Delayed Platelet Adhesion/Aggregation at Sites of Endothelial Injury in Mouse Cerebral Arterioles After Transient Elevations of Blood Pressure and Shear
Authors: Murata Shinji , Rosenblum William I. , Shimizu Takao , Nelson Guy H. 
Published: June 2011

Background and Purpose
Prior research showed that injection of angiotensin II (Ang II) produces a transient elevation of blood pressure (BP) and shear in pial arterioles. This inhibited platelet adhesion/aggregation at a site of subsequently injured endothelium. The present study attempted to confirm the Ang II finding with a different method of endothelial injury, to test the hypothesis that the effect on adhesion/aggregation was a consequence of prolonged release of “classic” endothelium-derived relaxing factor (released by acetylcholine [Ach]; EDRF
ACh
) produced by the preceding transient elevation in shear, and to show with the use of norepinephrine rather than Ang II that the effect of a preceding elevation of BP was independent of the pressor agent used.

Methods
Focal platelet adhesion/aggregation was elicited in cerebral surface (pial) arterioles by producing minimal endothelial damage with a helium-neon laser/Evans blue dye technique. Vessels were observed by intravital microscopy. We recorded the time required for the laser to elicit adhesion/aggregation in control mice and in mice given Ang II in a dose of 16 μg/25 g IP. This dose produces an abrupt and significant elevation of BP and shear, which return to baseline levels in less than 30 minutes. Laser/dye damage of endothelium and resultant adhesion/aggregation of platelets were not induced until after BP and shear returned to basal levels. The effect of topical Ang II on damage-induced adhesion/aggregation was also tested. In addition, mice injected with Ang II were treated with either topical indomethacin 40 μg/mL or topical
N
G
-monomethyl
l
-arginine (L-NMMA; 10
−6
mol/L) in an effort to prevent the preceding increase in shear from inhibiting subsequent adhesion/aggregation. Finally, norepinephrine instead of Ang II was used to transiently raise BP (and shear) in an effort to delay subsequently induced adhesion/aggregation.

Results
Platelet adhesion/aggregation at the injured site was significantly delayed by a prior transient rise in shear produced by either Ang II or norepinephrine. Locally applied Ang II failed to influence adhesion/aggregation, although a previous study showed that such Ang II reaches the endothelium. Locally applied indomethacin had no effect on inhibition of platelet adhesion/aggregation, but locally applied L-NMMA prevented the prior transient elevation of shear from inhibiting adhesion/aggregation at a subsequently injured site.

Conclusions
Elevation of BP with consequent elevation of shear inhibits local platelet adhesion/aggregation even when the latter is initiated by endothelial damage produced after return of shear to basal levels. The direct action of Ang II on endothelium is not responsible for the effect on adhesion/aggregation, and indeed the effect is independent of the pressor agent. The pharmacological data, together with the literature, support the hypothesis that increased shear causes an increased release of EDRF
ACh
, which may continue for at least some minutes after return of shear to normal levels.