Involvement of sarcoplasmic reticulum in changing intracellular calcium due to Na+/K+-ATPase inhibition in cardiomyocytes

Gestation changes sodium pump isoform expression, leading to changes in ouabain sensitivity, contractility, and intracellular calcium in rat uterus

Developmental and tissue‐specific differences in isoforms allow Na⁺, K⁺‐ATPase function to be tightly regulated, as they control sensitivity to ions and inhibitors. Uterine contraction relies on the activity of the Na⁺, K⁺ATPase, which creates ionic gradients that drive excitation‐contraction coupling. It is unknown whether Na⁺, K⁺ATPase isoforms are regulated throughout pregnancy or whether they have a direct role in modulating uterine contractility. We hypothesized that gestation‐dependent differential expression of isoforms would affect contractile responses to Na⁺, K⁺ATPase α subunit inhibition with ouabain. Our aims were therefore: (1) to determine the gestation‐dependent expression of mRNA transcripts, protein abundance and tissue distribution of Na⁺, K⁺ATPase isoforms in myometrium; (2) to investigate the functional effects of differential isoform expression via ouabain sensitivity; and (3) if changes in contractile responses can be explained by changes in intracellular [Ca²⁺]. Changes in abundance and distribution of the Na⁺, K⁺ATPase α, β and FXYD1 and 2 isoforms, were studied in rat uterus from nonpregnant, and early, mid‐, and term gestation. All α, β subunit isoforms (1,2,3) and FXYD1 were detected but FXYD2 was absent. The α1 and β1 isoforms were unchanged throughout pregnancy, whereas α2 and α3 significant decreased at term while β2 and FXYD1 significantly increased from mid‐term onwards. These changes in expression correlated with increased functional sensitivity to ouabain, and parallel changes in intracellular Ca²⁺, measured with Indo‐1. In conclusion, gestation induces specific regulatory changes in expression of Na⁺, K⁺ATPase isoforms in the uterus which influence contractility and may be related to the physiological requirements for successful pregnancy and delivery.

Resolvin D1 mitigates energy metabolism disorder after ischemia-reperfusion of the rat lung

BACKGROUND

Energy metabolism disorder is a critical process in lung ischemia-reperfusion injury (LIRI). This study was aimed to determine the effects of resolvin D1 (RvD1) on the energy metabolism in LIRI.

METHODS

Forty Sprague-Dawley rats were divided into the following groups: Sham group; untreated ischemia-reperfusion (IR) control; IR treated with normal saline (IR-NS); and IR treated with RvD1 (IR-RV) (100 μg/kg, iv). LIRI and energy metabolism disorder were determined in these rats.

RESULTS

The results revealed that the levels of interleukin (IL)-1β, tumor necrosis factor-α, IL-10, monocyte chemoattractant protein-1, macrophage inflammatory protein-2, cytokine-induced neutrophil chemoattractant-1, injured alveoli rate, apoptosis index, pulmonary permeability index, malondialdehyde, ADP, and lactic acid were increased, whereas the levels of ATP, ATP/ADP, glycogen, Na(+)-K(+)-ATPase, superoxide dismutase, glutathione peroxidase activity, pulmonary surfactant associated protein-A, and oxygenation index were decreased in rats with LIRI. Except for IL-10, all these biomarkers of LIRI and its related energy metabolism disorder were significantly inhibited by RvD1 treatment. In addition, histological analysis via hematoxylin-eosin staining, and transmission electron microscopy confirmed that IR-induced structure damages of lung tissues were reduced by RvD1.

CONCLUSION

RvD1 improves the energy metabolism of LIRI disturbance, protects the mitochondrial structure and function, increases the ATP, glycogen content and Na(+)-K(+)-ATPase activity of lung tissue, balances the ratio of ATP/ADP and finally decreases the rate of apoptosis, resulting in the protection of IR-induced lung injury. The improved energy metabolism after LIRI may be related to the reduced inflammatory response, the balance of the oxidative/antioxidant and the pro-inflammatory/anti-inflammatory systems in rats.

Drug-induced secretory diarrhea: A role for CFTR

Many medications induce diarrhea as a side effect, which can be a major obstacle to therapeutic efficacy and also a life-threatening condition. Secretory diarrhea can be caused by excessive fluid secretion in the intestine under pathological conditions. The cAMP/cGMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) is the primary chloride channel at the apical membrane of intestinal epithelial cells and plays a major role in intestinal fluid secretion and homeostasis. CFTR forms macromolecular complexes at discreet microdomains at the plasma membrane, and its chloride channel function is regulated spatiotemporally through protein-protein interactions and cAMP/cGMP-mediated signaling. Drugs that perturb CFTR-containing macromolecular complexes in the intestinal epithelium and upregulate intracellular cAMP and/or cGMP levels can hyperactivate the CFTR channel, causing excessive fluid secretion and secretory diarrhea. Inhibition of CFTR chloride-channel activity may represent a novel approach to the management of drug-induced secretory diarrhea.

Modification of Ca(2+)-handling in cardiomyocytes by redox sensitive mechanisms in response to ouabain

We examined the role of redox-sensitive signal transduction mechanisms in modifying the changes in [Ca(2+)](i) produced by ouabain upon incubating adult rat cardiomyocytes with antioxidants or inhibitors of different protein kinases and monitoring alterations in fura-2 fluorescence. Ouabain increased basal [Ca(2+)](i), augmented the KCl-induced increase in [Ca(2+)](i), and promoted oxyradical production in cardiomyocytes. These actions of ouabain were attenuated by an oxyradical scavenging mixture (superoxide dismutase plus catalase), and the antioxidants (N-acetyl-L-cysteine and N-(2-mercaptoproprionyl)glycine). An inhibitor of MAP kinase (PD98059) depressed the ouabain-induced increase in [Ca(2+)], whereas inhibitors of tyrosine kinase (tyrphostin and genistein) and PI3 kinase (Wortmannin and LV294002) enhanced the ouabain-induced increase in [Ca(2+)](i). Inhibitors of protein kinase C (calphostin and bisindolylmalaimide) augmented the ouabain-induced increase in [Ca(2+)](i), whereas stimulation of protein kinase C by a phorbol ester (phorbol 12-myristate 13-acetate) depressed the action of ouabain. These results suggest that ouabain-induced inhibition of Na (+)-K(+) ATPase may alter the redox status of cardiomyocytes through the production of oxyradicals, and increase the activities of various protein kinases. Thus, these redox-sensitive signal transduction mechanisms involving different protein kinases may modify Ca(2+)-handling sites in cardiomyocytes and determine the magnitude of net increase in [Ca(2+)](i) in response to ouabain.

Differentially regulated functional gene clusters identified in early hypoxic cardiomyocytes

Pathological stress including myocardial infarction and hypertension causes a negative effect on calcium regulation and homeostasis. Nevertheless, few studies reveal that Ca(2+) regulatory genes are related to pathological status in cardiomyocytes under early hypoxia. To determine the alteration of Ca(2+)-related gene in hypoxic myocytes, primary neonatal rat ventricular cardiomyocytes (NRVCMs) was isolated. Survival of hypoxic NRVCMs was significantly decreased in 6 h. We confirmed an increase of reactive oxygen species (ROS) generation and Ca(2+) overload in hypoxic NRVCMs by using 2',7'-dichlorodihydro-fluorescein diacetate (H2DCFDA) and FACS analysis. Furthermore, survival/apoptotic signals were also regulated in same condition. The expression profiles of more than 30,000 genes from NRVCMs that were subjected to early hypoxia revealed 630 genes that were differentially regulated. The intracellular Na(+) overload and Ca(2+) handling genes with at least two-fold changes were confirmed. The levels of Ca(2+)-handling proteins (calsequestrin, calmodulin, and calreticulin), ion channels (NCX, Na(+)-K(+)-ATPase, SERCA2a, and PLB), and stress markers (RyR2, ANP, and BNP) were significantly altered in early hypoxia. These results demonstrate that early hypoxia alters Ca(2+)-related gene expression in NRVCMs, leading to pathological status.

Ouabain increases iNOS-dependent nitric oxide generation which contributes to the hypertrophic effect of the glycoside: possible role of peroxynitrite formation

In addition to inotropic effects, cardiac glycosides exert deleterious effects on the heart which limit their use for cardiac therapeutics. In this study, we determined the possible contribution of ouabain-induced iNOS stimulation to the resultant hypertrophic as well as cytotoxic effects of the glycoside on cultured adult rat ventricular myocytes. Myocytes were treated with ouabain (50 μM) for up to 24 h. Ouabain significantly increased gene and protein levels of inducible nitric oxide synthase (iNOS) which was associated with significantly increased release of NO from myocytes as well as increased total release of reactive oxygen species (ROS), superoxide anion (O(2) (-)), and increased peroxynitrite formation as assessed by protein tyrosine nitration. Administration of ouabain was also associated with increased levels of myocyte toxicity as determined by myocyte morphology, trypan blue staining and lactate dehydrogenase (LDH) efflux. The nonspecific NOS inhibitor Nω-nitro-L: -arginine methyl ester and the more selective iNOS inhibitor 1400W both abrogated the increase in LDH release but had no significant effect on either morphology or trypan blue staining. Ouabain also significantly increased both myocyte surface area and expression of atrial natriuretic peptide indicating a hypertrophic response with both parameters being completely prevented by NOS inhibition. The effects of iNOS inhibitors were associated with diminished ouabain tyrosine nitration as well as abrogation of ouabain-induced p38 and ERK phosphorylation. Our study shows that ouabain is a potent inducer of NO formation, iNOS upregulation, and increased production of ROS. Inhibition of ouabain-dependent peroxynitrite formation may contribute to the antihypertrophic effect of iNOS inhibition possibly by preventing downstream MAPK activation.