The membranes of cultured rat brain astrocytes contain endothelin-converting enzyme activity

Both endothelins and their big-endothelin precursors were found capable of inducing the release of arachidonic acid from purified cultures of rat astrocytes. Their order of potency was as follows: big-endothelin-3 < big-endothelin-1 < endothelin-1 = endothelin-3. Mature endothelins induced the release of arachidonic acid in a rapid fashion. In contrast, much longer incubation times were required for big-endothelins to exert an effect, suggesting that their activity was dependent on their conversion. When big-endothelin-1 was added to the incubation medium of intact live astrocytes, it was converted into mature endothelin-1 in a time-dependent manner and the conversion was inhibited by phosphoramidon. This suggests that astrocytic endothelin-converting enzyme is (at least in part) an external membrane-bound metalloprotease. Some conversion of big-endothelin-3 into endothelin-3 also occurred. However, it was less efficient than the conversion of big-endothelin-1, which is compatible with the lower bioactivity of big-endothelin-3 vs. that of big-endothelin-1 in astrocytes.