and Ba2+ may enter into adrenergic nerve terminals mainly through depolarization and induce the output of noradrenaline when the Na+-K+ pump is inhibited.