Fractionation of rat ventricular nuclei

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²⁺ in adult cardiac myocytes

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

Cardiac myosin heavy chain gene regulation by thyroid hormone involves altered histone modifications

The antithetical regulation of cardiac α- and β-myosin heavy chain (MHC) genes by thyroid hormone (T(3)) is not well understood but appears to involve thyroid hormone interaction with its nuclear receptor and MHC promoters as well as cis-acting noncoding regulatory RNA (ncRNA). Both of these phenomena involve epigenetic regulations. This study investigated the extent that altered thyroid state induces histone modifications in the chromatin associated with the cardiac MHC genes. We hypothesized that specific epigenetic events could be identified and linked to cardiac MHC gene switching in response to a hypothyroid or hyperthyroid state. A hypothyroid state was induced in rats by propylthiouracil treatment (PTU), whereas a hyperthyroid (T(3)) was induced by T(3) treatment. The left ventricle was analyzed after 7 days for MHC pre-mRNA expression, and the chromatin was assessed for enrichment in specific histone modifications using chromatin immunoprecipitation quantitative PCR assays. At both the α-MHC promoter and the intergenic region, the enrichment in acetyl histone H3 at K9/14 (H3K9/14ac) and trimethyl histone H3 at K4 (H3K4me3) changed in a similar fashion. They were both decreased with PTU treatment but did not change under T(3), except at a location situated 5' to the antisense intergenic transcription start site. These same marks varied differently on the β-MHC promoter. For example, H3K4me3 enrichment correlated with the β-promoter activity in PTU and T(3) groups, whereas H3K9/14ac was repressed in the T(3) group but did not change under PTU. Histone H3K9me was enriched in chromatin of both the intergenic and α-MHC promoters in the PTU group, whereas histone H4K20me1 was enriched in chromatin of β-MHC promoter in the normal control and T(3) groups. Collectively, these findings provide evidence that specific epigenetic phenomena modulate MHC gene expression in altered thyroid states.

IP(3)R-mediated Ca(2+) release is modulated by anandamide in isolated cardiac nuclei

Cannabinoids (CBs) are known to alter coronary vascular tone and cardiac performance. They also exhibit cardioprotective properties, particularly in their ability to limit the damage produced by ischaemia reperfusion injury. The mechanisms underlying these effects are unknown. Here we investigate the intracellular localisation of CB receptors in the heart and examine whether they may modulate localised nuclear Ca(2+) release. In isolated cardiac nuclear preparations, expression of both the inositol 1,4,5-trisphosphate receptor type 2 (IP(3)R) and CB receptors (CB(1)R and CB(2)R) was demonstrated by immunoblotting. Both receptors localised to the nucleus and purity of the nuclear preparations was confirmed by co-expression of the nuclear marker protein nucleolin but absence of cytoplasmic actin. To measure effects of IP(3)R and CBR agonists on nuclear Ca(2+) release, isolated nuclei were loaded with Fluo5N-AM. This dye accumulates in the nuclear envelope. Isolated nuclei responded to IP(3) with rapid and transient Ca(2+) release from the nuclear envelope. Anandamide inhibited this IP(3)-mediated release. Preincubation of nuclear preparations with either the CB(1)R antagonist (AM251) or the CB(2)R antagonist (AM630) reversed anandamide-mediated inhibition to 80% and 60% of control values respectively. When nuclei were pre-treated with both CBR antagonists, anandamide-mediated inhibition of IP(3)-induced Ca(2+) release was completely reversed. These results are the first to demonstrate the existence of cardiac nuclear CB receptors. They are also the first to show that anandamide can negatively modulate IP(3)-mediated nuclear Ca(2+) release. As such, this provides evidence for a novel key mechanism underlying the action of CBs and CBRs in the heart.

A myocyte enhancer factor 2 (MEF2) site located in a hypersensitive region of the FGF16 gene locus is required for preferential promoter activity in neonatal cardiac myocytes

Fibroblast growth factor 16 (FGF16) is preferentially expressed in the heart after birth, suggesting its regulation is associated with tissue-specific chromatin remodeling and DNA-protein interactions. Here we have mapped the transcription initiation site of murine FGF16 to approximately 1.1 kilobases (kb) upstream of the translation start codon (ATG). Hybrid reporter genes directed by about 4.7 kb of upstream FGF16 DNA were expressed specifically in transfected neonatal rat cardiac myocytes, as well as in the heart of transgenic mice. A DNaseI hypersensitive site was mapped to a region about 1.2 kb upstream of the transcription initiation site in heart but not kidney tissue, and a nuclease protection assay gave evidence of a cardiac-specific protein-DNA interaction in this region. Deletion analysis indicated that a hybrid gene with 1205 bp but not 1054 bp of upstream DNA directed FGF16 promoter activity in transfected neonatal rat cardiac myocytes. We identified a putative myocyte enhancer factor 2 (MEF2)-binding site at nucleotides -1159/-1148, confirmed by electrophoretic mobility shift assay and MEF2 antibody binding. Mutation of the MEF2 site resulted in a blunting of FGF16 promoter activity in transfected neonatal rat cardiac myocytes. These data suggest that chromatin remodeling and MEF2 binding in the FGF16 promoter contribute to expression in the postnatal heart.

Myocardial lipidomics. Developments in myocardial nuclear lipidomics

The development of electrospray ionization mass spectrometry has been critical for the analyses of lipidomes from subcellular organelles. The myocardial nuclear lipidome likely has a key role in the molecular regulation of gene expression. In fact, recent studies have suggested that specific phospholipid classes bind and regulate specific transcription factors. The dynamic regulation of the myocardial nuclear lipidome may be critical in mediating long-term pathological responses to stresses such as ischemia, tachycardia, and hypertension. In this brief review, the preparation of myocardial nuclei is discussed, and the resulting nuclear lipidome from rat and rabbit are shown as examples. The rabbit myocardial nuclear lipidome contains relatively more plasmenylcholine/phosphatidylcholine molecular species in comparison to that ratio observed in the rat myocardial nuclear lipidome. The composition of the rat myocardial nuclear choline glycerophospholipid pool was relatively enriched with molecular species containing arachidonic acid and docosahexaenoic acid in comparison to that in the rabbit myocardial nuclear choline glycerophospholipid pool. While the ethanolamine glycerophospholipids of the rabbit myocardial nuclei are enriched with arachidonic acid and plasmalogens, the ethanolamine glycerophospholipid profile from rat myocardial nuclei show less plasmalogen and more species containing docosahexaenoic acid. Last, significant differences in the ethanolamine glycerophospholipid molecular species were observed in the rabbit heart lipidomes from the nucleus and the mitochondria. Quantitation of these lipid species in hearts subjected to pathophysiological stresses may provide important information on the role of the myocardial nuclear lipidome on long-term cardiac cell function.

Peripheral-type benzodiazepine receptor ligands: mitochondrial permeability transition induction in rat cardiac tissue

Strong evidence is emerging that mitochondrial permeability transition (MPT) may be important in certain physiological conditions and, above all, in the processes of cell damage and death. Reversible MPT, triggered by inducing agents in the presence of calcium ions, has resulted in the opening of a dynamic multiprotein complex formed in the inner mitochondrial membrane and has caused large-amplitude mitochondrial swelling. In the present work, the exposure of de-energized rat cardiac mitochondria to peripheral benzodiazepine receptor (PBR) ligands (1-(2-chlorophenyl-N-methyl-1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195), 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864), and diazepam) produced a dose-dependent and cyclosporin A (CSP)-sensitive loss of absorbance, which was indicative of mitochondrial swelling. By contrast, the addition of a high-affinity central benzodiazepine receptor ligand (clonazepam) was ineffective, even at the highest concentration tested. The ultrastructural changes associated with swelling were similar in mitochondria exposed either to PK 11195 or to calcium. Supporting the apoptotic role of PK 11195-induced swelling, supernatants from mitochondria that had undergone permeability transition caused apoptotic changes in isolated cardiac nuclei. In addition, ultrastructural abnormalities were observed in rat cardiac tissue following in vivo PK 11195 administration, with these abnormalities being prevented by CSP co-administration. These data indicate that PBR ligands induce mitochondrial permeability transition and ultrastructural alterations in isolated cardiac mitochondria as well as in myocardiocytes, suggesting a novel strategy for studying the implication of PBR ligands as apoptosis inducers, through a probable effect on the MPT pore.

Thyroid hormone regulation of myocardial Na/K-ATPase gene expression

Employing published methods for isolation of cardiac myocyte nuclei from adult rat ventricular myocardium with the use of mechanical disruption without digestive enzymes, we obtained transcriptionally active cardiac myocyte nuclei with sufficient yield and purity. The relative content of Na/K-ATPase subunit mRNAs (alpha 1, alpha 2, and beta 1) in ventricular myocardium of euthyroid rats closely matched the relative rates of transcription of the respective subunit genes determined by nuclear run-on assay. Treatment of hypothyroid rats with T(3)to elicit hyperthyroidism was associated with 2.9-, 7.5-, and seven-fold increases in the contents of alpha 1-, alpha 2, beta 1-mRNAs, respectively. In contrast, rates of transcription of the subunit genes were not changed significantly by T(3), while transcription of the 18 S ribosomal gene was stimulated identical with three-fold by the treatment. A quantitative reverse transcription-polymerase chain reaction assay for measurement of primary RNA transcripts of the beta 1 gene was developed employing a rat genomic DNA fragment that contains the first exon and part of the first intron of the beta 1 gene. The relative abundance of beta 1 primary transcripts did not change in RNA isolated from hypothyroid, euthyroid, and hyperthyroid rats. It is concluded that: (1) The relative contents of Na/K-ATPase subunit mRNAs in euthyroid adult myocardium is primarily controlled at the transcriptional level, and (2) T(3)-induced increases in the contents of Na/K-ATPase subunit mRNAs in the heart is not associated with increased rates of transcription of the subunit genes, and the effect is mediated at the post-transcriptional level.

Identification of specific nuclear protein kinase C isozymes and accelerated protein kinase C-dependent nuclear protein phosphorylation during myocardial ischemia

Protein kinase C (PKC) has been suggested to mediate, at least in part, multiple processes in the pathophysiological sequelae of myocardial ischemia. The present study demonstrates that the epsilon, eta and iota isozymes of PKC are translocated to nuclei in response to brief intervals of global ischemia as well as reperfusion of ischemic rat myocardium. Concomitant with the translocation of PKC isozymes to nuclei during ischemia, increased PKC-mediated nuclear protein phosphorylation was observed. Taken together, the present results demonstrate that nuclear signaling mechanisms are activated during myocardial ischemia that include PKC translocation and PKC-mediated nuclear protein phosphorylation.

Chromatin remodelling of the cardiac beta-myosin heavy chain gene

To investigate the role of chromatin structure in cardiac gene expression, we used the DNase I and micrococcal nuclease to probe the chromatin structure of the hamster cardiac beta-MyHC gene. Two cardiac-specific DNase I hypersensitive sites (DHS) were identified, one of which was mapped to the -2.3 kb (beta-2.3 kb) region and the other to the proximal promoter region of the beta-MyHC gene. The two sites were readily detectable using nuclei from neonatal hamster heart; however, the proximal promoter site disappeared when adult hamster heart nuclei were used, and the -2.3 kb site decreased in intensity. We were able to demonstrate the gradual disappearance of this proximal promoter DHS by comparing heart nuclei isolated from animals at late-gestation and 1-day-old stages. Furthermore, injecting thyroid hormone caused the disappearance of the proximal promoter DHS in late gestational fetal ventricular nuclei. Digestion of nuclei from various tissues by micrococcal nuclease revealed that the beta-MyHC gene proximal promoter exists in an array of three specifically-positioned nucleosomes only in fetal heart chromatin. The beta-MyHC gene proximal promoter is DNase I hypersensitive within one of the nucleosomal particles. Our data suggest that chromatin structure may participate actively in cardiac gene expression.

Multiple muscle-specific regulatory elements are associated with a DNase I hypersensitive site of the cardiac beta-myosin heavy-chain gene

Using nuclei isolated from neonatal cardiomyocytes, we have mapped the DNase I hypersensitive sites (DHSs) residing within the 5'-upstream regions of the hamster cardiac myosin heavy-chain (MyHC) gene. Two cardiac-specific DHSs within the 5 kb upstream region of the cardiac MyHC gene were identified. One of the DHSs was mapped to the -2.3 kb (beta-2.3 kb) region and the other to the proximal promoter region. We further localized the beta-2.3 kb site to a range of 250 bp. Multiple, conserved, muscle regulatory motifs were found within the beta-2.3 kb site, consisting of three E-boxes, one AP-2 site, one CArG motif, one CT/ACCC box and one myocyte-specific enhancer factor-2 site. This cluster of regulatory elements is strikingly similar to a cluster found in the enhancer of the mouse muscle creatine kinase gene (-1256 to -1050). The specific interaction of the motifs within the beta-2.3 kb site and the cardiac nuclear proteins was demonstrated using gel mobility-shift assays and footprinting analysis. In addition, transfection analysis revealed a significant increase in chloramphenicol acetyltransferase activity when the beta-2.3 kb site was linked to a heterologous promoter. These results suggest that previously undefined regulatory elements of the beta-MyHC gene may be associated with the beta-2.3 kb site.

A conserved GATA motif in a tissue-specific DNase I hypersensitive site of the cardiac alpha-myosin heavy chain gene

Transgenic analysis has indicated that far upstream regulatory elements of the cardiac alpha-myosin heavy chain (MyHC) gene are required for appropriate transgene expression [Subramaniam, Gulick, Neumann, Knotts and Robbins (1993) J. Biol. Chem. 268, 4331-4336]. In an attempt to identify these as-yet-undefined regulatory elements, we mapped the DNase I hypersensitive sites (DHSs) in the 4 kb upstream region of the hamster cardiac alpha-MyHC gene. When using nuclei isolated from late-gestational and adult heart ventricles, a strong DHS was identified in the -1.9 kb region (alpha-1.9 kb site). It cannot be detected in kidney, liver or cardiofibroblast nuclei. Within this site, we found a conserved GATA-motif that interacts specifically with GATA-binding factors in nuclear extracts of cardiomyocytes at various developmental stages. These data provide further evidence to support the role of GATA factors in the regulation of cardiac alpha-MyHC gene expression.

The nuclear membrane integrity assay

A novel technique is described for the evaluation of membrane integrity in isolated nuclei. Membrane integrity is assessed by measuring nuclear fragility in response to high salt conditions. Salt-induced disruption of the nuclear membrane is followed by spectrophotometric monitoring of released nucleotides. The assay is based on determining the amount of salt necessary to induce release of 50% of the total pool of releasable nucleotides. This allows semiquantitative comparison of relative nuclear membrane strength or integrity of different samples in response to high salt conditions. In this manner, the effects of altered nuclear membrane composition or metabolism on membrane integrity may be monitored.

Demonstration of potent lipid-lowering activity by a thyromimetic agent devoid of cardiovascular and thermogenic effects

A potent lipid-lowering thyromimetic (CGS 26214) devoid of cardiac and thermogenic activity was identified based on its ability to preferentially access and bind the nuclear fraction of hepatocytes over that of myocytes in culture. The difference in access achieved with CGS 26214 was at least 100-fold better for hepatocytes than for myocytes. This in vitro hepatoselectivity resulted in a compound with unprecedented in vivo lipid-lowering potency with a minimal effective dose of 1 microgram/kg in rats and dogs (approximately 25x that of L-T3). At the same time, CGS 26214 was free of any cardiovascular effects up to the highest dose tested of 25 mg/kg and 100 micrograms/kg in rats and dogs, respectively.

Insulin-induced phosphorylation of a 38 kDa DNA-binding protein in ventricular cardiomyocytes: possible implication of nuclear protein phosphatase activity

Ventricular cardiomyocytes isolated from adult rat heart were used to analyze the effect of insulin on the phosphorylation of DNA-binding nuclear proteins and to elucidate the potential involvement of protein phosphatase-1 (PP-1) and PP-2A in this hormonal action. Cells were labelled with [33P]orthophosphate, stimulated with insulin (1.7 x 10(-7) M) and processed for the isolation of nuclei and extraction of DNA-binding proteins. Insulin was found to induce a rapid and constant increase in the serine/threonine phosphorylation of a 38 kDa DNA-binding protein, reaching 150% of control after 15 min and 180% after 150 min. Immunoprecipitation and Western blotting experiments revealed the presence of phosphorylated numatrin in the nuclear extract, however, insulin did not modify its phosphorylation state. Treatment of cardiomyocytes with okadaic acid (1 microM) resulted in a large increase (246 +/- 30%) in the phosphorylation of the 38 kDa protein. Using 32P-labelled phosphorylase as a substrate, we observed a significant inhibition of nuclear PP-1 activity to 38.5 +/- 7% (n = 3) of control after incubation of cardiomyocytes with insulin for 15 min. PP-2A, which corresponds to about 25% of total phosphatase activity, was also inhibited to the same extent. These data show the presence of an insulin-responsive 38 kDa DNA-binding phosphoprotein in the nucleus of cardiomyocytes, which is at least partly regulated by nuclear phosphatase activity. It is suggested that inhibition of nuclear PP-1 and PP-2A represents a possible mechanism of insulin signalling to the nucleus of target cells.

Phorbol ester regulation of opioid peptide gene expression in myocardial cells. Role of nuclear protein kinase

Opioid peptide gene expression was characterized in adult rat ventricular cardiac myocytes that had been cultured in the absence or the presence of phorbol 12-myristate 13-acetate. The phorbol ester induced a concentration- and time-dependent increase of prodynorphin mRNA, the maximal effect being reached after 4 h of treatment. The increase in mRNA expression was suppressed by incubation of cardiomyocytes with staurosporine, a putative protein kinase C inhibitor, and was not observed when the cells were cultured in the presence of the inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate. Incubation of cardiac myocytes with phorbol 12-myristate 13-acetate also elicited a specific and staurosporine-sensitive increase in immunoreactive dynorphin B, a biologically active end product of the precursor, both in the myocardial cells and in the culture medium. In vitro run-off transcription assays indicated that transcription of the prodynorphin gene was increased both in nuclei isolated from phorbol ester-treated myocytes and in nuclei isolated from control cells and then exposed to phorbol 12-myristate 13-acetate. No transcriptional effect was observed when cardiac myocytes or isolated nuclei where exposed to 4 alpha-phorbol 12,13-didecanoate. The phorbol ester-induced increase in prodynorphin gene transcription was prevented by pretreatment of myocytes or isolated nuclei with staurosporine, suggesting that myocardial opioid gene expression may be regulated by nuclear protein kinase C. In this regard, cardiac myocytes expressed protein kinase C-alpha, -delta, -epsilon, and -zeta, as shown by immunoblotting. Only protein kinase C-delta and protein kinase C-epsilon were expressed in nuclei that have been isolated from control myocytes, suggesting that these two isotypes of the enzyme may be part of the signal transduction pathway involved in the effect elicited by the phorbol ester on opioid gene transcription in isolated nuclei. The incubation of myocardial nuclei isolated from control cells in the presence of a protein kinase C activator induced the phosphorylation of the myristoylated alanine-rich protein kinase C substrate peptide, a specific fluorescent substrate of the enzyme. The possibility that prodynorphin gene expression may control the heart function through autocrine or paracrine mechanisms is discussed.

Characterization of Zn(2+)-binding nuclear proteins present in the myocardium

Nonhistone nuclear proteins were isolated from 3-5 day old neonatal as well as 3 month-old adult myocardium. The nuclear proteins were separated and analyzed by two-dimensional polyacrylamide gel electrophoresis. Using a blot transfer technique equilibrated with 65Zn2+, at least four polypeptides exhibited Zn(2+)-binding activity over the spectrum of nonhistone nuclear proteins. A protein with a molecular weight of 68kDa pI7.8, which has been characterized for its involvement in nucleosome structure, consistently binds Zn2+ in both the neonatal and adult myocardium. This nuclear protein has now been further characterized by partial amino acid microsequencing. It was found that this novel polypeptide is distinct from the pore-complex lamina proteins. Three other polypeptides with M tau 90kDa, pI7.8, M tau 68kDa, pI6.5 and M tau 35kDa, pI7.5 exhibited increased Zn(2+)-binding activity in neonatal myocardium as compared to adult myocardium. Together with results from our previous studies, this study provides the first evidence implicating Zn(++)-binding nuclear proteins in the processes of growth and differentiation of myocardial development.

Non-radioactive reverse transcriptase/polymerase chain reaction for quantification of myosin heavy chain mRNA isoforms in various rabbit muscles

A method was established for measuring molecule numbers of three different myosin heavy chain (MHC) mRNA isoforms in total RNA preparations. The quantification was based on a combination of primer-directed reverse transcriptase and polymerase chain reactions with 5'-digoxigenin-labeled oligonucleotides, using external standards. The sensitivity of the method allowed the quantitation of mRNA amounts down to the range of 1,000 molecules (detection limit 50 molecules). The numbers determined for eight different rabbit muscles are in the range of 10(3)-10(9)/micrograms total RNA. In soleus muscle, the value of 1.11 x 10(9) MHCI mRNA molecules corresponds to approximately 8% of the total mRNA. With reference to myonuclei, this amount corresponds to 1-2 x 10(4) molecules/nucleus. A quantitative comparison of the two fast MHC mRNA isoforms with the distribution of different MHC isoforms at the protein level indicates that one of these two fast sequences is specific to MHCIIb and the other to MHCIId. However, our data point to the existence of additional MHCIId mRNA subtypes.

Regulation of rat cardiac nuclei-associated Mg(2+)-NTPase by phosphorylation

A nucleoside triphosphatase (NTPase) activity appeared to be associated with a highly purified nuclear preparation from rat cardiac ventricles. Different nucleoside triphosphates (UTP greater than GTP greater than ITP greater than CTP) supported this enzymic activity, which was stimulated by Mg2+ but not by Ca2+. The nuclear NTPase activity could be down regulated by endogenous phosphorylation of a 55,000 Mr protein. Maximal phosphorylation of the 55,000 Mr protein occurred in the presence of Mg(2+)-ATP. Addition of cAMP, cGMP, Ca2+, Ca2+/phospholipid, Ca2+/calmodulin, and catalytic subunit of cAMP-dependent protein kinase was not associated with any further phosphorylation of the 55,000 Mr protein. However, in the presence of Ca2+/calmodulin or the catalytic subunit of the cAMP-dependent protein kinase additional proteins became phosphorylated, but these had no effect on the Mg(2+)-NTPase activity. These results indicate that a protein with Mr 55,000 may be involved in the regulation the Mg(2+)-NTPase activity associated with rat cardiac nuclei.

Are the kinetics of technetium-99m methoxyisobutyl isonitrile affected by cell metabolism and viability?

To investigate the role of cell viability and metabolism on the myocardial kinetics of a new tracer, technetium-99m-methoxyisobutyl isonitrile (Tc-99m-MIBI), 250 microCi/l Tc-99m-MIBI was infused in isolated rat hearts under constant flow conditions. The hearts were studied after inducing irreversible damage by cytochrome c oxidase inhibitor sodium cyanide (n = 8) or sarcolemmal membrane detergent Triton X-100 (n = 8). The control hearts (n = 6) received no toxins. Mean Tc-99m-MIBI peak accumulation activity was significantly reduced after cyanide (51.1 +/- 44.2% of control, p less than 0.01) and Triton (13.8 +/- 2.7% of control, p less than 0.001) administration. Kinetic studies also showed marked reduction in accumulation rates and marked increase in clearance rates for cyanide (p less than 0.01) and Triton (p less than 0.01) groups compared with controls. Potential changes in regional flow distribution were assessed using microspheres. When peak accumulation activity was corrected for these changes, there remained significant differences between the groups. In the cyanide and Triton groups, irreversible cell injury was confirmed by creatine kinase and lactate dehydrogenase release, triphenyl tetrazolium chloride staining, and electron microscopy. All the cells were viable in the control group. We conclude that the accumulation and clearance kinetics of Tc-99m-MIBI are significantly affected by cell viability. Tc-99m-MIBI kinetics appear to be dependent on sarcolemmal integrity and to a lesser extent on aerobic metabolism.

RNA transcription in myocardial-cell nuclei during postnatal development. A study establishing an assay system for transcription in vitro

A system for RNA transcription in vitro was established in order to determine the relative rate of RNA synthesis in neonatal and adult rat myocardial cells. This assay system optimizes the incorporation of [3H]UMP into RNA by using 3.5 x 10(7) myocardial-cell nuclei, and minimizes RNA degradation for at least 1 h in transcription in vitro, by the addition of human placental RNAase inhibitor. A 100% increase in the incorporation of [3H]UMP into myocardial-cell RNA was found on addition of this inhibitor. Myocardial-cell nuclei derived from 5-, 10-, 15-, 20-, and greater than 100-day-old rat hearts indicated that there is a progressive decrease in RNA synthesis with age. A 3-fold increase in RNA synthesis in 5-day-old myocardial cell nuclei as compared with 20-day-old rat heart was found. RNA synthesis in the adult myocardial cell nuclei decreased more than 10-fold in comparison with the 5-day-old newborn. The incorporation of [3H]UMP into rat liver nuclear RNA was 3-fold greater than in the myocardial-cell nuclear RNA, even when compared with the highly active transcription of 12-day-old heart nuclei. In order to determine the relationship between total RNA synthesis and the extent of specific gene expression in myocardial-cell nuclei during development, two distinct cDNA probes were used for Northern-blot analysis. Our results indicate that myosin-heavy-chain gene expression is remarkably decreased with age, whereas the 'housekeeping' gene is continually expressed independently of age.

Hypothyroid-like effect of amiodarone in the ventricular myocardium of the rat

Due to the similar electrophysiological effects of amiodarone and hypothyroidism in the myocardium, the induction of a local hypothyroid state has been proposed as the mechanism of action of amiodarone. To examine this hypothesis we have studied the influence of amiodarone on the distribution of ventricular isomyosins--a sensitive parameter of the thyroid state in the rat heart--and the effects of amiodarone on 3,5,3'-triiodothyronine (T3) myocardial nuclear receptor binding in vivo. Amiodarone induced a dosage-dependent redistribution of isomyosins similar to hypothyroidism, while simultaneously inducing a low T3 syndrome at the higher dose level. In hypothyroid rats, which were pretreated with amiodarone, substitution of T3 (2 micrograms/100 g) led to a complete reversal of the myosin pattern not differing from control hypothyroid rats which were only given T3; the effect of T3 (0.5 microgram/100 g) was however partially inhibited by amiodarone. Nuclear receptor binding of T3 determined in hypothyroid rats in vivo was unaffected by amiodarone. We conclude that amiodarone induces a hypothyroid-like state in the ventricular myocardium of rats by inhibiting the action of T3--an effect which cannot be attributed to an antagonism at the T3 nuclear receptor level.

Changes in non-histone nuclear proteins during postnatal myocardial development

Myocardial-cell nuclei isolated from ventricles of rats between 4 days and 15 months of age showed a progressive loss of DNA-synthetic activity in vitro. Correlated with this loss is the differential appearance of three major groups of non-histone nuclear proteins. Group a, with pI5.7-6.5 and Mr 67000, appeared in detectable amounts in rats more than 10 days of age, whereas group b, consisting of proteins with pI6.8-7.2 and Mr 45000, appeared in rats older than 33 days. The third group (group c), with pI6.5-8.5 and Mr 32000-42000, was present throughout postnatal development, but the amount of these proteins appeared to be greater in the 10-day-old rat than in the 15-month-old rat. All groups of these specific proteins were localized within the nucleus and did not reflect cytoplasmic constituents. Furthermore, the amino acid compositions of these polypeptides were also analysed and compared with those of actin and tubulin.

Nonenzymatic separation of myocardial cell nuclei from whole heart tissue

A simple and rapid nonenzymatic method has been developed to isolate myocardial cell nuclei from whole heart tissue. This method consists of a controlled disruption of cells followed by isopycnic gradient centrifugation. We have reviewed and compared our method to others more lengthy and laborious. By using a number of criteria, such as morphometric measurements, chemical composition, functional studies, specific nuclear protein markers, and mathematical analysis, we show that the nonenzymatic digestion method provides a most useful technique for the study of the biochemistry of the myocardial cell nucleus.