Effect of High Glucose on Basal Intracellular Calcium Regulation in Rat Mesangial Cell

Abstract

Background: A number of cellular mechanisms are critically dependent on intracellular Ca²⁺ homeostasis. A sustained increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) is capable of activating a number of potentially harmful processes including phenotype change to secretory type, dysregulated cell proliferation, and cell injury and death. Mesangial cells (MCs) play an important role in the pathophysiology of diabetic nephropathy. Methods: We evaluated the effect of high glucose on basal [Ca²⁺]_i in the unstimulated state and identified its contributing pathways. MCs were isolated and cultured from Sprague-Dawley rats. [Ca²⁺]_i was measured by fluorometric technique with fura-2AM. Results: In a dose-dependent manner, superfusion of MCs with Tyrode’s solution containing high glucose (30 and 50 mM) induced a delayed spontaneous increase in [Ca²⁺]_i, which was not found in those with normal (5.5 mM) glucose or mannitol. The high glucose-induced increase in [Ca²⁺]_ioccurred through transmembrane influx of extracellular Ca²⁺ and was blocked by SKF96365, an inhibitor of store-operated Ca²⁺ influx. Na⁺-Ca²⁺ exchanger (NCX) activity, a major channel regulating basal [Ca²⁺]_i, and the clearing ability of intracellular Ca²⁺ were depressed after MCs were cultured in high-glucose medium. Western blot analysis revealed the decreased expression of a 70-kD NCX protein in MCs cultured in high-glucose medium. Conclusions: A high-glucose concentration induced a spontaneous increase in basal [Ca²⁺]_i of MCs without stimulation. There was a decrease in the activity of NCX in the high-glucose condition, which seems to occur at the level of protein expression. The present results provide a novel insight into the mechanisms of diabetic nephropathy in that intracellular Ca²⁺ homeostasis is an important secondary messenger and a mediator in hormonal signaling.