title: K(+) channels of squid giant axons open by an osmotic stress in hypertonic solutions containing nonelectrolytes.
abstract: In hypertonic solutions made by adding nonelectrolytes, K(+) channels of squid giant axons opened at usual asymmetrical K(+) concentrations in two different time courses; an initial instantaneous activation (I (IN)) and a sigmoidal activation typical of a delayed rectifier K(+) channel (I (D)). The current-voltage relation curve for I (IN) was fitted well with Goldman equation described with a periaxonal K(+) concentration at the membrane potential above -10 mV. Using the activation-voltage curve obtained from tail currents, K(+) channels for I (IN) are confirmed to activate at the membrane potential that is lower by 50 mV than those for I (D). Both I (IN) and I (D) closed similarly at the holding potential below -100 mV. The logarithm of I (IN)/I (D) was linearly related with the osmolarity for various nonelectrolytes. Solute inaccessible volumes obtained from the slope increased with the nonelectrolyte size from 15 to 85 water molecules. K(+) channels representing I (D) were blocked by open channel blocker tetra-butyl ammonium (TBA) more efficiently than in the absence of I (IN), which was explained by the mechanism that K(+) channels for I (D) were first converted to those for I (IN) by the osmotic pressure and then blocked. So K(+) channels for I (IN) were suggested to be derived from the delayed rectifier K(+) channels. Therefore, the osmotic pressure is suggested to exert delayed-rectifier K(+) channels to open in shrinking rather hydrophilic flexible parts outside the pore than the pore itself, which is compatible with the recent structure of open K(+) channel pore.
authors.affiliation: Department of Cell Physiology, National Institute for Physiological Sciences, National Institutes of Natural Sciences, Nishigonaka 38, Myodaiji, Okazaki 444-8585, Japan. kukita@nips.ac.jp
authors.forename: Fumio
authors.initials: F
authors.lastname: Kukita
mesh_terms: D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D049832:Decapodiformes; D004553:Electric Conductivity; D006982:Hypertonic Solutions / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009995:Osmosis / Q000187:drug effects / Q000502:physiology; D011188:Potassium / Q000378:metabolism; D024642:Potassium Channels, Voltage-Gated / Q000378:metabolism / Q000502:physiology*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*
chemical_list: D006982:Hypertonic Solutions; D024642:Potassium Channels, Voltage-Gated; D000644:Quaternary Ammonium Compounds; C009405:tetrabutylammonium; D011188:Potassium