Volume-activated, gadolinium-sensitive whole-cell currents in single proximal cells of frog kidney

Abstract

Stretch-activated channels (SACs) have been implicated in the control of epithelial cell volume. Such channels are generally sensitive to the trivalent lanthanide, gadolinium (Gd³⁺). In this study, using Gd³⁺ sensitivity and volume activation as indices, we have looked for ionic currents attributable to SACs using the wholecell-patch clamp technique in freshly isolated proximal tubule cells of the frog. Hypotonic shock caused a reversible increase in whole-cell conductance, which was inhibited by Gd³⁺. In conjunction with this increase in conductance, cell length (measured using an optical technique) also increased. We observed two types of volume and Gd³⁺-sensitive currents: voltage-dependentI _VD and voltage-independent I_VI. I_VD was found in all cells, activated by depolarisation and hypotonic shock, and was inhibited reversibly by 10 μM Gd³⁺. The conductance did not discriminate between Na⁺ and K⁺ but was slightly anion-selective and was Ca²⁺-permeable.I _VI was observed in only 50% of cells and was also inhibited by Gd³⁺. Although the inhibition was irreversible, it was dose-dependent, suggesting a specific effect of Gd³⁺ onI _VI. Cells that showed I_VI had a significantly higher conductance than those that did not (38.7±4.4,n=20, and 20.5±0.7,n=15, μS · cm⁻² respectively). In contrast toI _VD,I _VI was mildly cation-selective, Ca²⁺-permeable, and also selective for Na⁺ over K⁺. As withI _VD, volume-induced increases inI _VI were inhibited by Gd³⁺. Both of these currents are activated during hypotonic shock and may be involved in volume-regulatory processes in frog proximal cells.