Abstract 484: Augmented Myogenic Responses in Cerebral Arteries Following Interference with Peroxisome Proliferator-Activated Receptor γ (PPARγ) in Smooth Muscle via a Rho Kinase (ROCK)-Dependent Mechanism

Abstract

Myogenic responses by resistance vessels in brain play a crucial role in regulating cerebral blood flow and protecting the brain parenchyma and blood-brain barrier against potentially detrimental elevations in blood pressure. PPARγ is a transcription factor that may protect the vasculature by suppressing increases in vascular tone. Using transgenic mice expressing a dominant negative mutation of human PPARγ only in smooth muscle cells (S-P467L; TG), we have shown that augmented myogenic responses in mesenteric arteries are mediated by a mechanism that involved protein kinase C, but not ROCK. As ROCK is thought to play a major role in cerebrovascular disease, we hypothesized that interference with PPARγ in smooth muscle would augment myogenic responses in cerebral arteries via a ROCK dependent mechanism. We studied S-P467L TG mice and non-transgenic littermates (NT). Middle cerebral arteries (MCA) from TG mice generated more myogenic tone compared with NT (75 mmHg: 32±2% vs. 13±2% of max diameter). Myogenic reactivity was similarly augmented in MCA from TG mice from 15-150 mmHg whereas vessel diameter in Ca2+ free conditions was similar between groups. We used two approaches to evaluate the impact of ROCK in this model. First, the ROCK inhibitor, Y27632 (3 μM), abolished the augmented myogenic tone in MCA from TG mice (34±5% vs. 9±4% of max diameter). Second, treatment with a cell permeable molecule that modifies RhoA to make it constitutively active (CN-03) elevated myogenic tone to ~60% in both groups, which was reduced to ~10% by Y27632. The large conductance potassium (BK) channel modulates myogenic tone. The BK channel inhibitor, iberiotoxin, caused greater constriction in MCA from NT compared with TG (100 nM: NT, 14±2% vs. TG, 8±1% Δ tone at 75 mmHg). Although angiotensin II can activate ROCK, the AT1 receptor antagonist (losartan) did not affect myogenic tone in MCA from TG mice. Gene expression analysis revealed no change in mRNA for RhoA or ROCK1/2 expression in cerebral arteries from TG mice. These findings support the concept that in cerebral arteries, activity of ROCK has substantial effects on myogenic tone. The data also suggest that under normal conditions, PPARγ in smooth muscle inhibits ROCK, influencing myogenic tone in resistance arteries in brain.