Current activation by membrane hyperpolarization in the slowly adapting lobster stretch receptor neurone

1. A polarization-induced membrane current, IQ, was investigated in the slowly adapting lobster stretch receptor neurone using conventional electrophysiological techniques including intracellular ion measurements. 2. The current was readily blocked by Cs+ in a voltage-dependent manner, but proved to be unaffected by tetrodotoxin, tetraethylammonium and 4-aminopyridine. 3. From an analysis of the ionic basis of IQ, it appeared that the current is carried by both Na+ and K+ through a membrane channel whose permeability for K+ is about six times larger than that for Na+ in a normal ionic environment. In the presence of reduced external Na+ concentration the Q-channel increases its permeability for both Na+ and K+, but more so for Na+ than for K+. 4. Kinetically, IQ was found to be characterized by a steep sigmoidal relationship between membrane voltage and steady-state current activation, and by a bell-shaped relationship between membrane voltage and the time constant of the exponential phase of current activation or deactivation. A significant feature of the latter relationship is a tendency to level off at finite time-constant values in both strongly hyperpolarizing and strongly depolarizing voltage regions. 5. From the experiments a mathematical IQ model was inferred. This model was based on constant-field and channel-gating kinetics involving a voltage-dependent reaction step in series with a voltage-independent reaction step. The model was found to successfully reproduce IQ behaviour in the living preparation. 6. Functionally, the activation of IQ was found to play a role in setting the cell's resting polarization and membrane excitability. This function was inferred from experiments on unimpaled cells in which it was possible to demonstrate some overlap between the voltage ranges of IQ activation and impulse initiation. In addition, in impaled cells the activation of IQ was found to cause some shortening of post-tetanic membrane hyperpolarization and to accelerate, thereby, the post-tetanic restoration of membrane excitability to control levels.