Mechanism underlying the anticontractile effect of perivascular adipose tissue

This thesis, entitled ? Mechanism underlying the anticontractile effect of perivascular adipose tissue? , has been submitted by author Yiwen Dong for a degree of Doctor of Philosophy (PhD) in the Faculty of Medical and Human Sciences at the University of Manchester, September 2013. Most systemic blood vessels are surrounded by layers of adipocytes forming perivascular adipose tissue (PVAT). Healthy PVAT can act as an endocrine organ to release different factors which can reduce vessel contractility. The exact anticontractile mechanism is still unclear, although recent evidence suggests that PVAT-derived factors may activate the large conductance Ca2+-dependent K+ (BKCa) channels on the vascular myocytes possibly via AMP-activated protein kinase (AMPK). Additionally, cGMP-dependent protein kinase (PKG) within the adipocytes seemed to be important for the anticontractile function of PVAT. This project aims to investigate the mechanism of the anticontractile effect of PVAT by determining the functions of PKG within adipocytes, as well as to assess the importance of vascular AMPK and BKCa channels to the PVAT anticontractile mechanism. Experiments were carried out mainly with male rat mesenteric arteries and aortas. Whole vessels with or without PVAT were mounted on wire myographs to detect changes in vessel tension induced by pharmacological agents. Single myocytes were freshly-isolated from mesenteric artery or aorta and were voltage-clamped in the whole-cell configuration. BKCa channel current was triggered by voltage steps. Vascular tension experiments showed that sildenafil (an indirect PKG activator) did not enhance the anticontractile effect of PVAT. However, the vasorelaxant effect of sildenafil was reduced when endothelium was removed, but only in PVAT-denuded vessels. Nitric oxide (NO) synthase inhibition reversed the vasorelaxant effect of sildenafil regardless of the presence of endothelium or PVAT. In electrophysiology recordings, application of an AMPK activator A-769,662 increased BKCa channel current, an effect that was inhibited by iberiotoxin (a selective BKCa channel blocker), but not by dorsomorphin (an AMPK inhibitor). PT1 (an alternative AMPK activator) mimicked the effect of A-769,662 and increased the BKCa channel current. Again, this was not sensitive to dorsomorphin. In solution transfer experiments, the PVAT bath solution increased BKCa channel activity, an effect that was similarly observed with adiponectin. The effect of the PVAT bath solution was not inhibited by catalase. Furthermore, in tension studies, PVAT isolated from adiponectin deficient mouse had lost its anticontractile effect under basal conditions.The results indicate that, under basal conditions, PVAT exerts anticontractile effect by release NO to activate PKG in vascular myocytes. In addition, PVAT can also release an unknown factor which can activate the BKCa channels on the myocytes to cause membrane hyperpolarisation. The identity of this factor is unknown, but is unlikely to be hydrogen peroxide. AMPK activators as well as adiponectin, a putative AMPK activator, increased BKCa channel current, suggesting that AMPK activation can cause myocyte relaxation via opening BKCa channels. Thus, PVAT may release adiponectin which can cause vascular myocyte hyperpolarisation by opening of BKCa channels.