Postnatal development in the rat: Changes in Na⁺ flux, sodium pump molecular activity and membrane lipid composition

The cellular mechanisms underpinning changes in metabolism during postnatal development in young mammals have not been extensively examined. This study examines changes in sodium pump capacity (Na⁺, K⁺-ATPase activity), number and molecular activity, as well as, Na⁺ flux, cholesterol level and fatty acid composition in a number of major organs during postnatal development in the rat. In liver, Na⁺ flux was highest (2.6 times) in the youngest rats (3-day old) and decreased with increasing age, whereas Na⁺, K⁺-ATPase activity increased with age (up to 9-28 days) in liver, kidney and brain, but not in heart. Increases in Na⁺, K⁺-ATPase activity where primarily driven by increases in molecular activity, 4-fold in brain and 7-fold in kidney, rather than by increases in sodium pump number. Membrane polyunsaturation increased in both kidney and brain during development, with kidney becoming increasingly dominated by omega-6 (18:2n-6 and 20:4n-6) and brain by omega-3 (22:6n-3) fatty acids. Membrane reconstitution experiments support the concept that changes in membrane composition might underpin higher sodium molecular activities in the adult. In conclusion, at birth rats possess high Na⁺ flux but a lower sodium pump capacity that increases with age being driven by increases in molecular activities associate with changes in membrane lipid composition.