[3H]noradrenaline release from rabbit pulmonary artery: sodium-pump-dependent sodium-calcium exchange

Does Na⁺/Ca²⁺ exchanger, NCX, represent a new druggable target in stroke intervention?

Stroke causes a rapid cell death in the core of the injured region and triggers mechanisms in surrounding penumbra area that leads to changes in concentrations of several ions like intracellular Ca²⁺, Na⁺, H⁺, K⁺, and radicals such as reactive oxygen species and reactive nitrogen species. When a dysregulation of homeostasis of these messengers occurs, it can trigger cell death. In particular, it is widely accepted that a critical factor in determining neuronal death during cerebral ischemia is progressive dysregulation of Ca²⁺, Na⁺, K⁺, and H⁺ homeostasis that activate several death pathways, including oxidative and nitrosative stress, mitochondrial dysfunction, protease activation, and apoptosis. In the last decade, several seminal experimental works are markedly changing the scenario of research of principal players of an ischemic event. Indeed, some plasma membrane channels and transporters, involved in the control of Ca²⁺, Na⁺, K⁺, and H⁺ ion influx or efflux and, therefore, responsible for maintaining the homeostasis of these four cations, might function as crucial players in initiation of brain ischemic process. Indeed, these proteins, by regulating ionic homeostasis, may provide the molecular basis underlying glutamate-independent Ca²⁺ and Na⁺ overload mechanisms in neuronal ischemic cell death and, most importantly, may represent more suitable molecular targets for therapeutic intervention. Recently, a great deal of interest has been devoted to clarify the role of the plasma membrane protein known as Na⁺/Ca²⁺ exchanger, a transporter able to control Na⁺ and Ca²⁺ homeostasis. In this review, the pathophysiological role of NCX and its implication as a potential target in stroke intervention will be examined.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Mechanism of ouabain-induced contractions in guinea-pig tracheal rings

The aim of the present study was to analyse the mechanism that underlies the force development induced by ouabain (ED(100) = 100 micromol/L) in guinea-pig tracheal rings. The dose-response curve showed that concentrations of ouabain above 100 micromol/L evoked smaller contractions. Ouabain, at 100 micromol/L, produced two long-lasting consecutive transient contractions. The peak of the first contraction was 750 +/- 75 mg, whereas the peak of the second contraction was 280 +/- 46 mg. Both contractions induced by ouabain were dependent on extracellular Ca(2+). Consistent with this, verapamil (10 micromol/L) inhibited the first and second contractions by 77 and 59%, respectively. 3,4-Dichlorobenzamil (20 micromol/L) inhibited the first and second contractions by 68 and 97%, respectively. Simultaneous exposure to 15 mmol/L sodium solution and 100 micromol/L ouabain evoked only one transient contraction, larger (987 +/- 135 mg) than either of the ouabain-induced contractions. Inhibition of the sarcoendoplasmic reticulum Ca-ATPase with cyclopiazonic acid potentiated the first and second ouabain-induced contractions by 47 and 300%, respectively. Atropine (1 micromol/L) inhibited the first and second contractions by 44 and 76%, respectively. In conclusion, the results of the present study are relevant to the understanding of the mechanisms by which ouabain (100 micromol/L) contracts guinea-pig tracheal rings. At the muscular level, oubain induces Ca(2+) influx through L-type Ca(2+) channels and the reverse mode of the sodium-calcium exchanger. At the nerve terminals, ouabain promotes the release of acetylcholine secondary to the increase in Ca(2+) influx mediated by the reverse mode of the sodium-calcium exchanger.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Dependence of release of [3H]noradrenaline from rabbit pulmonary artery on internal sodium

1. [3H]Noradrenaline ([3H]NA) release from the isolated main pulmonary artery of the rabbit has been measured in the presence of uptake blockers (cocaine, 3 x 10(-5) M, and corticosterone, 5 x 10(-5) M) and after blocking the monoamine oxidase enzyme by pargyline (1.2 x 10(-4) M). 2. In normal Krebs solution Mn2+ (2 mM) significantly inhibited both [3H]NA release (approximately 80%; P < 0.001) and the contraction following 2 Hz field stimulation. 3. In Ca(2+)-free, EGTA (1 mM)-containing solution, the Na+ pump was inhibited by removal of K+ from the external medium. In Na+ pump-inhibited arteries, 2 mM Mn2+ (free Mn2+, 1 mM) increased the spontaneous release of [3H]NA according to the time of Na+ loading. TTX (10(-7) M) did not inhibit significantly the Mn(2+)-induced [3H]NA release from Na(+)-loaded preparations (percentage inhibition, approximately 24; P > 0.30). 4. Without Na+ loading (Ca2+ free, EGTA alone), Mn2+ failed to promote 3H release from arteries. 5. With constant Na+ loading (120 min 'K(+)-free' perfusion in Ca(2+)-free, 1 mM EGTA-containing solution), the release of 3H was also directly dependent on free Mn2+ concentration (0.2, 0.6 and 1 mM). 6. The Mn2+ (2 mM; free Mn2+, 1 mM)-induced 3H release from Na(+)-loaded nerves (120 min 'K(+)-free', perfusion) was further enhanced, when external Na+ was simultaneously reduced from 139.2 to 26.2 mM (choline+ or sucrose substitution). 7. Diphenylhydantoin (DPH, 10(-4) M) significantly reduced the Mn(2+)-evoked 3H release (approximately 44%; P < 0.02) when it was present during 'K(+)-free', perfusion. 8. Mn2+ was ineffective in releasing 3H if the Na+ pump was previously reactivated by readmission of K+ to Na(+)-loaded arteries. 9. It is concluded that in Ca(2+)-free solution Mn2+ releases neurotransmitter in a manner which depends on the degree of loading with internal Na+. The results suggest this depends at least partly on a block of Ca2+ efflux.

Depolarization promotes caffeine induced [3H]-noradrenaline release in calcium-free solution from peripheral sympathetic nerves

The transmitter releasing action of caffeine was studied in the absence of extracellular Ca2+ from the peripheral sympathetic nerves of the rabbit main pulmonary artery. Caffeine (10 mM) increased the release of [3H]-noradrenaline moderately, but not significantly in Ca2(+)-free (+1 mM EGTA) Krebs solution. When peripheral nerve endings/varicosities were depolarized by elevating extracellular K+ to 47.2 mM and 70.8 mM in Ca2(+)-free solution, the transmitter releasing effect of 10 mM caffeine became significant. Ca2+ removal itself transiently increased the [3H]-noradrenaline outflow. In the individual experiments the amount of the caffeine evoked transmitter release at 47.2 mM and 70.8 mM K(+)-depolarization was inversely correlated to the release evoked by Ca2(+)-removal. Our results suggest that caffeine-sensitive calcium stores are present in peripheral nerve terminals of rabbit pulmonary artery, and part of the caffeine sensitive calcium stores may discharge during Ca2(+)-removal from the extracellular solution.

Sodium-azide-evoked noradrenaline and catecholamine release from peripheral sympathetic nerves and chromaffin cells

1. The spontaneous release of [3H]noradrenaline [( 3H]NA) has been measured from rabbit pulmonary arteries and bovine chromaffin cells in the presence of neuronal uptake blocker cocaine (3 x 10(-5) M). 2. The Na+-pump inhibitor sodium-azide (NaN3, 2mM) produced a moderate increase of [3H]NA release from both preparations and relaxed the arteries. The [3H]releasing action of NaN3 was accompanied by a 30% inhibition of 86Rb-uptake into chromaffin cells. 3. In both preparations, ouabain (10(-4) M) markedly increased the release of [3H], contracted the arteries and inhibited the 86Rb-uptake of chromaffin cells by about 75%. A combined application of NaN3 and ouabain produced a similar inhibition of 86Rb-uptake of chromaffin cells and failed to increase further the release of [3H] in comparison to that found in response to ouabain alone. 4. Removal of K+ from the external medium increased both the release of [3H]NA and the tone of pulmonary arteries. NaN3 further increased the transmitter release in "K+-free" solution but relaxed the muscle. In the absence of external K+ and in the presence of azide, ouabain further enhanced the transmitter release but failed to produce significant contraction. 5. Reactivation of the Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the transmitter releasing action of NaN3 in arteries. 6. It is concluded that in peripheral sympathetic nerves and chromaffin cells, NaN3 inhibits the Na+-pump producing NA and CA release respectively and in nerves even if NA release had already been increased by K+-removal.(ABSTRACT TRUNCATED AT 250 WORDS)

A-23187 evoked transmitter release from rabbit pulmonary artery and its inhibition by reactivation of sodium-pump

1. The spontaneous [3H]-release has been measured from the isolated main pulmonary artery of the rabbit preloaded with [3H]noradrenaline in the presence of uptake blockers (cocaine, 3 x 10(-5) M; corticosterone, 5 x 10(-5) M). 2. The Ca-ionophore A-23187 (3 x 10(-7)-3 x 10(-5) M) increased the outflow of [3H] by a concentration dependent manner. 3. Inhibition of Na+-pump by removal of K+ from the external medium also increased the release of labelled noradrenaline. 4. In the absence of external K+, the applied A-23187 (3 x 10(-6) M; EC50) further increased the release of [3H]. 5. Reactivation of Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the [3H]-release which had previously been increased in "K+-free" solution. 6. The reactivated Na+-pump significantly inhibited the transmitter releasing action of A-23187. 7. This latter was antagonized by an increase of external Ca2+ (7.5 mM). 8. It is concluded that the reactivated Na+-pump caused re-establishment of Na+-gradient is capable to counteract the Ca-ionophore facilitated Ca2+-influx and release from internal stores, which can be antagonized by excess Ca2+.

Sodium dependent 3H-noradrenaline release from rat neocortical slices in the absence of extracellular calcium: presynaptic modulation by mu-opioid receptor and adenylate cyclase activation

In Ca2+-free EGTA (1 mmol/l)-containing medium veratrine (3 mumol/l) and ouabain (100 mumol/l) strongly enhanced the efflux of 3H-noradrenaline from superfused rat brain neocortical slices prelabelled with the radioactive amine. In both cases 3H-noradrenaline release was prevented by tetrodotoxin (1 mumol/l). These effects of veratrine and ouabain were virtually additive and independent of whether the noradrenaline uptake carrier was blocked with 1 mumol/l desipramine or not. The adenylate cyclase activator forskolin (10 nmol/l - 10 mumol/l) strongly enhanced veratrine- and ouabain-induced 3H-noradrenaline release, without affecting spontaneous tritium efflux. The release induced by both stimuli was profoundly inhibited by the selective mu-opioid receptor agonist [D-Ala, MePhe4, Gly-ol5]enkaphalin (DAGO, 3 nmol/l - 1 mumol/l) in a concentration-dependent manner. The inhibitory effects of 1 mumol/l DAGO were abolished by 1 mumol/l naloxone. On the other hand, preincubation of the slices for 1 h with the delta-opioid receptor-selective irreversible ligand fentanyl isothiocyanate (1 mumol/l) did not change the inhibitory effects of DAGO. These data show that veratrine- and ouabain-induced 3H-noradrenaline release from central noradrenergic nerve terminals is facilitated by increasing intracellular cyclic AMP levels and reduced by activation of presynaptic mu-opioid receptors, indicating the involvement of exocytotic neurotransmitter release. The results provide further evidence for the hypothesis that under these conditions neurotransmitter release from central noradrenergic neurons is triggerred by a Na+-induced efflux of Ca2+ ions from intracellular stores.