L. Maximilian Buja

Sites and mechanisms of localization of technetium 99m phosphorus radiopharmaceuticals in acute myocardial infarcts and other tissues

This study was performed to elucidate the localization at the cellular level of technetium-99m phosphorus ((99m)Tc-P) radiopharmaceuticals in acute myocardial infarcts and the mechanisms responsible for (99m)Tc-P uptake in acute myocardial infarcts and other tissues. In 20 dogs with proximal left anterior descending coronary arterial ligation for 1-3 days, elevated calcium levels were measured at all sites of increased (99m)Tc-P uptake (acute myocardial infarcts, necrotic thoracotomy muscle, lactating breast, and normal bone); however, a consistent linear relationship between (99m)Tc-P and calcium levels was not observed. A strong correlation (r = 0.95 and 0.99, n = 2 dogs) was demonstrated between levels of (3)H-diphosphonate and (99m)Tc-P in infarcted myocardium. Autoradiographic studies with (3)H-diphosphonate revealed extensive labeling in the infarct periphery which contained necrotic muscle cells with features of severe calcium overloading, including widespread hypercontraction as well as more selective formation of mitochondrial calcific deposits. Autoradiography also demonstrated labeling of a small population of damaged border zone muscle cells which exhibited prominent accumulation of lipid droplets and focal, early mitochondrial calcification. Cell fractionation studies revealed major localization of both (99m)Tc-P and calcium in the soluble supernate and membrane-debris fractions of infarcted myocardium and less than 2% of total (99m)Tc-P and calcium in the mitochondrial fractions; however, electron microscopic examination showed that mitochondria with calcific deposits were not preserved in the mitochondrial fractions. In vitro studies evaluating the role of serum protein binding on tissue uptake of (99m)Tc-P agents demonstrated that, in spite of significant complexing with serum proteins, serum (99m)Tc-P activity retained the ability to adsorp to calcium hydroxyapatite and amorphous calcium phosphate. In vivo studies showed that concentration of human serum albumin (labeled with iodine-131) in infarcted myocardium reached a maximum of only 3.8 times normal after a circulation time of 96 h, whereas (99m)Tc-P uptake was at least 10 times normal after a circulation time as short as 1 h. It is concluded that: (a) (99m)Tc-P uptake in acutely infarcted myocardium, and possibly other types of soft tissue damage, is limited to necrotic and severely injured cells; (b) concentration of (99m)Tc-P results from selective adsorption of (99m)Tc-P with various forms of tissue calcium stores, including amorphous calcium phosphate, crystalline hydroxyapatite, and calcium complexed with myofibrils and other macromolecules, possibly supplemented by calcium-independent complexing with organic macromolecules; and (c) lack of a linear relationship between (99m)Tc-P and tissue calcium levels mainly results from local differences in composition and physicochemical properties of tissue calcium stores and from local variations in levels of blood flow for delivery of (99m)Tc-P agents.

Conversion from chronic to acute coronary artery disease: Speculation regarding mechanisms

The factors that are primarily responsible for the conversion from chronic to acute coronary artery disease (CAD), including the development of unstable angina pectoris and acute myocardial infarction, remain uncertain. Some patients with angiographic evidence of extensive and severe coronary arterial stenoses anatomically do well with appropriate medical therapy, whereas others with similar or even anatomically less impressive coronary arterial stenoses have clinical manifestations of acute CAD. Improved insight into mechanisms responsible for the conversion of chronic CAD to acute CAD is essential for efforts directed at preventing acute myocardial infarction and sudden cardiac death. The purpose of this editorial is to emphasize recent developments in knowledge relating to alterations in prostaglandin production that may play a role in the development or perpetuation of acute CAD.

Gadolinium-DTPA-enhanced nuclear magnetic resonance imaging of reperfused myocardium: Identification of the myocardial bed at risk

Gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)-enhanced nuclear magnetic resonance (NMR) imaging can be useful in the identification of reperfused myocardium. However, the effects of dose and the time of administration and the relation of the extent of the region of enhancement to the myocardial bed at risk have not been evaluated. In this study, dogs were given Gd-DTPA (0.1 mM/kg body weight [n = 21] or 0.34 mM/kg [n = 7]) or saline solution (n = 5) after various periods of occlusion and reperfusion. Twenty-five dogs were killed after 1 or 2 h of reperfusion and the excised hearts were imaged. Images were analyzed for presence, intensity and extent of a region of increased signal. All images in dogs given Gd-DTPA had easily identifiable regions of increased signal in the distribution of the reperfused myocardial bed. Analysis of the extent of these regions in spin-echo images of excised hearts when Gd-DTPA was administered after 5 min of reperfusion demonstrated a correlation coefficient of 0.72 with the bed at risk as determined postmortem with a dye perfusion technique. These images consistently overestimated the infarct size. Signal intensity of the reperfused myocardium increased to a maximum of 1.67 times control (p less than 0.05) in spin-lattice relaxation time (T1)-weighted sequences as the dose of Gd-DTPA increased. This was due to a higher concentration of Gd-DTPA in the reperfused myocardium with resultant shortening of the T1 relaxation time. When Gd-DTPA was given after 90 min of reperfusion, NMR images did not identify the bed at risk.(ABSTRACT TRUNCATED AT 250 WORDS)

Free radicals alter ionic calcium levels and membrane phospholipids in cultured rat ventricular myocytes

Oxygen-derived free radicals have been implicated in damage to membrane phospholipids leading to alterations in membrane function. The purpose of this study was to investigate alterations in intracellular ionic calcium (Ca2+) levels and Ca2+ transients, cellular morphology, conjugated diene levels, arachidonate release, and lactate dehydrogenase release resulting from the exposure of cultured neonatal rat ventricular myocytes to a xanthine oxidase catalyzed free radical generating system capable of producing superoxide and hydroxyl radicals. The ability of alpha-tocopherol to prevent alterations due to free radical exposure was investigated. For measurements of Ca2+, myocytes grown on coverslips for 3-4 days were loaded with fura-2/AM and studied by microspectrofluorometry. Control myocytes superfused with a physiological buffer or buffer containing purine and iron-loaded transferrin exhibited Ca2+ transients associated with spontaneous contractions. For control, buffer perfused myocytes (n = 4), the fura-2 340/380 ratios were 0.5 +/- 0.1 (mean +/- S.E.) and 1.6 +/- 0.03 at the minimum and maximum, respectively, of the Ca2+ transient, after 1 h of perfusion. Exposure to the free radical generating solution (n = 14) altered intracellular Ca2+. The 340/380 minimum ratio was 639% of the control value after approximately 30-70 mins with cessation of normal Ca2+ transients. Bleb development was associated with increased Ca2+. Myocytes reperfused with control medium continued to exhibit an elevated minimum fura-2 ratio at 687% of control. Myocytes pretreated with 10 microM alpha-tocopherol (n = 13) for 18-24 h and exposed to free radicals did not exhibit increases in intracellular Ca2+, having a minimum 340/380 ratio of 0.5 +/- 0.1 after 60-90 mins, and although myocytes often ceased contracting, they resumed spontaneous Ca2+ transients with control medium reperfusion and also maintained normal structure. Exposure of myocyte cultures to free radical generating solutions resulted in increased levels of conjugated dienes and increased release of [3H]arachidonate and lactate dehydrogenase compared to control values after 1 h. alpha-Tocopherol treatment attenuated the increase in conjugated diene levels, and the release of [3H]arachidonate and lactate dehydrogenase. Thus, free radicals alter intracellular Ca2+, conjugated dienes and membrane structure indicating their ability to induce altered ionic homeostasis in association with myocardial membrane damage. alpha-Tocopherol decreased free radical mediated injury.

Detection and localization of recent myocardial infarction by magnetic resonance imaging

The potential of magnetic resonance imaging (MRI) to detect and localize acute myocardial infarction (AMI) in 27 patients a mean interval of 15 days after AMI was evaluated. Eighteen asymptomatic volunteers were also studied to determine the specificity of the observations. The diagnosis of AMI was established by conventional criteria; the infarct was localized by electrocardiography in all patients, technetium pyrophosphate scintigraphy in 19 and necropsy in 1 patient. MRI detected increased myocardial signal intensity in 88%, cavitary signal in 74% and regional wall thinning in 67% of the patients. At least 1 of these 3 features was seen in the area of the infarct in each patient. The sensitivity of these MRI observations was not influenced by location of the infarct or presence of Q waves. Asymptomatic volunteers also had increased myocardial signal in 83%, cavitary signal in 94% and wall thinning in 11% of cases. Some patients had these findings in myocardial segments not suspected of being involved by recent or remote AMI. It is concluded that AMI can be detected by MRI performed an average of 15 days after infarction. However, the hearts of normal volunteers and apparently normal myocardial segments of patients with AMI may have the MRI findings previously associated with AMI. Of these findings, wall thinning was the most predictive of and specific for AMI.

Assessment of myocardial systolic wall thickening using nuclear magnetic resonance imaging

A quantitative nuclear magnetic resonance (NMR) imaging method of evaluating regional left ventricular function was compared with histochemical evidence of infarction in dogs and functional measurements in patients. Short-axis images of the heart were obtained at end-diastole and at 100 ms intervals thereafter. Regional diastolic left ventricular wall thickness and maximal percent systolic wall thickening were measured at the level of the papillary muscles in each of six segments. In six normal dogs, the mean end-diastolic wall thickness was 9 +/- 1.6 mm, and the mean maximal percent thickening was 61 +/- 11%. In eight dogs with a 4 day old infarct, maximal percent thickening was 5 +/- 8% (p less than 0.001) in the infarcted segments. In 10 normal human volunteers, the mean end-diastolic wall thickness was 10.1 +/- 1 mm, and the mean maximal percent systolic wall thickening was 60 +/- 18%. Reduced maximal percent systolic wall thickening was defined as a value greater than or equal to 2 SD below the mean value obtained in normal volunteers. Seven patients with regional wall motion abnormalities were independently assessed by NMR imaging and biplane ventriculography. With a sensitivity of 94% and a specificity of 80%, NMR imaging demonstrated reduced maximal percent systolic wall thickening in the same segments identified as akinetic or dyskinetic by biplane ventriculography. Thus, abnormalities of regional systolic wall thickening are accurately identified with this quantitative imaging technique.

Unstable angina pectoris: a randomized study of patients treated medically and surgically.

Fifty patients with the clinical syndrome of unstable angina pectoris were evaluated. Twenty-seven were randomized into medical or surgical treatment groups and subsequently followed up. The results of the study reveal that: (1) there is approximately a 16 percent incidence rate of significant left main coronary artery disease in patients with this entity at our institution; (2) 10 percent of patients do not have angiographically significant coronary artery disease; (3) pain relief is better in the surgically treated patients, but the 1 1/2 year survival rate is not significantly different between the groups; (4) 50 percent of the medically treated patients again had the syndrome of unstable angina pectoris in the initial few months of the follow-up period; (5) the operative and late postoperative mortality rate in patients presenting with unstable angina pectoris and left main coronary artery disease in this small group of patients was 43 percent; and (6) four of six patients with this syndrome whose condition was deemed inoperable and who were not randomized died within the subsequent few months.

Failure of a slow channel calcium antagonist, verapamil, to retard atherosclerosis in the watanabe heritable hyperlipidemic rabbit: An animal model of familial hypercholesterolemia

Verapamil and other slow channel calcium antagonists have been reported to retard atherosclerosis in rabbits fed a high cholesterol diet. Because atherosclerosis in such a model may differ significantly from human atherosclerosis, experiments were conducted to prevent atherosclerosis with verapamil in the Watanabe heritable hyperlipidemic (WHHL) rabbit, which is a genetic, metabolic and pathologic model of homozygous familial hypercholesterolemia. At 2 months of age, 23 WHHL rabbits were divided into two groups since earlier studies showed no macroscopic atherosclerosis at 2 months. Group A (n = 11) was fed standard rabbit chow for 6 months. Group B (n = 12) received oral verapamil (46 mg/kg per day) absorbed in the identical chow as fed to Group A and subcutaneous verapamil (0.25 mg/kg twice daily 6 days a week). In Group B, mean serum verapamil concentrations (+/- SEM) averaged 16.9 +/- 1.9 ng/ml at 3 hours after subcutaneous injection. Sex ratios and serum cholesterol concentrations were the same in both groups. The percent of aortic surface area with visible plaque in Group A versus B was 49 +/- 7 versus 43 +/- 7%, respectively, of the entire aorta, and 61 +/- 5 versus 65 +/- 5%, respectively, of the proximal 3 cm of aorta (p = NS). Thus, verapamil did not suppress atherosclerosis in WHHL rabbits at serum drug levels greater than those reported to be effective in other models.

Iodine-123 phenylpentadecanoic acid: detection of acute myocardial infarction and injury in dogs using an iodinated fatty acid and single-photon emission tomography

The ability of an iodinated fatty acid, iodine-123 phenylpentadecanoic acid (1-123 PPA), and single-photon emission computed tomography (SPECT) to detect myocardium injured by temporary or permanent coronary arterial occlusion was evaluated. In 5 control dogs, 11 dogs that underwent 90 to 120 minutes of fixed left anterior descending coronary artery (LAD) occlusion, and 8 dogs that underwent 90 minutes of temporary LAD occlusion and up to 90 minutes of reflow, 2 to 6 mCi of I-123 PPA were injected and the dogs were imaged with SPECT. Control dogs showed relatively uniform uptake and clearance of I-123 PPA in similar left ventricular (LV) regions. Dogs with permanent LAD occlusion were identified by computer algorithm as having regions of decreased I-123 PPA uptake in the infarct-related area and a reduced rate of I-123 PPA clearance (-9.4% in infarct sectors [washin], +3.7% in sectors adjacent to the area of infarction, and +15.4% in control LV sectors [p less than 0.01]). Dogs with temporary LAD occlusion and reperfusion had decreased clearance of I-123 PPA from the regions with infarction; I-123 PPA clearance was -5.2 +/- 16.4% in infarct sectors, 12.7 +/- 7.4% in periinfarct zones, and 30.4 +/- 12% in control LV regions. These data demonstrate that tomographic analysis of I-123 PPA uptake and clearance permits the relatively noninvasive detection of LV myocardium injured by permanent or temporary LAD occlusion and reperfusion.

Differential enhancement of postischemic segmental systolic thickening by diltiazem

Prolonged depression of segmental systolic thickening after brief coronary artery occlusion may result principally from events during reperfusion rather than during the ischemic interval. Thus, cellular calcium overload at reperfusion may be a mediator of contractile dysfunction after brief ischemia, and reduction of calcium entry by diltiazem, a calcium channel antagonist, may enhance recovery of systolic thickening after brief periods of ischemia. Thirteen awake unsedated dogs instrumented with hemodynamic catheters, left anterior descending coronary artery occluders and five to six pairs of intramyocardial sonomicrometers underwent two 15 min coronary artery occlusions with 24 h reperfusion. The order of infusion of diltiazem (15 micrograms/kg per min) or saline solution was alternated. Systolic thickening, hemodynamic variables and regional myocardial blood flow were measured serially over 24 h. Despite equally severe ischemic dysfunction during coronary occlusion, diltiazem-treated segments with systolic thinning during ischemia recovered control segmental thickening significantly earlier than saline solution-treated segments (at 30 versus 180 min of reperfusion). Blood pressure was mildly decreased during diltiazem treatment; therefore, a second group of 10 dogs underwent a similar occlusion and reflow period during infusion of nitroprusside to lower mean arterial pressure equivalently. Decreases in blood pressure in this group resulted in some improvement in segmental systolic function; however, this did not reach statistical significance at any time. Regional myocardial blood flows were similar in the saline solution- and diltiazem-treated groups during ischemia and reflow. Thus, it is concluded that 1) diltiazem infusion significantly enhanced recovery of segmental systolic thickening after 15 min of ischemia and 24 h of reperfusion; 2) the enhancement in segmental systolic function could not entirely be attributed to decreased mean arterial pressure; 3) improvement in postischemic segmental ventricular function was seen only in those segments with systolic thinning during ischemia; thus, segments with the most severe ischemic dysfunction benefited most; and 4) there were no important differences in regional myocardial blood flow during ischemia and reperfusion between saline- and diltiazem-treated animals.