Chronic NO Restriction in Hypertensive Rats Increases Abdominal but Not Thoracic Aortic Intrinsic Stiffness via an Augmentation in Profibrotic Materials

Abstract

The spontaneously hypertensive rat model with reduced NO synthesis (SHRLN) shares features with aging and hypertension in humans, among other a severe aortic stiffening. The present in vivo study aimed to compare thoracic (TA) and abdominal (AA) aortic stiffness in the SHRLN (treated 5 weeks with L-NAME), SHR, and normotensive Wistar Kyoto (WKY). Dynamic properties of TA and AA were measured in the same rats, using echotracking recording of aortic diameter coupled with blood pressure (BP). Measurements were performed first at operating BP and then after BP reduction in hypertensive rats, thus in isobaric conditions. Histological staining and immunohistochemistry were used for structural analysis at both sites. At operating pressure, BP and pulse pressure (PP) were higher in SHRLN compared with SHR. Stiffness index was also increased and distensibility decreased in both TA and AA in SHRLN. At WKY-matched blood pressure, isobaric AA parameters remained specifically altered in SHRLN, whereas TA recovered to values identical to WKYs. Collagen, fibronectin, α5-selectin, and FAK were increased in SHRLN compared with SHR or WKY. Nevertheless, only the strong accumulations of fibronectin and collagen at the AA site in SHRLN were associated with intrinsic stiffening. In conclusion, we confirm that NO restriction associated with hypertension induces a severe pathological phenotype and shows that L-NAME induced stiffening is more pronounced in AA than in TA as a result of greater fibrosis.

Figure 1

(a) time-evolution of systolic blood pressure (SBP, triangles), diastolic blood pressure (DBP, circles), and pulse pressure (PP, squares) in spontaneously hypertensive rat (SHR, grey symbols, and n=8) and in SHR during 5-week L-NAME treatment (SHRLN, black symbols, and n=9) and (b) short-term blood pressure variability as ARV (average real variability) are shown with similar symbols. ∗: p<0.05 one way ANOVA and Dunnett's posttest on time effect.

Figure 2

Mean blood pressure (MBP) and stiffness index are compared in the three groups of rats. Black bars: spontaneously hypertensive rats treated with L-NAME (SHRLN), grey bars: SHR without treatment, and white bars: normotensive WKY. Thoracic aorta (left graph) and abdominal aorta (right graphs) parameters are shown. A: SHRLN, parameters at basal pressure, and n= 8; B: SHRLN n=5, at BP matched with that of SHR n=6; C: both SHRLN n=7 and SHR n=6 at BP matched with that of WKY n=6. N are similar at TA and AA except n=5 for WKY at AA. One way ANOVA and Tukey posttest comparison: ∗: p<0.05 compared to WKY and #: p<0.05 compared to SHR.

Figure 3

Changes in pressure and diameter over time emphasized the difference between thoracic aorta (TA) and abdominal aorta (AA) stiffening in hypertensive rats with reduced NO (SHRLN). The dynamic pressure wave and diameter distension waves throughout the cardiac cycle are shown for SHRLN TA and AA at basal mean blood pressure (MBP), n=7 and 8, respectively, and at reduced MBP (after clonidine administration), n=5 for each site. BP levels are similar for TA and AA whereas only TA distension recovers when BP decreases. Bars on the right quantify these data via measuring the area under the curve (AUC adjusted to heart rate). Despite similar pulse pressures, only the thoracic aorta is shown to increase when blood pressure is reduced. ∗: isobaric value differs from basal value (P < 0.05).

Figure 4

Comparison of fibronectin (a) and collagen (b) accumulation in the thoracic (TA) and abdominal (AA) aorta between SHRLN n=8, SHR n=6, and WKY n=6. The data show that AA levels of fibrosis markers were specifically higher in AA from SHRLN in agreement with stiffness data in vivo. Whilst collagen was also higher in AA in the SHR, these data show that fibronectin (Fn) content of the SHR AA remained similar to those of TA unlike the findings in the SHRLN. ∗: p<0.05 AA versus TA in the same group of rats. Statistics between groups are shown in Table 3.