Neuronal calcium signaling

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.

https://www.physiology.org/doi/pdf/10.1152/physrev.1999.79.3.763

Sodium/Calcium Exchange: Its Physiological Implications

An attempt is made to elucidate the cellular mechanisms which may account for the well-documented correlation between sodium metabolism and peripheral vascular resistance. As a starting point, the ...

https://www.physiology.org/doi/pdf/10.1152/ajpcell.1977.232.5.C165

Sodium ions, calcium ions, blood pressure regulation, and hypertension: a reassessment and a hypothesis

Most excitatory synaptic connections occur on dendritic spines. Calcium imaging experiments have suggested that spines constitute individual calcium compartments, but recent results have challenged this idea. Using two-photon microscopy to image fluorescence with high resolution in strongly scattering tissue, we measured calcium dynamics in spines from CA1 pyramidal neurons in slices of rat hippocampus. Subthreshold synaptic stimulation and spontaneous synaptic events produced calcium accumulations that were localized to isolated spines, showed stochastic failure, and were abolished by postsynaptic blockers. Single somatic spikes induced fast-peaking calcium accumulation in spines throughout the cell. Pairing of spikes with synaptic stimulation was frequently cooperative, that is, it resulted in supralinear calcium accumulations. We conclude: (1) calcium channels exist in spine heads; (2) action potentials invade the spines; (3) spines are individual calcium compartments; and (4) spines can individually detect the temporal coincidence of pre- and postsynaptic activity, and thus serve as basic functional units of neuronal integration.

Dendritic spines as basic functional units of neuronal integration.

Obligatory, coupled cotransport of Na+, K+, and Cl− by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion tra...

Sodium-Potassium-Chloride Cotransport

Calcium-induced calcium release in cerebellar purkinje cells

The Na+/Ca2+ exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca2+ fro...

https://www.physiology.org/doi/pdf/10.1152/physrev.00018.2005

Sodium/Calcium Exchanger: Influence of Metabolic Regulation on Ion Carrier Interactions

A method has been developed to measure Ca influx in internally dialyzed squid axons. This was achieved by controlling the dialyzed segment of the axon exposed to the external radioactive medium. The capacity of EGTA to buffer all the Ca entering the fiber was explored by changing the free EGTA at constant [Ca++]i. At a free [EGTA]i greater than 200 microM, the measured resting Ca influx and the expected increment in Ca entry during electrical stimulation were independent of the axoplasmic free [EGTA]. To avoid Ca uptake by the mitochondrial system, cyanide, oligomycin, and FCCP were included in the perfusate. Axons dialyzed with a standard medium containing: [ATP] = 2 mM, [Ca++]i = 0.06 microM, [Ca++]o = 10 mM, [Na+]i = 70 mM, and [Na+]o = 465 mM, gave a mean Ca influx of 0.14 +/- 0.012 pmol.cm-2.s-1 (n = 12. Removal of ATP drops the Ca influx to 0.085 +/- 0.007 pmol.cm-2.s-1 (n = 12). Ca influx increased to 0.35 pmol.cm-2,s-1 when Nao was removed. The increment was completely abolished by removing Nai+ and (or) ATP from the dialysis medium. At nominal zero [Ca++]i, no Nai-dependent Ca influx was observed. In the presence of ATP and Nai [Ca++]i activates the Ca influx along a sigmoid curve without saturation up to 1 microM [Ca++]i. Removal of Nai+ always reduced the Ca influx to a value similar to that observed in the absence of [Ca++]i (0.087 +/- 0.008 pmol.cm-2.s-1; n = 11). Under the above standard conditions, 50-60% of the total Ca influx was found to be insensitive to Nai+, Cai++, and ATP, sensitive to membrane potential, and partially inhibited by external Co++.

/pdf/calcium-influx-in-internally-dialyzed-squid-giant-axons-1lppjryvp6.pdf

Calcium influx in internally dialyzed squid giant axons.

Variations in intracellular calcium (Cai2+) levels play crucial roles in information processing in cerebellar Purkinje cells, since several forms of synaptic plasticity at their excitatory (Sakurai, 1990) and inhibitory inputs (reviewed by Marty & Llano, 1995) are triggered by [Ca2+]i rises. Transient changes in dendritic [Ca2+]i can be elicited by stimulation of the two excitatory synaptic inputs converging on Purkinje cells: climbing and parallel fibres (Miyakawa, Lev-Ram, Lasser-Ross & Ross, 1992; Denk, Sugimori & Llinas, 1995). Activation of voltage-gated Ca2+ channels leads to large increases in dendritic (Tank, Sugimori, Connor & Llinas, 1988; Lev-Ram, Miyakawa, Lasser-Ross & Ross, 1992) as well as in somatic [Ca2+]i (Kano, Schneggenburger, Verkhratsky & Konnerth, 1995), and these signals can be amplified by calcium-induced calcium release (CICR; Llano, DiPolo & Marty, 1994). Confocal imaging has shown that climbing fibre activation produces substantial increases in [Ca2+]i, not only in Purkinje cell dendrites, but also in the cell somata (Eilers, Callewaert, Armstrong & Konnerth, 1995).

Cerebellar Purkinje cells have powerful systems to control [Ca2+]i. Immunocytochemical studies demonstrate considerable amounts of Ca2+-binding proteins, particularly calbindin D28k and parvalbumin (Tolosa de Talamoni, Smith, Wasserman, Beltramino, Fullmer & Penniston, 1993). Calsequestrin, a well-known constituent of skeletal muscle sarcoplasmic reticulum which is not found in other neurones, has also been reported (Takei et al. 1992, and references therein). Concerning potential sources of Ca2+ release from intracellular stores (reviewed by Ogden, 1996), Purkinje cells express different forms of inositol trisphosphate receptors (InsP3Rs), as well as of ryanodine receptors (RyRs); they are unique among CNS neurones in expressing the skeletal type of RyRs. Rises in [Ca2+]i mediated by activation of InsP3Rs (Khodakhah & Ogden, 1995, and references therein) and RyRs (Llano et al. 1994) have been demonstrated in Purkinje cells.

In addition to these unusual characteristics, other ubiquitous systems involved in Cai2+ homeostasis have been described in Purkinje cells. By immunocytochemical techniques the presence of sarco-endoplasmic reticulum Ca2+ (SERCA) pumps has been recognized (Plessers, Eggermont, Wuytack & Casteels, 1991). SERCA pumps are particularly abundant at the level of the endoplasmic reticulum cisterns (Takei et al. 1992, and references therein). The plasma membrane Ca2+ (PMCA) pumps are ubiquitous in the plasma membrane of Purkinje cells (Tolosa de Talamoni et al. 1993).

Although several studies on dendritic Cai2+ dynamics in Purkinje cells have been performed (reviewed in Regehr & Tank, 1994), little is known of the kinetics of Cai2+ removal at the somata and dendrites of these neurones. In a previous paper we showed that Purkinje cells have a very high Ca2+-binding ratio which endows them with the ability to efficiently handle large Cai2+ loads (Fierro & Llano, 1996). Because buffers confiscate Ca2+, they decrease the effective extrusion rates of clearance mechanisms which are only effective on freely diffusing Ca2+ ions. Therefore, the presence of very strong buffers suggests that Purkinje cells extrude Ca2+ at slow rates, or else, that they should have particularly efficient clearance systems. In the present work, we study the mechanisms responsible for Cai2+ clearance at Purkinje cell somata and describe their respective contributions to the removal process in the range of [Ca2+]i increments pertaining to physiological activity.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7793.1998.499bk.x

Intracellular calcium clearance in Purkinje cell somata from rat cerebellar slices

We have used dialyzed squid axons to characterize the ouabain- and bumetanide-insensitive Na efflux components and their relation to the operation of the Na/Ca exchange mechanism. In axons dialyzed with solutions containing nearly physiological concentrations of K, Na, and Mg, three components of the Na efflux can be distinguished: Cai-activated, Cao-dependent Na efflux ("reverse" Na/Ca exchange); Cai-activated, Nao-dependent Na efflux; and Cai-independent, ATP-activated, Nao-dependent Na efflux. We have studied the effects of internal alkalinization, Mgi, Cao, and the ATP analogue [gamma-thio]ATP (ATP gamma S) on the different components of the Na efflux. The results show the following: (a) internal alkalinization activates both Cao- and Nao-dependent Na efflux components provided that Cai is present; (b) Mgi inhibits both the Cai-activated, Cao- and Nao-dependent Na efflux components; (c) Cao inhibits the Nao-dependent component by competition for a common site; (d) ATP gamma S activates both Nao- and Cao-dependent Na efflux components only in the presence of Cai; and (e) ATP activates the Nai/Nao and Nai/Cao exchanges, causing a 10-fold increase in the affinity of the reverse Na/Ca exchange toward Cai. In the absence of Cai, ATP stimulates an Nao-dependent Na efflux that is not affected either by internal alkalinization or high Cao. The ATP analogue does not activate the Cai-independent Na/Na exchange system. These experiments demonstrate that the Cai-activated Na/Na exchange is a mode of operation of the Na/Ca exchange mechanism that substantially contributes to Na movement during the activation of the Na/Ca antiporter. The experimental evidence obtained on the Cai-independent Na/Na exchange component shows that this system is not part of the Na/Ca exchange.

/pdf/characterization-of-the-reverse-na-ca-exchange-in-squid-19r4bip7i5.pdf

Reinaldo DiPolo

Papers

Calcium-induced calcium release in cerebellar purkinje cells

Sodium/Calcium Exchanger: Influence of Metabolic Regulation on Ion Carrier Interactions

Calcium influx in internally dialyzed squid giant axons.

Intracellular calcium clearance in Purkinje cell somata from rat cerebellar slices

Characterization of the reverse Na/Ca exchange in squid axons and its modulation by Cai and ATP. Cai-dependent Nai/Cao and Nai/Nao exchange modes.