Ion-Ion Interaction at the Multi-Ion TRPV1 Pore

Abstract

Proton released upon tissue injury, inflammation, or ischemia produces sustained pain that may be mediated by nociceptor TRPV1. Our previous study demonstrated that extracellular H+ not only activates the channel but also inhibits ion permeation. Voltage-dependence of H+ inhibition suggested that H+ moves halfway into the transmembrane electric field. The location of inhibitory H+ and the mechanism underlying H+ inhibition, however, remain unclear. To investigate the role of H+ in ion permeation, we examined the behavior of TRPV1 permeant ions using anomalous mole fraction effect (AMFE). For the experiment, two types of permeant ions mixed at a constant total concentration but at varying ratio were used to measure conductance of TRPV1. An AMFE exists when one of the mixed solutions yields the minimum conductance compared to pure solutions. We found that TRPV1 exhibits AMFE in mixtures of Na+ and Rb+, suggesting the presence of multiple permeant ions in the pore. This result is interesting given that the structure of TRPV1 ion selectivity filter appears much shorter than that of potassium channels, which is known to hold multiple ions in a single file. The AMFE under acidic conditions will be measured to test whether H+ interferes with permeant ions in the TRPV1 pore to produce inhibition.