Role of cAMP-Phosphodiesterase Isozymes in Pathogenesis of Murine Nephrogenic Diabetes Insipidus

Title: Role of cAMP-Phosphodiesterase Isozymes in Pathogenesis of Murine Nephrogenic Diabetes Insipidus
Authors: Homma, Sumiko; Gapstur, Susan M.; Coffey, Aline K.; Valtin, Heinz; Dousa, Thomas P.
Publisher: American Journal of Physiology
Date Published: August 01, 1991
Reference Number: 320
To test the hypothesis that rapid adenosine 3',5'-cyclic monophosphate (cAMP) catabolism via cyclic 3',5'-nucleotide phosphodiesterase (PDE) is a cause of the unresponsiveness to vasopressin (VP) in mice with hereditary nephrogenic diabetes insipidus (NDI), we investigated properties of PDEs and other aspects of the VP-dependent cAMP-signaling system in segments of collecting ducts [inner medullary (IMCD), cortical (CCD), and outer medullary (OMCD) ducts] microdissected from control mice and mice with NDI. The activity of cAMP-PDE, but not of cGMP-PDE, was markedly higher in IMCD (+109%), and to a lesser degree in OMCD (+41%) and CCD (+27%), of NDI mice than in normal controls. The cAMP-PDE in IMCD of NDI mice was more sensitive to inhibition by the PDE isozyme-specific inhibitors rolipram and cilostamide, but not by 3-isobutyl-1-methylxanthine, than was the cAMP-PDE in controls. Levels of cAMP in intact IMCD and CCD from NDI mice completely failed to increase in response to 10(-6) M VP. Incubation with rolipram alone, but not with cilostamide alone, restored VP-dependent cAMP accumulation in IMCD of NDI mice to the levels found in control mice; addition of cilostamide further enhanced the effect of rolipram. Analogous (but quantitatively lesser) anomalies of the VP-dependent cAMP system, including the effects of PDE inhibitors, were observed also in CCD of NDI mice. However, the activity of VP-stimulated adenylate cyclase assayed in permeabilized IMCD did not differ in NDI and control mice. These results indicate that anomalously high activities of low-Km cAMP-PDE isozymes account for the failure of collecting ducts of NDI mice to increase cAMP levels in response in VP.(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)

Kidney collecting ducts must respond to the antidiuretic hormone, arginine vasopressin (AVP), if they are to concentrate urine and reabsorb the body water flowing through them. Normally, AVP binds with the vasopressin-2 receptor, located in the principal cells of the kidney collecting duct. This elevates levels of adenylyl cyclase, and this elevates levels of cAMP. cAMP signals a process which results in water-transporting proteins called aquaporin-2s (AQP2s) being inserted in the membranes of the principal cells. This makes the membranes more water permeable, and that is how the kidney is able to concentrate urine and reabsorb the water flowing through the collecting duct.

The collecting duct (CD) has three sections: (1) the innermost section, called the inner medullary CD (IMCD), (2) the next most inner section, the outer medullary CD (OMCD), and (3) the outermost section, the cortical CD (CCD). Their previous research led Homma, et al., to test whether high levels of certain enzymes of nucleotide phosphodiesterase PDE, specifically PDE-III and PDE-IV, is what is responsible for the collecting ducts of mice with inherited nephrogenic diabetes insipidus (NDI) being unable to respond to AVP.

The authors speculated that PDE-III and PDE-IV broke down cAMP before it could carry out its step in the molecular sequence that allows water reabsorption and urine concentration in the collecting duct. To test their hypothesis, the authors dissected the kidneys of NDI mice, separating them into IMCD, OMCD and CCD. By separating the CD into its component parts, the authors could measure more precisely the level and type of PDE activity in each part. Kidneys from normal mice were prepared identically to serve as a control group.

The authors found there was a significantly higher level of PDE activity that rapidly degrades cAMP in NDI mice than in the control mice. This held true for all segments of the CD, especially the IMCD. Now the authors wanted to confirm that of all the different PDE enzymes, it was PDE-IV and PDE-III that was active in the CD. To do this, they infused the dissected CDs with rolipram, which inhibits PDE-IV; cilostamide, which inhibits PDE-III; and IBMX, which inhibits all PDE.

IBMX inhibited PDE to a similar degree in both control and NDI mice CDs. Rolipram significantly inhibited PDE activity in the IMCD of NDI mice. So did cilostamide, but to a lesser extent. There was no such effect in the control IMCD. The same pattern held true for the OMCD of NDI mice, though the difference was less distinct.

Next, the authors administered a stimulatory dose of AVP to both the NDI and control CD tissues. This did not increase cAMP levels in the NDI tissues, whereas it did in the control tissues. When they added rolipram to the NDI IMCD tissue, then administered AVP, there was a five-fold increase in cAMP accumulation. Though there was no cAMP increase when cilostamide was added, when both cilostamide and rolipram were added to the NDI mice tissue, the cAMP accumulation rose twice as much as with rolipram alone. Similar results were found in the CCD of NDI mice. This indicated that when PDE-IV and PDE-III were inhibited, cAMP was not degraded at an abnormally rapid rate

This showed that it was high levels of PDE-IV and, to a lesser extent, PDE-III that was degrading cAMP before it could signal AQP2s to insert themselves in CD cell membranes.