Loop of Henle functional differentiation: in vitro perfusion of the isolated thick ascending segment.

Abstract

Single segments of the loop of Henle, the cortical thick ascending limb (CTAL), were dissected from fresh renal slices (rabbit) and individually perfused in vitro. Direct analysis of transepithelial NaCl and water movement was carried out at two functional states of epithelial transport. I. Differentiating thick ascending loop of Henle (d-CTAL, day 2–10 postnatal,nt=31); II. Mature thick ascending loop of Henle (m-CTAL, day 25–30,nt=23). Nephron segments were dissected without collagenase. Perfusion rates were similar per unit surface area in both functional states. Perfusate was ultrafiltrate of rabbit serum or artificial solution identical to bath medium, except when osmotic hydraulic permeability or steady-state ion gradients were analyzed. Results: Osmotic hydraulic permeability (bath medium hyperosmotic) was 4.72 cm3 cm−2 min−1 kPa−1 10−8 [or 4.78±(S.D.) 1.15 cm3 cm−2 min−1 atm−1 10−6] in the d-CTAL, similar to the m-CTAL (5.19±1.21). Solute transport (ratio of collected to perfused osmolarity) was hyperosmotic in the m-CTAL (0.62±0.13), but only 0.91±0.07 in the d-CTAL. Net sodium transport across the d-CTAL was 2.13±0.37 mol cm−1s−110−12, and 11.6±1.3 in the m-CTAL. Steady-state sodium concentration in the single loop lumen was 65±8 mmol·l−1 in the m-CTAL, and 97±2.7 in the d-CTAL (bath medium: 148 mmol·l−1). Conclusions: Differentiation of the thick ascending loop of Henle is characterized by constant osmotic hydraulic permeability. Sodium absorption, transepithelial steady-state sodium concentration gradient, and transtubular osmotic disequilibrium suggest that the electrolyte transport capacity of the single cell of the diluting segment is the major determinant of renal medullary countercurrent differentiation.