Regulation of the Proton-Activated ASIC1 Channels by Extracellular Organic Cations

Abstract

Proton-activated channels (ASICs) are widely expressed in the central and peripheral nervous systems of mammals. ASIC1 and ASIC3 channels are potential targets for analgesia mainly due to their high expression in rat dorsal root ganglia neurons. In addition, ASIC1 is a potential target for stroke as it is highly expressed in the brain. Both channels belong to the epithelial amiloride-sensitive Na+ channel (ENaC/Degenerin) family of non-voltage gated Na+ channels. ASIC1 and ASIC3 currents can be evoked when extracellular pH drops to values observed during inflammatory response, stroke and ischemia. In HEK cell lines overexpressing human ASIC1 or ASIC3 channels, the current activated by sustained acidic pH rapidly (\u3c2 \u3es) inactivates due to channel desensitization. During prolonged (\u3e10 min) patch-clamp recording, accompanied with cell dialysis, current through ASIC1 (but not ASIC3) channels gradually decreases. This decrease or “rundown” of current is more pronounced at lower pH values. Here we demonstrate that extracellular polyamine spermine at micromolar concentrations has two effects on ASIC1 current: it drastically slows rundown and reduces its desensitization rate. Other polyamines, such as putrescine and spermidine do not mimic spermine’s effects. We propose a model where polyamine effects on ASIC1 current are explained by interaction with two different proton-binding sites. As spermine is a naturally occurring polyamine present both inside and outside of neurons, its effects on proton-gated channels may be physiologically relevant