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

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 whole-cell-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 Gd(3+)-sensitive currents: voltage-dependent IVD and voltage-independent IVI. IVD was found in all cells, activated by depolarisation and hypotonic shock, and was inhibited reversibly by 10 microM Gd3+. The conductance did not discriminate between Na+ and K+ but was slightly anion-selective and was Ca(2+)-permeable. IVI 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+ on IVI. Cells that showed IVI had a significantly higher conductance than those that did not (38.7 +/- 4.4, n = 20, and 20.5 +/- 0.7, n = 15, microS.cm-2 respectively). In contrast to IVD, IVI was mildly cation-selective, Ca(2+)-permeable, and also selective for Na+ over K+. As with IVD, volume-induced increases in IVI 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.