UNILATERAL EXPOSURE OF SHAKER-B POTASSIUM CHANNELS TO HYPEROSMOLAR SOLUTIONS

This study tests the hypothesis that ion channels will be affected dif ferently by external (extracellular) versus internal (cytoplasmic) exp osure to hyperosmolar media. We looked first for effects on inactivati on kinetics in wild-type Shaker B potassium channels. Although externa l hyperosmolar exposure did not alter the inactivation rate, internal exposure slowed both onset and recovery from fast inactivation. Differ ential effects on activation kinetics were then characterized by using a noninactivating Shaker B mutant. External hyperosmolar exposure slo wed the late rising phase of macroscopic current without affecting the initial delay or early rising phase kinetics. By contrast, internal e xposure slowed the initial steps in channel activation with only minim al changes in the later part of the rising phase. Neither external nor internal hyperosmolar exposure affected tail current rates in these n oninactivating channels. Additionally, suppression of peak macroscopic current was approximately twofold smaller during external, as compare d with internal, hyperosmolar exposure. Single-channel currents, obser ved under identical experimental conditions, showed a differential sup pression equivalent to that seen in macroscopic currents. Apparently, during unilateral hyperosmolar exposure, changes in macroscopic peak c urrent arise primarily from changes in single-channel conductance rath er than from changes in equilibrium channel gating. We conclude that u nilateral hyperosmolar exposure can provide information concerning the potential structural localization of functional components within ion -channel molecules.