Effect of Elevated Glucose on Endothelin-Induced Store-Operated and Non-Store-Operated Calcium Influx in Renal Mesangial Cells

Abstract

Early diabetic nephropathy exhibits renal glomerular hyperfiltration and an increase in renal plasma flow. The hyperfiltration is a dysfunctional state that may arise from a hyperglycemic-induced hypocontractility of glomerular mesangial cells that may be associated with depressed Ca²⁺ signaling events. The present study was designed to determine the effects of acute (minutes) and chronic (days) elevated glucose levels on endothelin-induced calcium signaling with a particular emphasis on the potential influence on stores and store-operated Ca²⁺ influx (SOCI; also called capacitative calcium entry) in glomerular mesangial cells. Primary cultures of rat mesangial cells were grown in either high (30 mM) or normal (5 mM) glucose-containing media and tested in the presence of either high (30 mM) or normal (5 mM) glucose levels. Intracellular calcium levels were monitored with the calcium-sensitive fluorophore fura-2 before and after treatment with either endothelin-1 (10 nM), to induce typical Ca²⁺ signals, or the endoplasmic reticulum (ER) Ca-ATPase inhibitor thapsagargin (1 μM), to unload ER Ca²⁺ stores. Both acute and chronic exposure to high glucose levels depressed the endothelin-induced calcium signal. However, neither release of Ca²⁺ from stores nor SOCI were depressed by high glucose levels. In contrast, an endothelin-induced calcium entry pathway (likely receptor-operated calcium influx), separate from SOCI, was markedly depressed in the presence of both acute and chronic high glucose levels. The depressant effect of high glucose was rapidly (minutes) reversible upon returning to normal glucose levels. It is concluded that high glucose levels depress endothelin-induced calcium signaling in rat mesangial cells by inhibiting non-SOCI Ca²⁺ entry pathways, namely the receptor-operated Ca²⁺ influx pathway. The glucose-induced alterations in the receptor-operated calcium influx pathway may, in part, contribute to the depressed contractile state of glomerular cells during periods of hyperglycemia.