Tunicamycin increases intracellular calcium levels in bovine aortic endothelial cells

Tunicamycin is anucleoside antibiotic that inhibits protein glycosylation andpalmitoylation. The therapeutic use of tunicamycin is limited inanimals because of its toxic effects, particularly in cerebralvasculature. Tunicamycin decreases palmitoylation of the endothelialisoform of nitric oxide synthase, stimulates nitric oxide synthesis,and increases the concentration of intracellular calcium(Ca²⁺]_i)in bovine aortic endothelial cells (B. J. Buckley and A. R. Whorton.FASEB J. 11: A110, 1997). In the present study,we investigated the mechanism by which tunicamycin altersCa²⁺]_iusing the Ca²⁺-sensitive dye fura2. We found that tunicamycin increasedCa²⁺]_iwithout increasing levels of inositol phosphates. When cells wereincubated in the absence of extracellularCa²⁺,Ca²⁺]_irapidly rose in response to tunicamycin, although a full response wasnot achieved. The pool of intracellularCa²⁺ mobilized by tunicamycinoverlapped with that mobilized by thapsigargin. Extracellular nickelblocked a full response to tunicamycin when cells were incubated in thepresence of extracellular Ca²⁺.The effects of tunicamycin onCa²⁺]_iwere partially reversed by washing out the drug, and the remainder ofthe response was inhibited by removing extracellularCa²⁺. These results indicate thattunicamycin mobilizes Ca²⁺ fromintracellular stores in a manner independent of phospholipase Cactivation and increases the influx ofCa²⁺ across the plasma membrane.