In this study, combinatorial use of insulin and vitamin C has successfully alleviated the CVS after SAH, though individual administration of either insulin or vitamin C fails to achieve the desired effects, which confirms the destructive effect of oxygen free radicals on vascular endothelial cells and the significant vasodilatory effect of insulin. These results are consistent with those from Hirashima et al. .
Insulin resistance is the state of tissues and organs with reduced sensitivity to insulin. Previous studies have confirmed that inhibition of insulin's vasoactive effect induces insulin resistance and hyperinsulinemia, which is the major cause of atherosclerosis and coronary artery spasm [5–7]. The present study has demonstrated that one day after blood injection, the cross-sectional area of basilar artery in experimental animals decreases significantly with apparent constriction of the blood vessels, which becomes mostly significant at the third day post blood injection (that is the fifth day of the study), relieves then gradually, and recovers at the fifth day. Although the glucose level only increases within 30 min after the blood injection, the plasma insulin level begins to rise continuously 30 min after the injection and reaches its maximum 5 days after the second injection, implying that CVS is associated with hyperinsulinemia. Experiments have confirmed that sustained high insulin level may promote the pathological changes of blood vessels, which forms an independent risk factor for vascular diseases. Firstly, hyperinsulinemia itself may downregulate insulin receptor by reducing the number and expression of insulin receptor in vascular endothelial cells, resulting in insulin resistance ; secondly, hyperinsulinemia activates protein kinase C (PKC) and abnormally increases its activity; thirdly, hyperinsulinemia promotes the proliferation of vascular smooth muscle cells and the synthesis and secretion of endothelin-1 (ET-1) in endothelial cells . These delayed pathophysiological changes are also characteristic in CVS after SAH [1, 10].
Therefore, according to the results of this study, it is reasonable to speculate that insulin resistance also exists in the present model of CVS after SAH. The number and expression of insulin receptor on the membrane of endothelial cells in the basilar artery with severe spasm have been significantly reduced, whereas the expression of insulin receptor on the endothelial cells of basilar artery without spasm is strongly positive. Downregulation of the number and expression of insulin receptor causes insulin resistance and inhibition of the various physiological effects of insulin, including its vasoactive effect . Vascular endothelial cells are the target cells for insulin to exert its vascular regulatory effects. Extensive studies have demonstrated that there are a large number of insulin receptors on vascular endothelial cells; upon binding with insulin, the downstream signaling pathways are activated to further stimulate the production of ET-1 and NO in endothelial cells, adjust the balance of vascular constriction, and hence exert the vasoactive effect . However, as shown in this study, the vascular endothelial cells are damaged after SAH; the number and expression of insulin receptor on these cells are reduced significantly, resulting in insufficient binding with insulin and failure in promoting the release of the vasodilatory factor NO, which hampers the vasoactive effect of insulin. Therefore, treatment with insulin alone fails to alleviate the CVS, whereas combinatorial use of insulin and vitamin C effectively relieve the symptom. Vitamin C is an antioxidant; although itself has no direct role on the vasomotor and hemodynamics , vitamin C can counteract the damaging effect of oxygen free radicals on the endothelial cells, improve cell function, reduce insulin resistance, increase the sensitivity to insulin in endothelial cells, promote the binding of insulin with its receptor, and thereby increase the release of vasodilatory factor NO to exert its vasoactive effect. In addition, vitamin C can inhibit the activity of oxygen free radicals and inhibit their binding to NO, thus facilitating the vasodilatory effect of NO [12, 13]. Therefore, combinatorial use of insulin and vitamin C has significantly improved CVS in animals of SAH + insulin + vitamin C group.
However, the most serious complication of insulin therapy is low blood glucose reaction. In this study, the blood glucose has been controlled at a level higher than 4.9 mmol/L (the normal range of blood glucose level in rabbit is 4.33 ~ 8.60 mmol/L), and there is no occurrence of twitch, coma, and other symptoms in the animals due to low blood glucose level.