Sodium-Calcium exchange: A possible target for drug development

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.

Vascular Na+/Ca2+exchanger: implications for the pathogenesis and therapy of salt-dependent hypertension

The Na+/Ca2+exchanger is an ion transporter that exchanges Na+and Ca2+in either Ca2+efflux or Ca2+influx mode, depending on membrane potential and transmembrane ion gradients. In arterial smooth muscle cells, the Na+/Ca2+exchanger is thought to participate in the maintenance of vascular tone by regulating cytosolic Ca2+concentration. Recent pharmacological and genetic engineering studies have revealed that the Ca2+influx mode of vascular Na+/Ca2+exchanger type-1 (NCX1) is involved in the pathogenesis of salt-dependent hypertension. SEA0400, a specific Na+/Ca2+exchange inhibitor that preferentially blocks the Ca2+influx mode, lowers arterial blood pressure in salt-dependent hypertensive models, but not in normotensive rats or other types of hypertensive rats. Furthermore, heterozygous mice with reduced expression of NCX1 are resistant to development of salt-dependent hypertension, whereas transgenic mice with vascular smooth muscle-specific overexpression of NCX1 readily develop hypertension after high-salt loading. SEA0400 reverses the cytosolic Ca2+elevation and vasoconstriction induced by nanomolar ouabain, as well as humoral factors in salt-loaded animals. One possibility is that circulating endogenous cardiotonic steroids may be necessary for NCX1-mediated hypertension. These findings help to explain how arterial smooth muscle cells in blood vessels contribute to salt-elicited blood pressure elevation and suggest that NCX1 inhibitors might be therapeutically useful for salt-dependent hypertension.

Sodium–calcium exchange inhibitors: therapeutic potential in cardiovascular diseases

Intracellular calcium ions (Ca2+) are the key regulators in cardiac and arterial functions during the contraction–relaxation cycle. Myocyte Ca2+ imbalance thus produces mechanical dysfunction, electrical instability (arrhythmia) and muscle remodeling. The sodium–calcium exchanger (NCX) is one of the major Ca2+-handling proteins in myocytes. Evidence is currently accumulating to suggest that NCX1 is upregulated in various cardiovascular diseases. Recently developed benzyloxyphenyl NCX inhibitors effectively prevent myocardial ischemia/reperfusion injury and salt-sensitive hypertension in animal models. Furthermore, several experiments with genetically engineered mice provide compelling evidence that these diseases are triggered by pathologic Ca2+ entry through NCX1 in cardiac and arterial myocytes, respectively. Thus, NCX inhibitors may have therapeutic potential as novel cardiovascular drugs for myocardial reperfusion injury and salt-sensitive hypertension. However, the efficacy of NCX inhibitors, as well as the role of NCX1, in heart failure or arrhythmias requires more detailed study.

Clinical potential of sodium-calcium exchanger inhibitors as antiarrhythmic agents

The Na(+)/Ca(2+) exchanger (NaCaX) plays an important role in calcium handling in myocytes, but in the setting of calcium overload NaCaX can also contribute to the activation of an arrhythmogenic transient inward current (I(ti)). Therefore, approaches to inhibit NaCaX could have potential antiarrhythmic effects in pathophysiological states such as heart failure (HF) or myocardial ischaemia and reperfusion. NaCaX typically functions in a forward (Ca(2+) extrusion) mode but can also function in a reverse (Ca(2+) influx) mode. The determining factors for the directionality of NaCaX ion movement are the electrochemical gradients of calcium and sodium, and membrane potential (E(m)). In HF, upregulated NaCaX plays a dual role: it decreases sarcoplasmic reticulum (SR) calcium load, which leads to contractile dysfunction, and it underlies the I(ti) responsible for delayed after-depolarisations (DADs) and ventricular arrhythmias. In myocardial ischaemia and reperfusion, increases in [Na(+)](i) (as a result of acidosis and activation of the Na(+)/H(+) exchanger [NHE]) lead to calcium overload via the NaCaX and arrhythmogenesis is probably mediated by I(ti) activation due to NaCaX. As such, inhibition of NaCaX could provide a novel therapeutic approach to the prevention and treatment of arrhythmias. Unfortunately, it is difficult to assess the efficacy of such an approach since there are no specific NaCaX inhibitors. Currently available agents are hampered by their nonspecific effects on other ion channels and carriers. The potential utility of specific inhibition of forward or reverse mode NaCaX as an antiarrhythmic approach in the settings of HF and ischaemia/ reperfusion is discussed within the context of current knowledge of myocyte calcium and sodium handling. NaCaX is a challenging and complex therapeutic target because of the delicate balance of SR calcium load (too little contributes to contractile dysfunction and too much leads to calcium overload and arrhythmogenesis). Further understanding of NaCaX function, [Na(+)](i) and [Ca(2+)](i) in HF and ischaemia/reperfusion, combined with the development and assessment of specific NaCaX inhibitors, will ultimately define the potential role of NaCaX inhibition in the prevention and treatment of ventricular arrhythmias.

Effect of amiloride and emopamil on intracranial pressure

The authors sought to determine whether amiloride or emopamil could reduce intracranial pressure in experimental brain edema of the rat. For this purpose the rats functionally nephrectomized and brain edema of the cytotoxic type induced by infusion of 100 ml aqua bidest/kg body weight. After the end of the infusion 10 or 20 ml mM amiloride/kg body weight or 50 microliters mM (s)-emopamil/kg body weight in 10 ml 150 mM NaCl/kg body weight or 10 ml isotonic saline/kg body weight were injected followed by continued recording of intracranial pressure (ICP) and systemic arterial pressure for at least 3 hours. The values of the ICP for the amiloride and s-emopamil treated animals are significantly (p < 0.05, Student's t-test for unpaired data) lower at any point after the injection of amiloride or (s)-emopamil. Amiloride and (s)-emopamil prevent the rise in ICP seen after the saline injection in the control group.

Amiloride blocks glutamate-operated cationic channels and protects neurons in culture from glutamate-induced death

The diuretic amiloride has been suggested as a specific inhibitor of T-type neuronal Ca2+ channels. The effects of amiloride on glutamate receptor-gated cationic channels and glutamate-induced. Ca2(+)-dependent neuronal death were investigated in primary neuronal cultures from neonatal rats. In primary cultures of cerebellar granule neurons of the rat, receiving 50 microM glutamate for 15 min, at 22 degrees C, in the absence of Mg2+, about 80% of neurons were killed in about 24 hr. Exposure of neurons to such a pulse of glutamate, in the presence of various concentrations of amiloride, resulted in a dose-dependent protection from neurotoxicity (EC50 300 microM, complete protection 1 mM). In voltage-clamped cortical and cerebellar neurons of neonatal rats in primary culture, 100 microM amiloride diminished (by about 25%) glutamate- and/or NMDA-evoked cationic currents, recorded in the whole-cell mode. About 80% of the NMDA-(20 microM) stimulated current was inhibited by 700 microM amiloride. The inhibitory effect of amiloride was not voltage-dependent. In outside-out membrane patches, excised from granule cells and held at -50 mV, 100 microM amiloride changed the NMDA-elicited single channel activity into a fast flickering between the open and closed states. The noise analysis of the data revealed that, although resembling the Mg2(+)-induced flickering, the amiloride-induced channel block was more similar to the effects described for the action of local anaesthetics on the nicotinic cholinergic channel. The pharmacological relevance of this action of amiloride requires further characterization; the data point out the necessity of a cautious use of amiloride in studying neuronal function.