Electrogenic Na(+)/Ca(2+) exchange. A novel amplification step in squid olfactory transduction

From synaptic input to muscle contraction: arm muscle cells of Octopus vulgaris show unique neuromuscular junction and excitation-contraction coupling properties

The muscular-hydrostat configuration of octopus arms allows high manoeuvrability together with the efficient motor performance necessary for its multitasking abilities. To control this flexible and hyper-redundant system the octopus has evolved unique strategies at the various levels of its brain-to-body organization. We focus here on the arm neuromuscular junction (NMJ) and excitation-contraction (E-C) properties of the arm muscle cells. We show that muscle cells are cholinergically innervated at single eye-shaped locations where acetylcholine receptors (AChR) are concentrated, resembling the vertebrate neuromuscular endplates. Na⁺ and K⁺ contribute nearly equally to the ACh-activated synaptic current mediating membrane depolarization, thereby activating voltage-dependent L-type Ca²⁺ channels. We show that cell contraction can be mediated directly by the inward Ca²⁺ current and also indirectly by calcium-induced calcium release (CICR) from internal stores. Indeed, caffeine-induced cell contraction and immunohistochemical staining revealed the presence and close association of dihydropyridine (DHPR) and ryanodine (RyR) receptor complexes, which probably mediate the CICR. We suggest that the dynamics of octopus arm contraction can be controlled in two ways; motoneurons with large synaptic inputs activate vigorous contraction via activation of the two routs of Ca²⁺ induced contraction, while motoneurons with lower-amplitude inputs may regulate a graded contraction through frequency-dependent summation of EPSP trains that recruit the CICR. Our results thus suggest that these motoneuronal pools are likely to be involved in the activation of different E-C coupling modes, thus enabling a dynamics of muscles activation appropriate for various tasks such as stiffening versus motion generation.

The Drosophila melanogaster Na+/Ca2+ Exchanger CALX Controls the Ca2+ Level in Olfactory Sensory Neurons at Rest and After Odorant Receptor Activation

CALX, the Na⁺/Ca²⁺ exchanger in Drosophila, is highly expressed in the outer dendrites of olfactory sensory neurons (OSNs) which are equipped with the odorant receptors (ORs). Insect OR/Orco dimers are nonselective cation channels that pass also calcium which leads to elevated calcium levels after OR activation. CALX exhibits an anomalous regulation in comparison to its homolog in mammals sodium/calcium exchanger, NCX: it is inhibited by increasing intracellular calcium concentration [Ca²⁺]_i. Thus, CALX mediates only Ca²⁺ efflux, not influx. The main goal of this study was to elucidate a possible role of this protein in the olfactory response. We first asked whether already described NCX inhibitors were capable of blocking CALX. By means of calcium imaging techniques in ex-vivo preparations and heterologous expression systems, we determined ORM-10962 as a potent CALX inhibitor. CALX inhibition did not affect the odor response but it affected the recovery of the calcium level after this response. In addition, CALX controls the calcium level of OSNs at rest.

The Na(+)/Ca(2+) exchanger NCKX4 governs termination and adaptation of the mammalian olfactory response

Sensory perception requires accurate encoding of stimulus information by sensory receptor cells. We identified NCKX4, a potassium-dependent Na(+)/Ca(2+) exchanger, as being necessary for rapid response termination and proper adaptation of vertebrate olfactory sensory neurons (OSNs). Nckx4(-/-) (also known as Slc24a4) mouse OSNs displayed substantially prolonged responses and stronger adaptation. Single-cell electrophysiological analyses revealed that the majority of Na(+)-dependent Ca(2+) exchange in OSNs relevant to sensory transduction is a result of NCKX4 and that Nckx4(-/-) mouse OSNs are deficient in encoding action potentials on repeated stimulation. Olfactory-specific Nckx4(-/-) mice had lower body weights and a reduced ability to locate an odorous source. These results establish the role of NCKX4 in shaping olfactory responses and suggest that rapid response termination and proper adaptation of peripheral sensory receptor cells tune the sensory system for optimal perception.

Imaging ensemble activity in arthropod olfactory receptor neurons in situ

We show that lobster olfactory receptor neurons (ORNs), much like their vertebrate counterparts, generate a transient elevation of intracellular calcium (Ca(i)) in response to odorant activation that can be used to monitor ensemble ORN activity. This is done in antennal slice preparation in situ maintaining the polarity of the cells and the normal micro-environment of the olfactory cilia. The Ca(i) signal is ligand-specific and increases in a dose-dependent manner in response to odorant stimulation. Saturating stimulation elicits a robust increase of up to 1 μM free Ca(i) within 1-2s of stimulation. The odor-induced Ca(i) response closely follows the discharge pattern of extracellular spikes elicited by odorant application, with the maximal rise in Ca(i) matching the peak of the spike generation. The Ca(i) signal can be used to track neuronal activity in a functional subpopulation of rhythmically active ORNs and discriminate it from that of neighboring tonically active ORNs. Being able to record from many ORNs simultaneously over an extended period of time not only allows more accurate estimates of neuronal population activity but also dramatically improves the ability to identify potential new functional subpopulations of ORNs, especially those with more subtle differences in responsiveness, ligand specificity, and/or transduction mechanisms.

Computational model of the insect pheromone transduction cascade

A biophysical model of receptor potential generation in the male moth olfactory receptor neuron is presented. It takes into account all pre-effector processes—the translocation of pheromone molecules from air to sensillum lymph, their deactivation and interaction with the receptors, and the G-protein and effector enzyme activation—and focuses on the main post-effector processes. These processes involve the production and degradation of second messengers (IP₃ and DAG), the opening and closing of a series of ionic channels (IP₃-gated Ca²⁺ channel, DAG-gated cationic channel, Ca²⁺-gated Cl⁻ channel, and Ca²⁺- and voltage-gated K⁺ channel), and Ca²⁺ extrusion mechanisms. The whole network is regulated by modulators (protein kinase C and Ca²⁺-calmodulin) that exert feedback inhibition on the effector and channels. The evolution in time of these linked chemical species and currents and the resulting membrane potentials in response to single pulse stimulation of various intensities were simulated. The unknown parameter values were fitted by comparison to the amplitude and temporal characteristics (rising and falling times) of the experimentally measured receptor potential at various pheromone doses. The model obtained captures the main features of the dose–response curves: the wide dynamic range of six decades with the same amplitudes as the experimental data, the short rising time, and the long falling time. It also reproduces the second messenger kinetics. It suggests that the two main types of depolarizing ionic channels play different roles at low and high pheromone concentrations; the DAG-gated cationic channel plays the major role for depolarization at low concentrations, and the Ca²⁺-gated Cl⁻ channel plays the major role for depolarization at middle and high concentrations. Several testable predictions are proposed, and future developments are discussed.

All sensory neurons transduce their natural stimulus, whether a molecule, a photon, or a mechanical force, in an electrical current flowing through their sensory membrane via similar molecular and ionic mechanisms. Olfactory receptor neurons (ORNs), whose stimuli are volatile molecules, are no exception, including one of the best known: the exquisitely sensitive ORNs of male moths that detect the sexual pheromone released by conspecific females. We provide a detailed computational model of the intracellular molecular mechanisms at work in this ORN type. We describe qualitatively and quantitatively how the initial event, the interaction of pheromone molecules with specialized receptors at the ORN surface, is amplified through a sequence of linked biochemical and electrical events into a whole cell response, the receptor potential. We detail the respective roles of the upward activating reactions involving a cascade of ionic channels permeable to cations, chloride and potassium, their control by feedback inactivating mechanisms, and the central regulatory role of calcium. This computational model contributes to an integrated understanding of this signalling pathway, provides testable hypotheses, and suggests new experimental approaches.

The Na+/Ca2+ exchanger inhibitor, KB-R7943, blocks a nonselective cation channel implicated in chemosensory transduction

The mechanism(s) of olfactory transduction in invertebrates remains to be fully understood. In lobster olfactory receptor neurons (ORNs), a nonselective sodium-gated cation (SGC) channel, a presumptive transient receptor potential (TRP)C channel homolog, plays a crucial role in olfactory transduction, at least in part by amplifying the primary transduction current. To better determine the functional role of the channel, it is important to selectively block the channel independently of other elements of the transduction cascade, causing us to search for specific pharmacological blockers of the SGC channel. Given evidence that the Na(+)/Ca(2+) exchange inhibitor, KB-R7943, blocks mammalian TRPC channels, we studied this probe as a potential blocker of the lobster SGC channel. KB-R7943 reversibly blocked the SGC current in both inside- and outside-out patch recordings in a dose- and voltage-dependent manner. KB-R7943 decreased the channel open probability without changing single channel amplitude. KB-R7943 also reversibly and in a dose-dependent manner inhibited both the odorant-evoked discharge of lobster ORNs and the odorant-evoked whole cell current. Our findings strongly imply that KB-R7943 potently blocks the lobster SGC channel and likely does so directly and not through its ability to block the Na(+)/Ca(2+) exchanger.

Plasma membrane Ca(2+)-ATPase in the cilia of olfactory receptor neurons: possible role in Ca(2+) clearance

Olfactory sensory neurons respond to odorants increasing Ca(2+) concentrations in their chemosensory cilia. Calcium enters the cilia through cAMP-gated channels, activating Ca(2+)-dependent chloride or potassium channels. Calcium also has a fundamental role in odour adaptation, regulating cAMP turnover rate and the affinity of the cyclic nucleotide-gated channels for cAMP. It has been shown that a Na(+)/Ca(2+) exchanger (NCX) extrudes Ca(2+) from the cilia. Here we confirm previous evidence that olfactory cilia also express plasma membrane Ca(2+)-ATPase (PMCA), and show the first evidence supporting a role in Ca(2+) removal. Both transporters were detected by immunoblot of purified olfactory cilia membranes. The pump was also revealed by immunocytochemistry and immunohistochemistry. Inside-out cilia membrane vesicles transported Ca(2+) in an ATP-dependent fashion. PMCA activity was potentiated by luminal Ca(2+) (K(0.5) = 670 nm) and enhanced by calmodulin (CaM; K(0.5) = 31 nm). Both carboxyeosin (CE) and calmidazolium reduced Ca(2+) transport, as expected for a CaM-modulated PMCA. The relaxation time constant (tau) of the Ca(2+)-dependent Cl(-) current (272 +/- 78 ms), indicative of luminal Ca(2+) decline, was increased by CE (2181 +/- 437 ms), by omitting ATP (666 +/- 49 ms) and by raising pH (725 +/- 65 ms), suggesting a role of the pump on Ca(2+) clearance. Replacement of external Na(+) by Li(+) had a similar effect (tau = 442 +/- 8 ms), confirming the NCX involvement in Ca(2+) extrusion. The evidence suggests that both Ca(2+) transporters contribute to re-establish resting Ca(2+) levels in the cilia following olfactory responses.

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.

Sodium/calcium exchanger expression in the mouse and rat olfactory systems

Sodium/calcium (Na(+)/Ca(2+)) exchangers are membrane transport systems that regulate Ca(2+)-homeostasis in many eukaryotic cells. In olfactory and vomeronasal sensory neurons ligand-induced olfactory signal transduction is associated with influx and elevation of intracellular Ca(2+), [Ca(2+)](i). While much effort has been devoted to the characterization of Ca(2+)-related excitation and adaptation events of olfactory chemosensory neurons (OSNs), much less is known about mechanisms that return [Ca(2+)](i) to the resting state. To identify proteins participating in the poststimulus Ca(2+)-clearance of mouse OSNs, we analyzed the expression of three potassium (K(+))-independent (NCX1, 2, 3) and three K(+)-dependent (NCKX1, 2, 3) Na(+)/Ca(2+) exchangers. In situ hybridization showed that mRNAs of all six Na(+)/Ca(2+) exchangers coexist in neurons of the olfactory and vomeronasal systems, and that some are already detectable in the embryo. Of these, NCX1 and NCKX1 represent the most and least abundant mRNAs, respectively. Moreover, immunohistochemistry revealed that the NCX1, 2, and 3 proteins are expressed in nearly all neurons of the olfactory epithelium, the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. These three exchanger proteins display different expression profiles in dendrites, knobs, and plasma membranes of OSNs and in sustentacular cells. Furthermore, we show that NCX1 mRNA in rat olfactory mucosa is expressed as 8 alternative splice variants. This is the first comprehensive analysis of Na(+)/Ca(2+) exchanger expression in the mammalian olfactory system. Our results suggest that Ca(2+)-extrusion by OSNs utilizes multiple different Na(+)/Ca(2+) exchangers and that different subtypes are targeted to different subcellular compartments.

Increased intracellular Ca++ in the macula densa regulates tubuloglomerular feedback

BACKGROUND

Tubuloglomerular feedback is initiated by an increase in NaCl at the macula densa lumen, which in turn increases intracellular Ca++. In the present study, we examined the role of increased intracellular Ca++ in tubuloglomerular feedback and the source of the increased Ca++. We hypothesized that an increase in intracellular Ca++ at the macula densa via the basolateral Na+/Ca++ exchanger, caused by an increase in luminal NaCl, initiates Ca++-mediated Ca++ release from intracellular stores, which is essential for tubuloglomerular feedback.

METHODS

Rabbit afferent arterioles and attached macula densas were simultaneously microperfused in vitro. Tubuloglomerular feedback was induced by increasing macula densa Na+/Cl- from 11/10 mmol/L (low) to 81/80 mmol/L (high) and was measured before and after treatment.

RESULTS

To investigate whether elevations in intracellular Ca++ are required for tubuloglomerular feedback, the calcium ionophore A23187 or the Ca++ chelator BAPTA-AM was added to the macula densa lumen. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.1 +/- 1.1 microm to 15.3 +/- 0.8 microm. Adding 2 x 10-6 mol/L A23187 to the low NaCl macula densa perfusate induced tubuloglomerular feedback; diameter decreased from 18.0 +/- 1.0 microm to 15.4 +/- 0.9 microm (N = 6; P < 0.01). After adding BAPTA-AM (25 micromol/L) to the macula densa lumen, tubuloglomerular feedback response was completely eliminated. We next studied the source of increased macula densa Ca++ in response to increased NaCl concentration. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.5 +/- 1.6 microm to 15.3 +/- 1.2 microm (N = 6; P < 0.01). After adding the Na+/Ca++ exchanger inhibitor 2'4'-dichlorobenzamil (10 micromol/L) or KB-R7943 (30 micromol/L) to the bath, the tubuloglomerular feedback response was blocked; however, the afferent arteriole response to angiotensin II or adenosine was not altered. Next, we tested the Ca++-adenosine triphosphatase (ATPase) inhibitor thapsigargin (0.1 micromol/L), which has been reported to inhibit sarcoplasmic reticulum Ca++-ATPase activity and prevent restoration of intracellular Ca++ stores. When thapsigargin was added to the macula densa lumen, it reduced the first tubuloglomerular feedback response by 33% and completely eliminated the second and third tubuloglomerular feedback responses. In the absence of thapsigargin, there was no significant decrease in the tubuloglomerular feedback responses (N = 6). Neither the L-type Ca++ channel blocker nifedipine (25 micromol/L), nor the T-type Ca++ channel blocker pimozide (10 micromol/L), inhibited tubuloglomerular feedback when added to the macula densa lumen.

CONCLUSION

We concluded that (1). increased intracellular Ca++ at the macula densa is required for the tubuloglomerular feedback response; (2). Na+/Ca++ exchange appears to initiate Ca++-mediated Ca++ release from intracellular stores; and (3). luminal L-type or T-type Ca++ channels are not involved in tubuloglomerular feedback.

Hypoxia activates a background conductance in freshly isolated pulmonary arterial endothelial cells of the rat

Utilising the patch-clamp recording technique we have demonstrated for the first time the effects of hypoxia on the background current in pulmonary arterial endothelial cells. Electrophysiological studies revealed the presence of a novel oxygen-sensitive, non-selective cation conductance (I(NSC)) in these cells. The inward component of I(NSC) was significantly potentiated by hypoxia. Both the inward and outward components of I(NSC) were inhibited by both La(3+) and Gd(3+). Hypoxic activation of I(NSC) may provide an important Ca(2+) influx pathway essential for the release of a pulmonary-selective vasoconstrictor pivotal to the sustained phase of hypoxic pulmonary vasoconstriction.

Na(+)/Ca(2+) exchanger in GABAergic presynaptic boutons of rat central neurons

Rat Meynert neurons were acutely isolated using a dissociation technique that maintains functional GABAergic presynaptic boutons. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded under voltage-clamp conditions using whole cell patch-clamp recordings. Using the frequency of mIPSCs as a measure of presynaptic terminal excitability, the existence of a Na(+)/Ca(2+) exchanger (NCX) in these GABAergic nerve terminals was clearly demonstrated. Both the frequency and the amplitude of mIPSCs were unaffected by replacement of extracellular Na(2+). However, in this Na(+)-free external solution, ouabain could now induce a transient increase of mIPSCs frequency, which was not inhibited by adding Cd(2+) or cyclopiazonic acid but was inhibited by removing external Ca(2+). This indicates that this transient potentiation was dependent on external Ca(2+), but that this Ca(2+) influx was not via voltage-dependent Ca(2+) channels. KB-R7943, an inhibitor of NCX, at a concentration of 3 x 10(-6) M, reduced this transient increase of mIPSCs frequency without affecting mIPSCs amplitude and the response to exogenous GABA. These results demonstrate the existence of NCX in these GABAergic nerve terminals. In zero external Na(+), ouabain causes an accumulation of intraterminal Na(+) and a resultant influx of Ca(2+) through the reversed mode operation of NCX. However, under more physiological conditions, NCX may also operate in a forward mode and serve to maintain low intracellular [Ca(2+)] in nerve terminals.

Calcium transients in the garter snake vomeronasal organ

The signaling cascade involved in chemosensory transduction in the VN organ is incompletely understood. In snakes, the response to nonvolatile prey chemicals is mediated by the vomeronasal (VN) system. Using optical techniques and fluorescent Ca(2+) indicators, we found that prey-derived chemoattractants produce initially a transient cytosolic accumulation of [Ca(2+)](i) in the dendritic regions of VN neurons via two pathways: Ca(2+) release from IP(3)-sensitive intracellular stores and, to a lesser extent, Ca(2+) influx through the plasma membrane. Both components seem to be dependent on IP(3) production. Chemoattractants evoke a short-latency Ca(2+) elevation even in the absence of extracellular Ca(2+), suggesting that in snake VN neurons, Ca(2+) release from intracellular stores is independent of a preceding Ca(2+) influx, and both components are activated in parallel during early stages of chemosensory transduction. Once the response develops in apical dendritic segments, other mechanisms can also contribute to the amplification and modulation of these chemoattractant-mediated cytosolic Ca(2+) transients. In regions close to the cell bodies of the VN neurons, the activation of voltage-sensitive Ca(2+) channels and a Ca(2+)-induced Ca(2+) release from intracellular ryanodine-sensitive stores secondarily boost initial cytosolic Ca(2+) elevations increasing their magnitude and durations. Return of intracellular Ca(2+) to prestimulation levels appears to involve a Ca(2+) extrusion mediated by a Na(+)/Ca(2+) exchanger mechanism that probably plays an important role in limiting the magnitude and duration of the stimulation-induced Ca(2+) transients.

Immunohistochemical evidence for the Na+/Ca2+ exchanger in squid olfactory neurons

The olfactory organs from the squid Lolliguncula brevis are composed of a pseudostratified epithelium containing five morphological subtypes of chemosensory neurons and ciliated support cells. Physiological recordings have been made from two of the subtypes and only the type 4 neuron has been studied in detail. Odour-stimulated increases in intracellular calcium and rapid activation of an electrogenic Na+/Ca2+ exchanger current in type 4 neurons suggest that the exchanger proteins are localized very close to the transduction machinery. Electrophysiological studies have shown that olfactory signal transduction takes place in the apical ciliary regions of olfactory neurons. Using polyclonal antiserum against squid Na+/Ca2+ proteins, we observed specific staining in the ciliary region of cells that resemble type 2, 3, 4 and 5 neurons. Staining was also observed in axon bundles, and in muscle tissue. Collectively, these data support the model that Na+/Ca2+ exchanger proteins are localized to transduction machinery in cilia of type 4 neurons and suggest that the other olfactory subtypes also use Ca2+ during chemosensory responses.