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Vasorelaxations Induced by Calcitonin Gene–related Peptide, Vasoactive Intestinal Peptide, and Acetylcholine in Aortic Rings of Endothelial and Inducible Nitric Oxide Synthase–Knockout Mice

The objective of this study was to determine if vasorelaxant responses to calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), and acetylcholine are altered in aortic rings of mice lacking genetic expression of endothelial nitric oxide synthase (eNOS) or inducible nitric oxide synthase (iNOS) genes (i.e., eNOS- and iNOS-knockout mice) as compared with control (wild-type) mice. Aortic rings from eNOS-knockout (eNOS (−/−)) mice did not relax in response to acetylcholine, thereby confirming previous reports. Aortic rings from iNOS-knockout (iNOS (−/−)) mice relaxed in response to acetylcholine in an endothelium-dependent manner. However, maximum relaxations in endothelium-intact rings were significantly (p < 0.05) larger than in control mice (85.3 ± 3.1% in iNOS (−/−) mice vs. 67.9 ± 5.6% in controls). CGRP caused concentration-dependent relaxations in aortas of all three types of mice: control mice, iNOS (−/−) mice, and eNOS (−/−) mice. Vasorelaxant responses to CGRP in control and iNOS (−/−) mice had identical relationships; both were partially dependent on endothelium. In eNOS (−/−) mice, dose-response curves of CGRP in endothelium-intact and endothelium-denuded rings were not significantly different, indicating loss of the partial dependence on endothelium. The vasorelaxant responses to VIP were completely dependent on endothelium in control and iNOS (−/−) mice. Maximum relaxations to VIP in iNOS (−/−) mice (77.4 ± 2.7%) were significantly greater than in control mice (64.0 ± 5.5%). Vasorelaxant responses to VIP in eNOS (−/−) aortic rings were also endothelium-dependent, but responses were greatly attenuated compared with wild-type mice. Relaxations induced by VIP (1 × 10−7M) in endothelium-intact aortic rings of eNOS (−/−) mice and control mice were 18.3 ± 5.4% and 64.0 ± 5.5%, respectively. These findings demonstrated that, in eNOS (−/−) mice, aortic vasorelaxant responses to CGRP were fully present but no longer dependent on the endothelium, and responses to VIP were greatly attenuated compared with control and responses to acetylcholine were abolished. In iNOS (−/−) mice, aortic vasorelaxant responses to VIP and acetylcholine were significantly greater than wild-type control, suggesting that induction of iNOS may have attenuated vascular responses to VIP and acetylcholine in wild-type controls.