It is well acknowledged that molecular oxygen (O2) constricts vascular tissue under physiological conditions. In hypoxia, the decrease in partial pressure of O2 (pO2) within the tissue may be due to either a reduction in O2 supply or an increased O2 demand. The pulmonary circulation responds to a decrease in O2 by inducing vasoconstriction, whereas the systemic circulation induces vasorelaxation. Systemic vasorelaxation in hypoxia occurs by one of two presumed mechanisms, direct, in which smooth muscle cells can no longer sustain adequate contraction, or indirect, in which vasodilatory molecules are produced. Since the early 1990’s, several laboratories worldwide hypothesised that the vasodilatory molecules released under hypoxic conditions originated from the red blood cell (RBC). Several mechanisms have been proposed to date, including nitrite (NO2-) reduction by haemoglobin (Hb) to nitric oxide (NO), S-nitrosation of Hb to S-nitrosohaemoglobin (HbSNO) and adenosine triphosphate (ATP) binding to P2Y receptors on the vascular endothelium. Although there has been extensive research within this field, a clear mechanism by which vasorelaxation occurs is yet to be fully elucidated. Therefore, the aims of this thesis were to determine the vasodilatory specie(s) released from RBCs and the mechanism by which vasorelaxation occurs. Myography experiments were conducted using dissected rabbit thoracic aortae. Rings were equilibrated at various O2 concentrations, directly influencing tissue pO2. Bolus administration of oxygenated RBCs, isolated Hb or Krebs-Henseleit (KH) buffer to pre-constricted hypoxic rings induced a transient relaxation which was immediately followed by a post-constriction of equivalent magnitude. Interestingly, oxygenated KH buffer alone could induce relaxation of aortic rings in a similar manner to RBCs and Hb, demonstrating that O2 itself relaxes hypoxic vascular tissue. In addition, the extent of vasorelaxation was inversely related to the tissue pO2. Oxygenated KH buffer alone induced vasorelaxation in hypoxic pre-constricted rings pre-incubated with NOS inhibitor, L-NMMA, indicating an endothelium-independent mechanism. Subsequent experiments investigated the role of soluble guanylate cyclase (sGC) in the context of these findings. A number of studies have shown that sGC does not bind O2. However, the results present herein demonstrate that O2 can stimulate an enhanced activity of soluble guanylate cyclase (sGC), increasing the production of cyclic guanosine monophosphate (cGMP). Importantly, this could occur in the absence of NO but was found to be dependent upon the presence of haem. In order to compare O2-induced vasorelaxation in a vessel with an alternative function, the left anterior descending (LAD) artery was dissected from porcine hearts. Hypoxic pre-constricted LAD rings relaxed 20% more to a bolus of oxygenated RBCs or KH buffer compared to rabbit aortic rings and this was not due to an increased expression of sGC within the smooth muscle. Further experiments aimed to show whether vessel size had an effect upon the magnitude of O2-induced vasorelaxation in hypoxia. O2 induced a greater vasorelaxation in rings of smaller inner diameter. In conclusion, the results within this thesis show a direct relaxant effect of O2 that is mediated via the sGC-cGMP pathway and suggest a role for O2 in response of vascular smooth muscle in acute hypoxia.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:590349 |
| Date | January 2013 |
| Creators | Dada, Jessica |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/57066/ |
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