Increased segmental resistance at different cerebrovascular levels is involved in cerebral blood flow regulation. The hypertensioninduced rise in resistance results from a fall in arterial/arteriolar internal diameter (ID), which could be due to functional (increased myogenic tone) or structural (decreased passive ID) remodeling. We evaluated ID changes in the middle cerebral artery (MCA) of spontaneously hypertensive rats (SHR, versus Wistar-Kyoto, WKY) in comparison with ID changes already published on the pial arteriole (open cranial preparation (Dupuis et al., J Hypertens 2005; 23: 1061-6).

Baseline ID of the pressurised MCA (60 % of systemic arterial mean blood pressure, SAMBP) and passive ID (following inactivation of smooth muscle cells, EDTA 2mM) were measured in vitro (small vessel arteriograph). The myogenic response was estimated as delta IDpassive-baseline/baseline, and compliance as length of the MCA segment (distance between the pipette tips) x delta IDpassive-baseline / MCA intraluminal pressure measured under EDTA (assuming initial intraluminal pressure was equal to zero).

Hypertension decreased baseline ID circa 25 % in both the MCA and the pial arteriole. In the MCA, this was associated with an increased myogenic tone, a decrease in compliance but no change in passive ID. In the pial arteriole, passive ID fell, wall thickness/ ID (WKY 0.013±0.004, SHR 0.028± 0.004^*) and distensibility (fall in elastic modulus/stress) increased.

In conclusion, although the hypertension-induced changes in segmental resistance at the two levels (MCA and pial arteriole) are similar, the functional and structural control systems differ with functional remodeling (increased myogenic tone) in the MCA and structural remodeling (increased wall thickness/ID) in the pial arteriole.