Exploring the Pore Size of Endogenous Channels in <em>Xenopus</em> Oocytes

Abstract

The ion selectivity of a current that is activated by extracellular calcium (Ca²⁺) in Xenopus oocytes has been studied using the two-electrode voltage-clamp technique, and the reversal potential for this current has been observed. In order to determine the ionic makeup of this current, work in the past has focused on replacing Na⁺ and K⁺ ions with large, supposedly impermeable organic cations. Here we seek to explore the pore size of the channels responsible for the Ca²⁺-dependent current by replacing Na⁺ ions with tetrapentylammonium bromide (TPAB) and tetrahexylammonium bromide (THAB), two very large organic salts. While THAB was found to be insoluble in water at the necessary concentrations, the application of the TPAB-containing solutions caused a significant change in the reversal potential of the current in question. This change is consistent with TPAB having a lower membrane permeability than Na+, although it does not necessarily imply that TPAB is impermeable to the membrane.