Type 2 Diabetes Alters Intracellular Ca²⁺ Handling in Native Endothelium of Excised Rat Aorta

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) plays a key role in controlling endothelial functions; however, it is still unclear whether endothelial Ca²⁺ handling is altered by type 2 diabetes mellitus, which results in severe endothelial dysfunction. Herein, we analyzed for the first time the Ca²⁺ response to the physiological autacoid ATP in native aortic endothelium of obese Zucker diabetic fatty (OZDF) rats and their lean controls, which are termed LZDF rats. By loading the endothelial monolayer with the Ca²⁺-sensitive fluorophore, Fura-2/AM, we found that the endothelial Ca²⁺ response to 20 µM and 300 µM ATP exhibited a higher plateau, a larger area under the curve and prolonged duration in OZDF rats. The "Ca²⁺ add-back" protocol revealed no difference in the inositol-1,4,5-trisphosphate-releasable endoplasmic reticulum (ER) Ca²⁺ pool, while store-operated Ca²⁺ entry was surprisingly down-regulated in OZDF aortae. Pharmacological manipulation disclosed that sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) activity was down-regulated by reactive oxygen species in native aortic endothelium of OZDF rats, thereby exaggerating the Ca²⁺ response to high agonist concentrations. These findings shed new light on the mechanisms by which type 2 diabetes mellitus may cause endothelial dysfunction by remodeling the intracellular Ca²⁺ toolkit.