Altered Angiotensinogen Amino Acid Sequence and Plasma Angiotensin II Levels in Genetically Hypertensive Rats

Abstract

Abstract The components of the renin-angiotensin system have been implicated in the development of primary hypertension in humans and genetically hypertensive rats. In humans a mutation in the angiotensinogen gene and elevated plasma angiotensinogen levels have been linked to primary hypertension. Although we had previously excluded a linkage of blood pressure to the angiotensinogen gene in the stroke-prone spontaneously hypertensive rat (SHRSP), elevated angiotensin II (Ang II) levels in this strain compared with the normotensive reference, the Wistar-Kyoto rat (WKY), prompted us to investigate further into the origins and effects of altered Ang II regulation using a range of physiological, biochemical, molecular, and genetic approaches. Ang II plasma levels determined by radioimmunoassay were confirmed to be significantly elevated in SHRSP compared with WKY. Sequence comparison among the two rat strains revealed a mutation in the coding region of the angiotensinogen gene that results in an isoleucine-to-valine substitution in SHRSP at amino acid position 154 ( I154V ). We performed a cosegregation analysis in an F 2 intercross cohort bred from SHRSP and WKY from the University of Heidelberg (SHRSP HD and WKY HD ) to address the following questions: (1) whether this or another mutation of the angiotensinogen gene may be causally related to the observed differential Ang II plasma levels, (2) whether Ang II plasma levels may be correlated with blood pressure or organ hypertrophy, and (3) whether genetic linkage to the renin or angiotensin-converting enzyme ( ACE ) gene loci (the two classic regulatory enzymes of the renin-angiotensin system) may provide an explanation for elevated Ang II plasma levels. We measured Ang II plasma concentrations and determined systolic and diastolic pressures as well as heart rate by indwelling arterial catheters in conscious rats before and after dietary sodium loading. We found (1) no linkage of the angiotensinogen gene mutation to Ang II plasma levels, (2) no cosegregation of Ang II plasma levels with blood pressure or morphometric parameters, and (3) no linkage of Ang II plasma levels to either the renin gene or the ACE gene locus, excluding a genetically determined regulatory effect on Ang II plasma levels of either enzyme. Of all other parameters measured in this cross, only plasma renin activity showed a significant correlation with Ang II plasma levels. Thus, the demonstrated angiotensinogen mutation has no effect on plasma Ang II levels; Ang II plasma levels do not account for, do not contribute to, and are not caused by elevated blood pressure in SHRSP HD ; and altered Ang II plasma levels in SHRSP HD are not genetically determined by the renin or ACE gene. We conclude that elevated Ang II levels in SHRSP HD are likely to represent an independent, unrelated phenomenon that is of no direct relevance to the pathogenesis of hypertension.