Biochemical evidence for cholinergic activity in retinal blood vessels

Blood vessels from human, cat, pig and bovine retinas were analyzed for their contents of choline acetyltransferase (ChAT) and muscarinic binding sites. ChAT was measured by the synthesis of 3H-acetylcholine in the presence of 3H-acetyl CoA and choline. Muscarinic binding sites were determined by the specific binding of 3H-quinuclidinyl benzylate (3H-QNB). Tissue levels of ChAT varied from 39-850 nmol/g/hr, the lowest values being in human tissues. Muscarinic binding sites were less different (0.5 to 1.8 fmol/mg net weight) among the species studied, being the highest values in human retinal vessels. It appears that retinal blood vessels not only have sites to bind acetylcholine, perhaps to mediate physiologic responses, but might be capable of supplying acetylcholine for local vascular tone control.