Effects of adrenal steroid hormones on mitochondrial maturation during the late fetal period

In the present work, we described the perinatal changes in mitochondrial maturation that contribute to metabolic development in the rat kidney. We focused on cytochrome-c oxidase activity and gene expression from the last three days of gestation to one day after birth. The role of adrenal steroids in the development of cytochrome-c oxidase expression and of mitochondrial DNA content was also investigated by studying the effects of fetal adrenalectomy. During the perinatal period, the developmental pattern of the cytochrome-c oxidase enzymatic complex exhibited parallel increases in transcript levels, protein content and enzyme activity, suggesting a transcriptional regulation of this enzyme. Adrenalectomy led to a decrease in fetal kidney cytochrome-c oxidase messengers and mtDNA content while administration of dexamethasone restored normal levels. In contrast, mtDNA content was unchanged in liver and heart after adrenalectomy whereas levels of cytochrome-c oxidase transcripts were controlled by adrenal steroids in liver but not in heart. These results indicate that adrenal steroid hormones contribute to the regulation of perinatal maturation of mitochondria in rat kidney and that these hormones are involved in the fetal mitochondrial biogenesis in a tissue-specific manner.

Effect of hyperglycemia on the polyol pathway in rat kidney during the perinatal period

The activity of the polyol pathway was studied in developing rat kidney. For this purpose, sorbitol content, aldose-reductase activity and sorbitol-dehydrogenase activity were determined in papilla from fetuses and 24-h-old neonates. After birth, no significant difference was observed in sorbitol content, whereas sorbitol-dehydrogenase activity decreased and aldose-reductase activity doubled. Changes in aldose-reductase activity were due to an increased number of enzymatic sites but not with a change in affinity. Low levels of sorbitol were found in fetal and neonatal medulla together with low levels of urine osmolarity. In neonates, sorbitol contents were tenfold lower than in the adult, probably as a result of a lower affinity and a lower number of enzymatic aldose-reductase sites. Attempts to increase the activity of polyol pathway in fetal kidney were made by means of hyperglycemic animals; this approach resulted in an increase of aldose-reductase activity without any change in sorbitol content. Our results indicate that, in fetal and neonatal kidneys, aldose-reductase activity is probably not the limiting factor for sorbitol synthesis; another parameter, such as the availability of NADPH, might explain the low efficiency of the polyol pathway during the perinatal period.

Perinatal maturation of rat kidney mitochondria

In the rat kidney, NaK-ATPase activity increased between days 19 and 20 of gestation (+50%) and between 1 and 24 h after birth (+20%), requiring an increased energy supply. In order to determine whether mitochondrial changes were involved, renal mitochondrial development was investigated from day 19 of gestation to 1 day after birth. Slot-blot analyses of mitochondrial-DNA/nuclear-DNA ratio and determination of citrate synthase activity showed a doubling in the mitochondrial pool between days 19 and 20 of gestation. In isolated mitochondria, oxygen consumption remained unchanged between days 19 and 20 of gestation, and then it was enhanced between days 20 and 21 of gestation (+70%) and between 1 and 24 h after birth (+50%). We also focused on one of the respiratory-chain complexes, ATP synthase, and measured its activity and content during the perinatal period. We demonstrated increases in both activity and content of ATP synthase between days 20 and 21 of gestation and between 1 and 24 h after birth, thus suggesting that changes in ATP synthase activity are ascribed to an increase in the mitochondrial density of ATP synthase complexes. Moreover, the mitochondrial ATP/ADP ratio only increased between 1 and 24 h (+90%), indicating a critical step in the renal respiratory-chain maturation at that time. We therefore conclude that the postnatal enhancement of renal mitochondrial oxidative capacity might depend on protein synthesis de novo and on changes in the adenine nucleotide concentrations.

Effect of maternal hyperglycemia on NaK ATPase activity in fetal rat kidney

The effect of moderate hyperglycemia on renal ATP production and ATPase activity of rat fetus was investigated using the experimental procedure of maternal continuous infusion of glucose during the last 5 days of gestation. Glucose-infused mothers and their fetuses showed a high level of glycemia (8.8 and 5.5 mM, respectively) and a high level of insulinemia (3 times higher than in controls). No change in either ATP or ADP concentration was detectable but an increase in NaK ATPase activity occurred without any change in Mg ATPase activity. These modifications should be the result of an enhanced Na/glucose cotransport leading to an enhanced extrusion of Na at the basolateral membrane. These results indicate that immature kidney is able to increase NaK ATPase activity to maintain Na homeostasis.

Effect of lipid diet on mitochondrial palmitoyl-l-carnitine oxidation in kidney at postnatal development

Oxygen consumption (VO2) and beta-hydroxyacyl-CoA dehydrogenase (beta OAC) activity were measured in isolated mitochondria of developing rat kidney from late fetal to adult age. In the presence of palmitoyl-L-carnitine, VO2 consumption was higher in suckling than in adult rats while beta OAC activity rose during the postnatal period and declined after weaning. During postnatal development, the high level of mitochondrial fatty acid oxidation was linked to the high level of fatty acid supply and any change in lipid diet altered mitochondrial fatty acid oxidation. By contrast at adult age, a high fat diet did not change either mitochondrial fatty acid oxidation or beta OAC activity measured in two nephron structures (PCT and mTAL). Dietary lipids seem to play an important role in the evolution of mitochondrial fatty acid oxidation in developing rat kidney.

Mitochondrial activity of rat kidney during ontogeny

The development of oxidative metabolism was studied from the late fetal to adult stages in mitochondria isolated from rat kidney. We used the oxygen consumption rate, as an index of inner membrane activity and citrate synthase and fumarase activities as an index of matrix activity and cytochrome c oxidase activity as an index of the number of mitochondria. Fumarase and citrate synthase activities displayed different developmental patterns, suggesting that these Krebs cycle enzymes did not mature synchronously. In fetal mitochondria, net oxygen consumption measured in the presence of succinate or glutamate as substrate, was low; it increased during the day after birth and reached adult level between days 10 and 15. During this period, the levels of citrate synthase and cytochrome c oxidase activity did not change significantly in the isolated mitochondrial fraction. However, in fetal and adult kidney homogenates, these levels increased four-fold, suggesting a corresponding increase in the number of mitochondria. Most of these increases occurred during the 15 days after birth. These results suggest that in rat kidney, mitochondrial maturation precedes the maturation of reabsorptive ion transport and does not limit its development.

Effects of birth on energy metabolism in the rat kidney

The oxygen-consumption rates and the activities of fumarase and beta-hydroxyacyl-CoA dehydrogenase were compared in mitochondria isolated from fetal- and neonatal-rat kidney. Whole-organ ATP, phosphocreatine and creatine contents were determined in parallel. Kidney mitochondrial respiratory rates in the presence of succinate, glutamate/malate and palmitoyl-L-carnitine increased between 21 days post coitum and 1 day post partum, together with activities of oxidative enzymes. However, this postnatal maturation of oxidative metabolism was not yet initiated in mitochondria isolated from kidney 1 h post partum. An increase in ATP and phosphocreatine was observed immediately after delivery; newborn-rat kidney ATP content then remained high, whereas phosphocreatine reserves decreased considerably between 6 h and 1 day post partum. It is concluded that the increase in high-energy phosphate compounds observed at birth is not initially related to an activation of oxidative phosphorylation, and probably involves a transient stimulation of anaerobic glycolysis, while a progressive mitochondrial maturation takes place in the rat kidney during the first day of newborn life.

Role of catecholamines in the control of newborn kidney adenine nucleotide content

During the 1st h of extrauterine life, the adenine nucleotide content of the rat kidney is modified: the ATP level increases (+30%) while ADP and AMP are lowered (-30 and -50%, respectively). This leads to a high value of energy charge in the newborn kidney (0.89 vs. 0.80 in the fetus). It was possible to obtain in utero a similar modification of ATP, ADP, AMP concentrations by injections to the fetuses of cAMP, dibutyryl cAMP, or isoprenaline. Conversely, the postnatal changes in adenine nucleotide content could be prevented by administration to the fetus, just before birth, of beta- or beta 1-adrenoreceptor antagonists. Therefore the rise of kidney energy charge following parturition appears to be under hormonal control. Glucagon had no effect on kidney adenine nucleotide content. It is strongly suggested that the catecholamines released at the time of parturition are the triggering factor of this evolution.

Changes in carnitine-palmitoyl-transferase and carnitine-acetyl-transferase activity in rat kidney during development; effects of fasting

Developmental changes in the activities of two enzymes catalysing transfer of fatty acid across the mitochondrial membrane (carnitine-palmitoyl-transferase and carnitine-acetyl-transferase) were studied in the kidneys of developing rats from late fetal life to 10 days post-partum and were compared to cortical adult value. The activities of carnitine-palmitoyl-transferase and carnitine-acetyl-transferase increased after birth to reach a maximal value on day 5. Thereafter both activities decreased to reach adult cortical value. The cytochrome c oxidase activity (index of mitochondrial activity) increases continuously from late fetal age to adult. In kidneys of fetuses from starved mothers the carnitine-palmitoyl-transferase activity is higher than that of controls while carnitine-acetyl-transferase activity is not changed. In postmature fetuses (23 days post-coïtum) carnitine-palmitoyl-transferase activity is the same as in 21 days post-coïtum old fetuses. These results are discussed in relation to variations in nutritional and hormonal changes occurring during the perinatal period.

Renal glycogen content and hormonal control of enzymes involved in renal glycogen metabolism

Fetal rat kidney showed glycogen deposition that reached a maximal value of 60 micrograms . mg prot-1 on day 18 and declined there-after. At birth glycogen concentration is reduced (20 micrograms . mg prot-1) but higher than adult one (cortex, 2.2 micrograms . mg prot-1 and medulla, 3.4 micrograms . mg prot-1). From day 17 to the birth, glycogen synthetase and phosphorylase activities increased slowly except for acid glucosidase activity which increased rapidly between day 18 to the birth (3-fold). Corticosteroid deprivation had no effect upon glycogen content but fetal decapitation on day 16 reduced glycogen content in kidney of 19-day-old fetuses.

Hormonal control of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities in the fetal rat kidney

Triamcinolone (20 microgram per fetus) administered in utero to term rat fetuses (day 21 of gestation) increases the activities of renal G-6-Pase and PEPCK. The absence (or marked decrease) of circulating corticosteroids in fetuses from adrenalectomized, metopirone-treated mothers has, however, no clear effect on the enzyme activities. Therefore, it is doubtful that corticosteroids play a role in the development of G-6-Pase and PEPCK activities during the fetal period of life. In 21-day-old fetuses entirely decapitated on day 18, both enzyme activities are lower than intact fetuses of the same litter (G-6-Pase, -48%; PEPCK, -36%). In partially decapitated fetuses, the activity of G-6-Pase remains at the control level, whereas the PEPCK activity is clearly reduced (-39%). These results strongly suggest that on the last day of gestation the hormone group of parathormone, calcitonin, or thyroxine controls the G-6-Pase activity, whereas the hypothalamo-hypophysis system is implied in the development of PEPCK activity. Parathormone (1 unit per fetus) administered to decapitated fetuses completely restores the G-6-Pase activity. Neither rat growth hormone, synacthene, nor arginine vasopressin has significant effects on the activity of PEPCK in the kidneys of decapitated fetuses. The administration of 0.5 mumole of dibutyryl-cAMP or cyclic adenosine 3':5'-monophosphate (cAMP) to decapitated fetuses completely restores the activity of renal PEPCK, suggesting that the development PEPCK is controlled by cAMP-dependent hormone. The same dose of dibutyryl-cAMP has no effect on the activity of G-6-Pase; cAMP produces a slight but significant increase of this enzyme activity.

Development of ammonia and glucose productions from glutamine in foetal rat kidney; effects of metabolic acidosis

Ammoniagenesis and gluconeogenesis have been studied in foetal rat kidneys during the five last days of gestation by measuring in vitro NH3 and glucose productions from glutamine and by assaying activities of soluble phosphoenolpyruvate carboxykinase(PEPCK) and glucose 6-phosphatase (G6Pase). These studies were carried out in normal (mean blood pH: 7.30) and acidotic (pH: 7.12) foetuses. In normal foetuses, NH3 production by kidney cortex slices remains constant throughout the studied period of development, at a level 10-fold lower than the maternal one. On day 20 of gestation, a low glucose production (20-fold lower than the maternal one) appears for the first time. This may be related to an increase of PEPCK and G6Pase activities which occurs between day 19 and 20. In 20 days old foetuses, NH4Cl induced acidosis stimulates NH3 production but has no effect on PEPCK activity and glucose production. A response of gluconeogenesis to acidosis is observed one day later (day 21).