Hypercapnia‐induced Shear‐mediated Dilation in the Internal Carotid Arteries Is Blunted in Healthy Older Adults
Authors: Iwamoto Erika , Bock Joshua M. , Casey Darren P. 
Published: June 2021

Shear‐mediated dilation (an index of vascular function) in peripheral conduit arteries is blunted with aging. However, the effects of aging on shear‐mediated dilation in carotid arteries are unknown. We hypothesized that hypercapnia‐induced shear‐mediated dilation in the common and internal carotid arteries (CCA and ICA, respectively) would be attenuated in healthy older adults relative to young adults. Hypercapnia‐induced shear‐mediated dilation in the CCA and ICA was assessed in ten young (5F/5M, 23±1 years) and ten older (6F/4M, 68±1 years) adults. Shear‐mediated dilation was induced by two levels of hypercapnia (Low and High; +5 and +10 mmHg from individual baseline end‐tidal CO2, respectively). Doppler measurements of the CCA and ICA at low and high levels of hypercapnia were performed in a randomized order. Each trial consisted of a two‐minute baseline and three minutes of hypercapnia. Percent change in diameter (shear‐mediated dilation) and shear rate (SR) were calculated as the percent rise in the peak value from the baseline value. Hypercapnia induced an increase in SR with increasing end‐tidal CO2 in both the CCA and ICA (main effect of hypercapnia; both P<0.05), while no differences were observed in the percent change in SR between young and older adults (Interaction and main effect of age, CCA; P=0.39 and P=0.74, ICA; P=0.31 and P=0.81, respectively). Shear‐mediated dilation in the CCA was increased from low to high levels of hypercapnia in both young and older adults (main effect of hypercapnia; both P<0.01) with no differences between age group (Low vs. High, Young; 2.8±0.6 vs. 4.5±0.8%, Older; 2.0±0.3 vs. 3.7±0.6%). In contrast, shear‐mediated dilation in the ICA was enhanced from low to high levels of hypercapnia in young (4.6±0.8 to 7.9±1.2%, P=0.01), but not older adults (3.6±0.4 to 4.5±0.5%, P>0.05). Consequently, shear‐mediated dilation in the ICA at the high level of hypercapnia was lower in older adults compared to the young adults (P=0.02). Lastly, shear‐mediated dilation was correlated to the percent change in SR in the young ICA (r=0.59, P<0.01) and older ICA (r=0.49, P=0.03); however, these relationships were not present in the young CCA (P=0.95) and older CCA (P=0.30). Our data indicate that although two levels of hypercapnia induced a similar increase in SR between young and older adults, shear‐mediated dilation of the ICA to the higher level of hypercapnia was attenuated in older adults, but not in the CCA. Age‐related differences in shear‐mediated dilation were not evident during the lower level of hypercapnia trials for either carotid artery. Additionally, vasodilation in the ICA is more strongly related to the increase in SR than the CCA in both young and older adults. Together, these data may provide insight into the age‐related changes in the regulation of cerebral blood flow in healthy adults.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.