Recent advances in studies on cardiac structure and metabolism最新文献

The pathobiology of the process of myocardial injury during ischemia comprises a series of events that results in the release of lysosomal enzymes from their subcellular locations within the myocardium. We have developed a canine model of acute myocardial ischemia in which the anterior descending coronary artery is ligated, myocardial blood flow is measured using radioactive microspheres, and tissues from subendocardium and subepicardium are assayed for activity of lysosomal hydrolases:N-acetyl-beta-glucosaminidase (NAG), beta-glucuronidase (beta-gluc), and acid phosphatase (AP). Particulate fractions of subendocardium revealed significant depletion of of total acid hydrolases (NAG, beta-gluc, and AP) after one and two hours of ischemia. In addition, after two hours of ischemia, the total activity of these three hydrolases in the subendocardial supernatant was decreased, correlating significantly with diminished myocardial blood flow (NAG: r =0.96; beta-gluc: r = 0.95; AP: r = 0.75). The diminished enzymatic levels in thesupernatant suggested "washout" of the hydrolases that was more efficient in those ischemic areas that had higher myocardial flow (greater than 20% of control). These changes in distribution of lysosomal hydrolases indicate early involvement of these enzymes in the pathobiology of myocardial injury and demonstrate the dynamic relationship of "washout" of acid hydrolases with the degree of diminished blood flow.

The protease inhibitor, pepstatin, inhibited acid protease in rat heart homogenates with a Ki of 7 x 10(-11) M by a noncompetitive mechanism, but it had no effect on myocardial acid protease activity when given in high doses to intact animals. Exposure of heart slices to 10(-7) M pepstatin for two hours also failed to produce demonstrable acid protease inhibition, suggesting that myocardial acid protease activity is unaffected by this potent inhibitor unless the integrity of the cell is disturbed.

Myocardial infarction was produced in dogs to investigate the behavior of cardiac lysosomal enzymes; acid protease and beta-glucuronidase activities were measured in whole myocardial homogenate and 30,000 X g supernatant fractions. Total activity of acid protease in the whole homogenate of the myocardium increased after 1 hr of infarction and showed maximal value after 6 hr, while in the 30,000 X g supernatant fraction the activity began to increase after 3 hr and reached the maximal level by 24 hr. Total beta-glucuronidase activity in the whole homogenate decreased during the initial 3 hr after myocardial infarction and markedly increased after 12 hr. The activity in the 30,000 X g supernatant fraction showed corresponding changes. These findings that the increase in whole homogenate activity preceded that in the 30,000 X g supernatant fraction may support the hypothesis that lysosomal enzymes may be newly synthesized and then solubilized in the infarcted myocardium.

In 18 patients, myocardial injury was estimated by mathematical analysis of the rise in plasma activity of alpha-hydroxybutyrate (alpha-HBDH) dehydrogenase as observed by multiple sampling after admission. This enzyme represents the lactate dehydrogenase isoenzymes LDH1 and LDH2. Changes in plasma volume were assessed by determining hematocrit values at the same time as the enzyme activities. Infarct size was expressed in IU/liter of plasma and in grams of heart muscle per liter of plasma (g/liter). Significant changes in plasma volume, reflected in hematocrit changes, occurred (average 12%): 10 patients without pulmonary edema showed an average change of 7%; for the group of eight patients with pulmonary edema, the change was 17%. When calculated infarct size was not correlated for plasma volume changes, a significant overestimation occurred (11.7%, range 0.7-29.7%). In the subgroup of patients with pulmonary edema the mean overestimation was 15.8%, and in patients without pulmonary edema, the mean overestimation was 8.5%. It is concluded that plasma volume changes after an acute myocardial infarction have to be taken into account when infarct size is calculated on the basis of plasma enzyme levels.

Coronary artery ligation of canine heart was performed to investigate the relationship between the lactate dehydrogenase (LDH) pattern in myocardium and the distribution of coronary flow, especially in the early stage of ligation and after reperfusion. In the myocardium of normal dogs, the LDH pattern was similar in the left ventricle, the interventricular septum, and the right ventricle; the average LD5:LD4 ratio was 1.1, 1.2, and 1.2, respectively, and consisted mainly of heart type. In the left and right auricles, however, the ratios were 0.3 and 0.2, respectively, lacking heart type. In the left ventricle, LD5:LD4 ratio in the subendocardium was different from that in the subepicardial layer. Blood flow distribution in canine myocardium was investigated by the fluorescent pattern on the cut surface of heart, in which 10% fluorescein sodium was injected into the cavity. By this method the evolution of the ischemic area from the endocardial layer to the epicardial side following coronary artery ligation and the effect of reperfusion on the ischemic area were clarified. Electron microscopic studies indicated that loss of mitochondrial function may account for the irreversibility of myocardial cell alteration. A new method for studying enzyme localization in tissues was introduced for studying LDH isoenzyme distribution in normal and injured myocardium.

Abnormal electrocardiographic changes were found in some patients on long-term glucocorticoid treatment. In experimental animals, chronic glucocorticoid administration resulted in an increase of amplitude of the QRS complex and abnormal ST and T changes. Changes of action potential were somewhat different in the subendocardial and the subepicardial layers. Diffuse mitochondrial alterations were found, particularly in the subepicardial layer. In rabbits treated with glucocorticoids, no significant changes were found in either serum or myocardial potassium content. Slightly decreased Vmax was the observed hemodynamic change.

Phase contrast microscopy of cultured embryonic heart cells showed the beating frequency decreased more rapidly and the regularity the rhythm of of the beating cells was lost sooner in heart cells from cardiomyopathic hamsters than from the control hamsters. Studies of cultured heart cells by differential interference contrast (with Nomarski's prism) and by electron microscopy revealed a significant impediment in the maturation of the sarcomeric units in the diseased animals compared to controls. The incorporation of [14C] leucine into acid-insoluble fractions was studied, and no significant difference in incorporation between the two groups was found. An analysis of polyacrylamide gel electrophoresis revealed the possible existence of a quantitative difference in one of the composing proteins of the erythrocyte membrane between the two groups. The protein kinase activity of ghosts from the control group was more sensitive to cAMP than that from the diseased animals. In addition, the binding of [3H] cAMP to the ghost was almost identical between the two. The morphological and biochemical observations lead one to the plausible supposition that there are some differences in the interaction of the so-called catalytic and regulatory subunits between the two groups and that there is an impairment of the higher arrangement of myofibrils from their building blocks in the diseased hamster. The significance of the existence of abundant corpuscles resembling neurosecretory granules was not established by this study. They may have an etiological significance or they may be related to a disturbed function in the cultured cells of the cardiomyopathic hamster.