George Lindesay

Pressure dependency of aortic pulse wave velocity in vivo is not affected by vasoactive substances that alter aortic wall tension ex vivo

Aortic stiffness, a predictive parameter in cardiovascular medicine, is blood pressure dependent and experimentally requires isobaric measurement for meaningful comparison. Vasoactive drug administration to change peripheral resistance and blood pressure allows such isobaric comparison but may alter large conduit artery wall tension, directly changing aortic stiffness. This study quantifies effects of sodium nitroprusside (SNP, vasodilator) and phenylephrine (PE, vasoconstrictor) on aortic stiffness measured by aortic pulse wave velocity (aPWV) assessed by invasive pressure catheterization in anaesthetized Sprague-Dawley rats (n = 7). This was compared with nondrug-dependent alteration of blood pressure through reduced venous return induced by partial vena cava occlusion. In vivo drug concentration was estimated by modeling clearance rates. Ex vivo responses of excised thoracic and abdominal aortic rings to drugs was measured using myography. SNP administration did not alter aPWV compared with venous occlusion (P = 0.21-0.87). There was a 5% difference in aPWV with PE administration compared with venous occlusion (P < 0.05). The estimated in vivo maximum concentration of PE (7.0 ± 1.8 ×10(-7) M) and SNP (4.2 ± 0.6 ×10(-7) M) caused ex vivo equivalent contraction of 52 mmHg (thoracic) and 112 mmHg (abdominal) and relaxation of 96% (both abdominal and thoracic), respectively, despite having a negligible effect on aPWV in vivo. This study demonstrates that vasoactive drugs administered to alter systemic blood pressure have a negligible effect on aPWV and provide a useful tool to study pressure-normalized and pressure-dependent aPWV in large conduit arteries in vivo. However, similar drug concentrations affect aortic ring wall tension ex vivo. Future studies investigating in vivo and ex vivo kinetics will need to elucidate mechanisms for this marked difference.

Age and hypertension strongly induce aortic stiffening in rats at basal and matched blood pressure levels

Age and hypertension are major causes of large artery remodeling and stiffening, a cardiovascular risk factor for heart and kidney damage. The aged spontaneously hypertensive rat (SHR) model is recognized for human cardiovascular pathology, but discrepancies appeared in studies of arterial stiffness. We performed experiments using a robust analysis via echo tracking in 20‐week adult (n = 8) and 80‐week‐old SHR (n = 7), with age‐matched normotensive Wistar Kyoto rats (WKY, n = 6;6) at basal and matched levels of blood pressure (BP). After anesthesia with pentobarbital, abdominal aortic diameter and pressure were recorded and BP was decreased by clonidine i.v. At basal BP, aortic pulse distension, compliance, and distensibility (AD) were reduced and stiffness index increased with age and hypertension and further altered with age + hypertension. When BP was adjusted in SHR to that of normotensive rats (130 mmHg), there was no difference between 20‐week‐old SHR and WKY. Importantly, the age effect was maintained in both WKY and SHR and accentuated by hypertension in old rats. At 130 mmHg, with similar pulse pressure in the four groups, AD (kPa−3) = 24.2 ± 1 in 20 weeks WKY, 19.7 ± 1.4 in 20 weeks SHR, 12.4 ± 1.3 in 80 weeks WKY and 6.6 ± 0.6 in 80 weeks SHR; distension = 7.6 ± 0.4%, 6.7 ± 0.6%, 3.7 ± 0.3%, and 1.8 ± 0.2% in the same groups. In conclusion, reduced distensibility, that is, stiffening due to age is clearly shown here in both WKY and SHR as well as a synergistic effect of age and hypertension. This technique will allow new studies on the mechanisms responsible and drug intervention.

138 IN-VITRO AND IN-VIVO USE OF VASOACTIVE AGENTS IN CHARACTERISING AORTIC STIFFNESS IN RATS: TESTING THE ASSUMPTIONS

Background: Arterial properties are studied in-vitro and in-vivo in rodents using vasoactive agents. In-vitro aortic ring experiments show a 1-2 fold change in active smooth muscle tension while the in-vivo use of the same agents assumes a singular action at the periphery, with minimal affect on aortic smooth muscle tension. This study investigates the validity of this assumption. Methods: Arterial stiffness was measured as pulse wave velocity (PWV) in anaesthetised (urethane) Sprague Dawley rats (12 weeks, n=6) using 1.6F pressure catheters. Sodium nitroprusside (SNP) and phenylephrine, (30 microg/kg/min i.v), lowered and raised mean arterial pressure (MAP). Partial occlusion of the inferior vena cava reduced venous return to lower pressure passively for comparison to the MAP range achieved with SNP. This was repeated during continuous infusion of phenylephrine to assess effect of phenylephrine on the aorta Results: No significant difference was found in arterial stiffness achieved with the vasoactive agents or with the venous occlusion methods of blood pressure manipulation. For MAP range 60–90 mmHg, PWV for SNP did not differ from PWV for venous occlusion (4.05±0.55 m/s, 4.05±0.51 m/s, p=0.99). For MAP 120–150 mmHg, PWV for phenylephrine did not differ from PWV with concurrent venous occlusion (6.37±0.87 m/s, 6.71±0.72 m/s, p=0.62). Conclusions: The results indicate the concentration of SNP and phenylephrine infused did not significantly affect aortic smooth muscle in-vivo and all pressure dependent changes in PWV were mediated through effects on total peripheral resistance and not via active effects on the smooth muscle in the aortic wall.

Differential Stiffening between the Abdominal and Thoracic Aorta: Effect of Salt Loading in Stroke-Prone Hypertensive Rats

Central artery stiffening is recognized as a cardiovascular risk. The effects of hypertension and aging have been shown in human and animal models but the effect of salt is still controversial. We studied the effect of a high-salt diet on aortic stiffness in salt-sensitive spontaneously hypersensitive stroke-prone rats (SHRSP). Distensibility, distension, and β-stiffness were measured at thoracic and abdominal aortic sites in the same rats, using echotracking recording of the aortic diameter coupled with blood pressure (BP), in SHRSP-salt (5% salted diet, 5 weeks), SHRSP, and normotensive Wistar-Kyoto (WKY) rats. Hemodynamic parameters were measured at BP matched to that of WKY. Histological staining and immunohistochemistry were used for structural analysis. Hemodynamic isobaric parameters in SHRSP did not differ from WKY and only those from the abdominal aorta of SHRSP-salt presented decreased distensibility and increased stiffness compared with WKY and SHRSP. The abdominal and thoracic aortas presented similar thickening, increased fibrosis, and remodeling with no change in collagen content. SHRSP-salt presented a specific increased elastin disarray at the abdominal aorta level but a decrease in elastin content in the thoracic aorta. This study demonstrates the pro-stiffening effect of salt in addition to hypertension; it shows that only the abdominal aorta presents a specific pressure-independent stiffening, in which elastin disarray is likely a key mechanism.

145 SYMPATHETICALLY MEDIATED AGE RELATED CHANGES IN ABDOMINAL AORTIC STIFFNESS PARAMETERS IN THE RAT

Background: Increased sympathetic activity is known as a driving factor behind increased blood pressure with age in spontaneously hypertensive rats (SHR). We have previously shown that sympathetic activity changes aortic stiffness. However, whether the increased sympathetic activity with age in the SHR invokes changes in aortic stiffness is unknown. Methods: SHR (10 weeks, n=7, 15 weeks, n=6, 20 weeks, n=5) were anaesthetized (urethane) and ventilated. Two high fidelity pressure transducers were introduced into the abdominal aorta via the carotid and femoral arteries for measurement of blood pressure and pulse wave velocity (PWV). Aortic diameters were measured via an ultrasound probe (6.2 Mhz) coupled with vessel tracking software (Art.Lab, Esaote). The recorded diameters and pulse pressures were used to calculate aortic compliance. Measurements were taken at basal blood pressure (138±3 mmHg) and again following sympathetic blockade with hexamathonium (i.v 20 mg/kg). Hexamethonium pressures were matched with basal blood pressures following a bolus of phenylephrine (i.v 0.3 ml 30 &mgr;g/ml). Results: Sympathetic blockade caused a greater change in compliance at 15 weeks (p<0.001) and 20 weeks (p<0.01) compared to the change induced at 10 weeks. There were no other significant differences between any measured changes in haemodynamic parameters with sympathetic blockade between ages. Table. No title available. Conclusions: These data suggest that as SHRs age there is an increase in the contribution of sympathetic activity towards aortic compliance, though the effect on abdominal aortic PWV is not evident. This may be due to differential effects on aortic wall thickness and diameter changes.