Interrelationship between glutamate and membrane-bound ATPases in nerve cells

Parallel dose-response studies of the voltage-dependent Na+ channel antagonist BW619C89, and the voltage-dependent Ca2+ channel antagonist nimodipine, in rat transient focal cerebral ischaemia

We have compared two classes of putative neuroprotectants, the voltage-dependent Na+ channel antagonist BW619C87 [4-amino-2-(4-methyl-1-piperazinyl)-5-(2,3,5-trichlorophenyl) pyrimidine], and the voltage-dependent Ca2+ channel antagonist nimodipine, in a rat model of transient focal cerebral ischaemia. BW619C87 (10-50 mg/kg) or nimodipine (10-100 microg/kg) were injected intravenously 5 min before induction of 2 h transient focal cerebral ischaemia via intraluminal thread occlusion of the middle cerebral artery. BW619C87 was a potent neuroprotectant over the range tested, maximally reducing the volume of hemispheric ischaemic damage by 51% at the 50 mg/kg dose. Nimodipine maximally reduced ischaemic damage by 33% at the 50 microg/kg dose, although the maximal level of neuroprotection afforded by BW619C89 and nimodipine was not significantly different. This is the first study to compare these two classes of drug directly in a model of middle cerebral artery occlusion with reperfusion, and it supports the effectiveness of both as neuroprotectants.

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.