Periendothelial acetylcholine synthesis and release in bovine cerebral cortex capillaries

Choline acetyltransferase (ChAT) activity is present in isolated cerebral capillaries, where it has been considered to be a marker for perivascular cholinergic nerve terminals. However, ChAT-like immunoreactivity has been visualized in endothelial cells. This finding raised the possibility that at least part of the biochemically detected ChAT has a nonneuronal origin. To evaluate the relative contribution of endothelial cells and nerve fibers to the total acetylcholine (ACh)-synthesizing capacity of cerebral capillaries, ChAT activity and ACh release were measured in capillaries and in purified endothelial cells isolated from bovine cerebral cortex. Isolated capillaries showed ChAT activity, which was inhibited by 2-benzoylethyl trimethylammonium to the same extent as cerebral ChAT. When preincubated with [3H]choline, these capillaries presented a calcium-dependent enhancement in tritium release upon electrical field stimulation. Purified endothelial cells had minor ChAT activity and lacked the ability to release tritium in response to electrical stimulation, although the endothelial markers alkaline phosphatase, gamma-glutamyltranspeptidase, and 1,1'-dioctadecyl-1,3,3',3'-tetramethyl-iodocarbocyanide perchlorate-labeled acetylated low-density lipoprotein uptake were fully preserved. These data indicate that, within isolated cerebral capillaries, ACh is synthesized and released by a periendothelial structure. The fact that ACh release is provoked by electrical stimulation and by a calcium-dependent mechanism strongly suggests that cerebrovascular ACh has a neuronal origin.