Cell-Volume Changes and Ion Conductances in Amphibian Gallbladder Epithelium

Abstract

Cell-volume regulation after hyposmotic swelling is common in transporting epithelial cells. Acute regulatory volume decrease is mediated in several cell types by loss of K⁺ and Cl^- via channels either activated by the swelling phenomenon or active in the cell membrane prior to swelling. In the last two decades, it has become clear that many features of cell-volume regulation mediated by ion fluxes vary considerably among cell types. In some instances, the ion channel activation, although demonstrable, is insufficient to account, on the basis of ion fluxes, for the measured cell volume changes. Here we review the case of a salt-transporting epithelium in which hyposmotic cell swelling activates plasma membrane K⁺ channels, but at the same time inhibits Cl^- channels, thus preventing an acute volume-regulatory response. The sequence of events following cell swelling appears to be as follows: K⁺ channels are activated by membrane stretch, the cell membrane hyperpolarizes (the voltage moves closer to the K⁺ equilibrium potential) and this hyperpolarization reduces the Cl^- conductance, likely by a reduction in open probability of voltage-sensitive Cl^- channels. The significance of this phenomenon may be related to the preservation of intracellular Cl^- rather than cell volume, or to a physiological ‘need’ to prevent shrinkage of cells stimulated by hormones or other mediators.

Molecular and functional aspects of anionic channels activated during regulatory volume decrease in mammalian cells*

“…There, hypoosmotic cell swelling activates plasma membrane K + channels and simultaneously inhibits Cl -channels. The physiological meaning is unclear but might be explained by the preservation of intracellular Cl -at the cost of a swollen cell [233]. Another example of an unique approach are freshly isolated inner medulla collecting duct (IMCD) cells and their primary cultures, which can even respond to a rapid decrease in the extracellular solute concentration with the uptake of K + and Cl - [14].…”

Mechanosensitive Channels in Prokaryotes

“…Mechanosensory transduction involves a wide range of physiological processes that include touch and pain sensation [1], hearing [2,3], blood pressure regulation [4], proprioception, turgor control in plant cells and osmoregulation in bacteria [5,6,7]), as well as cellular growth [8] and cell volume regulation [9,10,11]. Abnormalities in mechanotrans-duction have been implicated in dysfunctions such as cardiac arrhythmias [12,13], hypertension [14], and neuronal [15,16] and muscular degeneration [17].…”