Keith Ng

Aortic stiffness is associated with vascular calcification and remodeling in a chronic kidney disease rat model

Increased aortic pulse-wave velocity (PWV) reflects increased arterial stiffness and is a strong predictor of cardiovascular risk in chronic kidney disease (CKD). We examined functional and structural correlations among PWV, aortic calcification, and vascular remodeling in a rodent model of CKD, the Lewis polycystic kidney (LPK) rat. Hemodynamic parameters and beat-to-beat aortic PWV were recorded in urethane-anesthetized animals [12-wk-old hypertensive female LPK rats (n = 5)] before the onset of end-stage renal disease and their age- and sex-matched normotensive controls (Lewis, n = 6). Animals were euthanized, and the aorta was collected to measure calcium content by atomic absorption spectrophotometry. A separate cohort of animals (n = 5/group) were anesthetized with pentobarbitone sodium and pressure perfused with formalin, and the aorta was collected for histomorphometry, which allowed calculation of aortic wall thickness, medial cross-sectional area (MCSA), elastic modulus (EM), and wall stress (WS), size and density of smooth muscle nuclei, and relative content of lamellae, interlamellae elastin, and collagen. Mean arterial pressure (MAP) and PWV were significantly greater in the LPK compared with Lewis (72 and 33%, respectively) animals. The LPK group had 6.8-fold greater aortic calcification, 38% greater aortic MCSA, 56% greater EM/WS, 13% greater aortic wall thickness, 21% smaller smooth muscle cell area, and 20% less elastin density with no difference in collagen fiber density. These findings demonstrate vascular remodeling and increased calcification with a functional increase in PWV and therefore aortic stiffness in hypertensive LPK rats.

Heart Rate Dependence of Aortic Pulse Wave Velocity at Different Arterial Pressures in Rats

Arterial stiffness, as measured by aortic pulse wave velocity (PWV), is an independent marker of cardiovascular disease and events in both healthy and diseased populations. Although some cardiovascular risk factors, such as age and blood pressure, show a strong association with PWV, the association between heart rate (HR) and PWV is not firmly established. Furthermore, this association has not been investigated at different arterial blood pressures. To study effects of HR on aortic PWV at different mean arterial pressures (MAPs), adult (12 weeks; n=7), male, anesthetized Sprague-Dawley rats were randomly paced at HRs of between 300 and 450 bpm, at 50-bpm steps. At each pacing step, aortic PWV was measured across a physiological MAP range of 60 to 150 mmHg by infusing sodium nitroprusside and phenylephrine. When compared at the same MAP, increases in HR resulted in significant increases in PWV at all of the MAPs >80 mmHg (ANOVA, P<0.05), with the greatest significant change of 6.03±0.93% observed in the range 110 to 130 mmHg. The positive significant association between HR and PWV remained when PWV was adjusted for MAP (ANOVA, P<0.001). These results indicate that HR dependency of PWV is different at higher pressures than at lower pressures and that HR may be a confounding factor that should be taken into consideration when performing analysis based on PWV measurements.

Angiotensin-converting enzyme inhibitor limits pulse-wave velocity and aortic calcification in a rat model of cystic renal disease.

The effect of angiotensin-converting enzyme inhibition on function and structure of the aorta was studied in the Lewis polycystic kidney (LPK) rat model of cystic renal disease and Lewis controls. Pulse-wave velocity (PWV) was recorded under urethane anesthesia (1.3 g/kg ip) in mixed-sex animals aged 6 and 12 wk and in 12-wk-old animals treated with perindopril (3 mg·kg(-1)·day(-1) po) from age 6-12 wk. Tail-cuff systolic pressures were recorded over the treatment period. After PWV measurements, animals were euthanized and the aorta was removed for histomorphological and calcium analysis. Hypertension in LPK at 6 and 12 wk was associated with a shift of the PWV curve upward and to the right, indicating a decrease in aortic compliance, which was significantly reduced by perindopril. LPK demonstrated greater aortic calcification (6 wk: 123 ± 19 vs. 65 ± 7 and 12 wk: 406 ± 6 vs. 67 ± 6 μmol/g, P < 0.001, LPK vs. Lewis, respectively). This was reduced by treatment with perindopril (172 ± 48 μmol/g, 12 wk LPK P < 0.001). Medial cross-sectional area and elastic modulus/wall stress of the aorta were greater in LPK vs. Lewis control animals at 6 and 12 wk of age and showed an age-related increase that was prevented by treatment with perindopril (P < 0.001). Perindopril also ameliorated the degradation of elastin, increase in collagen content, and medial elastocalcinosis seen in 12-wk LPK. Overall, perindopril improved the structural and functional indices of aortic stiffness in the LPK rats, demonstrating a capacity for angiotensin-converting enzyme inhibition to limit vascular remodeling in chronic kidney disease.

Persistent effect of early, brief angiotensin-converting enzyme inhibition on segmental pressure dependency of aortic stiffness in spontaneously hypertensive rats.

Objective: The effect of brief, prehypertensive therapy by angiotensin-converting enzyme inhibition (ACEi) with perindopril on the structure and function of large arteries was studied in rats. Methods: Spontaneously hypertensive rats (SHR) received perindopril (3 mg/kg per day) between the ages of either 6 and 10 weeks (early) or 20 and 24 weeks (late) and compared with nonmedicated SHR and normotensive (Wistar–Kyoto) controls (n = 6 per group). Haemodynamic parameters and segmental aortic stiffness assessed by pulse wave velocity (PWV) were measured under urethane anaesthesia [1.3 g/kg, intraperitonealy (i.p.)] at 25 weeks. Aortic elastin and collagen content were determined by histomorphometry and calcium content by atomic absorption spectrophotometry. Results: Rats receiving brief, early perindopril therapy had lower tail-cuff SBP than nonmedicated SHR (121 ± 1 mmHg, 179 ± 5 mmHg, respectively, P < 0.05), reflected in lower thoracic and abdominal aortic pulse pressures. Aortic pulse pressure amplification was lower with early perindopril treatment (early treated 10 ± 5%, nonmedicated 20 ± 10%, P < 0.05). No blood pressure (BP) differences were observed between late treated and nonmedicated SHR. Isobaric thoracic PWV was lower when treated early but not later in life. Treatment did not alter abdominal PWV. Early treatment was associated with reduction in total aortic calcification (48%, P < 0.001) and reduction in medial cross-sectional area (40%, P < 0.05) of abdominal aorta compared with nonmedicated rats. Treatment increased the wall thickness/diameter ratio, but only early treated rats had a 46% higher elastin/collagen ratio compared with SHR controls. Conclusions: Early but not late brief ACEi decreased arterial pressure and isobaric wall stiffness in SHR. This was associated with marked alteration of wall structure and the effects persisted after cessation of early treatment. The structural mechanisms responsible for this change varied segmentally along the aortic trunk.

The dynamic response of intraocular pressure and ocular pulse amplitude to acute hemodynamic changes in normal and glaucomatous eyes.

PURPOSE To evaluate the effects of acute arterial blood pressure (ABP) and venous pressure changes on IOP in rats with experimental glaucoma. METHODS Unilateral experimental ocular hypertension was established in Sprague-Dawley rats by weekly intracameral injection of microbeads. Ocular pulse amplitude (OPA) and IOP were recorded from the anterior chamber using 1.2-Fr microsensors under urethane anesthesia. The effects on IOP during hemodynamic challenges using phenylephrine (PE) (50 μg/kg/min intravenous [IV]) and rapid saline loading (20 mL/kg/min IV) were studied. RESULTS Over an 8-week period, IOP was significantly elevated by 60% in the unilateral ocular hypertensive eyes. Both ABP and IOP were significantly increased by PE infusion. A significantly greater IOP increase was found in the experimental eyes compared with control eyes (1.32 ± 0.18 mm Hg vs. 0.90 ± 0.09 mm Hg). Correspondingly, higher OPAs and an amplification of the OPA during arterial hypertension were found in the experimental eyes. A sustained rise in IOP secondary to IV saline loading was observed, with a greater rise observed among experimental eyes (0.74 ± 0.13 mm Hg vs. 0.37 ± 0.005 mm Hg). CONCLUSIONS Sympathetic acceleration of ABP using PE resulted in surges in IOP and OPA. In contrast, increased venous pressure resulted in a more sustained rise in IOP but to a lesser extent. These responses were more pronounced in eyes with experimental glaucoma compared with control eyes, which may reflect the higher starting IOP contributing to a reduced ocular compliance. Our results suggest that eyes with ocular hypertension are more susceptible to IOP variability induced by hemodynamic fluctuations.

1026 EARLY BRIEF TREATMENT WITH ANGIOTENSIN CONVERTING ENZYME INHIBITOR, PERINDOPRIL, PREVENTS AORTIC CALCIFICATION IN ADULT SPONTANEOUSLY HYPERTENSIVE RATS

Background: Aortic calcification is a potent risk marker of cardiovascular events. Aortic calcification contributes to arterial stiffness and increased pulse pressure. Early angiotensin converting enzyme inhibition (ACEi) in spontaneously hypertensive rats (SHR) during a critical treatment window prevents aortic stiffness in adulthood. However, the effect of ACEi on aortic calcification is unknown. Method: ACEi (perindopril, 3 mg/kg/day, by gavage) was delivered during the pre- (6-10 weeks, SHR(Tx)y, n = 6) and established (20-24 weeks of age, SHR(Tx)o, n = ?) hypertensive phases. Vascular stiffness of the thoracic and abdominal aortic segments at their corresponding anaesthetised mean aortic pressure (MAP) was calculated from beat-to-beat pulse wave velocity (PWV) measurements in-vivo, using two 1.4F pressure catheters introduced to the thoracic and abdominal aorta, positioned a known distance apart. Aortas were harvested post-mortem and divided into thoracic and abdominal segments at the renal bifurcation for quantification of calcium content using atomic absorption spectrophotometry. Results were compared to untreated SHR, and the normotensive control strain, Wistar-Kyoto (WKY). Results: Perindopril significantly decreased MAP (-43%, P = 0.0005), isobaric thoracic PWV (-19%, P = 0.002) but not abdominal PWV. Thoracic aorta of the SHR(Tx)y group calcified 54% less than that of SHR controls (p = 0.001), and approximated values of the normotensive control strain, with no difference detected in the abdominal aorta (p = 0.09). The SHR(Tx)o group did not differ from SHR controls in any measured parameters. Conclusion: This study shows direct evidence of the persistent effect of early, but not late, ACEi in prevention of arterial stiffness associated with hypertension and corresponding aortic calcification.

CHARACTERIZATION OF SEGMENTAL AORTIC STIFFNESS AND PULSE PRESSURE AMPLIFICATION IN WISTAR-KYOTO RATS: PP.10.385

Objective: Arterial pulse pressure amplification (PPA) between proximal and distal sites is a result of progressive increase in arterial stiffness (AS) along the vascular tree and wave propagation phenomena, both dependent on mean arterial pressure (MAP). The aim of this study was to characterize aortic pulse wave velocity (PWV), a surrogate measure of AS, and PPA between the proximal and distal aorta in normotensive rats over a wide range of MAP. Methods: Experiments were performed in anaesthetized (Urethane, 1.3 g/kg, ip) Wistar Kyoto rats (n = 6). Beat-to-beat PWV and pulse pressure (PP) were measured in the proximal (pPWV, pPP) and distal (dPWV, dPP) aortic segments using simultaneous recordings of pressure with high fidelity catheter-tip sensors over segment lengths of 4–5 cm. MAP was increased and decreased by 30 s infusions of phenylephrine (50 μg/min) and sodium nitroprusside (10 μg/min) respectively. PPA (rdPP/pPP), pPWV, dPWV and whole aorta PWV were expressed as functions of MAP in the range 50 – 140 mmHg. Elastic non-uniformity (ENU) was calculated as ENU = dPWV-pPWV. Results: PWV-MAP plots for all aortic segments showed a progressive increase in PWV with MAP. dPWV showed a higher sensitivity to MAP (0.042 m/s/mmHg) compared to pPWV (0.0092 m/s/mmHg). PPA-MAP curves showed a curvilinear pattern, with peak PPA (1.21 ± 0.04 [SEM])) occurring around normal resting MAP (rMAP,106 ± 1.4[SEM] mmHg). This was associated with a general monotonic increase in dPP with MAP but a curvilinear pattern in pPP, with a minimal pPP around rMAP. For MAP < rMAP, PPA showed strong correlation with ENU (r2 = 0.91) with no significant correlation for MAP > rMAP. Conclusions: For pressures below resting MAP, PPA in rats corresponds almost entirely to elastic non-uniformity of the aorta. For higher pressure, PPA decreases, possibly due to increased effects of peripheral wave reflection. This may have implications in elucidation of mechanisms involved in aortic pulse pressure changes with antihypertensive interventions.