Development of cytochrome-c oxidase activity in rat heart: downregulation in newborn rats.

Cytochrome-c oxidase (COX) activity of the rat heart was two- to sevenfold activated when the membrane integrity was disrupted by hypotonic swelling, freezing-thawing, or a detergent, indicating that a large portion of COX capacity in intact mitochondria is not active. The effect of detergent was tested in heart mitochondria isolated from 1-, 5-, 15-, 29-, and 60-d-old rats; activation by detergent was up to 20-fold in 1-d-old animals, whereas in mitochondria from 60-d-old rats it was only 7-fold. Our data indicate that the rat heart exhibits significant developmental changes dependent on downregulation of COX activity in the neonatal period.

Developmental changes of cytochrome c oxidase and citrate synthase in rat heart homogenate.

Activity of cytochrome c oxidase and citrate synthase in rat heart homogenates was determined in 5-, 15-, 28- and 60-day-old rats. The activity of both enzymes increased during postnatal development but their changes followed different kinetics. The membrane-bound cytochrome c oxidase reached its adult values during the early postnatal period, i.e. between days 5 and 15, whereas soluble matrix-localized citrate synthase also continued to increase between days 15 and 60. Our data indicate a relative excess of cytochrome c oxidase in neonatal cardiocytes.

Ontogenetic development of energy-supplying enzymes in rat and guinea-pig heart.

The purpose of the present study was to compare the ontogenetic development of the activity of myocardial energy-supplying enzymes in two mammalian species, differing significantly in their level of maturation at birth. The animals were investigated during the late prenatal period and 2, 7, 14, 21, 25, 30, 63, 120 and 730 days after birth in the rat and 2, 21, 84 and 175 days in the guinea-pig. The following enzymes were assayed in the right and left ventricular myocardium: lactate dehydrogenase (LDH, lactate uptake and/or formation), triose phosphate dehydrogenase (TPDH, carbohydrate metabolism), glycerol phosphate dehydrogenase (GPDH, glycerol-P shuttle)), hexokinase (HK, glucose phosphorylation), malate dehydrogenase (MDH, tricarboxylic cycle), citrate synthase (CS, tricarboxylic cycle) and hydroxyacyl-CoA dehydrogenase (HOADH, fatty acid breakdown). The rat heart, highly immature at birth, exhibits three different developmental patterns of energy-supplying enzymes, identical in both ventricles: (i) two mitochondrial enzymes of aerobic metabolism (CS, HOADH) and GPDH have a relatively low activity at the end of prenatal life; thereafter their activity steadily increases, approaching the adult levels between the 3rd and 4th postnatal weeks. A significant decrease was observed between the 4th and 24th months. (ii) MDH and LDH: prenatal values were significantly higher as compared with the 2nd postnatal day; after this period the activities increased up to adulthood (4 months) and decreased during senescence. (iii) The activities of HK and TPDH are characterized by only moderate changes during development. HK differs from all other enzymes by the highest prenatal values, which exceed even adult values. In contradiction to the rat heart, the developmental differences in more mature guinea-pig heart were significantly less pronounced. The only ontogenetic differences observed were the lower activities of enzymes connected with aerobic metabolism at the end of the prenatal period. Our results point to possible differences in the development of adaptive metabolic pathways in animals with different levels of maturation at birth.

Optimal conditions for determination of cytochrome c oxidase activity in the rat heart.

The determination of cytochrome c oxidase (COX) activity represents an important indicator for the evaluation of cell oxidative capacity. However, it has been shown repeatedly that different factors modify the rate of COX activity under various experimental conditions. The most important concern the ionic concentrations of the medium and the application of various detergents for the solubilization of mitochondrial membranes. We found the highest activity of COX in rat heart homogenates and mitochondria at 40-60 mM potassium phosphate. The rate of COX activity is dependent on the detergent/protein (P) ratio. Using n-dodecyl-beta-D-maltoside (lauryl maltoside, LM) as the detergent, we obtained the highest activity at LM/P ratios of (50:100):1. By kinetic measurements of low-affinity binding sites in heart mitochondria, we found Vlim values of 4.3 and 22.2 micromol cytochrome c per min per mg P in the presence or absence of lauryl maltoside, respectively. The Km values were 16.7 micromol in the presence or absence of lauryl maltoside. Our results thus indicate that 1) the exact assessment of COX activity in heart homogenates and mitochondria requires the determination of optimum phosphate concentrations in the medium used, and 2) even small modifications of the experimental procedure may induce significant differences in the maximum values of COX activity.

Activity of cytochrome c oxidase in the right and left ventricular myocardium of male and female rats exposed to intermittent high altitude hypoxia.

The aim of the present study was to compare the capacity of the oxidative metabolism (total activity of cytochrome c oxidase, COX) in the right and left ventricular myocardium of adult rats exposed to intermittent high altitude (IHA) hypoxia simulated in a barochamber (5,000 m, 8 h/day, 5 days/wk, for a total of 32 exposures). In male and female rats, IHA induced significant increases of the right ventricular (RV) weight and protein content, whereas left ventricular (LV) weight and protein content remained unaffected. Consequently, the RV/LV ratio in both sexes markedly increased. Similarly, IHA induced an increase of the total activity of COX in RV in both sexes. The specific activities of COX in homogenate as well as in isolated mitochondria were not changed in IHA-exposed animals, which indicates that the increase of total activity of COX is proportional to the increase of total protein content and RV weight.