A B S T R A C T Renal micropuncture observations in the rat suggest that the entire "distal tubule" (defined by the micropuncturist as that portion of the renal tubule extending between the macula densa and its first junction with another (renal tubule) may be responsive to vasopressin. However, this portion of the renal tubule contains two segments that are morphologically dissimilar. The "early" distal tubule is lined by epithelium characteristic of the distal convoluted tubule, while the "late" distal tubule is lined by epithelium characteristic of the cortical collecting duct. Thus, the present study was initiated to identify the most proximal site of action of vasopressin in the distal renal tubule.

A B S T R A C T Previous studies in the mammalian proximal tubule have suggested that para-aminohippurate (PAH) secretion is -threefold greater in the straight segment, or pars recta, than in the convoluted segment, or pars convoluta. However, the possibility that the site of maximal PAH secretion might be related better to particular tubule segments as identified by cell type had not been explored. In addition, the presence or absence of differences in PAH secretion between morphologically identical regions of superficial (SF) vs. juxtamedullary (JM) proximal tubules has not been examined. These issues were studied using a combination of' histologic methods and measurement of' [3H]PAH secretion in isolated perfused tubules. Measturements of microdissected SF and JM proximal tubules from young and adult rabbits revealed that SF proximal tubules were slightly but significantly longer than JM tubtules ([young

A B S T R A C T Standard clearance