Michael Shea

Transient ST-segment depression as a marker of myocardial ischemia during daily life

Patients with angina and coronary artery disease (CAD) have many episodes of transient ST-segment depression during ordinary daily life, and these are often asymptomatic. To investigate this signal as a marker of myocardial ischemia, 30 patients with chronic stable angina and CAD underwent positron tomography, recording the regional myocardial uptake of rubidium-82, pain and ST-segment changes before, during and after 59 technically satisfactory exercise tests, 35 cold pressor tests and 22 episodes of unprovoked ST depression. Exercise resulted in 53 episodes of ST depression with angina and in 5 episodes without pain. After cold pressor tests, there were 3 episodes of ST depression and pain and 12 of painless ST depression. Only 9 episodes of unprovoked ST depression were accompanied by pain. Tomography showed independent evidence of ischemia in 63 (97%) of the total 65 episodes of ST depression with angina and in all 30 episodes of painless ST depression. In each patient perfusion defects occurred in the same myocardial segment during painful and painless ST depression and responses were significantly different from those in 16 normal subjects studied in the same way. These findings support the use of transient ST depression in continuous monitoring to assess the activity of CAD, but only in patients with typical angina pectoris, ST depression during exercise and proved CAD. They strengthen the evidence derived from ambulatory monitoring for a wider picture of the disease than is generally appreciated, with more frequent episodes of silent myocardial ischemia than of angina pectoris.

SILENT MYOCARDIAL ISCHAEMIA DUE TO MENTAL STRESS

Patients with angina and coronary disease have many episodes of symptomless transient myocardial ischaemia, most of which cannot be explained by physical exertion. 16 patients with typical stable angina pectoris were examined to test the hypothesis that these episodes can be triggered off by ordinary daily events, such as changes in mental activity. Regional myocardial perfusion and ischaemia were assessed by measurement of the uptake of rubidium-82 with positron tomography after mental arithmetic and physical exercise. With mental arithmetic, 12 (75%) patients had abnormalities of regional perfusion, accompanied in only 6 by ST-segment depression and in 4 of these 6 by angina, leaving 6 patients with perfusion abnormalities but neither pain nor electrocardiographic changes. After exercise, all the patients showed abnormal regional myocardial perfusion in the segments that became ischaemic with mental arithmetic. This was accompanied by ST depression in all and angina in 15. The association between mental activity and myocardial ischaemia may operate frequently during everyday life and may explain many of the transient and symptomless electrocardiographic changes in patients with coronary disease.

Increased uptake of 18F-fluorodeoxyglucose in postischemic myocardium of patients with exercise-induced angina.

Regional myocardial perfusion and exogenous glucose uptake were assessed with rubidium-82 (82Rb) and 18F-2-fluoro-2-deoxyglucose (FDG) in 10 normal volunteers and 12 patients with coronary artery disease and stable angina pectoris by means of positron emission tomography. In patients at rest, the myocardial uptake of 82Rb and FDG did not differ significantly from that measured in normal subjects. The exercise test performed within the positron camera in eight patients produced typical chest pain and ischemic electrocardiographic changes in all. In each of the eight patients a region of reduced cation uptake was demonstrated in the 82Rb scan recorded at peak exercise, after which uptake of 82Rb returned to the control value 5 to 14 min after the end of the exercise. In these patients, FDG was injected in the recovery phase when all the variables that were altered during exercise, including regional myocardial 82Rb uptake, had returned to control values. In all but one patient, FDG accumulation in the regions of reduced 82Rb uptake during exercise was significantly higher than that in the nonischemic regions, i.e., the ones with a normal increment of 82Rb uptake on exercise. In the nonischemic areas, FDG uptake was not significantly different from that found in normal subjects after exercise. In conclusion, myocardial glucose transport and phosphorylation seem to be enhanced in the postischemic myocardium of patients with exercise-induced ischemia.

Direct effects of smoking on the heart: silent ischemic disturbances of coronary flow

Cigarette smoking is strongly associated with ischemic heart disease and acute coronary events. The effect of smoking a single cigarette on regional myocardial perfusion was studied in 13 chronic smokers with typical stable angina pectoris using positron emission tomography and rubidium-82 (82Rb). Findings were compared with the effects of physical exercise. After exercise, 8 patients (61%) had angina, ST depression and abnormal regional myocardial perfusion. Uptake of 82Rb increased from 49 +/- 8 to 60 +/- 7 in remote myocardium, but decreased from 46 +/- 3 to 37 +/- 5 in an ischemic area. The remaining 5 patients (39%) had homogeneous increases in 82Rb uptake without angina or ST depression. After smoking, 6 of the 8 patients with positive exercise test responses had a decrease in 82Rb uptake, from 47 +/- 3 to 35 +/- 6 in the same segment of myocardium affected during exercise. However, in contrast to exercise, the events during smoking were largely silent. The absolute decreases in regional 82Rb uptake after smoking occurred at significantly lower levels of myocardial oxygen demand than after exercise. This suggests that an impairment of coronary blood supply is responsible. Thus, in smokers with coronary artery disease, each cigarette can cause profound silent disturbances of regional myocardial perfusion that are likely to occur frequently during daily life. Such repeated insults may represent an important mechanism linking smoking with coronary events.

Character of transient ischemia in angina pectoris

There is growing interest in the possible therapeutic and prognostic significance of silent myocardial ischemia in coronary artery disease (CAD) and its detection by ambulatory electrocardiographic (ECG) monitoring. In 100 apparently healthy normal subjects (20 with angiographically normal coronary arteries), Holter monitoring revealed significant ST-segment depression in only 2 (both over 40 years, one with positive treadmill test, the other with risk factor for CAD). No significant ECG changes were found in those with normal coronary vessels. In 30 patients with documented CAD, significant ST-segment depression during 1,934 episodes over 446 days of monitoring over 18 months was found. Only 24% of the episodes were associated with angina. Asymptomatic and symptomatic episodes were associated with comparable changes in perfusion detected by positron emission tomography. Heart rate increases greater than 10 beats/min preceding the onset of the ST-segment changes occurred in only 23% of the episodes. There was considerable variability in the ST-segment changes in the same patient monitored serially over long periods of time. The data indicate that it is extremely uncommon for patients without CAD to exhibit silent myocardial ischemia, whereas patients with stable angina exhibit frequent, variable and often asymptomatic ECG evidence of myocardial ischemia rarely triggered by increases in heart rate. These findings are likely to be of therapeutic and prognostic significance.

A family of aspartic proteases and a novel, dynamic and cell-cycle-dependent protease localization in the secretory pathway of toxoplasma gondii

Aspartic proteases are important virulence factors in pathogens like HIV, Candida albicans or Plasmodium falciparum. We report here the identification of seven putative aspartic proteases, TgASP1 to TgASP7, in the apicomplexan parasite Toxoplasma gondii. Bioinformatic and phylogenetic analysis of the TgASPs and other aspartic proteases from related Apicomplexa suggests the existence of five distinct groups of aspartic proteases with different evolutionary lineages. The members of each group share predicted biological features that validate the phylogeny. TgASP1 is expressed in tachyzoites, the rapidly dividing asexual stage of T. gondii. We present the proteolytic maturation and subcellular localization of this protease through the cell cycle. TgASP1 shows a novel punctate localization associated with the secretory system in non‐dividing cells, and relocalizes dramatically and unambiguously to the nascent inner membrane complex of daughter cells at replication, before coalescing again at the end of division.

Proteolytic activation of the essential parasitophorous vacuole cysteine protease SERA6 accompanies malaria parasite egress from its host erythrocyte.

Background: Malaria parasite egress from host erythrocytes requires cysteine protease activity, but the identity of key parasite proteases is unknown. Results: SERA6 is an essential parasite cysteine protease that resides in the parasitophorous vacuole and is activated by SUB1, a parasite serine protease. Conclusion: SERA6 may play a role in egress. Significance: SERA6 is a potential new antimalarial drug target. The malaria parasite replicates within an intraerythrocytic parasitophorous vacuole (PV). The PV and host cell membranes eventually rupture, releasing merozoites in a process called egress. Certain inhibitors of serine and cysteine proteases block egress, indicating a crucial role for proteases. The Plasmodium falciparum genome encodes nine serine-repeat antigens (SERAs), each of which contains a central domain homologous to the papain-like (clan CA, family C1) protease family. SERA5 and SERA6 are indispensable in blood-stage parasites, but the function of neither is known. Here we show that SERA6 localizes to the PV where it is precisely cleaved just prior to egress by an essential serine protease called PfSUB1. Mutations that replace the predicted catalytic Cys of SERA6, or that block SERA6 processing by PfSUB1, could not be stably introduced into the parasite genomic sera6 locus, indicating that SERA6 is an essential enzyme and that processing is important for its function. We demonstrate that cleavage of SERA6 by PfSUB1 converts it to an active cysteine protease. Our observations reveal a proteolytic activation step in the malarial PV that may be required for release of the parasite from its host erythrocyte.

Transient ischemia in angina pectoris: Frequent silent events with everyday activities

To help characterize episodes of transient myocardial ischemia, 80 patients with chronic stable angina and evidence of obstructive coronary disease were studied by ambulatory electrocardiographic (ECG) monitoring outside the hospital to detect both symptomatic and asymptomatic episodes of ST-segment depression. In addition, patients were tested on an outpatient basis by means of positron emission tomography to assess regional coronary blood flow under different conditions. All patients showed ECG evidence of transient ischemia, with or without symptoms, while active outside the hospital. In-hospital testing showed that symptomatic and asymptomatic disturbances in regional coronary blood flow occurred with normal everyday activities and were not caused by physical exertion involving marked increases in heart rate and blood pressure. Most of these provocations were followed by a decrease in coronary blood flow in a poststenotic segment of myocardium and, like the ischemic events monitored out of hospital, the majority were silent. Many of these features characterizing the activity of ischemic heart disease may not be apparent from a patients anginal history or results of hospital diagnostic testing.

Preparation of human serum [Methyl-11C] methylalbumin microspheres and human serum [Methyl-11C] methylalbumin for clinical use

Safely-injectable suspensions of human serum [methyl-11 C] methylalbumin microspheres have been prepared via the reaction of human serum albumin microspheres with [11C]methyl iodide, itself prepared in a novel one-pot synthesis from cyclotron-produced [11C]carbon dioxide. The preparation takes only 30 min from the end of radionuclide production and proceeds in 22% radiochemical yield based on the activity of [11C]carbon dioxide used and decay-corrected. It has been shown that such microspheres are highly stable in vivo and may be used as reference blood flow markers in positron emission tomography (PET). Similarly, safely-injectable and radiochemically pure solutions of human serum [methyl-11C] methylalbumin have been prepared in 31% radiochemical yield and in 40 min from the end of [11C]carbon dioxide production via the reaction of [11C]methyl iodide with human serum albumin. This radiopharmaceutical is intended for studies of lung permeability and blood-brain barrier permeability by PET.

Validation of quantitation of regional myocardial blood flow in vivo with 11C-labeled human albumin microspheres and positron emission tomography.

Use of radiolabeled microspheres is a standard method to measure regional myocardial perfusion in animals. Human albumin microspheres have been given safely to patients, but positron-emitting 68Ga-labeled human albumin microspheres are characterized by an unstable radiolabel. A new labeling procedure that covalently binds 11C (t1/2 = 20.3 min) to human albumin microspheres via 11CH3I was developed. Seven open-chest and two closed-chest dogs were studied. Reference and 11C-labeled human albumin microspheres (2 to 25 mCi) were both injected into the left atrium. Positron tomographic images were obtained of the myocardial distribution of the 11C-labeled microspheres. Timed arterial withdrawal was used for both reference gamma-labeled microspheres and 11C-labeled human albumin microspheres. Myocardial tissue samples matched to tomographic slices were well-counted for calculation of reference values of regional myocardial perfusion. Serial venous blood samples for residual 11C activity of 30 and 60 min after injection were less than 1% of the myocardial 11C concentration demonstrating a stable 11C bond to the human albumin microspheres. Regional myocardial perfusion calculated by this technique correlated well with values obtained with reference microspheres (r = .97) over a range of 0.2 to 3.5 ml/min/g. Correction for wall thickness improved the slope of the regression equation from y = 0.71 x -0.03 to y = 0.88 X -0.05. Thus, 11C human albumin microspheres are stable radiochemically and can be used as a quantitative measure of regional myocardial perfusion.