Thiazide-sensitive sodium chloride cotransport in early distal tubule

D. H. Ellison, H. Velazquez and F. S. Wright At least two pathways mediate sodium absorption across the luminal membrane of the renal distal tubule. One pathway is a conductive channel and the other appears to be a coupled Na-Cl cotransport pathway. The distal tubule comprises three segments: the distal convoluted tubule, the connecting tubule, and the initial collecting duct. To provide information about cellular locations of the proposed sodium transport pathways, we perfused early (14-38% of whole distal length) and late (61-83% of whole distal length) segments of whole distal tubules separately in vivo in anesthetized rats. When perfused with a solution that resembles fluid normally arriving at the distal tubule (75 mM Na, 68 mM Cl), rates of sodium absorption were similar in early and late segments (early 68 +/- 29.6, late 67 +/- 27.5 pmol X min-1 X mm-1). When perfused with a solution that resembles interstitial fluid (148 mM Na, 110 mM Cl), sodium transport was significantly higher in early than in late segments (276 +/- 28.4 vs. 113 +/- 29.7 pmol X min-1 X mm-1). Chlorothiazide (10(-3) M), which blocks sodium and chloride absorption in whole distal tubules, reduced sodium and chloride transport to zero in early distal tubules but had no significant effect in late distal tubules. Removing all chloride from perfusion solutions reduced sodium transport in early but not late distal segments.