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 Gd3+-sensitive currents: voltage-dependent I-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 mu M Gd3+. The conductance did not discriminate between Na+ and K+ but was slightly anion-selective and was Ca2+-permeable. I-IV 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 I-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, mu S.cm(-2) respectively). In contrast to I-VD, I-VI was mildly cation-selective, Ca2+-permeable, and also selective for Na+ over K+. As with I-VD, volume-induced duced increases in I-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.