Spermidine decreases Na+,K+-ATPase activity through NMDA receptor and protein kinase G activation in the hippocampus of rats

Spermidine is an endogenous polyamine with a polycationic structure present in the central nervous system of mammals. Spermidine regulates biological processes, such as Ca2+ influx by glutamatergic N-methyl-D-aspartate receptor (NMDA receptor), which has been associated with nitric oxide synthase (NOS) and cGMP/PKG pathway activation and a decrease of Na+,K+-ATPase activity in rats' cerebral cortex synaptosomes. Na+,K+-ATPase establishes Na+ and K+ gradients across membranes of excitable cells and by this means maintains membrane potential and controls intracellular pH and volume. However, it has not been defined whether spermidine modulates Na+,K+-ATPase activity in the hippocampus. In this study we investigated whether spermidine alters Na+,K+-ATPase activity in slices of hippocampus from rats, and possible underlying mechanisms. Hippocampal slices and homogenates were incubated with spermidine (0.05-10 mu M) for 30 min. Spermidine (0.5 and 1 mu M) decreased Na+,K+-ATPase activity in slices, but not in homogenates. MK-801 (100 and 10 mu M), a noncompetitive antagonist of NMDA receptor, arcaine (0.5 mu M), an antagonist of the polyamine binding site at the NMDA receptor, and L-NAME (100 mu M), a NOS inhibitor, prevented the inhibitory effect of spermidine (0.5 mu M). ODQ (10 mu M), a guanylate cyclase inhibitor, and KT5823 (2 mu M), a protein kinase G inhibitor, also prevented the inhibitory effect of spermidine on Na+,K+-ATPase activity. Spermidine (0.5 and 1.0 mu M) increased NO2 plus NO3 (NOx) levels in slices, and MK-801 (100 mu M) and arcaine (0.5 mu M) prevented the effect of spermidine (0.5 mu M) on the NOx content. These results suggest that spermidine-induced decrease of Na+,K+-ATPase activity involves NMDA receptor/NOS/cGMP/PKG pathway. (C) 2012 Elsevier B.V. All rights reserved.