Kinetics of Urea and Water Permeability Activation by Vasopressin in Rat Terminal IMCD

Title: Kinetics of Urea and Water Permeability Activation by Vasopressin in Rat Terminal IMCD
Authors: Wall, Susan M.; Han, Jin Suk; Chou, Chung-Lin; Knepper, Mark
Publisher: American Journal of Physiology
Date Published: June 01, 1992
Reference Number: 392
The vasopressin-regulated urea carrier and the vasopressin-regulated water channel are distinct transporters present in the apical membrane of the inner medullary collecting duct (IMCD) cells. To assess whether these transporters may be activated by common mechanisms, we investigated the time course of increase of urea and water permeability in response to vasopressin in isolated perfused terminal IMCD segments. The permeability responses were determined through the use of a specially designed continuous-flow fluorometer for rapid analysis of collected tubule fluid samples. The time courses of activation of the two transporters by vasopressin were virtually identical. Both urea and water permeability displayed a rapid initial increase for the first 10 min followed by a slower secondary response lasting at least 30 additional min. The lag periods between vasopressin addition and the initial rise in permeability were the same for urea (34.2 +/- 8.8 s) and water (34.8 +/- 8.9 s) transport activation. Furthermore, the initial rate of permeability increase (normalized by the total increase) was not significantly different for the two transport processes. The lag periods for the increase in urea permeability in response to 8-bromoadenosine 3',5'-cyclic monophosphate and vasopressin were not significantly different. The results are consistent with the view that the rate-limiting step in vasopressin-induced activation is the same for both the urea carrier and water channel and may lie at a step beyond generation of adenosine 3',5'-cyclic monophosphate.

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 kidney collecting duct (CD) has five segments recognized by researchers. The innermost section is called the terminal inner medullary CD (terminal IMCD). IMCD cells respond to the antidiuretic hormone, arginine vasopressin (AVP), by increasing the amount of water and urea that can flow through their apical membrane. This increase is called an increase in water permeability (Pf) and urea permeability (Purea), and it is an important step in the kidneys' urine concentrating process.

Although the terminal IMCD cells respond to AVP by increasing both apical membrane Pf and Purea, they do so by different means. Aquaporin-2(AQP2), an intrinsic membrane protein, is the means by which the cell increases its apical membrane Pf. Vasopressin-regulated urea transporters (VRUT), which is presumed to also be an intrinsic membrane protein, is the means by which the apical membrane's Purea is increased. Researchers don't know if AQP2 and VRUT share the same molecular pathway. That is, when AVP binds with the vasopressin-2 receptor on the basolateral membranes of the terminal IMCD cells, it sets up a molecular sequence which increases apical membrane Pf and Purea. But it is not known if the same sequence that results in the Pf increase also causes the Purea increase, or whether each transporter (AQP2 and VRUT) is activated by a different sequence.

To investigate whether these transporters share the same molecular sequence or not, Wall, et al., compared the effects of AVP on apical membrane Pf and Purea over a 60 minute period of time. The authors thought if both permeabilities changed equally (i.e. if the membranes increased their degree of Pf and Purea) over the same period of time, it would be an indication that the two transporters responsible for that change (AQP2 and VRUT) were regulated by the same molecular sequence.

The authors experimented on rat terminal IMCD cell preparations, exposing them to AVP and then measuring the effects on Pf and Purea over 60 minutes. The apical membrane Pf and Purea both displayed a rapid initial increase for the first 10 minutes after AVP exposure. This was followed by an almost identical slower secondary increase in Pf and Purea. Thus, the authors found that both AQP2 and VRUT increased the membrane permeability to water and urea by a highly similar rate and at virtually the identical time frame. Further, the lag time between when AVP was introduced to the terminal IMCD tissue and the beginning of the first rapid increase in permeabilities was the same for both urea and water transport activities.

These findings that the AVP-mediated increase in apical membrane PF and Purea occur at nearly the same rate over the same period of time indicated that the rate-limiting step (i.e. the key variable that allows the process to go forward) is the same for both VRUT and AQP2. As to what that rate-limiting step may be is unclear.

Researchers know that the rate-limiting barrier to water and urea movement across the terminal IMCD cells is the apical membrane. That is, the apical membrane normally doesn't let much water and urea through. Researchers know that when AVP binds with V2R it stimulates a Gs protein which in turn stimulates accumulation of cyclic adenosine monophosphate (cAMP). cAMP activates protein kinase C, which phosphorylates AQP2 vesicles, allowing them to travel to the apical membrane and fuse with it. Then the AQP2 is inserted into the apical membrane, allowing much more water than usual to pass through it. When AVP absents itself from the cell, the AQP2s are retrieved from the membrane and taken back into the cell interior in sacs called endosomes. So, the regulation of Pf is due to the addition and removal of AQP2s.

Researchers aren't as clear as to how VRUT is activated. Endosomes retrieved from the cells after AVP stimulation contain AQP2s but not VRUTs. This indicates that the two transporters are segregated during the vesicle cycling process. This indicates either that VRUTs are retrieved from the apical membrane by different vesicles or that regulation of VRUTs does not involve cycling them back and forth from the membrane.