Regulation of sodium-calcium exchange in intact myocytes by ATP and calcium

Sodium/calcium exchanger (NCX1) macromolecular complex

The sodium-calcium exchanger, NCX1, is a ubiquitously expressed membrane protein essential in calcium homeostasis for many cells including those in mammalian heart and brain. The function of NCX1 depends on subcellular ("local") factors, the phosphorylation state of NCX1, and the subcellular location of NCX1 within the cell. Here we investigate the molecular organization of NCX1 within the cardiac myocyte. We show that NCX1 is dynamically phosphorylated by protein kinase A (PKA)-dependent phosphorylation in vitro. We also provide evidence that the regulation of this phosphorylation is attributed to the existence of an NCX1 macromolecular complex. Specifically, we show that the macromolecular complex includes both the catalytic and regulatory subunits of PKA. However, only the RI regulatory subunit is found in this macromolecular complex, not RII. Other critical regulatory enzymes are also associated with NCX1, including protein kinase C (PKC) and two serine/threonine protein phosphatases, PP1 and PP2A. Importantly, the protein kinase A-anchoring protein, mAKAP, is found and its presence in the macromolecular complex suggests that these regulatory enzymes are coordinately positioned to regulate NCX1 as has been found in diverse cells for a number of channel proteins. Dual immunocytochemical staining showed the colocalization of NCX1 protein with mAKAP and PKA-RI proteins in cardiomyocytes. Finally, leucine/isoleucine zipper motifs have been identified as possible sites of interaction. Our finding of an NCX1 macromolecular complex in heart suggests how NCX1 regulation is achieved in heart and other cells. The existence of the NCX1 macromolecular complex may also provide an explanation for recent controversial findings.

Role of constitutively expressed heme oxygenase-2 in the regulation of guinea pig coronary artery tone

Carbon monoxide (CO) is well known as a relaxing substance in the vasculature, where it is released during the heme oxygenase (HO) reaction. Little is known about the tissue-specific targets of CO in smooth muscles. To date the functional role of CO in the coronary artery remains unclear. The expression of HO-2, the constitutive isoform of HO, but not of HO-1 (inducible HO isoform) was demonstrated by immunohistochemical reaction. Contractile studies, performed under isometrical conditions, showed that CO, as well as hemin (given as a substrate for HO), relax de-endothelized coronary smooth muscle after the blockade of neuronal transmission. The action of hemin was antagonized by preliminary treatment of the vessel with SnPPIX--a competitive inhibitor of HO. The relaxatory effects of hemin were abolished in the presence of guanylyl-cyclase or protein kinase G antagonists. Patch-clamp studies revealed that hemin caused activation of iberiotoxin-blockable K outward current (I(K)) via guanylyl-cyclase and protein-kinase-G-dependent mechanisms. This activation coincided with hyperpolarization of the plasma membrane of single coronary smooth muscle cells by 8+/-3 mV, which was prevented by preliminary exposure of cells to 10 microM SnPPIX. The I(K)-augmenting effect of hemin was not affected by pretreatment of cells with cyclopiazonic acid and/or ryanodine, blockers of phospholipase C or heparin (applied via pipette), but was not observed when ATP was omitted from the dialyzing solution, or in the presence of Na-free, ATP-containing pipette solution. The omission of Ca(2+) from the bath or the replacement of Na with Li in both pipette and bath media also prevented the I(K)-activating effect of hemin. These results suggest that the constitutive HO-2 in coronary artery smooth muscle cells plays role in the modulation of tone. At the level of smooth muscle cells CO and its precursor hemin may cause hyperpolarization of the plasma membrane by activation of iberiotoxin-sensitive I(K) presumably via PKG-dependent activation of the Na/Ca exchanger. This activation is thought to increase the submembrane Ca(2+) concentration in the vicinity of large-conductance, Ca(2+)-sensitive K channels, thus causing voltage-dependent inhibition of Ca(2+) entry and subsequent relaxation of the vessel.

Effects of docosahexaenoic acid on calcium pathway in adult rat cardiomyocytes

In this study we examined the effect of polyunsaturated fatty acids (PUFAs), in particular of docosahexaenoic acid (DHA), on calcium homeostasis in isolated adult rat cardiomyocytes exposed to KCl, ET-1 and anoxia. Free [Ca(2+)](i) in rat cardiomyocytes was 135.7 +/- 0.5 nM. Exposure to 50 mM KCl or 100 nM ET-1 resulted in a rise in free [Ca(2+)](i) in freshly isolated cells (465.4 +/- 15.6 nM and 311.3 +/- 12.6 nM, respectively) and in cultured cells (450.8 +/- 14.8 nM and 323.5 +/- 14.8 nM respectively). An acute treatment (20 minutes) with 10 microM DHA significantly reduced the KCl- and ET-1-induced [Ca(2+)](i) increase (300.9 +/- 18.1 nM and 232.08 +/- 11.8 nM, respectively). This reduction was greater after chronic treatment with DHA (72 h; 257.7 +/- 13.08 nM and 192.18 +/- 9.8 nM, respectively). Rat cardiomyocytes exposed to a 20 minute superfusion with anoxic solution, obtained by replacing O(2) with N(2) in gas mixture, showed a massive increase in cytosolic calcium (1200.2 +/- 50.2 nM). Longer exposure to anoxia induced hypercontraction and later death of rat cardiomyocytes. Preincubation with DHA reduced the anoxic effect on [Ca(2+)](i) (498.4 +/- 7.3 nM in acute and 200.2 +/- 12.2 nM in chronic treatment). In anoxic conditions 50 mM KCl and 100 nM ET-1 produced extreme and unmeasurable increases of [Ca(2+)](i.) Preincubation for 20 minutes with DHA reduced this phenomenon (856.1 +/- 20.3 nM and 782.3 +/- 7.6 nM, respectively). This reduction is more evident after a chronic treatment with DHA (257.7 +/- 10.6 nM and 232.2 +/- 12.5 nM, respectively). We conclude that in rat cardiomyocytes KCl, ET-1 and anoxia interfered with intracellular calcium concentrations by either modifying calcium levels or impairing calcium homeostasis. Acute, and especially chronic, DHA administration markedly reduced the damage induced by calcium overload in those cells.

Na-dependent changes in intracellular Ca in spontaneously hypertensive rat hearts

To determine whether Na/Ca exchange is altered in primary hypertension, Na-dependent changes in intracellular Ca, ([Ca]i), were measured in isolated perfused hearts from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Intracellular Na, (Nai, mEq/kg dry wt), and [Ca]i were measured by NMR spectroscopy. Control [Ca]i was less in WKY than SHR (176 +/- 18 vs 253 +/- 21 nmol/l; mean +/- S.E., P < 0.05), whereas Nai was not significantly different. One explanation for this is that net Na/Ca exchange flux is decreased in SHR. If this hypothesis is correct, the rate of Ca uptake in SHR should be less than WKY when Na/Ca exchange is reversed by decreasing the transmembrane Na gradient. The Na gradient was reduced by decreasing extracellular Na, ([Na]o) and/or by increasing [Na]i. To increase [Na]i, Na uptake was stimulated by acidification while Na extrusion by Na/K ATPase was inhibited by K-free perfusion. Seventeen minutes after acidification, Nai had increased but was not significantly different in SHR and WKY (18.0 +/- 2.3 to 57.4 +/- 7.6 vs 20.3 +/- 0.6 to 66.5 +/- 4.8 mEq/kg dry wt, respectively). Yet [Ca]i was greater in WKY than SHR (1768 +/- 142 vs 1201 +/- 90 nmol/l; P < 0.05). [Ca]i was also measured after decreasing [Na]o from 141 to 30 mmol/l. Fifteen minutes after reducing [Na]o, [Ca]i was greater in WKY than SHR (833 +/- 119 vs 425 +/- 94 nmol/l; P < 0.05). Thus for both protocols, decreasing the transmembrane Na gradient led to increased [Ca]i in both SHR and WKY, but less increase in SHR. The results are consistent with the hypothesis that Na/Ca exchange activity is less in SHR than WKY myocardium.

Significance of Na/Ca exchange for Ca2+ buffering and electrical activity in mouse pancreatic beta-cells

We have combined the patch-clamp technique with microfluorimetry of the cytoplasmic Ca2+ concentration ([Ca2+]i) to characterize Na/Ca exchange in mouse beta-cells and to determine its importance for [Ca2+]i buffering and shaping of glucose-induced electrical activity. The exchanger contributes to Ca2+ removal at [Ca2+]i above 1 microM, where it accounts for >35% of the total removal rate. At lower [Ca2+]i, thapsigargin-sensitive Ca2+-ATPases constitute a major (70% at 0.8 microM [Ca2+]i) mechanism for Ca2+ removal. The beta-cell Na/Ca exchanger is electrogenic and has a stoichiometry of three Na+ for one Ca2+. The current arising from its operation reverses at approximately -20 mV (current inward at more negative voltages), has a conductance of 53 pS/pF (14 microM [Ca2+]i), and is abolished by removal of external Na+ or by intracellularly applied XIP (exchange inhibitory peptide). Inhibition of the exchanger results in shortening (50%) of the bursts of action potentials of glucose-stimulated beta-cells in intact islets and a slight (5 mV) hyperpolarization. Mathematical simulations suggest that the stimulatory action of glucose on beta-cell electrical activity may be accounted for in part by glucose-induced reduction of the cytoplasmic Na+ concentration with resultant activation of the exchanger.

Calibration of intracellular Ca transients of isolated adult heart cells labelled with fura-2 by acetoxymethyl ester loading

A procedure for calibration of fluorescence signals from adult rat heart cells loaded with the -AM ester of fura-2 is described. Calibration is complicated by dye compartmentation and potentially incomplete dye hydrolysis. These problems were overcome by subtracting from fluorescence transients the non-cytosolic (mitochondrial) component of fura-2 fluorescence plus any Ca-insensitive component of dye fluorescence, after selectively and sequentially quenching cytosolic and non-cytosolic dye with Mn. The Kd of fura-2 in cells loaded by the -AM ester, in cells depleted of ATP and equilibrated with Ca buffers, was found to be 371 +/- 39 nM at 37 degrees C. We found that calibration values for RMAX and RMIN derived from previously measured cells were of general validity, removing the need to measure RMAX and RMIN on every cell. Once these calibration values are determined, the calibration procedure to measure cytosolic Ca on any cell is a five minute procedure to determine compartmentation, using just one non-toxic and inexpensive solution. Finally, we have calculated how the errors intrinsic to the measurements translate into errors of the calculated Ca concentration and transient peak heights. These calculations allow reasonable parameters for data acquisition to be set.