Irreversible ischemic injury occurs after coronary artery occlusion in vivo, first in the subendocardium and progressing toward the subepicardium over time. presumably due to transmural variations in collateral flow or wall tension. In this study, 10 left ventricular globally ischemic slabs were created that were free of wall tension and collateral flow. The onset and completion of ischemic contracture were identified by means of a new tissue compressibility gauge designed for these studies. Transmural samples were obtained at 15 min intervals for determination of high-energy nucleotide levels and for ultrastructural analysis. The results show that there is a statistically significant gradient of ATP depletion, with the subendocardium consistently showing accelerated energy utilization compared with the subepicardium (p < .05). Ultrastructural evidence of irreversible injury first appeared in the subendocardium at the onset of ischemic contracture and occurred when ATP levels declined to less than 1 ,umol/g wet weight. In summary, these data show that during total ischemia in vitro, cell death begins in the subendocardium at the onset of ischemic contracture and progresses toward the subepicardium over time. These changes occurred independent of variations in collateral flow or wall tension. The results suggest that the increased risk of the subendocardium to ischemic injury previously noted in vivo may occur not only because of variations in collateral flow and wall tension, but may also be secondary to an increased metabolic rate of the subendocardium resulting in faster ATP use during the period of ischemia. Circulation 68, No. 1, 190-202, 1983. GLOBAL myocardial ischemia, either in vivo or in vitro, eventually leads to the development of ischemic contracture or cardiac rigor mortis. There is significant evidence suggesting that the development of rigor or ischemic contracture is closely associated both with the severe depletion of high-energy phosphate compounds and the onset of cell death. 190sensitive indicator of the onset and the completion of this process.' However, contracture onset, which causes an increase in tissue stiffness, has been detected by the use of weights gently lowered onto the surface of the heart, with the distance traveled recorded by a linear transducer; ultrasonic crystals that indicate an increase in wall thickness with the onset of contracture2 I have also been used.A variety of interventions have been shown to increase or decrease the time period of global ischemia required to produce severe depletion of high-energy phosphate compounds and the subsequent formation of rigor complexes. Hypothermia, potassium cardioplegia, and decreased electrical activity all appear to delay the onset of contracture, whereas pacing and increased temperature appear to hasten the process.I 4 Thus this model system appears to be a simple experimental means to investigate interventions that delay or hasten the process of ischemic injury.Although the onset of ischemic contracture appears to ...

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THERE seems at present to be no general agreement amongst workers on liver catalase concerning either the best method of estimating the enzyme, or of preparing the homogenates in which the enzyme is measured. It is well known that the results of catalase activity measurements are considerably affected by such variables as temperature, hydrogen peroxide concentration, length of time for which the enzyme and substrate are allowed to remain in contact, and whether the measurements are of oxygen evolution or hydrogen peroxide disappearance. However, little attention seems to have been paid to the possibility that some discrepancies in the results obtained by various authors may depend as much on the method by which liver homogenates are prepared as on the method of enzyme assay.An example of an apparently complete disagreement in the literature concerns the question of a sex difference in liver catalase activity in mice and rats. Adams (1950, 1952) reported that a sex difference was present in the livers of the heterogenous strains of mice used, the catalase level in males being higher than in females. Hargreaves and Deutsch (1952) found similar results in their rats. However, Day, Gabrielson, and Lipkind (1954) measured liver catalase activities in both sexes of six pure line strains of mice and stated that the male level exceeded the female in only one strain. In three other strains there was no particular sex difference, and in the remaining two the female level was higher than the male. In view of these results Adams (1956) measured catalase activities in the livers of 14 strains of mice (including 10 pure lines) and three pure line strains of rats.All the rats, and all the mice with the exception of the related C57 black and brown strains, had significantly higher liver catalase activities in males than in females. The C57 black strain was stated by Greenstein and Andervont (1942) to be exceptional in that (a) the liver catalase activity was insensitive to S37 tumour growth, and (b) the activity was only about half that of the other strains studied. Adams (1956) found the catalase level in the C57 strains to be about half that of the other strains, in agreement with Greenstein and Andervont (1942).However, Day et al. (1954) stated that the catalase activity of their C57 brown mice was high, and equal to that of the other strains used. They also stated that the catalase levels in all their mice were considerably higher than those obtained by other authors. Greenstein and Andervont prepared their homogenates by grinding livers with sand, and Adams used a Ten Broeck grinder (see methods). Day et al., however, used the more drastic technique of homogenizing in a Waring blendor. It seemed possible therefore that the discrepancies between Greenstein and Andervont's (1942) and Adams ' (1950, 1952, 1956) results on the one hand,

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1. In the rat cerebral cortex net DNA synthesis ceases when the animal has reached about 25g. body weight (18 days of age). There is then little further change in the DNA content per cortex. 2. Nuclear and transfer RNA follow a similar pattern to DNA. 3. Microsomal and ribosomal RNA content increases up to 25g. body weight but then declines. The decrease in ribosomal and microsomal RNA content is associated with a change in RNA base composition. 4. Incorporation of [(14)C]orotic acid into nuclear RNA proceeds at a similar rate in 4-day-old and adult animals. However, there is a lag period of about 60min. in the young animals during which incorporation into the ribosome fractions proceeds slowly. In the adult animals the lag period is not seen.

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