Role of Inner Medullary Collecting Duct NaCl Transport in Urinary Concentration
| Title: | Role of Inner Medullary Collecting Duct NaCl Transport in Urinary Concentration |
|---|---|
| Authors: | Chandhoke, Paramjit S.; Saidel, Gerald M.; Knepper, Mark |
| Publisher: | American Journal of Physiology |
| Date Published: | November 01, 1985 |
| Reference Number: | 358 |
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.
This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)
The means by which this essential difference in solute concentrations is maintained differs at different parts of the nephron. For example, in that section of the kidney between the outermost part of the kidney (cortex) and the innermost section (inner medulla) called the outer medulla, the necessary solute concentration difference is generated by the thick ascending limb of Henle's loop (a section of the nephron) actively transporting salt (NaCl) out into the surrounding kidney tissue. The mechanics by which the differences in solution concentrations are generated in the inner medulla appear to be different, and researchers haven't settled on how the proper osmotic conditions are created.
In an attempt to determine how the concentration of solutes in the inner medulla interstitium (inner tissue) surrounding the medullary section of the nephron is generated, several researchers have examined the inner medullary collecting duct (IMCD). Here, researchers find evidence that the IMCD activity transfers NaCl from inside it to the inner medullary interstitium.
Chandhoke, et al., asked how the IMCD's active transport of NaCl affects the concentrating process in the inner medulla, and if it contributes to the urine concentrating process. In their study, they used mathematical modeling and simulation techniques to evaluate the role of active NaCl absorption by the IMCD in the urinary concentrating mechanism. They matched their simulations, all of which were based on different assumptions of the rate of NaCl absorption in the outer and inner medullary collecting duct, against experimental data from the experiments with studies on the urine concentration process in live rabbits. The authors used the mathematical simulations to identify what transport properties the IMCD must possess to explain results from the experiments on the rabbits. The results indicated that for the simulation to match the solute concentration profiles found in the rabbit kidneys the mathematical model must assume that the IMCD transport NaCl in a differentiated manner: from a very low level in the outer portion of the inner medulla and a very high level at its tip at the end.
The authors also conclude that active NaCl transport from the IMCD is important chiefly for efficient conservation of NaCl rather than for concentration of solute in the kidney's inner medulla.