Na-Ca exchanger as a calcium influx pathway in adrenal glomerulosa cells

Subtle mutation, far-reaching effects

Insufficient sodium-ion extrusion by mutated Na+,K+-ATPases causes hyperaldosteronism.

On the effect of hyperaldosteronism-inducing mutations in Na/K pumps

Mutated Na/K pumps in adrenal adenomas are thought to cause hyperaldosteronism via a gain-of-function effect involving a depolarizing inward current. The findings of Meyer et al. suggest instead that the common mechanism by which Na/K pump mutants lead to hyperaldosteronism is a loss-of-function.

Primary aldosteronism, a condition in which too much aldosterone is produced and that leads to hypertension, is often initiated by an aldosterone-producing adenoma within the zona glomerulosa of the adrenal cortex. Somatic mutations of ATP1A1, encoding the Na/K pump α1 subunit, have been found in these adenomas. It has been proposed that a passive inward current transported by several of these mutant pumps is a "gain-of-function" activity that produces membrane depolarization and concomitant increases in aldosterone production. Here, we investigate whether the inward current through mutant Na/K pumps is large enough to induce depolarization of the cells that harbor them. We first investigate inward currents induced by these mutations in Xenopus Na/K pumps expressed in Xenopus oocytes and find that these inward currents are similar in amplitude to wild-type outward Na/K pump currents. Subsequently, we perform a detailed functional evaluation of the human Na/K pump mutants L104R, delF100-L104, V332G, and EETA963S expressed in Xenopus oocytes. By combining two-electrode voltage clamp with [³H]ouabain binding, we measure the turnover rate of these inward currents and compare it to the turnover rate for outward current through wild-type pumps. We find that the turnover rate of the inward current through two of these mutants (EETA963S and L104R) is too small to induce significant cell depolarization. Electrophysiological characterization of another hyperaldosteronism-inducing mutation, G99R, reveals the absence of inward currents under many different conditions, including in the presence of the regulator FXYD1 as well as with mammalian ionic concentrations and body temperatures. Instead, we observe robust outward currents, but with significantly reduced affinities for intracellular Na⁺ and extracellular K⁺. Collectively, our results point to loss-of-function as the common mechanism for the hyperaldosteronism induced by these Na/K pump mutants.

Of channels and pumps: different ways to boost the aldosterone?

The mineralocorticoid aldosterone is a major factor controlling the salt and water balance and thereby also the arterial blood pressure. Accordingly, primary aldosteronism (PA) characterized by an inappropriately high aldosterone secretion is the most common form of secondary hypertension. The physiological stimulation of aldosterone synthesis in adrenocortical glomerulosa cells by angiotensin II and an increased plasma K⁺ concentration depends on a membrane depolarization and an increase in the cytosolic Ca²⁺ activity. Recurrent gain-of-function mutations of ion channels and transporters have been identified in a majority of cases of aldosterone-producing adenomas and in familial forms of PA. In this review, the physiological role of these genes in the regulation of aldosterone synthesis and the altered function of the mutant proteins as well are described. The specific changes of the membrane potential and the cellular ion homoeostasis in adrenal cells expressing the different mutants are compared, and their impact on autonomous aldosterone production and proliferation is discussed.

Interactions of endoplasmic reticulum and mitochondria Ca(2+) stores with capacitative calcium entry

Thiamine dependent enzymes are diminished in Alzheimer's disease (AD). Thiamine deficiency in vitro and in rodents is a useful model of this reduction. Thiamine interacts with cellular calcium stores. To directly test the relevance of the thiamine dependent changes to dynamic processes in AD, the interactions must be studied in cells from patients with AD. These studies employed fibroblasts. Mitochondrial dysfunction including reductions in thiamine dependent enzymes and abnormalities in calcium homeostasis and oxidative processes occur in fibroblasts from Alzheimer's Disease (AD) patients. Bombesin-releasable calcium stores (BRCS) from the endoplasmic reticulum (ER) are exaggerated in fibroblasts from patients with AD bearing a presenilin-1 (PS-1) mutation and in control fibroblasts treated with oxidants. ER calcium regulates calcium entry into the cell through capacitative calcium entry (CCE), which is reduced in fibroblasts and neurons from mice bearing PS-1 mutations. Under physiological conditions, mitochondria and ER play important and interactive roles in the regulation of Ca(2+) homeostasis. Thus, the interactions of mitochondria and oxidants with CCE were tested. Inhibition of ER Ca(2+)-ATPase by cyclopiazonic acid (CPA) stimulates CCE. CPA-induced CCE was diminished by inhibition of mitochondrial Ca(2+) export (-60%) or import (-40%). Different aspects of mitochondrial Ca(2+) coupled to CPA-induced-CCE were sensitive to select oxidants. The effects were very different when CCE was examined in the presence of InsP3, a physiological regulator of ER calcium release, and subsequent CCE. CCE under these conditions was only mildly reduced (20-25%) by inhibition of mitochondrial Ca(2+) export, and inhibition of mitochondrial Ca(2+) uptake exaggerated CCE (+53%). However, t-BHP reversed both abnormalities. The results suggest that in the presence of InsP3, mitochondria buffer the local Ca(2+) released from ER following rapid activation of InsP3R and serve as a negative feedback to the CCE. The results suggest that mitochondrial Ca(2+) modifies the depletion and refilling mechanism of ER Ca(2+) stores.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Effects of extracellular Na+ and Ca2+ ions and Ca2+ channel modulators on the cell-associated activity of 99mTc-MIBI and 99mTc-tetrofosmin in tumour cells

Our aim was to determine whether the Ca2+ ion or cell membrane Ca2+ and Na+/Ca2+ ion transport systems are involved in maintaining the cell-associated activity of technetium-99m-hexakis-methoxy-isobutyl-isonitrile (99mTc-MIBI) and technetium-99m-ethylene-bis[bis(2-ethoxyethyl)phosphin] (99mTc-tetrofosmin) in tumour cell lines. The cell-associated activities of 99mTc-MIBI and 99mTc-tetrofosmin were assessed in various buffers, with or without Na+ and/or with different concentrations of Ca2+, in Lewi's murine lung cell carcinoma and human glioma cell lines. Different Ca2+ channel modulators, such as verapamil, flunarizine and 3,4-dichlorobenzamil (DCB), were used to assess the effect of Ca2+ channels on the cell-associated activity of 99mTc-MIBI and 99mTc-tetrofosmin. Despite significant differences between cell lines, the cell-associated activity of 99mTc-MIBI was higher in buffers without extracellular Ca2+ and Na+. The cell-associated activity of 99mTc-MIBI was significantly lower in all buffers containing high concentrations of Ca2+ in both cell lines. The cell-associated activity of Tc-tetrofosmin was also significantly higher in buffers without Ca2+, and was significantly decreased in buffers with high concentrations of Ca2+. All modulators significantly increased the cell-associated activity of 99mTc-MIBI in both cell lines in all buffers. All modulators increased the cell-associated activity of 99mTc-tetrofosmin, particularly in buffers containing Ca2+. The cell-associated activities of both 99mTc-MIBI and 99mTc-tetrofosmin may be dependent on verapamil-, flunarizine- and DCB-sensitive Ca2+ channels.

Characterization of a Na+-Ca2+ exchanger NCX1 isoform in bovine fasciculata cells of adrenal gland

The Na+-Ca2+ exchanger (NCX) is well known to regulate intracellular Ca2+ concentration in many excitable cells. To date, three members of the NCX exchanger gene family have been identified: NCX1, NCX2 and NCX3. In zona fasciculata of the bovine adrenal gland, a Na+-Ca2+ exchanger has been recently found to display biochemical similarities with the NCX1 exchanger of renal basolateral cells. Although a Na+-Ca2+ exchanger has already been characterized in zona medulla of adrenal gland, such an exchanger has never been studied in the adrenal cortex. Thus, we have used a reverse transcriptase-polymerase chain reaction (RT-PCR) assay with specific NCX primers on fasciculata cell cultures. The sequencing of the amplified fragment revealed, for the first time, the presence of a NACA3 isoform of the NCX1 exchanger in the adrenal cortex, similar to the one found in adrenal chromaffin cells and in renal cortex. This isoform is 92 and 94% identical in the portion compared with kidney Na+-Ca2+ exchanger NACA3 of rat and pig, respectively.

Characterization of Na/Ca exchange in plasmalemmal vesicles from zona fasciculata cells of the bovine adrenal gland

The presence of an Na/Ca exchange system in fasciculata cells of the bovine adrenal gland was tested using isolated plasmalemmal vesicles. In the presence of an outwardly Na(+) gradient, Ca(2+) uptake was about 2-fold higher than in K(+) condition. Li(+) did not substitute for Na(+) and 5 mM Ni(2+) inhibited Ca(2+) uptake. Ca(2+) efflux from Ca(2+)-loaded vesicles was Na(+)-stimulated and Ni(2+)-inhibited. The saturable part of Na(+)-dependent Ca(2+) uptake displayed Michaelis-Menten kinetics. The relationship of Na(+)-dependent Ca(2+) uptake versus intravesicular Na(+) concentration was sigmoid (apparent K(0.5) approximately 24 mM; Hill number approximately 3) and Na(+) acted on V(max) without significant effect on K(m). Na(+)-stimulated Ca(2+) uptake was temperature-dependent (apparent Q(10) approximately 2.2). The inhibition properties of several divalent cations (Cd(2+), Sr(2+), Ni(2+), Ba(2+), Mn(2+), Mg(2+)) were tested and were similar to those observed in kidney basolateral membrane. The above results indicate the presence of an Na/Ca exchanger located on plasma membrane of zona fasciculata cells of bovine adrenal gland. This exchanger displays similarities with that of renal basolateral cell membrane.

Sodium/calcium exchange: its physiological implications

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.

Regional distribution in the rat central nervous system of a mRNA encoding a portion of the cardiac sodium/calcium exchanger isolated from cerebellar granule neurons

The cardiac Na+/Ca2+ exchanger is a bidirectional electrogenic ion transporter that exchanges three Na+ ions for each Ca2+ ion and plays a critical role in returning sarcolemma Ca2+ concentrations to their resting levels. Because of the importance that the Na+/Ca2+ exchanger may play in maintaining neuronal Ca2+ homeostasis in the central nervous system, we subcloned a 456 bp portion of the Na+/Ca2+ exchanger cDNA from RNA isolated from primary cultures of rat cerebellar granule neurons using the polymerase chain reaction (PCR). This cDNA fragment was sequenced and shown to share 91.4% sequence identity with the human and 88% sequence identity with the canine cardiac Na+/Ca2+ exchangers. The PCR amplification product was used to analyze the distribution of this portion of the Na+/Ca2+ exchanger mRNA in various regions of the CNS by both Northern blotting and in situ hybridization histochemistry. The Northern analysis showed that the rank order of abundance of this mRNA was: hippocampus > cortex > cerebellum > hypothalamus > midbrain > striatum. The in situ hybridization data indicated that the corresponding mRNA containing this portion of the exchanger was present in numerous brain regions including multiple cortical layers, the hippocampus, septal nuclei, various thalamic nuclei, cerebellum, hypothalamus, olfactory bulb, brainstem, in various regions of the thoracic spinal cord and to a lesser extent in the striatum. The differential distribution of the mRNA as revealed by the in situ hybridization pattern suggests that either additional molecular variants exist or that different Na+/Ca2+ exchange mechanisms may be operative in those cell types that contain low amounts of this fragment of the exchanger mRNA.

Na+ influx-induced decrease of (Na+ + K+)-ATPase activity in rat brain slices: role of Ca2+

Treatment of rat brain slices with veratrine and monensin decreased (Na+ + K+)-ATPase activity in the membranes in a dose-dependent manner. The effect of monensin, like that of veratrine, was accompanied by a decrease of maximal binding sites for ouabain. The inhibitory effect of monensin on the enzyme activity was dependent on external Ca2+ at low concentrations, but not at a high concentration. The decreased enzyme activity induced by monensin was restored by subsequent incubation of the slices in a Ca(2+)-free medium containing 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. The effect of monensin at a low concentration on enzyme activity was antagonized by amiloride (1 mM), bepridil (5 microM), quinacrine (30 microM) or verapamil (30 microM), but not by nifedipine (1 microM) or omega-conotoxin (1 microM). Furthermore, the inhibitory effect of monensin at a high concentration under Ca(2+)-free conditions was blocked by BAPTA-AM (30 microM) and by bepridil (100 microM) or diazepam (500 microM), inhibitors of mitochondrial Na(+)-Ca2+ exchange. Inhibitors of calmodulin, protein kinase C, phospholipase A2 and calpain did not affect the monensin-induced decrease of enzyme activity. Dithiothreitol (10 mM) blocked the effect of monensin on enzyme activity but did not affect the ionophore-induced influx of Ca2+ in the slices.

Verapamil inhibits proliferation but not steroidogenesis of regenerating rat adrenal cortex

The effect of verapamil, a calcium channel antagonist, on proliferation and steroidogenesis was investigated in regenerating rat adrenal cortex. Verapamil was given subcutaneously in two doses (1 and 5 mg/kg) to male Wistar rats subjected to adrenal enucleation combined with contralateral adrenalectomy. It was found that verapamil inhibited the mitotic activity of adrenocortical cells on the 4th and 8th day after surgery in a dose-dependent manner. However, no changes in corticosterone secretion were observed.