Relationship between calcium loading and impaired energy metabolism during Na+, K+ pump inhibition and metabolic inhibition in cultured neonatal rat cardiac myocytes

Enhanced cytotoxicity of a novel family of ATPase inhibitors in colorectal cancer cells with high NAT2 activity

Loss of heterozygosity (LOH) is a hallmark feature of cancer genomes that reduces allelic variation, thereby creating tumor specific vulnerabilities which could be exploited for therapeutic purposes. We previously reported that loss of drug metabolic arylamine N-acetyltransferase 2 (NAT2) activity following LOH at 8p22 could be targeted for collateral lethality anticancer therapy in colorectal cancer (CRC). Here, we report a novel compound CBK034026C that exhibits specific toxicity towards CRC cells with high NAT2 activity. Connectivity Map analysis revealed that CBK034026C elicited a response pattern related to ATPase inhibitors. Similar to ouabain, a potent inhibitor of the Na⁺/K⁺-ATPase, CBK034026C activated the Nf-kB pathway. Further metabolomic profiling revealed downregulation of pathways associated with antioxidant defense and mitochondrial metabolism in CRC cells with high NAT2 activity, thereby weakening the protective response to oxidative stress induced by CBK034026C. The identification of a small molecule targeting metabolic vulnerabilities caused by NAT2 activity provides novel avenues for development of anticancer agents.

Cardiomyocytes from postinfarction failing rat hearts have improved ischemia tolerance

Altered myocardial Ca(2+) and Na(+) handling in congestive heart failure (CHF) may be expected to decrease the tolerance to ischemia by augmenting reperfusion Ca(2+) overload. The aim of the present study was to investigate tolerance to hypoxia-reoxygenation by measuring enzyme release, cell death, ATP level, and cell Ca(2+) and Na(+) in cardiomyocytes from failing rat hearts. CHF was induced in Wistar rats by ligation of the left coronary artery during isoflurane anesthesia, after which cardiac failure developed within 6 wk. Isolated cardiomyocytes were cultured for 24 h and subsequently exposed to 4 h of hypoxia and 2 h of reoxygenation. Cell damage was measured as lactate dehydrogenase (LD) release, cell death as propidium iodide uptake, and ATP by firefly luciferase assay. Cell Ca(2+) and Na(+) were determined with radioactive isotopes, and free intracellular Ca(2+) concentration ([Ca(2+)](i)) with fluo-3 AM. CHF cells showed less increase in LD release and cell death after hypoxia-reoxygenation and had less relative reduction in ATP level after hypoxia than sham cells. CHF cells accumulated less Na(+) than sham cells during hypoxia (117 vs. 267 nmol/mg protein). CHF cells maintained much lower [Ca(2+)](i) than sham cells during hypoxia (423 vs. 1,766 arbitrary units at 4 h of hypoxia), and exchangeable Ca(2+) increased much less in CHF than in sham cells (1.4 vs. 6.7 nmol/mg protein) after 120 min of reoxygenation. Ranolazine, an inhibitor of late Na(+) current, significantly attenuated both the increase in exchangeable Ca(2+) and the increase in LD release in sham cells after reoxygenation. This supports the suggestion that differences in Na(+) accumulation during hypoxia cause the observed differences in Ca(2+) accumulation during reoxygenation. Tolerance to hypoxia and reoxygenation was surprisingly higher in CHF than in sham cardiomyocytes, probably explained by lower hypoxia-mediated Na(+) accumulation and subsequent lower Ca(2+) accumulation in CHF after reoxygenation.

Role of cardiac myocyte tissue factor in heart hemostasis

BACKGROUND

The tissue-specific pattern of tissue factor (TF) expression suggests that it plays a major role in the hemostatic protection of specific organs, such as the heart and lung. In support of this notion, we found that mice expressing very low levels of TF exhibit hemostatic defects in the heart and lung. Hemosiderosis and fibrosis are observed in the hearts of all low TF mice as early as 3 months of age. In contrast, TF(+/-) mice expressing approximately 50% of wild-type levels of TF had no detectable hemostatic defects.

OBJECTIVE AND METHODS

The objective of this study was to determine the threshold of TF that is required to maintain hemostasis under normal and pathologic conditions, and to investigate the specific role of cardiac myocyte TF in heart hemostasis using mice with altered levels of TF expression in cardiac myocytes.

RESULTS

First, we found that mice with 20% of wild-type levels of TF activity in their hearts had hemosiderosis and fibrosis by 6 months of age. Secondly, mice with a selective deletion of the TF gene in cardiac myocytes had a mild hemostatic defect under normal conditions but exhibited a significant increase in hemosiderosis and fibrosis after challenge with isoproterenol. Finally, we showed that cardiac myocyte-specific overexpression of TF abolished hemosiderin deposition and fibrosis in the hearts of low TF mice.

CONCLUSIONS

Taken together, our results indicate that TF expression by cardiac myocytes is important to maintain heart hemostasis under normal and pathologic conditions.

Phosphorylation and binding of AUF1 to the 3'-untranslated region of cardiomyocyte SERCA2a mRNA

Experimental animals and patients with cardiac hypertrophy and heart failure display abnormally slowed myocardial relaxation, which is associated with downregulation of sarco(endo)plasmic reticulum calcium ATPase 2a (SERCA2a), the cardiomyocyte sarcoplasmic reticulum Ca2+ pump. We previously showed that SERCA2a downregulation can be simulated in cultured neonatal rat ventricular myocytes (NRVM) by treatment with the hypertrophic agonist phorbol myristate acetate (PMA) or by overexpression of the novel protein kinase C (PKC) isoenzymes PKCdelta and PKCepsilon. PKC activation, in turn, decreased SERCA2a promoter activity and destabilized the SERCA2a mRNA. Here we demonstrate by using an RSV beta-galactosidase reporter system that a 609-nt fragment of the SERCA2a mRNA 3'-untranslated region (UTR), containing five adenylate-uridylate (AU)-rich regions, may be responsible for destabilizing the message following PMA treatment. UV cross-linking analysis demonstrated that several proteins found in the NRVM cell extracts bind to the 609-nt fragment. In addition, protein binding was transiently increased in response to PMA stimulation. 3'-UTR mRNA pull-down assays and Western blot analysis indicated that the AU binding protein AUF1 interacted with the SERCA2a 3'-UTR. AUF1 binding activity was predominantly found in the nuclear fraction, and PMA-induced AUF1 binding was associated with increased threonine phosphorylation of AUF1. These data suggest that the phosphorylation, binding, and location of AUF1 affect the posttranscriptional regulation of the SERCA2a message in NRVM.

Effects of Chan Su, a traditional Chinese medicine, on the calcium transients of isolated cardiomyocytes: cardiotoxicity due to more than Na, K-ATPase blocking

Extracts of Chan Su, a traditional Chinese medication used as a topical anesthetic and cardiac medication, were incubated with cardiomyocytes that had been loaded with a calcium specific fluorescent probe. Calcium transients were measured by real-time fluorescence spectrophotometry following treatment. The transients were rapidly abolished following addition of a moderate concentration of the extract (400 ng/ml), resulting in high levels of intracellular calcium, while the lower amount (40 ng/ml) blocked the sodium-potassium adenosine triphosphatase. Treatments with ouabain and nifedipine were also made, either prior to, or after the addition of the Chan Su, in an attempt to better delineate the site(s) of action. The moderate concentration of Chan Su (400 ng/ml) extract caused the myocytes to cease beating within seconds of addition, even in experiments when saturating concentrations of nifedipine or ouabain had been previously added to the cells. As expected bufalin, the active component of Chan Su has similar effects. Our findings indicate that this compound is extremely cardiotoxic, even in small dose and acts rapidly to alter intracellular calcium stores in cardiomyocytes and possibly acts at sites other than the Na(+)+K(+) ATPase, either directly, or indirectly via changes in calcium concentrations.

Calcium signaling mechanisms in dedifferentiated cardiac myocytes: comparison with neonatal and adult cardiomyocytes

Our studies focused on calcium sparking and calcium transients in cultured adult rat cardiomyocytes and compared these findings to those in cultured neonatal and freshly isolated adult cardiomyocytes. Using deconvolution fluorescence microscopy and spec trophotometric image capture, sequence acquisitions were examined for calcium spark intensities, calcium concentrations and whether sparks gave rise to cell contraction events. Observations showed that the preparation of dedifferentiated cardiomyocytes resulted in stellate, neonatal-like cells that exhibited some aspects of calcium transient origination and proliferation similar to events seen in both neonatal and adult myocytes. Ryanodine treatment in freshly isolated adult myocytes blocked the calcium waves, indicating that calcium release at the level of the sarcoplasmic reticulum and t-tubule complex was the initiating factor, and this effect of ryanodine treatment was also seen in cultured-dedifferentiated adult myocytes. However, experiments revealed that in both neonatal and cultured adult myocytes, the inositol triphosphate pathway (IP3) was a major mechanism in the control of intracellular calcium concentrations. In neonatal myocytes, the nucleus and regions adjacent to the plasma membrane we re major sites of calcium release and flux. We conclude: (1) culturing of adult cardiomyocytes leads them to develop mechanisms of calcium homeostasis similar in some aspects to those seen in neonatal cardiomyocytes; (2) neonatal myocytes rely on both extracellular and nuclear calcium for contractile function; and (3) freshly isolated adult myocytes use sarcoplasmic reticulum calcium stores for the initiation of contractile function.

Effects of hypokalemia on cardiac growth

In neonatal myocytes grown in culture, reductions in extracellular potassium concentration produced a hypertrophic response as assessed by induction of early response genes, atrial natriuretic peptide and skeletal actin, and repression of the alpha3 isoform of the sodium pump in a dose dependent manner. The degree of alpha3 repression appeared to be dose dependent with decreases in media (K). Similarly, decreases in media potassium concentrations caused increases in cytosolic calcium concentration in a dose dependent manner; moreover these increases in cytosolic calcium concentration correlated quite well with repression of alpha3 expression. In contrast, although moderate reductions of potassium concentration induced upregulation of skACT and ANP, severely reduced potassium concentrations caused repression of skACT and ANP expression. In parallel studies performed in vivo, 3-5 weeks dietary K restriction induced molecular phenotypical changes similar to that seen in the neonatal myocyte model without demonstrable growth as assessed by the heart weight/body weight ratio. However, when rates subjected to dietary K restriction were subsequently subjected to acute aortic constriction, cardiac growth was greater than in rats fed a control diet. These data suggest that hypokalemia may produce molecular phenotypic alterations consistent with cardiac hypertrophy as well as contribute to hypertrophy in an in vivo model.

Dietary fish oil promotes positive inotropy of ouabain in the rat heart

We tested the hypothesis that a fish oil (FO) diet promotes positive inotropy of ouabain without increased toxicity. For 2 mo, two groups of adult male rats were fed 1) a regular food diet supplemented with dietary long-chain polyunsaturated fatty acid from FO or 2) a regular food diet (control). The responsiveness to ouabain was evaluated for the two groups in Langendorff-perfused hearts, by (31)P nuclear magnetic resonance spectroscopy, and on purified membrane-bound Na-K-ATPase. The maximum positive inotropy achieved with ouabain was nearly two times higher in the FO than in the control group and was not associated with significant changes in energetics. Alteration of function and energetic metabolism and inhibition of Na-K-ATPase in response to 3 x 10(-4) M ouabain were delayed in the FO group. This study demonstrates that dietary FO, by a cardiac membrane incorporation of n-3 polyunsaturated fatty acid, promotes positive inotropy of ouabain without toxicity and changes in cardiac metabolism.

The small heat shock proteins Hsp20 and alphaB-crystallin in cultured cardiac myocytes: differences in cellular localization and solubilization after heat stress

Hsp20, a recently described new member of the small heat shock protein superfamily, is abundant in heart, skeletal muscle types and smooth muscle. We investigated the intracellular localization of Hsp20 in cultured rat neonatal cardiac myocytes, under normal conditions and after stress. These cellular characteristics of Hsp20 were compared with those of its closest relative, alphaB-crystallin, which is also highly expressed in heart. Like alphaB-crystallin, Hsp20 is normally located in the cytoplasm of the cardiac myocytes. After a heat stress, a subpopulation of Hsp20 migrates into the nucleus, while another part remains in the cytoplasm. In very few cells a faint sarcomeric association of Hsp20 is observed. In contrast, as previously reported, alphaB-crystallin displays a very distinct cross-striated sarcomeric staining after the heat shock, but no nuclear migration. Also at the level of Triton solubility, differences exist between the two related proteins; while alphaB-crystallin, like other small heat shock proteins, becomes insoluble upon heat stress, Hsp20 remains largely soluble. Our results indicate that Hsp20 and alphaB-crystallin, despite their structural similarities, display conspicuous functional differences.

Cytokines increase neonatal cardiac myocyte calcium concentrations: the involvement of nitric oxide and cyclic nucleotides

Neonatal rat cardiac myocytes were treated with cytokines, with or without the nitric oxide synthase (NOS) inhibitors N-monomethyl-L-arginine (LNMMA) and N-nitro-L-arginine methyl ester (LNAME), and systolic and diastolic calcium levels were measured by fluorescence spectrophotometry and confocal microscopy. Time-dependent changes following interferon-gamma (IFN-gamma) treatment revealed a continuing increase in intracellular calcium, which was reduced with LNMMA, but not with LNAME. Increases in calcium also occurred with interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha), but not to the extent seen with IFN-gamma. Increased cyclic guanosine monophosphate (cGMP) was involved in the results described with short-term (2 hr) TNF-alpha and long-term (18 hr) IFN-gamma treatments. Short-term exposure to IFN-gamma produced an increase in cyclic adenosine monophosphate (cAMP) and also an initial increase in the myocyte-bearing rate, with calcium levels either (i) subsequently returning to control levels while maintaining a fast beating rate or (ii), retaining a high systolic calcium level, but beating at control rates. Treatment with both IL-1beta and IFN-gamma stabilized the beating rate of the cells on some occasions. Shortening of myocytes increased with isoproterenol and following treatment with IFN-gamma, while isoproterenol stimulation of IFN-gamma-treated cells revealed increased contractile activity after short, but not long, treatment. LNMMA, but not reduced the increased contractile response with short-term IFN-gamma treatment. Our findings suggest that TNF-alpha acts via a cGMP-dependent pathway, whereas the actions of IFN-gamma involve adenylate cyclase, and possibly a NO-forming mechanism and cGMP pathway as well. It is also apparent that the two NO inhibitors function via different mechanisms or that LNMMA has a direct effect on the calcium-signaling pathway.

Physical, contractile and calcium handling properties of neonatal cardiac myocytes cultured on different matrices

Extracellular matrix components play a vital role in the determination of heart cell growth, development of spontaneous contractile activity and morphologic differentiation. In this work we studied the physical and contractile changes in neonatal rat cardiac myocytes over the first four days of growth on three different extracellular matrices. We compared commercial laminin and fibronectin, plus a fibroblast-derived extracellular matrix, which we have termed cardiogel. Myocytes cultured on cardiogel were characterized by greater cellular area and volume when compared to cells cultured on the other single-component matrices. Spontaneous contractile activity appeared first in the cells grown on cardiogel, sometimes as early as the first day post-plating, in contrast to day three in the cells cultured on laminin. Measurements of cardiac myocyte contractility i.e. percent shortening and time to peak contraction, were made on each of the first four days in each culture. Myocytes cultured on cardiogel developed maximum shortening more rapidly than the other cultures, and an earlier response to electrical pacing. Histochemical staining for myocyte mitochondrial content, revealed that the cardiogel-supported cells exhibited the earliest development of this organelle and, after four days, the greatest abundance. This reflects both a greater cell size, as well as response to increasing energy demands. Due to the increase in volume and contractile activity exhibited by the cardiogel grown myocytes, we employed calcium binding and uptake experiments to determine the comparative cellular capacities for calcium and as an indicator of sarcoplasmic reticulum development. Also whole cell phosphorylation in the presence of low detergent was assayed, to correlate calcium uptake with phosphorylation, in an attempt to examine possible increases in calcium pump number and other phosphorylatable proteins. In agreement with our physical and contractile data, we found that the cells grown on cardiogel showed a greater calcium uptake over the first four days of culture, and increased phosphorylation. However, calcium binding was not dramatically different comparing the three culture matrices. Based on our data, the fibroblast-derived cardiogel is the matrix of choice supporting earliest maturation of neonatal cardiomyocytes, in terms of spontaneous contractions, calcium handling efficiency, cell size and development of a subcellular organelle, the mitochondrion.

Effect of vacuolar proton ATPase on pHi, Ca2+, and apoptosis in neonatal cardiomyocytes during metabolic inhibition/recovery

Recently, we found that vacuolar proton ATPase (VPATPase) operates in cardiomyocytes as a complementary proton-extruding mechanism. Its activity was increased by preconditioning with resultant attenuation of intracellular acidification during ischemia. In this study, we examined whether VPATPase-mediated proton efflux during metabolic inhibition/recovery may spare Na+ overload via Na+-H+ exchange, attenuate Na+-Ca2+ exchange, and decrease apoptosis. Neonatal rat cardiomyocytes were subjected to 2- to 3-hour metabolic inhibition with cyanide and 2-deoxyglucose and 24-hour recovery. The effect of VPATPase inhibition by 50 nmol/L bafilomycin A1 on apoptosis, pHi, and [Ca2+]i was studied by flow cytometry with propidium iodide, seminaphthorhodafluor (SNARF)-1-AM, and indo-1-AM staining, respectively. VPATPase inhibition increased the amount of apoptosis measured after 24 hours of recovery and abrogated the protective effect of inhibition of Na+-H+ exchange by (5-N-ethyl-N-isopropyl)amiloride (EIPA). Dual blockade of VPATPase and Na+-H+ exchange was additive in effect with EIPA on pHi during metabolic inhibition/recovery and recovery from the acid challenge with sodium propionate. VPATPase blockade increased the rate of accumulation of intracellular Ca2+ at the beginning of metabolic inhibition and abrogated the delaying effect of EIPA on intracellular Ca2+ accumulation. These results indicate that VPATPase plays an important accessory role in cardiomyocyte protection by reducing acidosis and Na+-H+ exchange-induced Ca2+ overload.

alpha B-crystallin and hsp25 in neonatal cardiac cells--differences in cellular localization under stress conditions

Two members of the small heat shock protein family, alpha B-crystallin and hsp25, occur at high levels in the mammalian heart. To try and understand any differences in functioning, we compared their properties in cultured rat neonatal cardiac myocytes. Both proteins are stress-inducible, but the level of hsp25 is only slightly increased in cultured cardiac myocytes subjected to hyperthermic stress, while alpha B-crystallin levels even remain unchanged. Phosphorylation of alpha B-crystallin and to a lesser extent also of hsp25 is induced after the heat shock. Directly after heat stress, alpha B-crystallin and hsp25 are partly found in detergent-insoluble fractions, representing cytoskeletal/nuclear structures. Additionally, we show by confocal laser scanning microscopy that alpha B-crystallin and hsp25 become associated with sarcomeric structures directly after the heat shock, indicating a cytoskeletal protective function. Four to six hours after the heat shock, both proteins reoccupy their original positions in the cytoplasm again. In contrast to alpha B-crystallin, hsp25 not only translocates to the cytoskeleton but also migrates to positions inside the nucleus. Despite the fact that both proteins are normally part of the same complex, their behavior in neonatal cardiac myocytes appears to be very different. The sarcomeric association of alpha B-crystallin occurs under milder conditions and persists for a longer period of time in comparison with hsp25. Our findings suggest that alpha B-crystallin and hsp25 are both involved in protection of the cytoskeleton during stress situations in the heart, although in different manners. In addition, hsp25 also plays a role inside the nucleus.

Digoxin's Minimal Inotropic Effect Is Not Limited by Sodium-Calcium Exchange in the Intact Immature Rabbit Heart

BACKGROUND: In the intact immature heart, how much digoxin can drive sodium-calcium exchange has not been studied in the context of sodium-calcium exchanger abundance. METHODS AND RESULTS: The effects of digoxin and low potassium on contractility in the intact, paced and isovolumically contracting immature rabbit heart were studied in both the absence and presence of L-type calcium channel blockade. Without calcium channel blockade, digoxin increased contractility minimally and only at 10(_6) M/L. In contrast, low potassium (2.2 mM/L) substantially increased contractility in all experiments, a result indicating abundant sodium-calcium exchanger activity. During nifedipine-induced calcium channel blockade, digoxin (10(_6) M/L) allowed modest recovery of contractility, whereas digoxin and low potassium together allowed complete recovery as assessed by dP/dt(max); however, all hearts so perfused subsequently developed ventricular fibrillation, presumably because of calcium overload. CONCLUSIONS: In intact immature rabbit heart, digoxin can drive sodium-calcium exchange and thus increase contractility to only a minimal extent. This effect does not appear to be limited by intrinsic exchanger activity, which appears abundant in this preparation. Rather, digoxin's inability to drive the sodium-calcium exchanger may be due to developmental differences in binding to the sodium pump. The sodium-calcium exchanger itself seems capable not only of providing enough intracellular calcium for normal contraction, but also of overloading the myocardium with calcium, despite L-type calcium channel blockade.

Role of Ca2+ in metabolic inhibition-induced norepinephrine release in rat brain synaptosomes

Ischemia and simulated ischemic conditions induce enhanced release of norepinephrine (NE) in the brain and the heart. Although studies with neuronal preparations demonstrated a rise in [Ca2+]i under energy-depleted conditions, such release of NE in the heart appears to be predominantly Ca2+ independent. Since Ca2+ overload occurs in ischemia or energy depletion and since a rise in [Ca2+]i triggers exocytosis without membrane depolarization, we tested the possibility, using brain synaptosomes, that increased NE release could be, at least in part, a consequence of raised [Ca2+]i. Brain synaptosomes were incubated with Krebs-Henseleit medium, and ischemia was mimicked by treatment with metabolic inhibitors. NE content in incubation medium (supernatant) and synaptosomes was analyzed chromatographically. Treatment with metabolic inhibitors reduced ATP content by 75% and increased [Ca2+]i by more than fourfold within minutes. Metabolic inhibition elicited NE release, which started within 10 minutes and reached a maximum after 30 minutes, with a corresponding 55% reduction in synaptosomal NE content after 40 minutes. NE release, together with a marked increase in [Ca2+]i, was also induced in energy-depleted synaptosomes by Ca2+ repletion after incubation with the Ca(2+)-free medium. Effects on NE release of various interventions to prevent Ca2+ overload were tested. Omission of Ca2+ from the incubation medium or loading synaptosomes with the Ca2+ chelator BAPTA-AM (20 and 100 mumol/L) prevented NE release, indicating a Ca(2+)-dependent mechanism. Inhibition of Ca2+ channels with omega-conotoxin, cadmium, or nifedipine had no effect on NE release during energy depletion. In contrast, nickel and 3,4-dichlorobenzamil, Na(+)-Ca2+ exchange inhibitors, dose-dependently inhibited NE release. In conclusion, this study provides evidence that under energy-depleted conditions, Ca2+ overload in synaptosomes of noradrenergic neurons from the brain is an important mechanism for the enhanced release of NE and that a reversal of Na(+)-Ca2+ exchange may be the key pathway leading to intraneuronal Ca2+ overload.

Ricin disturbs calcium homeostasis in the rabbit heart

Ricin, a toxic lectin from the castor bean, affects the cardiovascular system. Because calcium is very important in cardiotoxicity and cell intoxication, we studied the effects of ricin pretreatment to rabbits on basal intracellular calcium levels and calcium uptake and release from isolated papillary muscle, microsomes, and mitochondria. An increase in basal intracellular calcium levels was observed. Ricin pretreatment nearly doubled the intracellular-free Ca2+ concentration as measured by fura-2 fluorescence microscopy in isolated myocytes (p = 0.002). Ricin did not alter basal calcium efflux in isolated papillary muscles. However, ricin inhibited the NE-induced calcium efflux (expressed as fractional efflux ratios) in papillary muscles from rabbits receiving the minimum lethal dose of ricin at 25-35 minutes (p = 0.002 and 0.003, respectively). Ricin depressed basal calcium uptake into isolated papillary muscles at 5 minutes (mean +/- SEM, mumol/g wet weight) (control: 3.68 +/- 0.57; ricin: 2.31 +/- 0.28, p = 0.045, n = 6). Ricin pretreatment significantly depressed calcium uptake into microsomes (mean +/- SEM, mumol/g protein) (control: 9.9 +/- 1.9; ricin: 3.1 +/- 1.9, p = 0.025, n = 6). Calcium uptake into mitochondria was increased at the beginning (2 minutes, p = 0.048), but not thereafter. Thus, administration of ricin disturbed calcium homeostasis in the rabbit heart, which may be at least partially responsible for altering cardiac function and myocardial cell death.

Effect of pH on intracellular free Mg2+ in isolated adult rat cardiomyocytes

Changes in intracellular pH (pHi) alters the cytosolic concentrations of many electrolytes including Ca2+ and Na+. The present studies determined the effect of pHi on intracellular Mg2+ ([Mg2+]i) activity in isolated adult rat ventricular myocytes. Intracellular magnesium, [Mg2+]i, and calcium, [Ca2+]i, concentrations were measured with microfluorometry. Basal intracellular [Mg2+]i was 634 +/- 27 microM, n = 42 cells, and was not changed following electrical stimulation (0.5 Hz/s) which resulted in transient increases in [Ca2+]i and cell contractions. An NH4Cl pulse was used to rapidly alkalize and acidify the cytosol. Intracellular Mg2+ concentration within single cells decreased by 129 +/- 13 microM with rapid alkalinization of pH from basal levels of 7.1 to 7.6 following a NH4+ pulse. Removal of NH4Cl bathing solution resulted in cytosolic acidification, pH 6.9, and an increase in [Mg2+]i, from 467 +/- 47 to 569 +/- 41 microM. Intracellular [Ca2+] rose with acidification 80 +/- 4 to 149 +/- 19 nM, n = 5, and returned to normal levels, 89 +/- 5 nM, following recovery of pHi. Intracellular acidosis following exposure to 5% CO2/20 mM HCO3- solutions resulted in a significant increase in [Mg2+]i, to 778 +/- 63 microM. These results indicate that intracellular pH may have significant effects on [Mg2+]i in adult cardiomyocytes.

Simultaneous measurement of intracellular Na+ and Ca2+ during K(+)-free perfusion in isolated myocytes

To study the relationship between intracellular Na+ concentration ([Na+]i) and intracellular Ca2+ concentration ([Ca2+]i), guinea pig ventricular myocytes were loaded with both the Na(+)-sensitive probe, sodium-binding benzofuran isophthalate (SBFI), and the Ca(2+)-sensitive probe, fluo 3. [Na+]i was measured from the ratio image at 510 nm when excited at 340/380 nm. [Ca2+]i, expressed as the percent change of corrected fluo 3 fluorescence, was measured at 540 nm when excited at 500 nm. The fluorescent spectra of these probes were sufficiently different to allow for simultaneous measurement. After 30 min perfusion of K(+)-free solution, [Na+]i of rod-shaped cells increased from 6.4 +/- 0.5 to 20.6 +/- 2.6 mM, and [Ca2+]i increased to 256 +/- 36% of the control. [Ca2+]i was higher in spontaneously contracting cells and shortened cells than in rod-shaped cells at similar levels of [Na+]i. When Ca(2+)-free solution or Ni2+ (5 mM) was applied, [Ca2+]i was lower than in cells perfused with K(+)-free solution alone. It was suggested that extracellular Ca2+ and the Na(+)-Ca2+ exchange were involved in the increase in [Ca2+]i. In conclusion, we have developed a new method for the simultaneous measurement of [Na+]i and [Ca2+]i in isolated myocytes, which should be useful to study the relation between [Na+]i and [Ca2+]i.

Induction of HSP70 in cultured rat neonatal cardiomyocytes by hypoxia and metabolic stress

BACKGROUND

A cultured neonatal rat cardiomyocyte model is used to investigate the expression of the inducible heat shock protein 70 (HSP70i) during hypoxia/reoxygenation and metabolic stress.

METHODS AND RESULTS

The major HSP70i is increased in its expression at the mRNA and protein level in myocytes exposed to hypoxia/reoxygenation and metabolic stress by the addition of 2-deoxyglucose and sodium cyanide, which are inhibitors known to block ATP production. Surprisingly, the appearance of HSP70 mRNA precedes the intracellular ATP depletion caused by hypoxia, which is contrary to what we observe when the cardiomyocytes are subjected to metabolic stress.

CONCLUSIONS

It has been postulated recently that the decrease in intracellular ATP content in cells under stress may be the trigger that leads to the induction of HSP70i by reducing the pool of free HSP70, thus activating the stress response. Our results indicate that although this may be the case during metabolic stress, another route of activation must be used during the early stages of hypoxia in cardiomyocytes. The induction of HSP70i also appears to precede the onset of cellular damage as measured by the release of cytoplasmic enzymes and preincorporated arachidonic acid. This indicates that cardiomyocytes are able to respond to hypoxia/reoxygenation and metabolic stress with increased HSP70i production and points to a potential protective role of heat shock proteins during ischemia/reperfusion injury.

Quick-freezing of cultured cardiac cells in situ with special attention to the mitochondrial ultrastructure

A new method has been developed which allows quick-freezing in situ of primary, cardiac cell cultures grown to confluence on gas-permeable membranes (Petriperm dishes). Small pieces of the growth substratum, with rhythmically beating myocardial cells, were slam-frozen, without cryoprotectants, against the surface of a helium-cooled copper block at approximately 16 K. The quality of the cellular cryopreservation, as judged by ultrastructural criteria, was studied in freeze-substituted specimens processed for transmission electron microscopy. The ultrastructure of cryofixed cardiac cells was compared with that of unfrozen, chemically fixed samples. The severity of cryodistortions increased progressively with increasing distance from the point of first impact. Of particular interest were the dramatic alterations of the mitochondrial ultrastructure. The concept that the reticular and the outer mitochondrial membranes are intimately and strongly associated was clearly demonstrated. Optimally frozen material revealed cryopreserved ultrastructure of high quality. The method described appears to offer an ideal model system for correlating the information gained by phase-contrast microscopy of living cell cultures with the ultrastructure of the same samples fixed in situ by chemical or physical techniques. Cryofixation would be particularly useful for studying dynamic cellular processes associated with physiological and pathophysiological conditions, e.g. metabolic inhibition, anoxia and substrate deprivation.

Subcellular electrolyte alterations during progressive hypoxia and following reoxygenation in isolated neonatal rat ventricular myocytes

This study characterizes the sequential alterations of, and relations between, multiple electrolytes in cytoplasm, mitochondria, and whole cells during hypoxia and on reoxygenation in isolated neonatal rat ventricular myocytes. Subcellular electrolyte content and distribution were measured by electron probe x-ray microanalysis, membrane phospholipid degradation by tritiated arachidonic acid release, and cell morphology by electron microscopy. At 1-2 hours of hypoxia, the myocyte population showed a loss of cytoplasmic potassium, magnesium, and chlorine without alteration of cytoplasmic sodium or calcium. Mitochondria showed increased potassium with unchanged magnesium content. There was no morphological evidence of cell injury or tritiated arachidonic acid release. At 3-5 hours of hypoxia, the myocyte population showed a further loss of cytoplasmic potassium and magnesium and an increase in cytoplasmic sodium, chlorine, and calcium. At a single-cell level, the increase in cytoplasmic sodium preceded the increase in cytoplasmic calcium. Mitochondria showed increased sodium and chlorine and decreased magnesium before increased calcium content; potassium loss was manifest only at 5 hours of hypoxia. At 3-5 hours of hypoxia, there was also tritiated arachidonic acid release and morphological evidence of cell injury. Reoxygenation for 1 hour after 5 hours of hypoxia partially reversed the mean alterations of all electrolytes, except calcium, in the cytoplasm of the myocyte population, whereas analysis was required at a single-cell level to show a partial reversal in calcium levels in cytoplasm of reoxygenated cells. Reoxygenation for 1 hour after 5 hours of hypoxia partially reversed the mean alterations of all electrolytes, including calcium, in the mitochondria of the myocyte population. Recovery of potassium in the cytoplasm correlated with reduction of mitochondrial calcium content on reoxygenation and best predicted recovery of cellular homeostasis of sodium, chlorine, magnesium, and calcium. This study demonstrates that in this experimental model of hypoxia 1) initial losses of cytoplasmic potassium and magnesium occur in the absence of cell injury; 2) increases of sodium, chlorine, and calcium occur in association with cell injury, with sodium increasing before calcium; 3) membrane phospholipid degradation and electrolyte derangement, including increased calcium, may contribute to reversible and irreversible phases of cell injury; 4) analysis of calcium at a subcompartmental level and at a single-cell level is required to correlate reduction of calcium on reoxygenation with recovery of cell homeostasis; 5) reduction of calcium content in mitochondria may predict recovery of cell homeostasis; and 6) recovery of potassium on reoxygenation best predicts recovery of cell membrane function and cell homeostasis.

Relation between glycolysis and calcium homeostasis in postischemic myocardium

This study examined the hypothesis that glycolysis is required for functional recovery of the myocardium during reperfusion by facilitating restoration of calcium homeostasis. [Ca2+]i was measured in isolated perfused rabbit hearts by using the Ca2+ indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA) and 19F nuclear magnetic resonance spectroscopy. In nonischemic control hearts, inhibition of glycolysis with iodoacetate did not alter [Ca2+]i. In hearts subjected to 20 minutes of global zero-flow ischemia, [Ca2+]i increased from 260 +/- 80 nM before ischemia to 556 +/- 44 nM after 15 minutes of ischemia (p less than 0.05). After reperfusion with 5 mM pyruvate as a carbon substrate, [Ca2+]i increased further in hearts with intact glycolysis to 851 +/- 134 nM (p less than 0.05 versus ischemia) during the first 10 minutes of reperfusion, before returning to preischemic levels. In contrast, inhibition of glycolysis during the reperfusion period resulted in persistent severe calcium overload ([Ca2+]i, 1,380 +/- 260 nM after 15 minutes of reperfusion, p less than 0.02 versus intact glycolysis group). Furthermore, despite the presence of pyruvate and oxygen, inhibition of glycolysis during early reperfusion resulted in greater impairment of functional recovery (rate/pressure product, 3,722 +/- 738 mm Hg/min) than did reperfusion with pyruvate and intact glycolysis (rate/pressure product, 9,851 +/- 590 mm Hg/min, p less than 0.01). Inhibition of glycolysis during early reperfusion was also associated with a marked increase in left ventricular end-diastolic pressure during reperfusion (41 +/- 5 mm Hg) compared with hearts with intact glycolysis (16 +/- 2 mm Hg, p less than 0.01). The detrimental effects of glycolytic inhibition during early reperfusion were, however, prevented by initial reperfusion with a low calcium solution ([Ca]o, 0.63 mM for 30 minutes, then 2.50 mM for 30 minutes). In these hearts, the rate/pressure product after 60 minutes of reperfusion was 12,492 +/- 1,561 mm Hg/min (p less than 0.01 versus initial reflow with [Ca]o of 2.50 mM). These findings indicate that the functional impairment observed in postischemic myocardium is related to cellular Ca2+ overload. Glycolysis appears to play an important role in restoration of Ca2+ homeostasis and recovery of function of postischemic myocardium.

Intracellular calcium transients and arrhythmia in isolated heart cells

Intracellular calcium ([Ca2+]i) elevation may mediate cardiac arrhythmias. However, direct measurement of the rapid alterations of [Ca2+]i on a beat-to-beat basis using fast temporal resolution and without signal averaging in the spontaneously beating in vivo heart is lacking. Furthermore, data from an isolated spontaneously beating myocyte preparation that develops arrhythmia similar to that in the in vivo heart are unavailable. We measured rapid changes of [Ca2+]i with fast temporal resolution in isolated spontaneously beating neonatal rat ventricular myocytes with cell-to-cell communication and characterized the interrelation between [Ca2+]i and arrhythmia. An elevated extracellular calcium ([Ca2+]o) concentration of 10.8 mM induced premature beats, a rapid beating rate (tachyarrhythmia), and chaotic or fibrillatory beating activity in a small group of myocytes. [Ca2+]i levels during systole increased from the nanomolar to micromolar concentration range before arrhythmia development. Spontaneous oscillations of [Ca2+]i during diastole could evoke a spontaneous tachyarrhythmia. In the presence of [Ca2+]i elevation, a spontaneous tachyarrhythmia could induce severe [Ca2+]i overload. Reduction of [Ca2+]i with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid AM (5 microM) in the presence of 10.8 mM [Ca2+]o reversed the arrhythmia. In single ventricular myocytes superfused with 10.8 mM [Ca2+]o, oscillations of membrane potential characteristic of transient inward current occurred that were prevented by ryanodine (0.1 microM), an inhibitor of Ca2+ flux across the sarcoplasmic reticulum. This study characterizes 1) an isolated multicellular myocyte model of arrhythmia similar to that evident in in vivo hearts, 2) elevation of [Ca2+]i with systolic [Ca2+]i levels of 1-3 microM and diastolic [Ca2+]i oscillations before the initiation of arrhythmia, 3) tachyarrhythmia as a cause of severe [Ca2+]i overload, which may be important in the perpetuation and degeneration of arrhythmias, and 4) reversal of arrhythmia with reduction of [Ca2+]i. The results in the isolated myocyte model may have relevance to the generation and perpetuation of certain cardiac arrhythmias associated with calcium overload.

Role of increased cytosolic free calcium in the pathogenesis of rabbit proximal tubule cell injury and protection by glycine or acidosis

To assess the role of increased cytosolic free calcium (Caf) in the pathogenesis of acute proximal tubule cell injury and the protection afforded by exposure to reduced medium pH or treatment with glycine, fura-2-loaded tubules were studied in suspension and singly in a superfusion system. The Ca2+ ionophore, ionomycin, increased Caf to micromolar levels and rapidly produced lethal cell injury as indicated by loss of lactate dehydrogenase to the medium by suspended tubules and accelerated leak of fura and failure to exclude Trypan blue by superfused tubules. Decreasing medium Ca2+ to 100 nM prevented the ionomycin-induced increases of Caf and the injury. Reducing medium pH from 7.4 to 6.9 or adding 2 mM glycine to the medium also prevented the cell death, but did not prevent the increase of Caf to micromolar levels. Cells treated with 1799, an uncoupler of oxidative phosphorylation which produced severe adenosine triphosphate (ATP) depletion, did not develop increases of Caf until just before loss of viability. Preventing these increases of Caf with 100 nM Ca2+ medium did not protect 1799-treated cells. Reduced pH and glycine protected 1799-treated cells without ameliorating the increases of Caf. These data demonstrate the toxic potential of increased Caf in the proximal tubule and show that Caf does sharply increase prior to loss of viability in an ATP depletion model of injury, but this increase does not necessarily contribute to the outcome. The potent protective actions of decreased pH and glycine allow the cells to sustain increases of Caf to micromolar levels in spite of severe, accompanying cellular ATP depletion without developing lethal cell injury.

Continuous fluorimetric assessment of the changes in cytoplasmic calcium concentration during exposure of rat isolated myocardium to conditions of simulated ischaemia

1. The cytoplasmic calcium concentration ([Ca]c) of rat isolated atrial myocardium was assessed with the dye indo-1. Dye-loaded atria were superfused with physiological salt solution and excited with radiation at 360 nm, while epifluorescence emissions were collected simultaneously at 400 nm and 500 nm. The ratio of these emissions was used as a measure of [Ca]c. 2. Dye-loaded atria showed a phasic rise and fall in [Ca]c with each applied electrical pacing stimulus. The amplitudes of systolic increments in tension and [Ca]c were augmented by the presence of isoprenaline. 3. Atria superfused with a solution the composition of which resembled that found extracellularly in regions of ischaemia rapidly lost systolic increments in tension and [Ca]c, while end-diastolic [Ca]c and tension gradually rose. 4. The presence of lactate (20 mM) or flufenamate (5 microM) in the superfusate during simulated ischaemia aggravated the rises in both end-diastolic tension and end-diastolic [Ca]c. Inclusion in the superfusate of sulphinpyrazone (50 microM) or glucose (20 mM) protected against some of the deleterious effects of lactate seen during simulated ischaemia.

Association between inhibition of arachidonic acid release and prevention of calcium loading during ATP depletion in cultured rat cardiac myocytes

The development of irreversible myocardial ischemic injury is associated with progressive degradation of membrane phospholipids, accumulation of arachidonate and other free fatty acids, and electrolyte derangements, including calcium accumulation. To study the relationship between arachidonate release and calcium loading during adenosine triphosphate (ATP) depletion in cardiac myocytes, the effects of two purported phospholipase inhibitors, mepacrine and U26,384, were evaluated. Cultured neonatal rat ventricular myocytes were pretreated for 90 minutes with 5 to 10 microM U26,384 (a steroidal diamine) or 10 to 50 microM mepacrine (an alkyl acridine) and then treated for 3 hours with 30 microM of the metabolic inhibitor, iodoacetic acid (IAA), with or without an additional dose of drug. IAA treatment resulted in a marked reduction in ATP level and a several-fold increase in free fatty acid radioactivity released from myocytes prelabeled with tritiated arachidonic acid (3H-AA). U26,384 produced substantial inhibition of the increased 3H-AA release, and was effective when given as a single pretreatment dose before IAA exposure or as continuous treatment before and during IAA exposure (for example, with 5 microM U26,384, the percentage of 3H-AA release versus IAA alone was 8% +/- 2% [SEM] [N = 15] for pretreatment only and 13% +/- 4% [N = 10] for continuous treatment). Mepacrine also resulted in significant reduction in 3H-AA release, but was more effective when given as continuous treatment (for example, with 50 microM mepacrine, the percentage of 3H-AA release versus IAA alone was 43% +/- 9% [N = 6] for pretreatment only and 22% +/- 7% [N = 9] for continuous treatment). More detailed analysis showed that U26,384 and mepacrine blocked the IAA-induced redistribution of 3H-AA into free fatty acids from other lipid species. Electron probe x-ray microanalysis of freeze-dried cryosections revealed marked electrolyte derangements in myocytes exposed to IAA, including a 24-fold increase in cellular Ca, a four fold increase in cellular Na, and a seven fold decrease in cellular K, and associated changes in cytoplasm and mitochondria. U26,384 treatment markedly reduced these electrolyte abnormalities, and maintained normal Ca levels in some protocols. Mepacrine treatment was less effective, but did produce normal Ca levels in 50% of myocytes. Prevention of IAA-induced cellular hypercontraction and blebbing also was observed. These data support the hypothesis that reduction of free fatty acid accumulation by inhibition of accelerated phospholipid degradation is associated with protection of myocytes from calcium loading and morphologic damage during inhibition of ene