Dual Actions of Vasopressin and Oxytocin in Regulation of Water Permeability in Terminal Collecting Duct

Title: Dual Actions of Vasopressin and Oxytocin in Regulation of Water Permeability in Terminal Collecting Duct
Authors: Han, Jin Suk; Maeda, Yoshitaka; Knepper, Mark
Publisher: American Journal of Physiology
Date Published: July 01, 1993
Reference Number: 354
We conducted studies in isolated perfused terminal inner medullary collecting ducts (IMCD) from rats to investigate the roles of oxytocin and vasopressin in the regulation of osmotic water permeability. Vasopressin and oxytocin were found to have both stimulatory effects (at 0.1 nM) and inhibitory effects (at 10 nM) on osmotic water permeability. Measurements of adenosine 3',5'-cyclic monophosphate (cAMP) production demonstrated that both vasopressin and oxytocin increase cAMP production. Both the selective oxytocin-receptor agonist [Thr4,Gly7]oxytocin (10 nM) and the selective V1b agonist [deamino1,D-3-(pyridyl)Ala2,Arg8]vasopressin (10 nM) inhibited vasopressin-stimulated osmotic water permeability. In contrast, the selective V1a vasopressin-receptor agonist [Phe2,Ile3,Orn8]vasopressin (10 nM) had no effect on vasopressin-stimulated osmotic water permeability. These effects on water permeability correlated with the ability of the agents to transiently increase intracellular free calcium. The oxytocin/vasopressin-receptor antagonist [des-glycinamide9,d(CH2)5(1),O-Me-Tyr2,Thr4,Orn8]vasot ocin, which almost completely blocks vasopressin-induced calcium mobilization, also blocked the ability of 10 nM vasopressin to inhibit osmotic water permeability relative to that found with 0.1 nM vasopressin. We conclude the following. 1) Oxytocin, like vasopressin, has dual effects on osmotic water permeability, increasing it at subnanomolar concentrations and inhibiting it at suprananomolar concentrations. 2) Oxytocin, like vasopressin, can increase cAMP production, perhaps accounting for the increase in water permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

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 current view of the molecular process leading to the reabsorption of body water flowing through the inner medullary collecting duct (IMCD) gives primacy to the antidiuretic hormone, arginine vasopressin (AVP). This view holds that when AVP binds to the vasopressin-2 receptor (V2R) located on the basolateral membranes of the principal cells of the kidney collecting duct, a molecular sequence begins which increases the water permeability (Pf) of the collecting duct cells: AVP binds with V2R. This stimulates the enzyme complex, adenylyl cyclase (AdC) which, in turn, elevates levels of cyclic adenosine monophosphate (cAMP). This induces aquaporin-2 to travel to and insert itself into the apical membrane of the collecting duct principal cells. This dramatically increases their Pf, and this is how the kidney is able to reabsorb water flowing through the IMCD.

Another way of saying this is that when AVP binds with V2R, it stimulates the AdC pathway to increase IMCD Pf. However, AVP also produces other effects in IMCD cells. It stimulates temporary increases in intracellular free calcium; it increases the production of inositol trisphosphate; and it stimulates prostaglandin synthesis. AVP does this by binding with other vasopressin receptors other than V2R and then stimulating, not the AdC pathway, but the phosphoinositide signaling pathway in IMCD cells. These receptors may be vasopressin receptors such as V1a, but they also might include oxytocin (OXT) receptors.

OXT is one of the two major hormones (the other being AVP) made in the magnocellular hypothalamic neurons and stored in the posterior lobe of the pituitary. It helps in uterine contraction and breast milk ejection, and it contributes to the second stage of labor. But, the author's work suggests that it may be worthwhile to investigate whether OXT plays a role as an antidiuretic hormone as well.

Jin Suk Han, et al., studied the role of AVP-mediated activation of the phosphoinositide pathway to see if it played a role in the overall regulation of osmotic Pf in rat IMCD cells. Previous studies revealed that concentrations of 10nM of AVP inhibited rabbit collecting duct Pf due to the activation of the phosphoinositide signaling pathway by AVP, which was associated with transient increases in intracellular calcium in the collecting duct cells.

The authors treated laboratory cultures of rat IMCD with two different concentrations of AVP and OXT, a subnanomolar concentration (0.1nM) and a suprananomolar concentration (10 nM). Then they determined whether or not the IMCD Pf was increased or not. They found both AVP and OXT increased IMCD Pf at subnanomolar concentrations, and both inhibited IMCD Pf at suprananomolar concentrations. The authors expressed the view that suprananomolar concentrations of OXT and AVP inhibit osmotic Pf by activating the phosphoinostide pathway and through the associated temporary increases of intracellular calcium.

The authors also demonstrated that OXT, like AVP, can increase cAMP production in IMCD cells (which perhaps accounts for its ability to increase IMCD Pf). However, it can only do this at suprananomolar concentrations. OXT appears to be nearly as potent as AVP in increasing IMCD Pf. Since it does this, and since it can increase cAMP production, it is likely that OXT can bind to a receptor that is coupled to AdC. It could be that OXT simply binds to the V2R, the same receptor that AVP binds with to initiate increased Pf in IMCD cells. Studies with the cloned V2R show that OXT binds with V2R only at concentrations of at least 10nM. This could explain why OXT could stimulate cAMP only at 10nM in the current study.

However, the picture is not entirely clear. OXT can increase IMCD Pf at 0.1 nM (the subnanomolar concentration), but can only increase cAMP levels at 10nM. cAMP must be stimulated after AVP binds with V2R if an increase in IMCD is going to occur. Does OXT take a different signaling pathway to increase water permeability? Or are the authors' tests not sufficiently sensitive to measure small increases in cAMP that may be sufficient to increase Pf? The authors believe the latter is possible, but do not exclude that OXT's ability to increase IMCD Pf may not require cAMP.

Then the authors tested to determine the relative roles of OXT and AVP receptors in AVP's Pf stimulating action. (Receptors are molecular structures that form weak, reversible chemical bonds with specific extracellular agents like hormones. When the receptor and hormone bind, it initiates specific molecular sequences that have a specific physiological result.)

To determine the relative roles of the receptors, they authors systematically prevented each receptor (The OXT receptor (OXTR), the V1b receptor (V1bR), and the V1aR) from functioning, and then they measured the Pf of the IMCD cells in response to AVP. When OXTR was blocked, and when V1bR was blocked, IMCD cell Pf was inhibited. When V1aR was blocked, there was no inhibition of AVP-stimulated Pf. In a companion study, the authors argued that the rat IMCD possesses an OXT receptor and a non-V1a, non-V2 AVP receptor, both of which are coupled to intracellular calcium mobilization pathways. The authors propose that one or both of these receptors mediate the inhibitory effects of suprananomolar concentrations of AVP and OXT. As to the possibility that OXT could act as another antidiuretic hormone, that remains to be determined by further research.

The authors conclude that the inhibition of Pf by suprananomolar concentrations of AVP and OXT is associated with their ability to cause intracellular calcium mobilization (and presumably activation of the phosphoinositide pathway). Their inhibitory effect on Pf is also associated with the binding of AVP or OXT to either an OXT receptor, a novel vasopressin receptor, or both.