Does ischemia cause acute neuronal damage by converting the Na+/K+ pump into a channel?

Abstract

The gray matter of the higher brain undergoes spreading depolarization in response to ischemia, which increases metabolic demand and so promotes acute neuronal injury. The molecular mechanism linking ischemic failure of the Na+/K+ pump to the subsequent onset of a large inward current in neurons has remained a mystery because blockade of any conventional voltage- or ligand- gated channel does not prevent ischemic or “anoxic” depolarization (AD) propagating across grey matter. Recently our laboratory became aware of a marine poison whose molecular action is well characterized but has escaped scrutiny by neuroscientists. We hypothesized that this toxin could provide insight as to how ischemia acutely damages neurons at the molecular level. Palytoxin (PTX) specifically binds the Na+/K+ ATPase molecule at nanomolar concentrations, converting it from an ATP-requiring transporter to an open cationic channel. The result is sudden neuronal Na+ influx and K+ efflux. The double jeopardy of pump failure with the induction of a strong inward current should induce dramatic AD-like activity. Using light transmittance (LT) imaging, we show that bath application of 10 nM PTX to live coronal brain slices induces a propagating depolarization at 246±14 s in the neocortex, similar to AD induced by oxygen/glucose deprivation (OGD) at 289±10 s. In the neocortex, a distinct negative DC shift was recorded as the elevated LT front induced by OGD (4.2±1.0 mV) or PTX (4.1±0.8 mV) passed by an extracellular recording pipette, indicating strong depolarization of cells at the electrode. Both treatments induced strong spreading depolarization in the same higher brain regions and weaker events in lower brain gray matter. We also tested the drugs dibucaine and carbetapentane, which potently delay AD onset induced by OGD. One µM dibucaine pretreatment increased depolarization latency induced by OGD by 44±11 % and PTX by 61±20 % while 30 µM carbetapentane pretreatment increased depolarization latency induced by OGD by 51±16 % and PTX by 129±29 %. All of the above findings support our proposal that, like most biological poisons, palytoxin mimics (and takes advantage of) a natural process. In this case the process is ischemia itself where low ATP conditions might open the Na+/K+ pump to evoke AD and the brain damage that ensues.