Inositol phosphate metabolism during myocardial ischemia

Increases in [IP3]i aggravates diastolic [Ca2+] and contractile dysfunction in Chagas' human cardiomyocytes

Chagas cardiomyopathy is the most severe manifestation of human Chagas disease and represents the major cause of morbidity and mortality in Latin America. We previously demonstrated diastolic Ca2+ alterations in cardiomyocytes isolated from Chagas' patients to different degrees of cardiac dysfunction. In addition, we have found a significant elevation of diastolic [Na+]d in Chagas' cardiomyocytes (FCII>FCI) that was greater than control. Exposure of cardiomyocytes to agents that enhance inositol 1,4,5 trisphosphate (IP3) generation or concentration like endothelin (ET-1) or bradykinin (BK), or membrane-permeant myoinositol 1,4,5-trisphosphate hexakis(butyryloxy-methyl) esters (IP3BM) caused an elevation in diastolic [Ca2+] ([Ca2+]d) that was always greater in cardiomyocytes from Chagas' than non- Chagas' subjects, and the magnitude of the [Ca2+]d elevation in Chagas' cardiomyocytes was related to the degree of cardiac dysfunction. Incubation with xestospongin-C (Xest-C), a membrane-permeable selective blocker of the IP3 receptors (IP3Rs), significantly reduced [Ca2+]d in Chagas' cardiomyocytes but did not have a significant effect on non-Chagas' cells. The effects of ET-1, BK, and IP3BM on [Ca2+]d were not modified by the removal of extracellular [Ca2+]e. Furthermore, cardiomyocytes from Chagas' patients had a significant decrease in the sarcoplasmic reticulum (SR) Ca2+content compared to control (Control>FCI>FCII), a higher intracellular IP3 concentration ([IP3]i) and markedly depressed contractile properties compared to control cardiomyocytes. These results provide additional and convincing support about the implications of IP3 in the pathogenesis of Chagas cardiomyopathy in patients at different stages of chronic infection. Additionally, these findings open the door for novel therapeutic strategies oriented to improve cardiac function and quality of life of individuals suffering from chronic Chagas cardiomyopathy (CC).

FoxO proteins and cardiac pathology

The FoxO family of transcription factors mediate a wide range of cellular responses from cell death to cell survival, growth inhibition and glucose utilization. This complex array of responses is regulated by an equally complex regulatory system, involving phosphorylation, ubiquitinization and acetylation, in addition to interactions with other transcription factors and transcriptional modifiers. In heart, FoxO proteins have been shown to be involved in development in limiting hypertrophic growth responses and in cardioprotection provided by silent information regulator 1 (Sirt1). However, the range of responses mediated by FoxO proteins and the clear evidence for involvement of FoxO regulators in cardiac pathology, suggest that further pathological actions of FoxO family members remain to be elucidated.

Phosphoinositide signalling and cardiac arrhythmias

Arrhythmias arise from a complex interaction between structural changes in the myocardium and changes in cellular electrophysiology. Electrophysiological balance requires precise control of sarcolemmal ion channels and exchangers, many of which are regulated by phospholipid, phosphatidylinositol(4,5)bisphosphate. Phosphatidylinositol(4,5)bisphosphate is the immediate precursor of inositol(1,4,5)trisphosphate, a regulator of intracellular Ca2+ signalling and, therefore, a potential contributor to arrhythmogenesis by altering Ca2+ homeostasis. The aim of the present review is to outline current evidence that this signalling pathway can be a player in the initiation or maintenance of arrhythmias.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous intracellular messenger regulating diverse functions in almost all mammalian cell types. It is generated by membrane receptors that couple to phospholipase C (PLC), an enzyme which liberates IP(3) from phosphatidylinositol 4,5-bisphosphate (PIP(2)). The major action of IP(3), which is hydrophilic and thus translocates from the membrane into the cytoplasm, is to induce Ca(2+) release from endogenous stores through IP(3) receptors (IP(3)Rs). Cardiac excitation-contraction coupling relies largely on ryanodine receptor (RyR)-induced Ca(2+) release from the sarcoplasmic reticulum. Myocytes express a significantly larger number of RyRs compared to IP(3)Rs (~100:1), and furthermore they experience substantial fluxes of Ca(2+) with each heartbeat. Therefore, the role of IP(3) and IP(3)-mediated Ca(2+) signaling in cardiac myocytes has long been enigmatic. Recent evidence, however, indicates that despite their paucity cardiac IP(3)Rs may play crucial roles in regulating diverse cardiac functions. Strategic localization of IP(3)Rs in cytoplasmic compartments and the nucleus enables them to participate in subsarcolemmal, bulk cytoplasmic and nuclear Ca(2+) signaling in embryonic stem cell-derived and neonatal cardiomyocytes, and in adult cardiac myocytes from the atria and ventricles. Intriguingly, expression of both IP(3)Rs and membrane receptors that couple to PLC/IP(3) signaling is altered in cardiac disease such as atrial fibrillation or heart failure, suggesting the involvement of IP(3) signaling in the pathology of these diseases. Thus, IP(3) exerts important physiological and pathological functions in the heart, ranging from the regulation of pacemaking, excitation-contraction and excitation-transcription coupling to the initiation and/or progression of arrhythmias, hypertrophy and heart failure.

Inositol phosphates and phosphoinositides in health and disease

In the past two decades, considerable progress has been made toward understanding inositol phosphates and PI metabolism. However, there is still much to learn. The present challenge is to understand how inositol phosphates and PIs are compartmentalized, identify new targets of inositol phosphates and PIs, and elucidate the mechanisms underlying spatial and temporal regulation of the enzymes that metabolize inositol phosphates and PIs. Answers to these questions will help clarify the mechanisms of the diseases associated with these molecules and identify new possibilities for drug design.

Inositol-1,4,5-trisphosphate-dependent Ca(2+) signalling in cat atrial excitation-contraction coupling and arrhythmias

Inositol-1,4,5-trisphosphate (IP(3))-dependent Ca(2+) release represents the major Ca(2+) mobilizing pathway responsible for diverse functions in non-excitable cells. In the heart, however, its role is largely unknown or controversial. In intact cat atrial myocytes, endothelin (ET-1) increased basal [Ca(2+)](i) levels, enhanced action potential-evoked [Ca(2+)](i) transients, caused [Ca(2+)](i) transients with alternating amplitudes (Ca(2+) alternans), and facilitated spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) in the form of Ca(2+) sparks and arrhythmogenic Ca(2+) waves. These effects were prevented by the IP(3) receptor (IP(3)R) blocker aminoethoxydiphenyl borate (2-APB), suggesting the involvement of IP(3)-dependent SR Ca(2+) release. In saponin-permeabilized myocytes IP(3) and the more potent IP(3)R agonist adenophostin increased basal [Ca(2+)](i) and the frequency of spontaneous Ca(2+) sparks. In the presence of tetracaine to eliminate Ca(2+) release from ryanodine receptor (RyR) SR Ca(2+) release channels, IP(3) and adenophostin triggered unique elementary, non-propagating IP(3)R-dependent Ca(2+) release events with amplitudes and kinetics that were distinctly different from classical RyR-dependent Ca(2+) sparks. The effects of IP(3) and adenophostin were prevented by heparin and 2-APB. The data suggest that IP(3)-dependent Ca(2+) release increases [Ca(2+)](i) in the vicinity of RyRs and thus facilitates Ca(2+)-induced Ca(2+) release during excitation-contraction coupling. It is concluded that in the adult mammalian atrium IP(3)-dependent Ca(2+) release enhances atrial Ca(2+) signalling and exerts a positive inotropic effect. In addition, by facilitating Ca(2+) release, IP(3) may also be an important component in the development of Ca(2+)-mediated atrial arrhythmias.

Inositol polyphosphate 1-phosphatase is a novel antihypertrophic factor

Activation of G(q)-coupled alpha(1)-adrenergic receptors leads to hypertrophic growth of neonatal rat ventricular cardiomyocytes that is associated with increased expression of hypertrophy-related genes, including atrial natriuretic peptide (ANP) and myosin light chain-2 (MLC), as well as increased ribosome synthesis. The role of inositol phosphates in signaling pathways involved in these changes in gene expression was examined by overexpressing inositol phosphate-metabolizing enzymes and determining effects on ANP, MLC, and 45 S ribosomal gene expression following co-transfection of appropriate reporter gene constructs. Overexpression of enzymes that metabolize inositol 1,4,5-trisphosphate did not reduce ANP or MLC responses, but overexpression of the enzyme primarily responsible for metabolism of inositol 4,5-bisphosphate (Ins(1,4)P(2)), inositol polyphosphate 1-phosphatase (INPP), reduced ANP and MLC responses associated with alpha(1)-adrenergic receptor-mediated hypertrophy. Similarly overexpressed INPP reduced ANP and MLC responses associated with contraction-induced hypertrophy. In addition, overexpression of INPP reduced the increase in ribosomal DNA transcription associated with both hypertrophic models. Hypertrophied cells from both cell models as well as ventricular tissue from mouse hearts hypertrophied by pressure overload in vivo contained heightened levels of Ins(1,4)P(2), suggesting reduced INPP activity in three different models of hypertrophy. These studies provide evidence for an involvement of Ins(1,4)P(2) in hypertrophic signaling pathways in ventricular myocytes.

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.

Activation of the Na(+)/H(+) exchanger is required for reperfusion-induced Ins(1,4,5)P(3) generation

Post-ischemic reperfusion causes a change in inositol phosphate responses to norepinephrine from primary generation of inositol(1,4) bis phosphate (Ins(1,4)P(2)) to generation of inositol(1,4,5) tris phosphate (Ins(1,4,5)P(3)) that is required for the initiation of reperfusion arrhythmias. The current study was undertaken to investigate the role of Na(+)/H(+)exchange in facilitating this transient change in inositol phosphate response. Rat hearts were subjected to 20 min ischemia followed by 2 min reperfusion and Ins(1, 4,5)P(3)content was measured by mass analysis or by anion-exchange HPLC following [(3)H]inositol labeling. Reperfusion caused generation of [(3)H]Ins(1,4,5)P(3)(1732+/-398 to 3103+/-214, cpm/g tissue, mean+/-S.E.M., n=5, P<0.01) and the development of arrhythmias. Inhibition of Na(+)/H(+)exchange, by reperfusing at pH 6.3 or by pretreating with HOE-694 (10 n M-3 microM) or HOE-642 (3 microM) prevented the [(3)H]Ins(1,4,5)P(3)generation, without causing any suppression of norepinephrine release. Increases in Ins(1,4,5)P(3)mass were similarly reduced by inhibition of Na(+)/H(+)exchange. Thus, activation of Na(+)/H(+)exchange is required for the enhanced Ins(1,4,5)P(3)response observed under reperfusion conditions, and prevention of Ins(1,4,5)P(3)generation may be an important contributor to the anti-arrhythmic actions of inhibitors of Na(+)/H(+)exchange.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Reduced reperfusion-induced Ins(1,4,5)P3 generation and arrhythmias in hearts expressing constitutively active alpha1B-adrenergic receptors

Reperfusion of globally ischemic rat hearts causes the generation of inositol(1,4,5)trisphosphate [Ins(1,4,5)P3] and the initiation of arrhythmias. These responses are mediated by alpha1-adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic conditions, hearts from transgenic animals expressing constitutively active alpha1B-ARs in heart (alpha1B-constitutively active mutant [CAM]) showed higher [3H] inositol phosphate responses to norepinephrine (2.3-fold) than hearts from nontransgenic animals (alpha1B-WT) (1.6-fold). alpha1B-WT hearts responded to 2 minutes of reperfusion after 20 minutes of global ischemia by generation of Ins(1,4,5)P3 (5301+/-1310 to 11 413+/-1597 CPM/g tissue; mean+/-SEM; n=6; P<0.01 in [3H] labeling studies and 3.8+/-0.2 to 6.3+/-0.6 nmol/g by mass analysis, n=6; P<0.05). In contrast to findings in normoxia, hearts from alpha1B-CAM animals showed no Ins(1,4,5)P3 response in early reperfusion. In parallel studies, alpha1B-WT hearts developed ventricular tachycardia and ventricular premature beats (VPB) during 5 minutes of reperfusion after 20 minutes of ischemia. The incidence of these arrhythmias was reduced in the alpha1B-CAM hearts (95% to 62% for VPB and 47% to 12% for ventricular tachycardia; both P<0.05). The resistance of the alpha1B-CAM hearts was not due to alpha1B-AR-mediated preconditioning, as the Ins(1,4,5)P3 response to thrombin receptor activation during reperfusion was not different between the 2 groups. To investigate the possibility of reduced alpha1A-receptor activity in the alpha1B-CAM hearts, expression of the mRNA for alpha1A- and alpha1B-receptors was measured. alpha1B-WT hearts contained mRNA for both receptor subtypes, but the levels of alpha1B-receptor mRNA were 5-fold higher than alpha1A-receptor mRNA. alpha1B-CAM hearts contained very high levels of alpha1B-receptor mRNA (26-fold increase), but the expression of mRNA for the alpha1A-receptors (0.141+/-0.035 amol/ microg RNA; mean+/-SEM; n=6) was reduced by 50% relative to alpha1B-WT controls (0.276+/-0.046 amol/ microg RNA; n=6; P<0.01). The reduction in arrhythmogenic and Ins(1,4,5)P3 responses in alpha1B-CAM hearts provides evidence that these response are not mediated by alpha1B-receptors.