CA2-DEPENDENT NITRIC-OXIDE RELEASE IN ENDOTHELIAL BUT NOT R3230AC RATMAMMARY ADENOCARCINOMA CELLS()

We have characterized the ability of several cell types associated wit h the microvasculature of solid tumors to release nitric oxide (NO .) in response to increases in cytosolic Ca2+ concentration ([Ca2+](c)). EA.hy926 immortalized human umbilical vein endothelial cells (EC), rat fibroblasts (RFL), and tumorigenic cells isolated from R3230Ac rat ma mmary adenocarcinoma (MaC) were treated with thapsigargin (TG), an inh ibitor of Ca2+-ATPase. NO . output was measured via a chemiluminescenc e detection system. Baseline NO . output was detectable only for EC. T G caused a significant increase in EC NO . output that could be blocke d with N-G-manomethyl-L-arginine and restored with L-arginine. TG did not stimulate NO . release from RFL or MaC cells, despite elevating [C a2+](c) in all cells. A Ca2+-dependent isoform of NO synthase (eNOS) w as detected by immunoblot only in EC. These data indicate that EC, but not RFL or MaC, are capable of Ca2+-dependent NO . release and sugges t that any Ca2+-dependent NO release within this tumor is primarily of endothelial (and not tumorigenic cell) origin.