Role of inner medullary collecting duct NaCl transport in urinary concentration.

Mathematical modeling and simulation techniques were used to analyze the role of medullary collecting duct NaCl transport in the urinary concentrating process. The mathematical model incorporated experimentally determined epithelial transport parameters and anatomical parameters obtained chiefly from experiments in rabbit kidneys. The simulations predict that solute concentration profiles along the medullary collecting ducts are highly sensitive to the rate and pattern of active NaCl absorption along the length of the collecting duct system. When active NaCl absorption was assumed to be zero in the outer medullary collecting duct and to increase along the inner medulla to a very high value at the papillary tip, the simulated solute concentration profiles in the medullary collecting ducts as well as relative concentrations between different inner medullary structures agreed well with experimental data. However, despite optimal choice of collecting duct transport parameters and the use of experimentally determined permeability coefficients, only modest total solute gradients could be generated axially in the inner medullary interstitium, and passive luminal dilution did not occur in the thin ascending limb. We conclude: 1) Axial heterogeneity of transport properties along the inner medullary collecting duct must be assumed to explain in vivo findings from micropuncture and microcatheterization studies. 2) Active NaCl transport from the inner medullary collecting ducts is important chiefly for efficient conservation of NaCl rather than for concentration of solutes in the renal inner medulla. 3) Important inconsistencies exist between several previously reported experimental observations and the theoretical requirements for passive luminal dilution in the thin ascending limb of Henle's loop.