Kinetic studies of the Na⁺-K⁺-ATPase enzyme system in brain and heart of aging rats

Kinetic analyses of the Na⁺-K⁺-adenosine triphosphatase (ATPase) system were performed in brain and heart preparations from young mature (4 mo old) and healthy aged (25 mo old) rats. The K⁺-activated p-nitrophenylphosphatase (K⁺-pNPPase) method was used to assess activity of the enzyme system. Ouabain inhibition of K^-pNPPase activity was also examined. A significant age-related decrease in maximal velocity (V(max)) was found in cardiac K⁺-pNPPase activity, but no changes were seen in the K⁺ concentration for half-maximal velocity (K(0.5)). No age differences in V(max) or K(0.5) were seen for brain. No differences in ouabain inhibition were found in either brain or heart. In a second experiment, the major component of the age-related decline in cardiac K⁺-pNPPase activity was found to occur between 5 and 14 mo of age, a period during which plasma thyroxine had previously been found to decline in the same animals. Since peripheral Na⁺-K⁺-ATPase activity is partly thyroid hormone dependent, the age-dependent decrease in cardiac enzyme activity appears to be secondary to neuroendocrine changes.