The Effects of in Vitro Hyperglycemic Incubation on A23187-Mediated Contractile Responses of Rat Thoracic Aorta

Objective: Vascular tonus has been controlled by several factors secreted from endothelium in physiological conditions. In the present study, the possible changes on endothelium-derived contractile responses in normoglycemic and hyperglycemic conditions in terms of exposure-time dependency has been investigated. Methods: To assess the possible alterations under acute hyperglycemia with different incubation periods on endothelium-derived contractile responses in isolated rat thoracic aorta, A23187-mediated contractile responses were performed in a cumulative manner for isometric tension measurements. Results: Incubation for 3 hours with Krebs solution containing high glucose increased the A23187-mediated contraction of rat thoracic aorta. The A23187- induced contraction significantly decreased in response to the same incubation period with normoglycemic conditions and totally abolished with incubation for 6 hours. The possible effects of osmotic pressure induced by high glucose content checked with mannitol. Conclusion: Our results indicated that the A23187-mediated contractile response was increased by acute hyperglycemia. These data has shown the detrimental effects of short term hyperglycemia on endothelium-derived contractile factors. Also the incubation period-related characteristics of A23187-mediated contractile responses as observed in normoglycemic conditions indicate the unstable properties of endothelium-derived contractile factors and/or related signaling pathway(s). in the endothelium mediates contraction by activating thromboxane-prostanoid (TP) receptors in vascular smooth muscle (10). The effect of EDCF can be seen not only in pathologies like hypertension and chronic diabetes in which endothelial dysfunction has been observed but also in conditions like acute hyperglycemia, hypoxia and aging in which the production of reactive oxygen species (ROS) has increased (11). The aim of the present study is to evaluate the possible effects of acute hyperglycemia on A23187-mediated contractile responses in rat thoracic aorta. vascular smooth muscle contractility: the activation of endothelial NOS and production of EDCF by COX activation. COX-mediated products like endoperoxides easily diffuse from endothelium to vascular smooth muscle and mediate contraction by TP-receptor activation. The mechanisms which can be responsible from the reduced A23817-mediated contractility in response to exposure period as presented in the current study may be the inhibition of endothelial COX activation and/or hyporesponsiveness of TP-receptors located on vascular smooth muscle. According to the best of our knowledge, there is no data for inhibition of COX activation by exposure periods and/or extracellular conditions in normoglycemic conditions. However, it has been demonstrated that endothelium-dependent contractions to acetylcholine are pH-dependent unlike endothelium-dependent acetylcholine-induced relaxations (24). The endothelium-dependent contractions are found to be increased in alkaline (pH 7.8) than the control solution (pH 7.4) and the contractions induced by acetylcholine is depressed at acidic conditions (pH 7). It has been also shown that the pH changes have no effect on the release of EDCF, however alterations in the extracellular pH affect the responsiveness of TP receptors on vascular smooth muscle (24). The results of the mentioned study are quite exciting because the authors showed that pH of the extracellular solution affects only TP receptor responsiveness without any alteration in endothelium signaling which can the case for the present data. In our experimental settings, the formulation and oxygenation protocol of the extracellular solution was validated according to the pH 7.4. causes enhanced A23187-mediated contractions of rat thoracic aorta in the presence of NOS inhibition. In normoglycemic conditions, the in vitro incubation of preparations reduced the A23187-induced conractile responses. Therefore, the nature of A23187-mediated contractile responses any alteration in physiological the enhanced the A23187-mediated through activation.