The objective of this work was to explore whether measureable changes in carotid artery mechanical properties occur following an abrupt, sustained increase of blood flow. Results indicate that statistically significant alterations occur rapidly but then subside with time. Initial exploratory studies suggest that MMPs may contribute to the observed changes. These findings lend new insight into how blood vessels respond to changes in their mechanical environment.
To our knowledge, the reported rapid change in vessel properties, followed by a return to baseline behavior, has not been previously reported. While this temporal pattern mirrors previously reported expression of MMP-9 in the same model, it is not clear which components of the vessel wall are responsible for the changes in properties. In discussing this question, it is important to note that smooth muscle activity was not chemically eliminated in experiments. Tests were performed in a standard physiological salt solution (DMEM), considered appropriate by many for measurement of passive properties (Humphrey 2002
), and in the absence of flow and, thus, any endothelium-dependent smooth muscle signaling. Data for determining properties were also obtained following preconditioning, eliminating some smooth muscle contraction (Fridez et al 2001
), but these conditions don’t remove the possibility of a myogenic response at the basal tone level.
previously suggested that remodeling in response to a change in flow occurs in two phases – an immediate vascular smooth muscle response followed by later structural alterations in the media. The observed increase in circumferential stretch indicates that carotid arteries previously exposed to flow augmentation expanded more under physiological pressure in pressure-diameter tests than did arteries in which flow was normal before resection. The observed increase in circumferential stretch at 1 day appears to be consistent with smooth muscle-induced dilation in response to increased flow, but it is not clear why this dilation would decrease over the next few days while the increased level of flow persisted.
Consistent with Langille’s timeline, no structural remodeling was observed during the period studied, but this would not rule out all structural alteration. Given the increase in circumferential stretch, it is notable that Eiv
, the stiffness of the circumferential stress-stretch curve at 100 mmHg, did not change significantly with flow augmentation. These two results together are consistent with a shift of the circumferential stress-stretch curve to the right, a lengthening of the toe region of the curve without significant alteration of the rest of its shape. Because elastin has been shown to define the low-stiffness response of blood vessels (Roach and Burton 1957
), it may be that the mechanism producing the observed alterations affects elastin fibers more than other structural elements of the vascular wall, such as collagen. Given the long time required for elastin synthesis (Humphrey 2009
), however, the rapid recovery of control behavior could not be accomplished through replacement of elastin, but contributions from other components may compensate.
Increase in circumferential stretch at 1 day could also be the result of damage to the vessel wall by overload. Cope and Roach (1977)
observed changes in toe-region elastic properties of isolated human cerebral vessels subjected to over-pressures in vitro
, though the pressures applied in their study were much higher than those present here. Other investigators have reported changes in the internal elastic lamina following increased flow in the carotid or basilar arteries (Hoi et al 2008
; Tronc et al 2000
; Wong and Langille 1996
), though large flow changes were required to produce fragmentation of the lamina. Hoi et al (2008)
reported remodeling activity in the basilar artery of rabbits following both unilateral and bilateral ligation of the carotid arteries, but only bilateral ligation resulted in internal elastic lamina fragmentation. In contrast, Nuki et al (2009)
showed that luminal diameter increased only slightly during the first 5 minutes after ligation in the current model, so it is unlikely that passive overload occurred. Additionally, it is unlikely that a significantly damaged vessel could return to baseline behavior over a small number of days. Nevertheless, histological study of these vessels would help rule out such damage in future work.
Given these considerations, it is not currently possible to determine the cause for the observed change in properties. It appears, however, that inflammation resulting from the rapid change in mechanical environment produces an alteration in either expected smooth muscle response or the way structural components of the wall interact with one another, or both; related work has suggested that structures coupling extracellular matrix to smooth muscle cells may be disrupted in vascular injury (Jamal et al 1992
). Future investigation should include appropriate chemicals to manipulate smooth muscle contributions and allow clear distinction between active and passive response.
While the reported data suggest a trend for maximum increase in circumferential stretch at day 1, followed by a decrease back toward baseline levels, the existence and timing of this trend should be further explored with experiments at earlier and later times. Because results at 1 and 2 days were not statistically different from each other and no measurements were made earlier than 1 day, it may be that maximum change occurs at a time other than one day following flow augmentation. In order to more fully understand the vascular response, it is also important to define whether the trend of recovery suggested by the data actually occurs.
Although neither inhibitor treatment demonstrated a statistically significant effect on mechanical properties, both tended to produce lower circumferential stretch ratios than comparable controls, with doxycycline being more influential than GM6001. In contrast to GM6001, doxycycline is less selective and produces effects other than MMP inhibition, including modulation of various aspects of inflammation (Beekman et al 1997
; Kim et al 2005
; Lee et al 2003
), possibly contributing to its relatively greater influence.
The potential significance of such a rapid change in vessel properties is not known. The observed increase in circumferential stretch for a given level of pressure, however, could indicate a loss of vascular integrity with potential for subsequent dysfunction or initiation of disease. Although none of the carotid arteries tested in this study displayed any tendency to not recover from the increase in compliance, mechanical thresholds for the initiation of such instability should be defined. Additionally, while the initiating mechanical event in this study was an abrupt change in shear stress, the approach of this research should be extended to examine response to other types of changes in mechanical environment, such as those that occur with trauma.
In summary, mechanical properties of carotid arteries change rapidly following exposure to augmented flow. Trends suggest that the affected properties may return to baseline values over time and that the timing of changes may be mediated by MMPs. The reported findings further define how blood vessels respond to changes in their mechanical environment and are believed to lend insight into processes associated with the development of vascular disease and dysfunction.