Abstract 15153: Rap1: A Key Regulator Controlling Vascular Tone and Blood Pressure
Vascular tone is controlled by smooth muscle (SM)-mediated vessel contraction and endothelial cell (EC)-dependent vasodilation. Increased vascular tone is one of the major factors underlying hypertension, a major health problem. Rap1, a small GTPase with two isoforms, 1a and 1b, is implicated in EC angiogenic responses and Rho-mediated contractility in SM cells. We observed grossly enlarged hearts in Rap1b-knockout (KO) mice and increased heart to body weight ratio (p=0.001; n≥10), accompanied by cardiac remodeling consistent with pathological hypertrophy. However, cardiac hypertrophy marker expression was not increased while blood pressure was elevated (131±7 vs. 105±4 mm Hg in wild type (WT) controls, n≥9). Therefore, we hypothesized that cardiac hypertrophy in Rap1b-KO mice is secondary to increased vascular tone. To address this hypothesis we measured contractile responses of aortic segments isolated from SM-restricted Rap1-KO (SM22-Cre+rap1af/+rap1bf/f) mice, which were increased in response to 100nM U46619 compared with Cre- controls (57±7.5 vs. 36±5.5%, p=0.01, n=6; expressed as % maximal force). A cAMP analog specific for Rap1 GEF, Epac, suppressed these contractile responses significantly more in the Cre- controls than in the Rap1-KO aortae, supporting a previously proposed model in which Rap1 mediates cAMP-induced desensitization of SM contraction. To determine Rap1’s role in EC function, we examined acetylcholine-induced dilation of preconstricted aortic segments and found that nitric oxide (NO)-dependent vasodilation was significantly decreased in Rap1b-KO mice (39±5 vs. 56±2% of max. constriction, WT controls; n=3). Strikingly, NO-dependent vasodilation was abrogated in segments from EC-restricted Rap1-KO mice (-6±2 vs. 17±7%, WT; n≥4), indicating that the defect was EC-autonomous. Because the decrease in NO bioavailability was not accompanied by a decrease in NO sensitivity of SM, we measured shear stress-induced NO release from ECs and found that it was reduced in Rap1b-KO cells (1.7±0.5 vs. 4.1 ±1.0 nmol/mg, WT; n=2). In conclusion, Rap1-deficiency leads to EC dysfunction and increased SM contractility and, therefore, Rap1 function in both of these tissues is critical for maintenance of normal vascular tone.
- © 2012 by American Heart Association, Inc.