CELL SHRINKAGE ACTIVATES A CATION CONDUCTANCE IN PRINCIPAL CELLS OF RAT CORTICAL COLLECTING DUCT

We examined whether a nonselective cation (NSC) conductance is activat ed in the freshly isolated rat cortical collecting duct (CCD) by osmot ically induced cell shrinkage, Membrane voltages (V-m), whole cell con ductances (G(c)) and single-channel currents were measured with the pa tch-clamp method, Cellular Ca2+ ([Ca2+](i)) and Na+ ([Na+](i)) activit ies were estimated fluorimetrically with fura-2 and sodium-binding ben zofuran isophthalate, Increasing extracellular osmolality by 50 or 100 mosm/l depolarized V-m by 6 +/- 1 (n = 26) and 11 +/- 3 mV (n = 12), respectively, G(c) was increased under these conditions by 10 +/- 7 (n = 9) and 46 +/- 17% (n = 6), respectively, The depolarization was par tly inhibited by either amiloride (n = 6) or Gd3+ (n = 4, each 10 mu m ol/l) and completely inhibited by amiloride plus Gd3+ (n = 13) or remo val of Na+ (n = 6), Cell-attached NSC channels could neither be observ ed under control conditions nor after increasing osmolality, [Ca2+](i) did not significantly change upon increasing osmolality by 50 mosm/l (n = 11) and decreased slightly by 15 +/- 3 nmol/l (n = 11) when the o smolality was increased by 100 mosm/l, [Na+](i) was significantly incr eased upon increasing osmolality (+100 mosm/l) 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 mu mol/1), to 11 +/- 3 mmol/l (n = 8) in the presence of Gd3+ (10 mu mol/l) and to 6 +/- 1 mmol/l (n = 7) w ith both inhibitors, These data indicate that osmotically induced cell shrinkage activates a NSC conductance in rat CCD cells.