A guanidine dietary supplement influences pH sensitivity and NSAID activity in acid-sensing ion channels

The acid-sensing ion channels (ASICs) are proton sensitive, sodium channels that belong to the epithelial sodium channel/ Degenerin family of ligand-gated ion channels. Activation of the ASIC1a subtype in the central nervous system increases neurodegeneration after ischemic stroke while ASIC3 subtype in the peripheral nervous system is involved in perception of pain. They are emerging targets for ischemic stroke, pain and inflammation. However, we lack selective ligands to target ASICs. In order to gain a better understanding of the channel and to develop selective ligands we must first determine how ASICs are modulated by synthetic as well as endogenous guanidine compounds. This study investigates whether a guanidine dietary supplement, creatine, modulates ASICs. Creatine has been shown to protect from ischemia and benefits patients suffering from muscular dystrophy, osteoarthritis, and fibromyalgia. Furthermore, pain medications such as nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit ASICs. Since supplements and NSAIDs are available over-the-counter, the significant amount of the population would consume them simultaneously.However, the interactions of combination of creatine and NSAIDs on ASICs still remain elusive. Here we sought to determine if creatine would modulate ASIC1a and ASIC3 proton sensitivity and if the combination of creatine and NSAIDs would inhibit ASIC3. Our results indicate that, creatine reduces human ASIC1a (hASIC1a) steady-state desensitization and increases their recovery from desensitization. Creatine also slows down the open-state desensitization of hASIC1a. The efficacy of hASIC1a is increased by creatine at higher concentrations. This indicates that, creatine increases channel's reactivation from desensitization by stabilizing the closed conformation of hASIC1a. Creatine's effect on rat ASIC3 (rASIC3) was calcium dependent. Creatine reduced proton sensitivity of rASIC3 in the nominal calcium environment. As previously reported, NSAIDs inhibited steady-state current of rASIC3 which is involved in pain perception. However, creatine reduced NSAIDs efficacy on rASIC3. To summarize, the creatine's effect depends on the desensitized state of hASIC1a and creatine increases the availability of channels for opening. While in rASIC3, creatine reduces proton sensitivity in nominal Ca2+ and antagonizes NSAIDs inhibitory effect. Thus, the use of creatine should be monitored in diseased states and when it is consumed along with NSAIDs.