HIGH-ACTIVITY OF LOW-MICHAELIS-MENTEN CONSTANT 3',5'-CYCLIC ADENOSINE MONOPHOSPHATE-PHOSPHODIESTERASE ISOZYMES IN RENAL INNER MEDULLA OF MICE WITH HEREDITARY NEPHROGENIC DIABETES-INSIPIDUS

Our previous studies on microdissected kidney tubule segments indicate that the failure of vasopressin (VP) to increase cAMP content in collecting ducts of mice with hereditary nephrogenic diabetes insipidus (NDI mice) is due to abnormally rapid cAMP catabolism via cyclic-3',5'-nucleotide phosphodiesterases (PDE). Furthermore, the VP-stimulated cAMP accumulation can be restored by addition of PDE isozyme-specific inhibitors. To elucidate the biochemical basis of the NDI syndrome, we analyzed PDE activities in extracts from inner medullary tissues of NDI mice and from control mice separated with use of ionex fast protein liquid chromatography on a Mono-Q column. In extracts of inner medullary tissues from either control or NDI mice, the low Michaelis-Menten constant (K(m)) cAMP-PDE activity specific for cAMP as a substrate (cAMP-PDE) was eluted from a Mono-Q column with linear sodium acetate gradient as peak 3 at Na-acetate concentration (0.75-0.93 M) and was well separated from fractions containing the Ca2+-calmodulin sensitive PDE. The cAMP-PDE activity in peak 3 was significantly higher in NDI mice (> DELTA + 100%) than in controls. The sensitivity to effect of cAMP-PDE isozyme-specific inhibitors, rolipram and cilostamide, indicates that peak 3 consists predominantly (approximately 75%) of the rolipram-sensitive PDE-IV isozyme and a minor portion (approximately 25%) of cilostamide-sensitive PDE-III isozyme in both control and NDI mice. Higher activity of PDE-IV in NDI mice was due to 2.4 times higher apparent maximum velocity compared to controls, whereas the apparent K(m) for cAMP was not different. Our results show that low K(m) cAMP-PDE activities, predominantly PDE-IV, are higher in inner medulla of NDI mice. We suggest that the higher activity of PDE-IV, and to a lesser degree perhaps also PDE-III, accounts for rapid cAMP hydrolysis, which prevents the increase of cAMP generated in the response to VP in collecting ducts of NDI mice.