Maturational changes in rabbit renal cortical phospholipase A₂ activity

Several studies have demonstrated that the neonatal kidney has a markedly attenuated response to parathyroid hormone (PTH); however, the cause for this blunted response is unknown. PTH stimulated cAMP production by 215 ± 18% in neonatal proximal tubule suspensions compared to a 35 ± 7% increase in adult proximal tubules. Thus, neonatal proximal tubules have functioning PTH receptors and a greater adenylate cyclase response than the adult segment. In adult proximal tubules, PTH stimulates phospholipase A₂ (PLA₂) activity and the inhibition of Na,K-ATPase activity by PTH is blocked by inhibitors of PLA₂. We examined whether maturational changes in renal cortical activity could play a role in the attenuated response to PTH in the neonatal proximal tubule. Com pared to adults, neonates had a lower renal cortical cytosolic PLA₂ (cPLA₂) activity, assessed as the release of ¹⁴C- arachidonic acid (AA) from labeled phosphatidyl choline (0.44 ± 0.10 vs. 0.74 ± 0.06% ¹⁴C-AA released/min/mg protein, P < 0.05) and microsomal PLA₂ activity (0.32 ± 0.03 vs. 1.20 ± 0.13% ¹⁴C-AA released/min/mg protein, P < 0.001). The protein abundance of cPLA₂ was not different between the neonatal and adult renal cortex as assessed by immunoblot assay. Thus, the difference in activities must be due to a difference in regulation of cPLA₂. Annexin 1 (lipocortin 1) has been shown to inhibit PLA₂ activity by binding to phospholipid substrate. Annexin 1 protein abundance was higher in neonatal than in adult renal cortex (P < 0.001). Thus, the lower activity of PLA₂ in the neonatal tubules may be due in part to higher expression of annexin 1. PLA₂ activation by PTH, 8-bromo-cAMP and PMA was assessed as ³H- AA release from prelabeled suspensions of neonatal and adult proximal tubules. PTH (10^-7 M), 8-bromo-cAMP (10^-4 M) and PMA (5 x 10^-8 M) significantly increased ³H-AA release from adult tubules (P < 0.05) but had no effect on neonatal tubules (P = NS). Thus, PTH, 8-bromo-cAMP and PMA stimulated PLA₂ in adult but not neonatal proximal tubules. In conclusion, the maturational changes in renal cortical PLA₂ activity may be a factor in the blunted response of neonatal proximal tubules to PTH.