Hyperpolarization slowly activates a potassium current in locust skeletal muscle.

An inward current activated by hyperpolarization, IK,H, was studied under voltage clamp in locust skeletal muscle. The dependence of its reversal potential on [K+]o and its insensitivity to changes in [Na+]o indicate that the underlying conductance is a K+ conductance. The instantaneous current-voltage (I-V)-relationship exhibits outward rectification. Activation and deactivation take seconds and have complex time courses. At 10 mM [K+]o activation seems to start at a voltage greater than or equal to 20 mV more positive than the resting potential (approximately -68 mV). Ba++ blocks IK,H strongly; so do Rb+ and Cs+, the latter in a voltage dependent manner. The slow inward current bears similarities to anomalous rectifiers as well as to mixed, hyperpolarization-activated K+/Na+ currents in other tissues.