Ion (H+, Ca2+, Co2+) and temperature effects on a hyperpolarization-activated membrane current in the lobster stretch...

The effects of some ions (H+, Ca2+, Co2+) and temperature on the Q-channel (a channel which conducts a hyperpolarization-activated membrane current, IQ) was investigated in the slowly adapting lobster stretch receptor neurone. It was found: (1) that increases in pH cause an increase in the maximum channel conductance; (2) that Co2+ (1 mM) and increases in extracellular Ca2+ produce a positive shift of the relationship between the membrane voltage and the steady-state channel activation along the voltage scale; (3) that Co2+ and decreases in extracellular pH cause a decrease in rate of the voltage-dependent step of the channel's gating reaction; (4) that none of the ions has an effect on the rate of the voltage-independent step of the gating reaction; and (5) that Co2+ and K+ do not interfere with each other's actions on the channel's gating reaction and maximum ion conductivity. Increases in temperature from 13 to 23 degrees C were found: (1) to cause an increase in the maximum Q-channel conductance by a factor of 1.7; (2) to have no effect on the voltage dependence of the channel's gating reaction; but (3) to lead to an increase in rate of the gating reaction by a factor of about five for the voltage-dependent, and by a factor of about three for the voltage-independent gating step. The temperature sensitivity of the IQ mechanism proved to be mathematically reproducible by a previously developed, and slightly modified Q-channel model (Edman et al. 1987, Edman & Grampp 1989), and to be of no significance in the control of membrane excitability in a thermally unstable environment.