The Milan hypertensive rat as a model for studying cation transport abnormality in genetic hypertension

Altered expression of renal aquaporins and α-adducin polymorphisms may contribute to the establishment of salt-sensitive hypertension

BACKGROUND

Sodium-sensitive hypertension is caused by renal tubular dysfunction, leading to increased retention of sodium and water. Previous findings have suggested that single-nucleotide polymorphisms of the cytoskeletal protein, α-adducin, are associated with increased membrane expression of the Na/K pump and abnormal renal sodium transport in Milan hypertensive strain (MHS) rats and in humans. However, the possible contribution of renal aquaporins (AQPs) to water retention remains undefined in MHS rats.

METHODS

Kidneys from MHS rats were analyzed and compared with those from age-matched Milan normotensive strain (MNS) animals by quantitative-PCR, immunoblotting, and immunoperoxidase. Endocytosis assay was performed on renal cells stably expressing AQP4 and co-transfected either with wild-type normotensive (NT) or with mutated hypertensive (HT) α-adducin.

RESULTS

Semiquantitative immunoblotting revealed that AQP1 abundance was significantly decreased only in HT MHS whereas AQP2 was reduced in both young pre-HT and adult-HT animals. On the other hand, AQP4 was dramatically upregulated in MHS regardless of the age. These results were confirmed by immunoperoxidase microscopy. Endocytosis assays clearly showed that the expression of mutated adducin strongly reduced the rate of constitutive AQP4 endocytosis, thereby increasing its abundance at the plasma membrane.

CONCLUSIONS

We provide here the first evidence that AQP1, AQP2, and AQP4 are dysregulated in the kidneys of MHS animals. In particular, we provide evidence that α-adducin mutations may be responsible for AQP4 upregulation. The downregulation of AQP1 and AQP2 and the upregulation of AQP4 may be relevant for the onset and maintenance of salt-sensitive hypertension.

Glomerulus number and blood pressure in the Prague hypertensive rat

The kidney has long been attributed a key role in the pathogenesis of hypertension. Reduction of filtration area by glomerular loss is regarded currently as a major causative mechanism. Here we analyze the relationship between glomerulus number and blood pressure (BP) in a new model of genetic hypertension and the Prague hypertensive rat (PHR) and the Prague normotensive rat (PNR). Glomerular numbers were determined in 7- to 53-week-old PNR and PHR, and the correlation with conscious systolic BP was analyzed. PHR had significantly higher BP but 19% fewer glomeruli than PNR. Glomerular number correlated (partial correlation analysis, controlling for effects of body weight, age, and kidney weight) significantly (P < 0.01, r2 = 0.46) with BP in male PHR but not in female PHR or in PNR. Moreover, subgroups of PHR and PNR selected for the same mean BP showed the same differences in glomerular counts, and subgroups selected for the same mean glomerular count showed the same differences in BP as the whole group. Reduced glomerular numbers and BP seem not to be causally related to BP in PHR older than seven weeks. Other mechanisms, such as genetically determined changes in transporter and receptor proteins, vascular abnormalities, and humoral mechanisms, must be considered.

Molecular cloning of an adducin-like protein: evidence of a polymorphism in the normotensive and hypertensive rats of the Milan strain

Differences genetically associated with the development of hypertension in a strain of genetically hypertensive rat (MHS) were described in ion transport across erythrocyte membranes compared to normotensive control (MNS). Antibodies against the MNS ghost proteins were raised in the MHS, producing an immunoreaction against a 105 KDa protein later identified as adducin. A clone coding for a portion of mouse adducin was isolated with these antibodies. Using this clone, overlapping cDNA clones coding for a 63 KDa adducin-like protein were isolated. A family of related mRNAs of about 3500, 3800, 4200 nt, was found to be present in spleen, kidney and heart tissues. Similar mRNAs and an additional tissue specific 8000 nt mRNA are present in brain. All mRNAs seem to be generated by alternative splicing from the transcript of a single gene. An interesting polymorphism, a Gln to Arg substitution, was detected in the carboxiterminal area of rat adducin 63.