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.

Preferential Solvation: Dividing Surface vs Excess Numbers

“…In the preceding section, there was no restriction on the solute size, and the KB theory can now be applied to systems much larger than those studied before, such as ion channels and actin polymerization. − ,− …”

“…The answer for the adsorption theory is the Gibbs dividing surface. ,,,, By appropriately positioning the dividing surface, the surface excess of water can be made zero. ,,,, The surface-tension change due to the introduction of the cosolvents can therefore be attributed entirely to the surface excess of the cosolvent. ,,− The indeterminate problem can thus be circumvented elegantly by introducing the dividing surface.…”

“…In OST, the modulation of the solvation free energy is entirely attributed to the excess number of water. ,,− Hence, the number of water molecules released upon binding or allosteric effect can be directly obtained. ,,− This was made possible, albeit implicitly in the beginning, by introducing the dividing surface into the preferential solvation theory. , This major progress from the older binding perspective, however, has stirred significant controversy, − as will be reviewed in the following.…”

“…How many water molecules are released when a ligand binds a protein or when an allosteric transition takes place? This number, according to OST, can be experimentally accessible by measuring the “volume of hydration”, namely, the dependence of the reaction Gibbs energy change upon the “osmotic pressure” (whose meaning will be clarified below). ,,− To this end, OST exploits “inert” cosolvents called osmolytes, such as sugars, polyols, or polyethylene glycols. , Since these osmolytes are strongly excluded from biomolecular surfaces (such as grooves, cavities, binding sites, and channels), the dividing surface can be placed outside the hydration shell, into which the osmolytes do not enter. , Hence, the hydration shell is subjected to the “osmotic pressure”. , It is for this purpose that the dividing surface is used as the theoretical foundation for the estimation of hydration changes.…”

Reduction of RNA A‐to‐I editing in Drosophila acclimated to heat shock

“…Temperature change impacts the function of the nervous system by affecting synaptic gain, as well as synaptic and conduction delays [17]. In their pioneering study of the electrical activity of the squid giant axon, Hodgkin and Katz found that the falling phase of the action potential was particularly sensitive to temperature [18], which was later shown to be caused by the temperature sensitivity of the gating property and closing kinetics of the potassium channel [19]. In cold temperatures, in the absence of functional adjustment through RNA editing, the delayed closing of the potassium channel would broaden the action potential, thereby limiting the firing frequency [20].…”