Ovine aquaporin-2: cDNA cloning, ontogeny and control of renal gene expression

The aim of this study was to test the hypothesis that the relative insensitivity of the ovine fetal kidney to arginine vasopressin (BVP) is due to low levels of expression of the gene for aquaporin-2 (AQP(2)) which encodes the AVP-regulated water channel. We report the cloning of the cDNA for the ovine AQP(2) which has a major transcript at 4.2 kilobases (kb) and a minor transcript at 1.5 kb, resembling the human gene transcripts. At 40-60 days' (term = 145-150 days'), mRNA levels are very low, detectable only by reverse transcriprion-polymerase chain reaction (RT-PCR). By Northern blot analysis AQP(2) mRNA is detectable at 75 days'. The ratio of AQP(2)/glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA increases approximately 2.4-fold between 100 and 140 days' when it is about 41% of adult values. Both glucocorticoids and the renin-angiotensin system are involved in maturation of renal function. When fetuses at 75 or 85 days of gestation were exposed to high levels of dexamethasone for 2-3 days, mRNAs for both GAPDH and AQP(2) doubled, but the ratio was unchanged. Angiotensin I, infused for 3 days at 115-120 days' gestation, increased the AQP(2)/GAPDH mRNA ratios by twofold (major transcript) and sixfold (minor transcript), which were highly significant (P<0.001). The increasing sensitivity of the ovine fetal kidney to AVP, from 100-140 days of gestation, is largely due to increasing AQP(2) gene expression over this period.