Adrenergic modulation of the type 1 IP3 receptors in the rat heart

Apoptosis induced clustering of IP(3)R1 in nuclei of non-differentiated PC12 cells

Inositol 1,4,5-trisphosphate (IP(3)) receptors are emerging as key sites for regulation by pro- and anti-apoptotic factors. Induction of apoptosis for 3 h increased mRNA and protein levels of type 1 IP(3) receptors in non-differentiated (ND), but not in differentiated (D) PC12 cells. Inhibitors of the IP(3) R's calcium release-2-aminoethoxydiphenyl borate (2-APB) and xestospongin-completely prevented Bax and caspase-3 mRNA increase after treatment with the apoptosis inducer set (AIK), and this reinforces the importance of IP(3) R1 in the apoptosis of ND PC12 cells. Apoptosis induction not only increases the IP(3) R1 protein, but it also causes formation of IP(3) R1 clusters in the nucleus which most likely result from fusion of the nucleoplasmic reticulum and/or IP(3) R1 translocation to the nucleus. This is quite similar to the observations noted after overexpression of IP(3) R1 in PC12 cells. The amount of IP(3) induced calcium release was higher in control than in AIK-treated cells. From our results we propose that after the apoptosis induction the amount of intranuclear calcium decreased dramatically due to the increase of calcium permeability of the nuclear calcium store vesicles. Therefore, increase of the calcium permeability may result from IP(3) receptors translocation to nuclei that can boost the calcium transport through IP(3) receptors.

Chemical sympathetic denervation, suppression of myocardial transient outward potassium current, and ventricular fibrillation in the rat

Sympathetic denervation is frequently observed in heart disease. To investigate the linkage of sympathetic denervation and cardiac arrhythmia, we developed a rat model of chemical sympathectomy by subcutaneous injections of 6-hydroxydopamine (6-OHDA). Cardiac sympathetic innervation was visualized by means of a glyoxylic catecholaminergic histofluorescence method. Transient outward current (Ito) of ventricular myocytes was recorded with the whole-cell configuration of the patch clamp technique. We observed that sympathectomy (i) decreased cardiac sympathetic nerve density and norepinephrine level, (ii) reduced the protein expression of Kv4.2, Kv1.4, and Kv channel-interacting protein 2 (KChIP2), (iii) decreased current densities and delayed activation of Ito channels, (iv) reduced the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP response element-binding protein (CREB), and (v) increased the severity of ventricular fibrillation induced by rapid pacing. Three weeks after 6-OHDA injections, which allowed time for sympathetic regeneration, we found cardiac sympathetic nerve density, norepinephrine levels, expression levels of Kv4.2 and KChIP2 proteins, and I(to) densities were partially normalized and ventricular fibrillation severity was decreased. We conclude that chemical sympathectomy downregulates the expression of selective Kv channel subunits and decreases myocardial I(to) channel activities, contributing to the elevated susceptibility to ventricular fibrillation.

Type 1 and 2 IP3 receptors respond differently to catecholamines and stress

Type 1 and 2 inositol 1,4,5-trisphosphate (IP3) receptors have been found in cardiac tissue, although they are localized in different types of cells. While the type 1 predominates in neuronal cells and cardiac ganglia, type 2 IP3 receptor is localized mainly in cardiomyocytes. In the heart, gene expression of the type 1 IP3 receptor is modulated by catecholamines, while type 2 is not affected. In this study we compared the effects of two stressors--cold exposure and hypoxia--on gene expression and protein levels of type 1 and 2 IP3 receptors. We found that both types of IP3 receptors were increased by these stressors, although the response of the type 2 IP3 receptor was delayed. We propose that regulation of these types of IP3 receptors in the heart might differ in control conditions and during stress.

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.

Adrenergic and calcium modulation of the heart in stress: from molecular biology to function

There is strong evidence about the importance of catecholamines and calcium signaling in heart function. Also, interaction of these two systems is well documented. Catecholamines signal through adrenergic receptors, and further activate calcium transport either from the extracellular space, or from the intracellular calcium stores. This review summarizes current knowledge on catecholamine production in the heart, with special focus on the final enzyme in the catecholamine synthesizing pathway, phenylethanolamine N-methyltransferase (PNMT), in different cell types in the heart. Further, signaling through different types of adrenergic receptors in physiological conditions and after exposure to different stressors is discussed. Also, part of this review considers activation of an intracellular calcium transport system via inositol 1,4,5-trisphosphate receptor and to possible functional consequences in control and stress conditions.

Changes in ultrastructure and endogenous ionic channels activity during culture of HEK 293 cell line

Human embryonic kidney (HEK) 293 cells were characterised as an expression system for voltage-activated cationic channels. Current density for cationic channels intrinsically expressed in HEK 293 cells as well as cell ultrastructure was described after 7-11, 29-30 and 49-63 days of cell culture. Slowly activating outward potassium current with the current density varying between +10 and +26 pA/pF was observed in 72% to 95% of investigated cells. Rapidly inactivating outward potassium current with the current density varying between +7 and +10 pA/pF was present in 38% to 48% of all cells. 30% of cells exhibited voltage-activated calcium channel with the current density less than -1 pA/pF. Tetrodotoxin-sensitive sodium current with amplitudes between -1.4 and -2.2 pA/pF was initially present in 5% of cells, nevertheless, after 49-63 days of cell culture this proportion increased to 35%. Ultrastructure of HEK 293 cell surface, but not of cell's interior changed during cell culture. The longer the time after thawing the more microvilli and protrusions appear on the cell surface. Irregular cell contours hinder the cells to appose and only small patches of membranes form attachments. Staining of cells with a polycationic dye ruthenium red initially increased and decreased again following prolonged period of time in culture indicating regression of negatively charged layers of the cell surface coat. We suggest that the optimal time window for patch clamp experiment is between days 7 and 63 of cell culture due to alterations of cell surface.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

First molecular evidence that inositol trisphosphate signaling contributes to infarct size reduction with preconditioning

Considerable attention has focused on the role of protein kinase C (PKC) in triggering the profound infarct-sparing effect of ischemic preconditioning (PC). In contrast, the involvement of inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)], the second messenger generated in parallel with the diacylglycerol-PKC pathway, remains poorly understood. We hypothesized that, if Ins(1,4,5)P(3) signaling [i.e., release of Ins(1,4,5)P(3) and subsequent binding to Ins(1,4,5)P(3) receptors] contributes to PC-induced cardioprotection, then the reduction of infarct size achieved with PC would be attenuated in mice that are deficient in Ins(1,4,5)P(3) receptor protein. To test this concept, hearts were harvested from 1) B6C3Fe-a/a-Itpr-1(opt+/-)/J mutants displaying reduced expression of Ins(1,4,5)P(3) receptor-1 protein, 2) Itpr-1(opt+/+) wild types from the colony, and 3) C57BL/6J mice. All hearts were buffer-perfused and randomized to receive two 5-min episodes of PC ischemia, pretreatment with d-myo-Ins(1,4,5)P(3) [sodium salt of native Ins(1,4,5)P(3)], the mitochondrial ATP-sensitive K(+) channel opener diazoxide, or no intervention (controls). After the treatment phase, all hearts underwent 30-min global ischemia followed by 2 h of reperfusion, and infarct size was delineated by tetrazolium staining. In both wild-type and C57BL/6J cohorts, area of necrosis in hearts that received PC, d-myo-Ins(1,4,5)P(3), and diazoxide averaged 28-35% of the total left ventricle (LV), significantly smaller than the values of 52-53% seen in controls (P < 0.05). In contrast, in Itpr-1(opt+/-) mutants, protection was only seen with diazoxide: neither PC nor d-myo-Ins(1,4,5)P(3) limited infarct size (52-58% vs. 56% of the LV in mutant controls). These data provide novel evidence that Ins(1,4,5)P(3) signaling contributes to infarct size reduction with PC.