Elevation of bovine endothelial cell angiotensin converting enzyme by cationophores and inhibition by ouabain

Abstract

We recently reported that calcium ionophore A23187 causes a several-fold elevation of angiotensin converting enzyme (ACE) activity of bovine pulmonary artery endothelial cells in culture and that this elevation is dependent upon extracellular calcium. Now we have observed that monensin, a sodium ionophore, also elevates the ACE activity of these cells. This elevation in ACE was not inhibited by 0.2 mM EGTA or the calcium channel inhibitor nifedipine, and monensin did not alter intracellular calcium as measured by fluorimetric assessment of fura-2 AM-loaded cells. When confluent endothelial cells were incubated with monensin or A23187 in the presence of 10–20 nM ouabain, a specific inhibitor of Na + K +- ATPase , the elevation in ACE produced by both of the ionophores was totally eliminated. Concentrations of ouabain greater than 10 nM also inhibited baseline levels of ACE activity. Ca 2+ measurements of fura-2 AM-loaded cells showed that ouabain had no effect on the influx of Ca 2+ produced by A23187. The elevation of ACE seemed to require new protein synthesis, since 0.1 μg/ml cycloheximide inhibited the elevation produced by monensin and A23187. Other sodium transport inhibitors such as amiloride or bumetanide had no effect on ACE elevation caused by monensin. These results suggest that ACE levels of bovine endothelial cells in culture are under cation regulation and may be modulated by ouabain-sensitive Na + K +- ATPase .