title: Sodium-independent active potassium reabsorption in proximal tubule of the dog.
abstract: Prior studies of proximal tubule reabsorption have failed to distinguish conclusively between a separate active K+ transport system and K+ movement linked to Na+ reabsorption. To attempt to dissociate movement of K+ from Na+ and Ca++, recollection micropuncture experiments were performed in proximal tubules of intact and thyroparathyroidectomized (TPTX) dogs under two different conditions known to inhibit Na+ reabsorption: saline expansion to 5% body wt, and 5 mg/kg acetazolamide. A control hydropenic group was also studied. Tubular concentrations of K+, Na+, and Ca++ were measured by electron probe analysis. During initial collections, mean+/-SEM tubular fluid/plasma (TF/P)(K+) was 1.07 +/-0.05, 1.05+/-0.05, and 1.00+/-0.03 in intact hydropenic (n = 7), saline (n = 6), and acetazolamide (n = 8) groups; fractional reabsorption (FR) of K+ in proximal tubules was 0.35, 0.39, and 0.31 respectively. After saline, (TF/P)(Inul in) fell from 1.81 to 1.34 (P < 0.01); (TF/P)(K+), (TF/P)(Na+), and tubular fluid/ultrafiltrate, (TF/UF)(Ca++) did not change, so that FR of all three ions fell proportionately. After acetazolamide, however, despite a 24% inhibition of FR of Na+ and Ca++, (TF/P)(K+) fell to 0.85+/-0.04 (P < 0.005) so that FR of K+ was unchanged at 0.34. In three corresponding groups of TPTX dogs, similar results were obtained. Acetazolamide (n = 7) inhibited FR of Na+ and Ca++ by 41%, but (TF/P)(K+) fell from 1.03+/-0.03 to 0.89+/-0.04 (P < 0.005) so that FR of K+ was unchanged (0.36-0.34).A separate uphill transport system for K+ in proximal tubules is therefore unmasked by acetazolamide, a drug which selectively inhibits Na+ (and Ca++) reabsorption. Saline, on the other hand, inhibits net reabsorption of all three ions, probably by increasing passive backflux via intercellular channels.
mesh_terms: D000086:Acetazolamide / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004285:Dogs; D004577:Electron Probe Microanalysis; D005260:Female; D007684:Kidney Tubules / Q000378:metabolism*; D007687:Kidney Tubules, Proximal / Q000187:drug effects / Q000378:metabolism; D008657:Metabolic Clearance Rate; D010280:Parathyroid Glands / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D011677:Punctures; D012964:Sodium / Q000378:metabolism; D012965:Sodium Chloride / Q000494:pharmacology; D013961:Thyroid Gland / Q000502:physiology; D014867:Water / Q000378:metabolism
chemical_list: D014867:Water; D012965:Sodium Chloride; D012964:Sodium; D000086:Acetazolamide; D011188:Potassium; D002118:Calcium