TRANSPORT DEFECTS OF RABBIT INNER MEDULLARY COLLECTING DUCT CELLS IN OBSTRUCTIVE NEPHROPATHY

Urinary obstruction markedly reduces collecting duct Na+ reabsorption. To define the cellular mechanisms of this derangement in Na+ reabsorp tion in inner medullary collecting duct (IMCD) of obstructed kidneys, suspensions of intact IMCD cells and inner medulla plasma membranes (I MPM) were prepared from 24 h obstructed and untreated control kidneys. Oxygen consumption (QO2) studies revealed marked reductions in both a miloride-sensitive and ouabain-sensitive QO2 but not ouabain-insensiti ve QO2 in intact IMCD cells from obstructed, compared with control ani mals, indicating a reduction in oxygen-dependent transport activities of both the Na+ channel and the Na+-K+-adenosinetriphosphatase (ATPase ). Amiloride-sensitive conductive Na-22+ uptake in intact IMCD cells f rom obstructed kidneys was significantly decreased by 45% at 10 s, 30 s, and 1-5 min (10 s: 2.42 +/- 0.63 vs. 4.49 +/- 0.64 nmol Na+ flux/mg protein, n = 7, P < 0.05; 1 min: 4.65 +/- 0.7 vs. 8.27 +/- 0.98 nmol Na+ flux/mg protein, n = 7, P < 0.05), indicating decreased activity o f amiloride-sensitive Na+ channels in these cells. However, immunoblot s of IMPM with antibodies to Na+ channel proteins did not show signifi cant differences in content of Na+ channel proteins between membranes from obstructed and control groups. Ouabain-sensitive Na+-K+-ATPase ac tivity in IMPM of obstructed kidneys was also reduced (61.1 +/- 18.1 v s. 152.6 +/- 25.8 nmol ATP degradation . min-1.mg protein-1, n = 6, P < 0.02), and immunoblots with monoclonal antibodies against the alpha1 - and beta-subunits of rabbit Na+-K+-ATPase showed a 51 +/- 7% reducti on of both subunits in IMPM from obstructed kidneys (n = 4). In summar y, ureteral obstruction reduces the activities of apical Na+ channel a nd basolateral Na+-K+ATPase of the IMCD, probably contributing to the salt wasting that characterizes obstructive nephropathy. However, the lack of change in the amount of Na+ channel protein in contrast to the decreased numbers of Na+-K+-ATPase subunits suggests distinct regulat ory mechanisms for these two transporters in obstructed kidneys.