No abstract

The fatty acid content of rat renal phospholipids was examined during development. An increase in the arachidonic acid content of a particular fraction of phosphatidylcholine (PC2), phosphatidylethanolamine and phosphatidylinositol between 10 and 20 days of age could be observed in the papilla, and these levels were maintained into adulthood, while in the 20:4 content of phosphatidylserine changes were found between 30-day-old rats and adults. In the other fraction of phosphatidylcholine (PCI), saturated fatty acids such as 16:0 and 14:0 decreased, while no changes occurred in the stearic acid (18:0) content. The pattern found for the medulla did not differ significantly from that of the papilla. In contrast, the cortex content of arachidonic acid at 10 days of age was higher than that for papilla and medulla. Levels increased between 10 and 20 days of age, returning to the original values by 30 days with no further variations in the adult. These changes in arachidonic acid content with age in addition to the differences found between the three kidney zones might explain some causes of incomplete renal function in newborns.

The biosynthesis of prostaglandins (PGs) from the endogenous and exogenous precursor, arachidonic acid (AA), in renal papilla, medulla and cortex from neonatal to adult rats was investigated. Rat renal papilla and medulla incubated in the presence of [1-¹⁴C]AA released radioactive PGE₂, PGF_2α and PGD₂ which increased with age. No radioactive prostaglandins were found in the supernatants of renal cortex at any age studied. The amount of total prostaglandins released from the endogenous precursor also increased from 10 to 70 days of age, PGD₂ being the prostaglandin that showed the most important rise. In the cortex, only PGE₂ release increased with age. Cyclooxygenase (COX) activity was measured in papillary, medullary and cortical homogenates by using [1-¹⁴C]AA as substrate. Papillary and medullary COX activity increased after 10 days of age and continued to rise up to day 30 thereafter remaining unaltered until adulthood. Cortical COX activity was very low and decreased with age. These findings indicate the low capacity of the neonatal rat kidney to synthesize PGs.

Changes in endogenous levels of phospholipid arachidonic acid and in its radioactive incorporation can be detected in the rat kidney medulla during maturation. In an effort to explain this phenomenon, the arachidonoyl-CoA synthetase activity in the neonatal (10 days of age) and adult kidney (70-day-old rats) were studied. The neonatal kidney enzyme showed greater affinity, but less capacity than the adult enzyme to incorporate arachidonic acid into the membrane. The apparent K_m values were 27.8 and 73.9 µM while the v_max was 3.68 and 15.7 nmol/min/mg protein, for 10 and 70 days of age, respectively. Affinity for ATP was found to be almost 7-fold greater in adults when compared to neonates. The v_max for ATP also increased during development, with values of 1.04 and 2.87 at 10 and 70 days of age, respectively. Affinity for coenzyme A did not vary between the two stages studied, though the v_max increased 10-fold from 10 to 70 days of age. Since arachidonic acid availability is low in the perinatal stage because of its restricted endogenous synthesis, the higher enzyme affinity for this fatty acid at 10 days of age described here, could be of great importance in helping to capture the low but indispensable amount of arachidonic acid present in maternal milk during lactation.