Cell Shrinkage Activates a Cation Conductance in Principal Cells of Rat Cortical Collecting Duct

Abstract

We examined whether a nonselective cation (NSC) conductance is activated in the freshly isolated rat cortical collecting duct (CCD) by osmotically induced cell shrinkage. Membrane voltages (Vm), whole cell conductances (Gc) and single-channel currents were measured with the patch-clamp method. Cellular Ca2+ ([Ca2+]i) and Na+ ([Na+]i) activities were estimated fluorimetrically with fura-2 and sodium-binding benzofuran isophthalate. Increasing extracellular osmolality by 50 or 100 mosm/1 depolarized Vm by 6 ± 1 (n = 26) and 11 ± 3 mV (n = 12), respectively. Gc was increased under these conditions by 10 ± 7 (n = 9) and 46 ± 17% (n = 6), respectively. The depolarization was partly inhibited by either amiloride (n = 6) or Gd3+ (n = 4, each 10 µmol/l) and completely inhibited by amiloride plus Gd3+ (n = 13) or removal of Na+ (n = 6). Cell-attached NSC channels could neither be observed under control conditions nor after increasing osmolality. [Ca2+]i did not significantly change upon increasing osmolality by 50 mosm/1 (n = 11) and decreased slightly by 15 ± 3 nmol/l (n = 11) when the osmolality was increased by 100 mosm/1. [Na+]i was significantly increased upon increasing osmolality (+100 mosm/1) by 14 ± 2 mmol/l (n = 23). This increase in [Na+]i was reduced to 8 ± 2 mmol/l (n = 9) in the presence of amiloride (10 µmol/ 1), to 11 ± 3 mmol/l (n = 8) in the presence of Gd3+ (10 µmol/l) and to 6 ± 1 mmoI/1 (n = 7) with both inhibitors. These data indicate that osmotically induced cell shrinkage activates a NSC conductance in rat CCD cells.