The opposite correlation between calcium ion and cyclic-AMP regarding the activation of microsomal triglyceride transfer protein in rat liver

In this study, the effects of Ca²⁺ and cyclic adenosine monophosphate (cAMP) on microsomal triglyceride (TG) transfer protein (MTP) activity were investigated in rat liver. MTP activity was high when liver contained low levels of cAMP, which was induced by administration of glucose, or high levels of total Ca²⁺ and TG. However, MTP activity increased by high levels of Ca²⁺ and TG was reduced in a dose-dependent manner by treatment with dibutyryl-cAMP (db-cAMP), a cAMP analogue. Conversely, when homogenates of liver from normal rats, with low levels of total Ca²⁺ and high levels of cAMP, were incubated with thapsigargin, a Ca²⁺-inducer, MTP activity was increased in a dose-dependent manner compared to control. Therefore, our results suggest that high levels of Ca²⁺ cause hypertriglyceridemia through the elevation of MTP activity, as opposed to high levels of cAMP, which suppress MTP activity and inhibit hypertriglyceridemia.