Fundamental properties of the Na-Ca exchange. An overview

Pre- and post-synaptic effects of manipulating surface charge with divalent cations at the photoreceptor synapse

Persistence of horizontal cell (HC) light responses in extracellular solutions containing low Ca2+ plus divalent cations to block Ca2+ currents (ICa) has been attributed to Ca2+-independent neurotransmission. Using a retinal slice preparation to record both ICa and light responses, we demonstrate that persistence of HC responses in low [Ca2+]o can instead be explained by a paradoxical increase of Ca2+ influx into photoreceptor terminals arising from surface charge-mediated shifts in ICa activation. Consistent with this explanation, application of Zn2+ or Ni2+ caused a hyperpolarizing block of HC light responses that was relieved by lowering [Ca2+]o. The same concentrations of Zn2+ and Ni2+ reduced the amplitude of ICa at the rod dark potential and this reduction was relieved by a hyperpolarizing shift in voltage dependence induced by lowering [Ca2+]o. Block of ICa by Mg2+, which has weak surface charge effects, was not relieved by low [Ca2+]o. Recovery of HC responses in low [Ca2+]o was assisted by enhancement of rod light responses. To bypass light stimulation, OFF bipolar cells were stimulated by steps to -40 mV applied to presynaptic rods during simultaneous paired recordings. Consistent with surface charge theory, the post-synaptic current was inhibited by Zn2+ and this inhibition was relieved by lowering [Ca2+]o. Nominally divalent-free media produced inversion of HC light responses even though rod light responses remained hyperpolarizing; HC response inversion can be explained by surface charge-mediated shifts in ICa. In summary, HC light responses modifications induced by low divalent cation solutions can be explained by effects on photoreceptor light responses and membrane surface charge without necessitating Ca2+-independent neurotransmission. Furthermore, these results suggest that surface charge effects accompanying physiological changing divalent cation levels in the synaptic cleft may provide a means for modulating synaptic output from photoreceptors.

Caloxin: a novel plasma membrane Ca2+ pump inhibitor

Plasma membrane (PM) Ca2+ pump is a Ca+-Mg2+-ATPase that expels Ca2+ from cells to help them maintain low concentrations of cytosolic Ca2+ . There are no known extracellularly acting PM Ca2+ pump inhibitors, as digoxin and ouabain are for Na+ pump. In analogy with digoxin, we define caloxins as extracellular PM Ca2+ pump inhibitors and describe caloxin 2A1. Caloxin 2A1 is a peptide obtained by screening a random peptide phage display library for binding to the second extracellular domain (residues 401-413) sequence of PM Ca2+ pump isoform 1b. Caloxin 2A1 inhibits Ca2+-Mg2+-ATPase in human erythrocyte leaky ghosts, but it does not affect basal Mg2+-ATPase or Na+-K+-ATPase in the ghosts or Ca2+-Mg2+-ATPase in the skeletal muscle sarcoplasmic reticulum. Caloxin 2A1 also inhibits Ca2+-dependent formation of the 140-kDa acid-stable acylphosphate, which is a partial reaction of this enzyme. Consistent with inhibition of the PM Ca2+ pump in vascular endothelium, caloxin 2A1 produces an endothelium-dependent relaxation that is reversed by N(G)-nitro-L-arginine methyl ester. Thus caloxin 2A1 is a novel PM Ca2+ pump inhibitor selected for binding to an extracellular domain.

Functional role of sodium-calcium exchange in the regulation of renal vascular resistance

Our study aimed to assess a possible functional role of the Na(+)/Ca(2+) exchanger in the regulation of renal vascular resistance (RVR). Therefore, we investigated the effects of an inhibition of the Na(+)/Ca(2+) exchanger either by lowering the extracellular sodium concentration ([Na(+)](e)) or, pharmacologically on RVR, by using isolated perfused rat kidneys. Graded decreases in [Na(+)](e) led to dose-dependent increases in RVR to 4.3-fold (35 mM Na(+)). This vasoconstriction was markedly attenuated by lowering the extracellular calcium concentration, by the L-type calcium channel blocker amlodipine or by the chloride channel blocker niflumic acid. Further lowering of [Na(+)](e) to 7 mM led to an increase in RVR to 7.5-fold. In this setting, amlodipine did not influence the magnitude but did influence the velocity of vasoconstriction. Pharmacological blockade of the Na(+)/Ca(2+) exchanger with KB-R7943, benzamil, or nickel resulted in significant vasoconstriction (RVR 2.5-, 1.8-, and 4.2-fold of control, respectively). Our data suggest a functional role of the Na(+)/Ca(2+) exchanger in the renal vascular bed. In conditions of partial replacement of [Na(+)](e), vasoconstriction is dependent on chloride and L-type calcium channels. A total replacement of [Na(+)](e) leads to a vasoconstriction that is nearly independent of L-type calcium channels. This might be due to an active calcium transport into the cell by the Na(+)/Ca(2+) exchanger.

Renal sodium/calcium exchange; a vasodilator that is defective in salt-sensitive hypertension

The Na+ : Ca2+ exchanger is an important plasma membrane ion transport pathway that plays a major role in controlling [Ca2+]i. In smooth muscle cells, it may function as a Ca2+ extrusion pathway and may help lower [Ca2+]i in response to vasoconstrictor-induced increases in [Ca2+]i. It may also extrude [Ca2+]i and lead to vasodilation in response to vasodilators. Our recent studies have been performed to determine the existence and regulation of the Na+ : Ca2+ exchanger in renal contractile cells which include afferent and efferent arterioles and mesangial cells. Exchanger activity is present in all three of these contractile elements but is higher in afferent arterioles vs. efferent arterioles. We have also examined the role of altered regulation of the exchanger in the SHR and in salt-sensitive hypertension. With the establishment of high blood pressure, Na+ : Ca2+ exchanger activity is reduced in afferent but not in efferent arterioles in both models of hypertension. Other works in cultured mesangial cells and freshly dissected afferent arterioles, have shown that protein kinase C (PKC) up-regulates the Na+ : Ca2+ exchanger from Dahl/Rapp salt-resistant rats while it fails to do so in arterioles and mesangial cells from salt-sensitive rats. This defect in PKC regulation of Na+ : Ca2+ exchange is the result of a loss of PKC-mediated translocation of the exchanger to the plasma membrane in S mesangial cells. Thus, a defect in the PKC-Na+ : Ca2+ exchanger-translocation pathway may cause dysregulation of [Ca2+]i and help explain the dramatic decrease in GFR that occurs in this model of hypertension.

Altered protein kinase C activation of Na+/Ca2+ exchange in mesangial cells from salt-sensitive rats

The purpose of these studies was to determine whether there is a defect in protein kinase C (PKC) regulation of the Na+/Ca2+ exchanger in cultured mesangial cells (MC) from Dahl/Rapp salt-sensitive (S) and salt-resistant (R) rats. R and S MCs were cultured, grown on coverslips, and loaded with fura 2 for measurement of single cell cytosolic calcium concentration ([Ca2+]i) in a microscope-based photometry system. Studies were performed in cells that were exposed to serum (serum fed) and in cells that were serum deprived for 24 h. Baseline [Ca2+]i values measured in a Ringer solution containing 150 mM NaCl were similar between R and S MCs in both serum-fed and serum-deprived groups, although baseline [Ca2+]i values were uniformly higher in the serum-deprived groups. Exchanger activity was assessed by reducing extracellular Na (Nae) from 150 to 2 mM, which resulted in movement of Na+ out of and Ca2+ into these cells (reverse-mode Na+/Ca2+ exchange). PKC was activated in these cells with 15-min exposure to 100 nM phorbol 12-myristate 13-acetate (PMA). In the absence of PMA, the change in [Ca2+]i (Delta[Ca2+]i) with reduction in Nae was similar between R and S MCs in both serum-fed and serum-deprived groups, although the magnitude of Delta[Ca2+]i was enhanced by serum deprivation. In both serum-fed and serum-deprived groups, PMA significantly increased Delta[Ca2+]i in R but not S MCs. Upregulation of exchanger activity in R MCs could be abolished by prior 24-h exposure to PMA, a maneuver that downregulates PKC activity. Other studies were performed to evaluate exchanger protein expression using monoclonal and polyclonal antibodies. Immunoblots of PMA-treated cells revealed an increase in the levels of 70- and 120-kDa proteins in the crude membrane fraction of R but not S MCs, an increase which was abrogated by prior 24-h PMA pretreatment and corresponded to reduction in the 70-kDa protein in the crude cytosolic fraction. These data demonstrate that PKC enhances Na+/Ca2+ exchange activity in MCs from R but not from S rats, suggesting that there may be a defect in the PKC-Na+/Ca2+ exchange regulation pathway in MCs of S rats.

Heat shock- and ethanol-induced ionic changes in C6 rat glioma cells determined by NMR and fluorescence spectroscopy

The effects of two different stressors, heat shock (HS; 44 degrees C, 20 min) and ethanol (1.2 M, 60 min), on ion content and membrane potential were investigated in C6 rat glioma cells. Both treatments were previously shown to induce the HS response [26]. Intracellular pH (pH(i)), sodium ion concentration ([NA+]i), potassium ion concentration ([K+]i) and membrane potential were determined by means of continuous 31P and 23Na nuclear magnetic resonance (NMR), continuous fluorescence spectroscopy and 86Rb uptake. Lactate extrusion was determined in addition with respect to pH(i) regulation. The aim of this study was a detailed picture of HS and ethanol-induced ion changes in a single cell type, because stress-induced changes in the intracellular ionic balance may be important factors for determining proliferation, stress response and apoptosis. HS lowered the pH(i) from 7.38 +/- 0.04 to about 7.05 +/- 0.04. [Na+]i decreased during HS to 50% of the control and recovered to normal level 95 min after HS treatment. During HS, [K+]i remained constant but increased after HS. The membrane potential hyperpolarized from -83 mV to -125 mV and returned to initial values during HS treatment. Lactate extrusion increased 3-fold after HS. Ethanol (1.2 M) lowered the pH(i) from pH 7.38 +/- 0.04 to pH 7.0 +/- 0.04, but in contrast to heat strongly increased [Na]i. It hyperpolarized the membrane potential from -83 to -125 mV. Ethanol also increased lactate extrusion similar to HS. Also in contrast to the effect of HS, the potassium concentration decreased during ethanol treatment. The Na(+)-H+ exchanger monensin was used to overcome the apparent inhibition of the cellular Na(+)-H+ exchanger by HS. At normal pH(e) (7.4) monensin increased [Na+]i and pH(i) considerably. A subsequent HS reduced [Na+]i only minimally. Acidification of the cells by low pH(e) (6.2) prior to HS did not abolish the HS-induced drop of pH(i), indicating that the Na(+)-H+ exchanger was also inhibited at low pH(i). At low pH(e), monensin transports H+ into the cell. A subsequent HS decreased pH(i) only little, showing the importance of inhibition of the Na(+)-H+ exchanger for the HS-induced pH(i) decrease. 100 microM amiloride reduced pH(i) and [Na+]i in a similar way as HS, but did not change pH(i) and [Na+]i much during a HS. These results indicate that some of the HS-induced ionic changes are mediated by inhibition of the Na(+)-H+ exchanger, activation of Na(+)-K(+)-ATPase and changes of membrane conductance for ions.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Why are endocrine pituitary cells excitable?

Since 1975, endocrine pituitary cells have been known to be excitable neuronlike cells. Using powerful single-cell approaches, in particular the patch clamp electrophysiological recording technique and the monitoring of Ca(2+) with fluorescent probes, solid evidence has been provided in the last 10 years that intracellular Ca(2+) signals are produced by stimulators and inhibitors of secretion via the modulation of action potentials in isolated pituitary cells. As cytosolic Ca(2+) changes are thought to control numerous cellular functions (for example, secretion, protein synthesis, gene expression, and proliferation) over a long time scale-milliseconds to hours-it is now time to address the long-standing question of what functions would be physiologically controlled by electrical excitability in intact pituitary tissue.

Role of sodium/calcium exchange in the mechanism of myocardial stunning: protective effect of reperfusion with high sodium solution

OBJECTIVES

This study was conducted to elucidate the role of sodium/calcium (Na+/Ca2+) exchange in the mechanism of myocardial stunning.

BACKGROUND

Cellular Ca2+ overload mediated by Na+/Ca2+ exchange during reperfusion has been proposed as a mechanism for myocardial stunning. Because no specific pharmacologic inhibitors of the exchanger are available, we increased extracellular sodium concentration ([Na]o) during the early phase of reperfusion to decrease the driving force for Ca2+ influx through the pathway.

METHODS

Isovolumetric left ventricular pressure and phosphorus-31 nuclear magnetic resonance spectra were measured in isolated perfused ferret hearts. Hearts were reperfused with different solutions after 15 min of total global ischemia at 37 degrees C.

RESULTS

Hearts reperfused with standard solution ([Na]o = 140 mmol/liter; the stunned hearts, n = 8) showed only 69 +/- 3% (mean +/- SEM) recovery of developed pressure relative to preischemic control developed pressure. In contrast, hearts reperfused with a high [Na]o solution ([Na]o = 268 mmol/liter) during the initial 5 min, followed by a gradual decrease of [Na]o to the standard level over 25 min (the high [Na]o group, n = 8) showed significantly better recovery of developed pressure (85 +/- 2%, p < 0.05 vs. the stunned hearts). In contrast, reperfusion with solutions in which the additional Na was substituted either by 256 mmol/liter sucrose or 128 mmol/liter choline chloride did not improve functional recovery, indicating that the beneficial effects of high [Na]o reperfusion are not due to either high ionic strength or high osmolarity. Phosphorus-31 nuclear magnetic resonance spectra showed no correlation between functional recovery and intramyocardial contents of phosphorus compounds or pH.

CONCLUSIONS

High [Na]o reperfusion protects against stunning, supporting the concept that Na+/Ca2+ exchange plays an important role in the mechanism of stunned myocardium.