Vasopressin Effects on Urea and H20 Transport in Inner Medullary Collecting Duct Subsegments
| Title: | Vasopressin Effects on Urea and H20 Transport in Inner Medullary Collecting Duct Subsegments |
|---|---|
| Authors: | Sands, Jeff M.; Nonoguchi, Hiroshi; Knepper, Mark |
| Publisher: | American Journal of Physiology |
| Date Published: | November 01, 1987 |
| Reference Number: | 395 |
The inner medullary collecting duct (IMCD) is widely viewed as a single renal tubule segment with homogeneous properties. However, recent morphological and functional studies have raised the possibility that the initial and terminal parts of the IMCD may differ. To test this possibility further and to localize sites of action of arginine vasopressin (AVP) along the IMCD, we measured osmotic water permeability (Pf) and urea permeability (Purea) in isolated perfused rat IMCDs. In the initial third of the IMCD, 10 nM AVP increased Pf from 16 +/- 8 to 148 +/- 50 micron/s. The terminal two-thirds of the IMCD had a significantly higher basal Pf (70 +/- 12 micron/s), which increased to 186 +/- 25 micron/s with AVP. The initial IMCD had a relatively low basal Purea (3 +/- 1 X 10(-5) cm/s), which did not change with AVP. The terminal IMCD had a significantly higher basal Purea (17 +/- 4 X 10(-5) cm/s), which increased to a very high value (69 +/- 15 X 10(-5) cm/s) with AVP. The results support the premise that (from the point of view of vasopressin effects on water and urea transport) there are two functionally distinct parts of the inner medullary collecting duct: an initial part that resembles the cortical or outer medullary portions of the mammalian collecting duct and a terminal part that resembles the toad urinary bladder. The significance of these findings for the urinary concentrating mechanism is discussed.
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)
Researchers recognize five segments of the CD, the innermost segment of which is called the inner medullary collecting duct (IMCD). Until recently, researchers thought that the IMCD was uniform in structure and function. But more recent study implied there were two distinct structural and functional parts to the IMCD: the initial IMCD and the terminal IMCD.
Sands, et al, studied rat IMCDs to see if there were differences between the initial and terminal IMCD in terms of arginine vasopressin (AVP)-mediated water and urea permeability of the IMCD apical cell membranes. AVP is the antidiuretic hormone that increases water and urea permeability in the IMCD apical cells. This is an important part of the urine concentrating process, and the authors' study sought to further clarify the understanding of it.
They found the initial IMCD had low rates of urea and water permeability. That is, the apical membranes did not let much water or urea pass through them. When AVP was added to the initial IMCD, the degree of apical membrane water permeability (Pf) dramatically increased, but the degree of urea permeability (Purea) did not. The terminal IMCD had a significantly higher level of apical membrane Pf in the absence of AVP than did the initial IMCD. When AVP was added to the terminal IMCD, the membrane Pf increased significantly. The terminal IMCD had a significantly higher level of Purea than the initial IMCD, and when AVP was added to the terminal IMCD, there was a very high increase in Purea. So, as measured by AVP-mediated effects on water and urea transport, there are two functionally distinct parts of IMCD.
Knowledge of this functional difference can provide a basis for a clearer understanding of the urine concentrating process. The properties of the initial and terminal IMCDs are such that urea and water reabsorption will be distributed along the IMCD in such a way that the osmotic condition between the IMCD and the surrounding kidney tissue will be optimum for both urination and conservation of body water.
During diuresis (urination), AVP is not present and osmotic Pf and Purea are low in the initial IMCD so neither much water or urea is being reabsorbed there. However, in the terminal IMCD, Pf is high despite the absence of AVP. This allows a high level of water absorption in the terminal IMCD. This absorption is aided by the relatively high level of Purea in the terminal IMCD even without the presence of AVP.
During antidiuresis (the suppression of urinary excretion) AVP is present creating high Pf in the initial IMCD. Because AVP doesn't increase Purea in the initial IMCD, the water absorption concentrates urea within the initial IMCD. This increase creates the osmotic conditions between the IMCD and the kidney tissue surrounding it which will help reabsorb urea from the terminal IMCD when AVP significantly increases an already substantial Purea level. Water absorption from the terminal IMCD is less than that which occurs in the initial IMCD.