Repetitive stunning, hibernation, and heart failure: contribution of PET to establishing a link

Acquisition, Processing, and Interpretation of PET 18F-FDG Viability and Inflammation Studies

PURPOSE OF REVIEW

This article reviews the acquisition protocols and image interpretation for ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) imaging with positron emission tomography (PET) applied to the evaluation of myocardial viability and inflammation.

RECENT FINDINGS

Cardiac PET with ¹⁸F-FDG provides essential information for the assessment of myocardial viability and inflammation and is usually combined with PET perfusion imaging using ⁸²Rb or ¹³N-ammonia. Viable myocardium maintains glucose metabolism which can be detected via the uptake of ¹⁸F-FDG by PET imaging. The patient is prepared for viability imaging by shifting the metabolism of the heart to maximize the uptake of glucose and hence of ¹⁸F-FDG. Comparison of the ¹⁸F-FDG and myocardial perfusion images allows distinction between regions of the myocardium that are hibernating and thus may recover function with intervention, from those that are infarcted. Increased glucose utilization in the inflammatory cells also makes ¹⁸F-FDG a useful imaging technique in conditions such as cardiac sarcoidosis. Here, suppression of normal myocardial uptake is essential for accurate image interpretation. ¹⁸F-FDG PET broadens the scope of information potentially available through a cardiac PET study. With careful patient preparation, it provides valuable insights into myocardial viability and inflammatory processes such as sarcoidosis.

Vitamin D status and coronary flow reserve measured by positron emission tomography: a co-twin control study

CONTEXT

Vitamin D insufficiency is associated with increased cardiovascular events in the general population. Additionally, low serum 25-hydroxyvitamin D [25(OH)D] is associated with endothelial dysfunction and arterial stiffness. However, little is known about the association between serum 25(OH)D level and myocardial blood flow.

OBJECTIVE

Our objective was to examine the association between serum 25(OH)D levels and coronary flow reserve (CFR) measured by (13)N-positron emission tomography in asymptomatic middle-aged male twins.

DESIGN

The Emory Twin Study is a cross-sectional study of soldiers from the Vietnam Era Registry.

SETTING

The study was conducted at the General Clinical Research Center, Emory University, Atlanta, GA.

PARTICIPANTS

A total of 368 middle-aged male twins were enrolled for the study. Serum 25(OH)D levels were measured in all subjects and classified as vitamin D insufficiency [25(OH)D <30 ng/ml] or sufficiency [25(OH)D ≥30 ng/ml]. Positron emission tomography with [(13)N]ammonia was used to evaluate myocardial blood flow at rest and after adenosine stress. CFR was measured as the ratio of maximum to rest myocardial blood flow.

MAIN OUTCOME MEASURE

Primary outcome was CFR measurement.

RESULTS

Mean overall serum 25(OH)D concentration was 37.0 ± 21.4 ng/ml; 167 twins (45%) were vitamin D insufficient. CFR was significantly lower in subjects with vitamin D insufficiency compared with subjects with vitamin D sufficiency (2.41 vs. 2.64; P = 0.007), even after adjustment for traditional cardiovascular risk factors, serum PTH, calcium, and phosphorus levels, and season. An abnormal CFR (CFR <2) was more prevalent in subjects with vitamin D insufficiency than with vitamin D sufficiency (31 vs. 20%; P = 0.03). In addition, in vitamin D status-discordant twin pairs, CFR was significantly lower in the vitamin D-insufficient twin than in the vitamin D-sufficient co-twin (2.35 vs. 2.58; P = 0.037).

CONCLUSION

Vitamin D insufficiency is associated with lower CFR in men. This association may help explain some of the increased cardiovascular risk reported in individuals with vitamin D insufficiency.

Cell-based vasculogenic studies in preclinical models of chronic myocardial ischaemia and hibernation

INTRODUCTION

Coronary artery disease commonly leads to myocardial ischaemia and hibernation. Relevant preclinical models of these conditions are essential to evaluate new therapeutic options such as cell-based vasculogenic therapies.

AREAS COVERED

In this article, the authors first review basic concepts of myocardial ischaemia/hibernation and relevant techniques to assess myocardial viability. Then, preclinical models of chronic myocardial ischaemia and hibernation, induced by devices such as ameroid constrictors, Delrin stenosis, hydraulic occluders, and coils/stents are described. Lastly, the authors discuss cell-based vasculogenic therapy, and summarise studies conducted in large animal models of chronic myocardial ischaemia and hibernation.

EXPERT OPINION

Approximately one-third of patients with viable myocardium do not undergo revascularisation; however, this population is at high risk for cardiac events and would surely benefit from effective cell-based therapy. Because of the modest benefits in clinical studies, preclinical models accurately representing clinical myocardial ischemia/hibernation are necessary to better understand and appropriately direct regenerative therapy research.

Does remote ischemic conditioning salvage left ventricular function after successful primary PCI?

The translation of ischemic preconditioning to a viable therapy that benefits patients has been slow. This has been largely due to the difficultly in preempting when ischemia will occur. Recent advances in the field have demonstrated that cardioprotection from brief episodes of ischemia is possible when applied immediately after reperfusion (ischemic postconditioning) or remotely in another tissue during myocardial ischemia, prior to reperfusion (remote ischemic conditioning). This has facilitated the therapeutic application to patients presenting with acute myocardial infarction. In this article, we will discuss the results of a recent study published by Munk et al., concerning the application of remote ischemic conditioning during primary percutaneous coronary intervention to salvage myocardial function following ST-elevation myocardial infarction.

Assessment of myocardial ischaemia and viability: role of positron emission tomography

In developed countries, coronary artery disease (CAD) continues to be a major cause of death and disability. Over the past two decades, positron emission tomography (PET) imaging has become more widely accessible for the management of ischemic heart disease. Positron emission tomography has also emerged as an important alternative perfusion imaging modality in the context of recent shortages of molybdenum-99/technetium-99m ((99m)Tc). The clinical application of PET in ischaemic heart disease falls into two main categories: first, it is a well-established modality for evaluation of myocardial blood flow (MBF); second, it enables assessment of myocardial metabolism and viability in patients with ischaemic left ventricular dysfunction. The combined study of MBF and metabolism by PET has led to a better understanding of the pathophysiology of ischaemic heart disease. While there are potential future applications of PET for plaque and molecular imaging, as well as some clinical use in inflammatory conditions, this article provides an overview of the physical and biological principles behind PET imaging and its main clinical applications in cardiology, namely the assessment of MBF and metabolism.

Assessment of Myocardial Scar; Comparison Between F-FDG PET, CMR and Tc-Sestamibi

Objective:

Patients with heart failure and ischaemic heart disease may obtain benefit from revascularisation if viable dysfunctional myocardium is present. Such patients have an increased operative risk, so it is important to ensure that viability is correctly identified. In this study, we have compared the utility of 3 imaging modalities to detect myocardial scar.

Design:

Prospective, descriptive study.

Setting:

Tertiary cardiac centre.

Patients:

35 patients (29 male, average age 70 years) with coronary artery disease and symptoms of heart failure (>NYHA class II).

Intervention:

Assessment of myocardial scar by ⁹⁹Tc-Sestamibi (MIBI), ¹⁸F-flurodeoxyglucose (FDG) and cardiac magnetic resonance (CMR).

Outcome Measure:

The presence or absence of scar using a 20-segment model.

Results:

More segments were identified as nonviable scar using MIBI than with FDG or CMR. FDG identified the least number of scar segments per patient (7.4 +/− 4.8 with MIBI vs. 4.9 +/− 4.2 with FDG vs. 5.8 +/− 5.0 with CMR, p = 0.0001 by ANOVA). The strongest agreement between modalities was in the anterior wall with the weakest agreement in the inferior wall. Overall, the agreement between modalities was moderate to good.

Conclusion:

There is considerable variation amongst these 3 techniques in identifying scarred myocardium in patients with coronary disease and heart failure. MIBI and CMR identify more scar than FDG. We recommend that MIBI is not used as the sole imaging modality in patients undergoing assessment of myocardial viability.

An in vivo multimodal imaging study using MRI and PET of stem cell transplantation after myocardial infarction in rats

PURPOSE

The purpose of the study is to track iron-oxide nanoparticle-labelled adult rat bone marrow-derived stem cells (IO-rBMSCs) by magnetic resonance imaging (MRI) and determine their effect in host cardiac tissue using 2-deoxy-2-[F-18]fluoro-D: -glucose-positron emission tomography (FDG-PET).

PROCEDURES

Infarcted rats were randomised to receive (1) live IO-rBMSCs by direct local injection, or (2) dead IO-rBMSCs as controls; (3) sham-operated rats received live IO-rBMSCs. The rats were then imaged from 2 days to 6 weeks post-cell implantation using both MRI at 9.4T and FDG-PET.

RESULTS

Implanted IO-rBMSCs were visible in the heart by MRI for the duration of the study. Histological analysis confirmed that the implanted IO-rBMSCs were present for up to 6 weeks post-implantation. At 1 week post-IO-rBMSC transplantation, PET studies demonstrated an increase in FDG uptake in infarcted regions implanted with live IO-rBMSC compared to controls.

CONCLUSIONS

Noninvasive multimodality imaging allowed us to visualise IO-rBMSCs and establish their affect on cardiac function in a rat model of myocardial infarction (MI).

Cumulative exercise-induced left ventricular systolic and diastolic dysfunction in hypertrophic cardiomyopathy

The phenomenon of cumulative exercise-induced left ventricular function impairment was studied in 40 patients with non-obstructive hypertrophic cardiomyopathy with resting normal left ventricular function and no increase in ejection fraction on exercise. All patients underwent two symptom-limited exercise tests one-hour apart. Cumulative myocardial dysfunction was seen in 13 patients (group I) but not in the remaining 27 patients (group II). During follow-up, group I showed more commonly than group II a deterioration in symptoms (67% vs 22%, P=0.025) and left ventricular function (50% vs 9%, P=0.019). In conclusion, cumulative exercise-induced myocardial dysfunction can occur in hypertrophic cardiomyopathy and may be associated with clinical deterioration and worse outcome.

Reversible 18-FDG-uptake defects on myocardial PET: Is this myocardial resurrection?

Because it can accurately detect preserved glucose metabolism even in the hypoperfused or stunned myocardium, 18-FDG-PET is considered as the gold standard of myocardial viability assessment. In tako-tsubo cardiomyopathy, a presumed condition of stunning, absence of glucose metabolism however is not a marker of death. This sheds a critical light on 18-FDG-PET as a gold standard for viability.

Protein kinase D2 mediates lysophosphatidic acid-induced interleukin 8 production in nontransformed human colonic epithelial cells through NF-kappaB

The signaling pathways mediating lysophosphatidic acid (LPA)-stimulated PKD(2) activation and the potential contribution of PKD(2) in regulating LPA-induced interleukin 8 (IL-8) secretion in nontransformed, human colonic epithelial NCM460 cells were examined. Treatment of serum-deprived NCM460 cells with LPA led to a rapid and striking activation of PKD(2), as measured by in vitro kinase assay and phosphorylation at the activation loop (Ser706/710) and autophosphorylation site (Ser876). PKD(2) activation induced by LPA was abrogated by preincubation with selective PKC inhibitors GF-I and Ro-31-8220 in a dose-dependent manner. These inhibitors did not have any direct inhibitory effect on PKD(2) activity. LPA induced a striking increase in IL-8 production and stimulated NF-kappaB activation, as measured by NF-kappaB-DNA binding, NF-kappaB-driven luciferase reporter activity, and IkappaBalpha phosphorylation. PKD(2) gene silencing utilizing small interfering RNAs targeting distinct PKD(2) sequences dramatically reduced LPA-stimulated NF-kappaB promoter activity and IL-8 production. PKD(2) activation is a novel early event in the biological action of LPA and mediates LPA-stimulated IL-8 secretion in NCM460 cells through a NF-kappaB-dependent pathway. Our results demonstrate, for the first time, the involvement of a member of the PKD family in the production of IL-8, a potent proinflammatory chemokine, by epithelial cells.

Qdot nanocrystal conjugates conjugated to bombesin or ANG II label the cognate G protein-coupled receptor in living cells

Quantum dots (Qdot Nanocrystal Conjugates; Quantum Dot, Hayward, CA) exhibit high fluorescence and low photobleaching compared with organic dyes, properties that should enhance their detection at low densities. In view of the properties of Qdots and the biological and pharmaceutical importance of G protein-coupled receptors (GPCRs), we attempted to use Qdots to label GPCRs in a variety of live cell types. An agonist consisting of biotinylated bombesin or ANG II was conjugated to Qdot Nanocrystal Conjugates coated with streptavidin through a biotin-streptavidin linkage (Qdot agonist). Herein we demonstrate that Qdot-bombesin conjugate can label the bombesin-preferring GPCR in living mouse Swiss 3T3 cells and in Rat-1 cells. Similarly, we used the Qdot-ANG II conjugate to label GPCR in intact rat intestinal epithelial cells (IEC)-18 and in a human pancreatic adenocarcinoma cell line of ductal origin, HPAF-II cells. We demonstrate that Qdot-ANG II is brighter and more photostable than agonist labeled with the organic dye Cy3. Our results demonstrate that Qdot technology can be adapted to monitor ligand binding to GPCRs. Combined with the narrow and symmetric emission profile of Qdot Nanocrystal Conjugates, this information suggests the potential for a new multiplex strategy to determine the effect of agonists and/or antagonists on agonist binding to several GPCRs simultaneously in living cells.

EGF receptor transactivation mediates ANG II-stimulated mitogenesis in intestinal epithelial cells through the PI3-kinase/Akt/mTOR/p70S6K1 signaling pathway

The role of epidermal growth factor receptor (EGFR) tyrosine kinase and its downstream targets in the regulation of the transition from the G0/G1 phase into DNA synthesis in response to ANG II has not been previously investigated in intestinal epithelial IEC-18 cells. ANG II induced a rapid and striking EGFR tyrosine phosphorylation, which was prevented by selective inhibitors of EGFR tyrosine kinase activity (e.g., AG-1478) or by broad-spectrum matrix metalloproteinase (MMP) inhibitor GM-6001. Pretreatment of these cells with either AG-1478 or GM-6001 reduced ANG II-stimulated DNA synthesis by approximately 50%. To elucidate the downstream targets of EGFR, we demonstrated that ANG II stimulated phosphorylation of Akt at Ser473, mTOR at Ser2448, p70S6K1 at Thr389, and S6 ribosomal protein at Ser(235/236). Pretreatment with AG-1478 inhibited Akt, p70S6K1, and S6 ribosomal protein phosphorylation. Inhibition of phosphatidylinositol (PI)3-kinase with LY-294002 or mTOR/p70S6K1 with rapamycin reduced [3H]thymidine incorporation by 50%, i.e., to levels comparable to those achieved by addition of either AG-1478 or GM-6001. Utilizing Akt small-interfering RNA targeted to Akt1 and Akt2, Akt protein knockdown dramatically inhibited p70S6K1 and S6 ribosomal protein phosphorylation. In contrast, AG-1478 or Akt gene silencing exerted no detectable inhibitory effect on ANG II-induced extracellular signal-regulated kinase 1/2 phosphorylation in IEC-18 cells. Taken together, our results demonstrate that EGFR transactivation mediates ANG II-stimulated mitogenesis through the PI3-kinase/Akt/mTOR/p70S6K1 signaling pathway in IEC-18 cells.

Bombesin and nutrients independently and additively regulate hormone release from GIP/Ins cells

Glucose-dependent insulinotropic polypeptide (GIP) regulates glucose homeostasis and high-fat diet-induced obesity and insulin resistance. Therefore, elucidating the mechanisms that regulate GIP release is important. GIP is produced by K cells, a specific subtype of small intestinal enteroendocrine (EE) cell. Bombesin-like peptides produced by enteric neurons and luminal nutrients stimulate GIP release in vivo. We previously showed that PMA, bombesin, meat hydrolysate, glyceraldehyde, and methylpyruvate increase hormone release from a GIP-producing EE cell line (GIP/Ins cells). Here we demonstrate that bombesin and nutrients additively stimulate hormone release from GIP/Ins cells. In various cell systems, bombesin and PMA regulate cell physiology by activating PKD signaling in a PKC-dependent fashion, whereas nutrients regulate cell physiology by inhibiting AMPK signaling. Western blot analyses of GIP/Ins cells using antibodies specific for activated and/or phosphorylated forms of PKD and AMPK and one substrate for each kinase revealed that bombesin and PMA, but not nutrients, activated PKC, but not PKD. Conversely, nutrients, but not bombesin or PMA, inhibited AMPK activity. Pharmacological studies showed that PKC inhibition blocked bombesin- and PMA-stimulated hormone release, but AMPK activation failed to suppress nutrient-stimulated hormone secretion. Forced expression of constitutively active vs. dominant negative PKDs or AMPKs failed to perturb bombesin- or nutrient-stimulated hormone release. Thus, in GIP/Ins cells, PKC regulates bombesin-stimulated hormone release, whereas nutrients may control hormone release by regulating the activity of AMPK-related kinases, rather than AMPK itself. These results strongly suggest that K cells in vivo independently respond to neuronal vs. nutritional stimuli via two distinct signaling pathways.

Relation between wall thickening on gated perfusion SPECT and functional recovery after coronary revascularization in patients with previous myocardial infarction

PURPOSE

This study aimed to evaluate whether wall thickening analysis by gated perfusion single-photon emission computed tomography (SPECT) is useful in predicting functional recovery after revascularization.

METHODS

Forty-one patients with previous myocardial infarction and left ventricular (LV) dysfunction (ejection fraction, EF, 36+/-6%) who were scheduled for revascularization underwent rest 99mTc-sestamibi gated SPECT.

RESULTS

Of 131 akinetic or dyskinetic segments at baseline echocardiography, 82 (63%) recovered after revascularization. Compared with wall thickening analysis, perfusion imaging provided higher sensitivity (78% vs 50%, P<0.0001) and specificity (80% vs 71%, P<0.0005). Among segments with > or =55% sestamibi uptake (viable), those with detectable wall thickening had a higher likelihood of functional recovery than those with absent wall thickening (95% vs 77%, P<0.05). In segments with improved function, the absence of wall thickening was associated with lower sestamibi activity than was observed when detectable wall thickening was present (58+/-14% vs 71+/-13%, P<0.0005). An increase in EF of > or =5% was detectable in 22 (54%) patients. For the prediction of EF improvement, perfusion imaging provided a higher sensitivity than wall thickening analysis (68% vs 41%, P<0.05), while specificity was not significantly different (68% vs 74%). The prevalence of patients with functional recovery did not change when wall thickening analysis was considered in addition to perfusion status (73% in patients with detectable wall thickening and 70% in those without; P=NS).

CONCLUSION

In patients with coronary artery disease, wall thickening analysis by gated perfusion SPECT provides additional information compared with perfusion data for the prediction of segmental functional recovery. However, on a patient basis, wall thickening assessment seems to be of more limited value than perfusion status.

Ultrastructural evidence of increased tolerance of hibernating myocardium to cardioplegic ischemia-reperfusion injury

OBJECTIVES

The goal of this study was to investigate the effects of ischemia-reperfusion on myocardial ultrastructure in patients with and without hibernating myocardium.

BACKGROUND

It is generally accepted that chronically dysfunctional, hibernating myocardium may remain nonetheless viable for a long time. It has been postulated that hibernating myocytes may survive, despite being subtended by a severe coronary artery stenosis, as they might be less susceptible to ischemic insults. However, whether hibernating myocardium is indeed more resistant to ischemia has never been investigated.

METHODS

Myocardial biopsies were taken before cardiac arrest and after reperfusion from the anterior wall of the left ventricle in patients undergoing coronary artery bypass surgery, divided according to presence (n = 7) or absence (n = 7) of hibernating myocardium. Ultrastructural changes were studied by electron microscopy. Because ischemia-reperfusion injury is related to oxidative stress, we also evaluated coronary sinus concentration of the antioxidants alpha-tocopherol, beta-carotene, and ubiquinol, and of lipid peroxidation products pre-ischemia and after reperfusion.

RESULTS

Both groups were similar with respect to length of ischemia and changes in the various indexes of oxidative stress. In normally contracting myocardium, ischemia/reperfusion induced moderate overall ultrastructural changes, and marked alterations at the mitochondrial level. In contrast, post-reperfusion biopsies of hibernating myocardium displayed only minor overall ultrastructural changes, and scored significantly better on mitochondrial damage.

CONCLUSIONS

Despite similar severity of ischemia/reperfusion, hibernating myocardium showed significantly less ultrastructural evidence of cell injury compared with normally contracting myocardium. These data indicate that human hibernating myocardium is intrinsically more resistant to ischemia/reperfusion injury.

Contrast-enhanced MR Delineation of Stunned Myocardium with Administration of MnCl2in Rats

PURPOSE

To determine whether stunned myocardium can be delineated at magnetic resonance (MR) imaging with differential cellular uptake of manganese ions.

MATERIALS AND METHODS

Twenty-one adult Sprague-Dawley rats underwent either (a) a sequence of three episodes of 10 minutes of coronary artery occlusion and 12 minutes of reflow (group 1, n = 9); (b) a single episode of 10 minutes of occlusion followed by reflow (group 2, n = 6), designed to produce different degrees of myocardial stunning; or (c) a single episode of 2 minutes of occlusion followed by reperfusion (group 3, n = 6), designed to produce no stunning. Ventricular wall thickening was measured on spin-echo (SE) MR images. MnCl2 (0.025 mmol/kg) was intravenously infused for 10 minutes. Highly T1-sensitive inversion-recovery (IR) SE images were obtained to detect subtle regional differences in manganese accumulation. Hearts were stained at sacrifice to define area at risk and to test for myocardial infarction. Significance of differences in mean values was evaluated with repeated-measures analysis of variance.

RESULTS

All hearts were free of infarction, as detected with triphenyltetrazolium chloride staining. On IR SE images, the hearts from rats in groups 1 and 2 exhibited clearly delineated regions of diminished manganese uptake in the expected territory of the occluded artery. The circumferential extent of the manganese-defined defect (45.5% +/- 5.6) was similar to that of the area at risk (46.8% +/- 7.5). Systolic wall thickening in the defect was significantly (P <.01) less than in the nonischemic myocardium (2.7% +/- 3.3 vs 31.2% +/- 7.5 and 10.0% +/- 4.8 vs 28.6% +/- 6.5, respectively, for groups 1 and 2). The hearts from rats in group 3 demonstrated no wall thickening deficit or abnormal zone on manganese-enhanced images.

CONCLUSION

Stunned myocardium was delineated with MnCl2-enhanced MR imaging as a hypoenhanced zone. This finding suggests that Ca2+ channel activity is diminished in stunned myocardium.

Dobutamine responsiveness, PET mismatch, and lack of necrosis in low-flow ischemia: is this hibernation in the isolated rat heart?

The clinical hallmarks of hibernating myocardium include hypocontractility while retaining an inotropic reserve (using dobutamine echocardiography), having normal or increased [18F]fluoro-2-deoxyglucose-6-phosphate (18FDG6P) accumulation associated with decreased coronary flow [flow-metabolism mismatch by positron emission tomography (PET)], and recovering completely postrevascularization. In this study, we investigated an isolated rat heart model of hibernation using experimental equivalents of these clinical techniques. Rat hearts (n = 5 hearts/group) were perfused with Krebs-Henseleit buffer for 40 min at 100% flow and 3 h at 10% flow and reperfused at 100% flow for 30 min (paced at 300 beats/min throughout). Left ventricular developed pressure fell to 30 +/- 8% during 10% flow and recovered to 90 +/- 7% after reperfusion. In an additional group, this recovery of function was found to be preserved over 2 h of reperfusion. Electron microscopic examination of hearts fixed at the end of the hibernation period demonstrated a lack of ischemic injury and an accumulation of glycogen granules, a phenomenon observed clinically. In a further group, hearts were challenged with dobutamine during the low-flow period. Hearts demonstrated an inotropic reserve at the expense of increased lactate leakage, with no appreciable creatine kinase release. PET studies used the same basic protocol in both dual- and globally perfused hearts (with 250MBq 18FDG in Krebs buffer +/- 0.4 mmol/l oleate). PET data showed flow-metabolism "mismatch;" whether regional or global, 18FDG6P accumulation in ischemic tissue was the same as (glucose only) or significantly higher than (glucose + oleate) control tissue (0.023 +/- 0.002 vs. 0.011 +/- 0.002 normalized counts. s-1x g-1x min-1, P < 0.05) despite receiving 10% of the flow. This isolated rat heart model of acute hibernation exhibits many of the same characteristics demonstrated clinically in hibernating myocardium.

ANG II stimulates PKC-dependent ERK activation, DNA synthesis, and cell division in intestinal epithelial cells

PKC, a major target for the tumor-promoting phorbol esters, has been implicated in the signal transduction pathways that mediate important functions in intestinal epithelial cells, including proliferation and carcinogenesis. With the use of IEC-18 cells arrested in G0/G1, addition of phorbol esters resulted in a modest increase in [3H]thymidine incorporation and a slight shift toward the S and G2/M phases of the cell cycle, whereas the combination of EGF and phorbol 12,13-dibutyrate (PDB) synergistically stimulated DNA synthesis. To investigate the effects of receptor-mediated PKC activation on mitogenesis, we demonstrated that ANG II induced ERK activation, a response completely blocked by pretreatment with mitogen/extracellular signal-regulated kinase inhibitors or specific PKC inhibitors. Furthermore, ANG II stimulated an over threefold increase in [3H]thymidine incorporation that was corroborated by flow cytometric analysis of the cell cycle to levels comparable to that achieved by the combination of EGF and PDB. Taken together, our results indicate that receptor-mediated PKC activation, as induced by ANG II, transduces mitogenic signals leading to DNA synthesis and cell proliferation in IEC-18 cells.

Evidence that stunning can be cumulative in man

Myocardial stunning and hibernation are two entities that have become increasingly recognised as clinically important causes of reversible left ventricular (LV) dysfunction. Their occurrence is important as resting myocardial dysfunction, which was once thought to be irreversible, may recover if ischaemia is lessened or abolished. Recent evidence has suggested that cumulative stunning can occur in man and may in fact be responsible for the phenomenon of hibernation. In this chapter we will review the evidence supporting the occurrence of cumulative stunning in man.