In the present study, we firstly demonstrated a significant hypotensive effect of captopril following a 10-day treatment. This acute duration treatment showed a 20% hypotensive response, whereby the baseline blood pressure of 191 mmHg was reduced to 151 mmHg (Table 1). When the sera of SHR were subjected to 2-DE and compared to similar profiles generated from those of SDR, the levels of five protein spots were found to be significantly altered. The spots were subsequently identified as those of the RBP4, C3, a 19.9 kDa fragment of ALB, A1MG and A1AT (Table 2). All proteins have been previously identified to have some association with hypertension. The levels the proteins were earlier reported to be altered in the SHR as well as patients with hypertension, although the experiments were carried out on individual proteins. In this study, the different altered levels of the five serum proteins were detected simultaneously using the gel-based proteomics approach. Interestingly, the altered expression of A1MG and A1AT appeared to be abrogated in the SHR that were treated with captopril as their serum levels were no longer significantly different from those of the SDR.
RBP4 is responsible of delivering retinol to tissues . Many studies have associated the serum/plasma protein with insulin resistance and diabetic complications [10, 11]. RBP4 was reported to be elevated in patients with pregnancy-induced hypertension, possibly as a result of perturbed maternal glucose metabolism , and in women with hypertension caused by insulin resistance . In contrast to these reports, our results demonstrated a significant reduction of RBP4 in both the non-treated and captopril-treated SHR groups. Treatment of the SHR with captopril did not appear to induce a significant change in the reduced serum levels of RBP4.
C3 is a complement component involved in both the humoral and innate immunity. The levels of C3 were found to be altered in the sera of patients with idiopathic pulmonary arterial hypertension , although the nature of this association is unclear. Studies performed by Lin et al. correlated the differential expression of C3 with the enhanced growth of arterial smooth muscle cells from SHR, prior to the development of hypertension . C3 apparently mediates the arterial smooth muscle cell growth in the rats. The data of our study further confirms the altered levels of C3 in SHR. Like RBP4, however, the levels of C3 in the sera of SHR that were treated with captopril remains significantly different from that expressed in the SDR.
Serum ALB levels have been previously associated with hypertension. In an epidemiological investigation, increasing albumin concentration in the serum within the physiological range was found to correlate with the increase in systolic and diastolic blood pressure in men and women in all age groups . However, the albumin spot that was significantly enhanced in both non-treated and captopril-treated SHR compared to SDR in our study appeared to be a 19.9 kDa fragment of the serum protein. These fragments may be a result of the proteolysis of serum albumin, although currently no information is available on the association of serum albumin with hypertension in rats.
A1MG, which is identical to the α2-macroglobulin that is present on the vascular endothelial cells in humans, binds to a group of serine proteases called tonins in rat tissues . Tonin acts on angiotensin I (Ang I), as well as angiotensinogen (AG) and other peptides presenting the N-terminal sequence of AG to form the vasoconstrictor peptide, angiotensin II (Ang II). The tonin-α1-macroglobulin complex can generate Ang II from Ang I despite complete inhibition of the Ang I converting enzyme . Studies have demonstrated the capability of tonin to release bradykinin directly, thus suggesting that the protease is involved in the kinintensin system that generates both the pressor (AngII) and depressor (bradykinin) . When taken together, these findings suggest that A1MG may play a role in the regulation of blood pressure. In the present study, the low levels of A1MG detected in the SHR compared to SDR provide some explanation for the increase in blood pressure in the SHR. The lowered levels were apparently normalised when the SHR were treated with captopril, which indicates an inverse correlation of the serum A1MG levels with hypertension.
A1AT is a serine protease inhibitor (serpin) that functions as an antitrypsin as well as an antithrombin . Its primary target includes elastase, plasmin and thrombin. A1AT protects the connective tissues (elastin) from inflammatory enzymes such as elastase in the lungs and pulmonary system, as well as helps to prevent blood coagulation. Levels of the serum acute-phase protein were shown to correlate positively with blood pressure in humans .
Similarly, our results indicated that the levels of A1AT were increased in the sera of SHR compared to SDR, suggesting an acute-phase response to the increase of blood pressure in the SHR. Treatment of the SHR with captopril appeared to demonstrate an apparent abrogation of the altered levels of A1AT in sera of the rats, which further suggests a direct correlation of A1AT levels with hypertension.