Functional differentiation of the medullary collecting tubule: influence of vasopressin.

Medullary collecting tubules of rabbits were dissected from the outer zone at different stages of ontogenetic evolution and perfused in vitro. The hydraulic conductivity coefficient (Lp) was measured in the presence of either hypotonic perfusate and isotonic bath or isotonic perfusate and hypertonic bath. Basal Lp (cm s-1 atm-1 10(-7) was 85 +/- 34 (N = 17) during early functional differentiation of the outer medullary collecting tubule (e-OMCT), 36 +/- 6 (N = 8) in the intermediate state (i-OMCT), and 10 +/- 7 (N = 11) in the final, mature state of function (m-OMCT). Addition of supramaximal concentrations of arginine-vasopressin (AVP) to the bath increased Lp in i-OMCT (250 +/- 36) and m-OMCT (327 +/- 63) but did not activate the osmotic hydraulic conductance in e-OMCT (105 +/- 27). In 11 studies, OMCT were analyzed using isotonic solutions as the perfusate and bath medium. The spontaneous transtubular voltage (PD) was lumen positive, 1.71 +/- 0.3 in e-OMCT, lumen negative 2.43 +/- 0.3 in i-OMCT, and 6.1 +/- 0.4 in m-OMCT. AVP had no effect on PD in e- and i-OMCT but increased the PD significantly (P less than 0.025) to 9.2 +/- 0.6 in m-OMCT. The results indicate that both hydraulic conductivity coefficient and transtubular voltage in the medullary collecting tubule are dependent upon epithelial ontogeny and, particularly, display differential responses to the antidiuretic hormone. The data suggest that the differentiation of water and ion transport in the medullary collecting tubule may contribute to the increasing efficacy of the medullary countercurrent system.