Sodium-calcium exchange. Ins and outs of Ca2+ transport

Functional expression of the Na+/Ca2+ exchanger in the embryonic mouse heart

The Na(+)/Ca(2+) exchanger (NCX) is one of the earliest functional genes and is currently assumed to compensate at least in part for the rudimentary sarcoplasmic reticulum in the developing mouse heart. However, to date little is known about the functional expression of NCX during development. This prompted us to investigate the NCX current (I(NCX)) in very early (embryonic day E8.5-E9.5 post coitum), early (E10.5-E11.5), middle (E13.5) and late (E16.5) stage mouse embryonic cardiomyocytes. For standard I(NCX) measurements, [Ca(2+)](i) was buffered to 150 nmol/l and voltage ramps were applied from +60 mV to -120 mV. At very early stages of development, we observed a prominent role of the I(NCX) Ca(2+) inward mode in elevating the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Accordingly, a high I(NCX) density was observed (+60 mV: 4.6+/-0.7 pA/pF, n=14). Likewise, we found a strong Ca(2+) outward mode of I(NCX) (-120 mV: -3.9+/-0.7 pA/pF, n=14). At later stages, however, I(NCX) Ca(2+) inward mode was reduced by 54+/-6% (n=15, p<0.0001) in ventricular and 68+/-10% (n=9, p<0.0006) in atrial cells. For the outward mode, a reduction by 43+/-10% (n=15, p<0.01) in ventricular and 62+/-11% (n=9, p<0.004) in atrial cardiomyocytes was observed. By contrast, NCX isoform expression and the reversal potential did not significantly change during development. Thus, NCX displays a prominent Ca(2+) inward and outward mode during early embryonic heart development pointing to its important contribution to maintain [Ca(2+)](i) homeostasis. The functional and protein expression of NCX declines during further development.

Cardiometabolic syndrome: pathophysiology and treatment

The cardiometabolic syndrome, an interesting constellation of maladaptive cardiovascular, renal, metabolic, prothrombotic, and inflammatory abnormalities, is now recognized as a disease entity by the American Society of Endocrinology, National Cholesterol Education Program, and World Health Organization, among others. These cardiovascular and metabolic derangements individually and interdependently lead to a substantial increase in cardiovascular disease (CVD) morbidity and mortality, making the cardiometabolic syndrome an established and strong risk factor for premature and severe CVD and stroke. Established and evolving treatment strategies including moderate physical activity, weight reduction, rigorous blood pressure control, correction of dyslipidemia, and glycemic control have proven beneficial in reversing these abnormal responses and decreasing the CVD risk.

Na(+)/Ca(2+) exchange and its role in intracellular Ca(2+) regulation in guinea pig detrusor smooth muscle

The role of Na(+)/Ca(2+) exchange in regulating intracellular Ca(2+) concentration ([Ca(2+)](i)) in isolated smooth muscle cells from the guinea pig urinary bladder was investigated. Incremental reduction of extracellular Na(+) concentration resulted in a graded rise of [Ca(2+)](i); 50-100 microM strophanthidin also increased [Ca(2+)](i). A small outward current accompanied the rise of [Ca(2+)](i) in low-Na(+) solutions (17.1 +/- 1.8 pA in 29.4 mM Na(+)). The quantity of Ca(2+) influx through the exchanger was estimated from the charge carried by the outward current and was approximately 30 times that which is necessary to account for the rise of [Ca(2+)](i), after correction was made for intracellular Ca(2+) buffering. Ca(2+) influx through the exchanger was able to load intracellular Ca(2+) stores. It is concluded that the level of resting [Ca(2+)](i) is not determined by the exchanger, and under resting conditions (membrane potential -50 to -60 mV), there is little net flux through the exchanger. However, a small rise of intracellular Na(+) concentration would be sufficient to generate significant net Ca(2+) influx.

Participation of PI3K and atypical PKC in Na+-K+-pump stimulation by IGF-I in VSMC

The activity of the Na+-K+-pump is intricately linked to the maintenance of vascular tone. Here we demonstrate that insulin-like growth factor I (IGF-I) increases Na+-K+-pump activity in the vascular smooth muscle cell (VSMC) clone A7r5 in a time- and dose-dependent manner. This stimulatory effect of IGF-I was prevented by the tyrosine kinase inhibitor genistein (5 microM) and by the specific phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin (100 nM) and LY-294002 (25 microM). IGF-I activated a wortmannin-sensitive PI3K and its purported effector, the atypical protein kinase C (PKC)-zeta. Stimulation of PKC-zeta was prevented by the generic PKC inhibitor GF109203x (bisindolylmaleimide, 10 microM). Downregulation of diacylglycerol-sensitive (conventional and novel) PKCs by 24-h pretreatment with 1 microM phorbol 12-myristate 13-acetate had no effect on IGF-I-stimulated Na+-K+-pump activity. Similarly, inhibition of only conventional and novel PKCs with GF109203x (1 microM) had no effect on IGF-I-stimulated Na+-K+-pump activity. In contrast, a concentration of GF109203x (10 microM) that also inhibits the atypical PKCs abolished Na+-K+-pump stimulation by IGF-I. Neither the Na+-K+-2Cl- cotransporter inhibitor bumetanide (100 microM) nor the Na+/H+ exchanger inhibitor HOE-694 (5 microM) affected the Na+-K+-pump stimulation by IGF-I, suggesting that a rise in intracellular Na+ concentration is not necessary for increased Na+-K+-pump activity. These results suggest that IGF-I directly stimulates the Na+-K+ pump via a signaling pathway involving PI3K and atypical PKC (zeta).

Isoform-specific regulation of the Na+/Ca2+ exchanger in rat astrocytes and neurons by PKA

The Na+/Ca2+ exchanger is a major transporter of Ca2+ in neurons and glial cells. The Na+/Ca2+ exchanger gene NCX1 expresses tissue-specific isoforms of the Na+/Ca2+ exchanger, and the isoforms have been examined here quantitatively using primary cultures of astrocytes and neurons. We present a PCR-based quantitative method, quantitative end-labeled reverse transcription-PCR (QERT-PCR), to determine the relative amounts of the NCX1 isoforms present in these cells. Six exons (A, B, C, D, E, and F) are alternatively spliced to produce the known NCX1 isoforms. Three exon B-containing isoforms (BDEF, BDF, and BD) are the predominant transcripts in primary rat cortical astrocytes and in C6 glioma cells. In contrast, exon A-containing isoforms (ADF and AD) are the predominant transcripts in primary rat hippocampal neurons. Functional differences between full-length constructs of NCX1 containing either the astrocyte isoform BD or the neuron isoform AD were examined in a Xenopus oocyte expression system. Although both isoforms function normally, the activity of the AD isoform can be increased 39% by activation of protein kinase A (PKA), whereas that of the BD isoform is not affected. We conclude that specific NCX1 isoforms are expressed in distinct patterns in astrocytes and neurons. Furthermore, the activity of a neuronal (but not glial) isoform of the Na+/Ca2+ exchanger can be altered by the activation of the PKA pathway.

Exposure of vascular allografts to insulin-like growth factor-I (IGF-I) increases vascular expression of IGF-I ligand and receptor protein and accelerates arteriosclerosis in rats

BACKGROUND

Accelerated arteriosclerosis limits the survival of transplanted hearts. We hypothesized that insulin-like growth factor-I (IGF-I) is crucial in accelerating transplant arteriosclerosis. Recently, we reported that exposure to IGF-I prior to transplantation accelerates transplant arteriosclerosis in the rat aorta allograft model. Here, we studied the mechanism whereby IGF-I exposure accelerates transplant arteriosclerosis.

METHODS

The abdominal aorta was harvested from male Brown Norway rats and exposed to 0, 200, or 500 ng/ml of IGF-I at 37 degrees C for 30 min prior to transplantation to the abdominal position of male Lewis rats. The allografts were harvested 14 days later and processed for immunohistochemical staining for alpha-actin, growth factors (IGF-I, IGF-I receptor, platelet-derived growth factor-BB, and basic fibroblast growth factor), and immunological markers (major histocompatibility complex class II antigen, macrophage, and CD4- and CD8-positive T cells).

RESULTS

By 14 days, the ex vivo IGF-I donor aorta treatment with IGF-I increased in a concentration-dependent manner the expression of IGF-I and IGF-I receptor in both the intima and the adventitia. In contrast, the expression of platelet-derived growth factor-BB was decreased in a concentration-dependent manner in the intima while basic fibroblast growth factor remained unchanged. The cell-mediated immune response was not affected by IGF-I at 14 days after transplantation, which suggests that the immune events associated with acceleration of transplant arteriosclerosis may occur at an earlier time.

CONCLUSION

Acceleration of transplant arteriosclerosis by exposure to IGF-I is associated with increased IGF-I ligand and receptor expression in the allograft vascular wall. These data further suggest that IGF-I may be a major factor in mediating graft arteriosclerosis.

The release of human chorionic gonadotrophin and placental lactogen by placental explants can be stimulated by Ca2+ entry through a Na(+)-Ca2+ exchange process

Isosmotical replacement of extracellular Na+ ([Na+]o) by K+, choline and to a lesser extent by saccharose stimulated the release of chorionic gonadotrophin and placental lactogen from human term placental explants. The effect of [Na+]o removal on the release of both hormones was concentration-dependent and was inhibited in the absence of extracellular Ca2+ or in the presence of 0.5 mM Co2+, a Ca2+ entry blocker. Blockers of the voltage-sensitive Ca2+ channels (20 microM nifedipine and 50 microM methoxyverapamil) or Na+ channels (5 microM tetrodotoxin) did not affect the stimulatory effects of [Na+]o omission. By contrast, Mg2+ and Sr2+ (10 mM) as well as amiloride (2 mM) and its analogue 2',4'-dimethylbenzamil (50 microns), all known to affect the Na(+)-Ca2+ exchange, markedly reduced the increase in hormone release elicited by [Na+]o removal. Lastly, the secretory responses to [Na+]o deprivation were increased in the presence of 2 mM ouabain, an inhibitor of the Na(+)-K+ ATPase. These results indicate for the first time that [Na+]o omission provokes a Ca(2+)-dependent stimulation of human chorionic gonadotrophin and placental lactogen releases. The pharmacological dissection of the secretory effects of [Na+]o removal supports the existence of a process of Na(+)-Ca2+ exchange in placental cells.

Coupling of the Na+/Ca2+ exchanger, Na+/K+ pump and sarcoplasmic reticulum in smooth muscle

The Na+/Ca2+ exchanger, driven by a transmembrane Na+ gradient, plays a key role in regulating Ca2+ concentration in many cells. Although the exchanger influences Ca2+ concentration, its activity in smooth muscle appears to be closely coupled to Ca2+ availability from intracellular stores. This linkage might result if the exchanger were positioned close to Ca2+ storage sites within the sarcoplasmic reticulum. To test this hypothesis we have developed methods to assess the relative three-dimensional distribution of proteins involved in Na+/K+ pumping, Na+/Ca2+ exchange, Ca2+ storage within the sarcoplasmic reticulum, and attachment of contractile filaments to the membrane in smooth muscle. Here we report that the Na+/Ca2+ exchanger is largely co-distributed with the Na+/K+ pump on unique regions of the plasma membrane in register with, and close to, calsequestrin-containing regions of the sarcoplasmic reticulum in sites distinct from the sites where contractile filaments attach to the membrane. This molecular organization suggests that the plasma membrane is divided into at least two functional domains, and appear to provide a mechanism for the strong linkage seen in smooth muscle between Na+/K+ pumping and Na+/Ca2+ exchange, and between Na+/Ca2+ exchange and Ca2+ release from the sarcoplasmic reticulum.

Na(+)-Ca2+ exchange, myoplasmic Ca2+ concentration, and contraction of arterial smooth muscle

Na(+)-Ca2+ exchange is proposed to be an important regulator of myoplasmic intracellular Ca2+ concentration ([Ca2+]i) and contraction in vascular smooth muscle. We investigated the role of Na(+)-Ca2+ exchange in regulating [Ca2+]i in swine carotid arterial tissues that were loaded with aequorin to allow simultaneous measurement of [Ca2+]i and force. Reversal of Na(+)-Ca2+ exchange, by reduction of extracellular Na+ concentration ([Na+]o) to 1.2 mM, induced a large increase in aequorin-estimated [Ca2+]i and a low [Ca2+]i sensitivity. The contraction induced by 1.2 mM [Na+]o was partially caused by depolarization and opening of L-type Ca2+ channels because 10 microM diltiazem partially attenuated the 1.2 mM [Na+]o-induced increases in [Ca2+]i. High dose ouabain (10 microM), a putative endogenous Na+,K(+)-ATPase inhibitor, increased both [Ca2+]i and force. However, the increases in [Ca2+]i and force were mostly blocked by 10 microM phentolamine, suggesting the predominant effect of ouabain was to increase norepinephrine release from nerve terminals. In the presence of 10 microM phentolamine, 10 microM ouabain slightly accentuated 1 microM histamine-induced increases in [Ca2+]i and force. The ouabain dose necessary to induce contraction in the absence of phentolamine was significantly less than the ouabain dose necessary to accentuate histamine-induced contractions in the presence of phentolamine. These results suggest that Na(+)-Ca2+ exchange exists in swine arterial smooth muscle. These data also suggest that ouabain (which should increase [Na+]i and inhibit Na(+)-Ca2+ exchange) primarily enhances contractile function in the swine carotid artery by releasing catecholamines from nerve terminals; direct action of Na+,K(+)-ATPase inhibitors on smooth muscle appears to occur only with very high doses.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.