Ion permeation through hyperpolarization-activated membrane channels (Q-channels) in the lobster stretch receptor...

In the lobster stretch receptor neurone it is possible to demonstrate a hyperpolarization-activated membrane current, IQ, which appears to be carried by Na+ and K+ in combination. The ion permeability of the membrane channel conducting this current (Q-channel) was investigated using conventional electrophysiological techniques including intracellular ion concentration measurements. It was found that none of the ions choline, protonated Tris, Rb+, NH4+, Li+, and protonated hydroxylamine was able to pass through the Q-channel which, thus, appears to be permeable to Na+ and K+ only. With increasing extracellular Na+ concentrations, IQ was increased up to a saturation level. This behaviour could be described by a one-site-two-barriers version of the Eyring rate theory, assuming that the permeant ions are turned over at specific saturable channel sites which 'sense' 70% of the transmembrane potential difference. With increasing extracellular K+ concentrations, IQ was increased in accordance with a simple first-order dose-response relationship. This finding can be accounted for by assuming that K+ increases all rates of turn-over of the permeant ions at their specific sites by similar relative amounts. Changes in extracellular Na+ and K+ concentrations were found to have no effect on the gating properties of the Q-channel.