On the regulation of tone in vasa vasorum.

OBJECTIVE The vasa vasorum form a network of microvessels in and around the walls of large blood vessels and are thought to be necessary to delivery oxygenated blood to the outer parts of the vessel wall that are inadequately nourished by diffusion from luminal blood. This study was undertaken to investigate directly the mechanisms which control tone in the vasa vasorum. METHODS Arterial vasa vasorum were dissected from the walls of porcine or bovine thoracic aorta and mounted in a tension myograph. Concentration-response curves were constructed to vasoconstrictors; endothelin-1(ET-1), noradrenaline (NA) angiotensin II (Ang II) and thromboxane A2-mimetics (U44069 or U46619) or vasodilators; substance P (SP) bradykinin (BK), calcitonin gene-related peptide (CGRP) or isoprenaline. Strips of porcine aorta were mounted in 25 ml organ baths. RESULTS Potent concentration-dependent contraction of vasa vasorum was produced by ET-1. NA was a weak constrictor, Ang II had no effect or produced contraction that underwent tachyphylaxis and thromboxane A2-mimetics had no effect. In contrast NA, Ang II, U-44069 and ET-1 all produced potent concentration-dependent contraction of aortic strips. SP and BK produced endothelium-dependent relaxation while CGRP produced endothelium-independent relaxation of ET-1-precontracted vasa vasorum. Isoprenaline had no relaxant effect. CONCLUSIONS We have demonstrated functional responses of arterial vasa vasorum to vasodilators and vasoconstrictors. Additionally these microvessels appear to respond to constrictors differently from the large host vessel.