Modulating factors of intracellular calcium dynamics in vascular smooth muscle cell : a numerical study
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The prevalence of diabetes, a metabolic disorder characterized by hyperglycemia, is increasing rapidly worldwide. According to the world health organization (WHO), almost 422 million people in the world have diabetes. Vascular dysfunction, that is distinctively observed in diabetic patients, also leads to the high chance of cardiovascular diseases. The cellular mechanisms of vascular dysfunction in diabetes are not completely elucidated because of the high-level complexity involved. There are some recent studies in literature about the vascular smooth muscle cell (VSMC) dysfunction associated with diabetes. These studies imply that the dysfunction of diabetic VSMC is related to the alterations of functions of gap junctional intercellular communication (GJIC) and calcium handling proteins such as sarcoplasmic reticulum calcium ATPase (SERCA), inositol trisphosphate receptor (IP3R), and ryanodine receptor (RyR). The main purpose of the thesis is to explore the importance of SERCA, IP3R, RyR and GJIC as modulating factors of intracellular calcium Ca2+ cyt dynamics in VSMCs and investigate the related consequences in the perspective of diabetes. As a powerful tool to investigate extremely complex biological systems, we have developed a mathematical model of VSMC to accomplish our research aims. The sarcoplasmic reticulum (SR) was modelled as a single luminally continuous store with homogeneous luminal calcium. None of the VSMC mathematical models has used sequential binding of calcium and IP3 for the activation of IP3R. As calcium is the central attention of the study, we have incorporated a simplified 4-state sequential-binding IP3R model. The change of SERCA, IP3R or RyR level shifted the oscillatory state of intracellular calcium to steady and vice versa at a constant agonist concentration. Especially, increasing SERCA level move the oscillatory region to high agonist concentrations whereas increasing IP3R or RyR level shift the oscillatory region to lower agonist concentrations. The shift of oscillatory region was significant at low SERCA levels and at low IP3 levels. The model results indicate that using the same agonist concentration for studying the existence of intracellular concentrations in the absence of IP3R or RyR may lead to contradictory findings because of the shift of oscillatory region. The model results showed that Vm-mediated signalling mechanism plays dominant role in the propagation of calcium rise whereas the activation of IP3R acts as a limiting step to make the IP3-mediated calcium propagation ineffective. It seems that the penetration depth and the wave velocity of intracellular calcium oscillations in coupled VSMCs are strongly related to the GJIC if the regenerative propagation was not extended to all the non-stimulated cells. The downregulation of IP3R and RyR reduced the penetration of intracellular calcium propagation in VSMCs. The SERCA regulation of the frequency of intracellular calcium oscillations and penetration of regenerative propagation are biphasic. Based on the model results, our hypothesis is that if an external environment influences the agonist-induced calcium transients, the VSMC may alter the levels of SERCA, IP3R and/or RyR expressed in the SR to restore intracellular calcium transients matched with the functional need. The reduced SERCA level is a possible mechanism for the “smoothed" calcium transients in diabetic VSMCs. The SERCA level tends to be the regulating factor of intracellular calcium in both the activation and relaxation phases of oscillations. Also, the model results indicate that the functional changes due to altered level of IP3R in diabetic VSMCs might be regulated by altering the level of the RyR protein and vice versa. However, the impaired intracellular calcium dynamics due to reduced SERCA levels can not be restored by changing IP3R or RyR levels because of the reduced intra-SR calcium load. These findings lead to the fact that reduced SERCA level is probably the primary factor responsible for the reduced intracellular calcium transients and the reduced contractility in diabetic VSMCs even though all the three proteins, SERCA, IP3R and RyR, are varied.