Mechanical properties of rat middle cerebral arteries with and without myogenic tone

The inner diameter and wall thickness of rat middle cerebral arteries (MCAs) were measured in vitro in both a pressure-induced, myogenically-active state and a drug-induced, passive state to quantify active and passive mechanical behavior. Elasticity parameters from the literature (stiffness derived from an exponential pressure-diameter relationship, [beta], and elasticity in response to an increment in pressure, [E.sub.inc-p]) and a novel elasticity parameter in response to smooth muscle cell (SMC) activation, [E.sub.inc-a], were calculated. [beta] for all passive MCAs was 9.11 [+ or -] 1.07 but could not be calculated for active vessels. The incremental stiffness increased significantly with pressure in passive vessels; [E.sub.inc-p] ([10.sup.6] dynes/[cm.sup.2]) increased from 5.6 [+ or -] 0.5 at 75 mmHg to 14.7 [+ or -] 2.4 at 125 mmHg, (p