Renal Expression of Angiotensinogen Protein in Angiotensin II-Infused Hypertensive Rats Maintained on High Salt Diet
Chronic infusion of angiotensin (Ang) II leads to the development of hypertension and enhances intrarenal AngII content to levels greater than can be explained from the circulating concentrations. We previously reported that angiotensinogen mRNA is enhanced in AngII-dependent hypertension and may contribute to augmented intrarenal AngII levels. It has been shown that high salt diet suppresses renal expression of angiotensinogen mRNA. The purpose of the present study was to test the hypothesis that AngII augments renal angiotensinogen protein synthesis under condition of suppressed renal angiotensinogen production by high salt diet. We examined the effect of chronic infusion of AngII on angiotensinogen protein levels in male Sprague-Dawley rats (BW 158±10 g, n=12) maintained on an 8% salt diet. AngII was administered via osmotic minipumps (40 ng/min) to one group (n=6) while the remaining rats were sham-operated (n=6). High salt diet alone did not alter systolic BP in sham animals (109±6 mmHg at day 12); however, combination of AngII infusion and high salt diet significantly increased systolic BP (167±7 at day 12) and intrarenal AngII content (459±107 fmol/g vs. 270±42) in spite of a marked suppression of plasma renin activity (0.88±0.22 ng AngI/mL/h vs. 2.79±1.31). Western blot analysis of protein extracts from kidney (15 μg) showed an immunoreactive band at ∼52 kDa, while plasma protein (1.25 μg) showed two bands at ∼52 kDa and ∼64 kDa using a specific angiotensinogen polyclonal antibody (1:5000). Densitometric analysis of the immunoreactive bands showed that the combination of AngII infusion and high salt diet significantly increased kidney (88%) and plasma (61% for 52 kDa and 4.0 fold for 64 kDa) angiotensinogen protein compared with sham animals. Thus, AngII infusion to rats fed high salt diet elicited progressive hypertension and elevated intrarenal AngII through enhanced kidney and plasma angiotensinogen protein expression despite suppressed plasma renin activity. These data suggest that the augmented intrarenal AngII in AngII-dependent hypertension occurs, in part, by activated renal expression of angiotensinogen protein.