[Effects of acetylcholine and carbachol on the iris sphincter].

We compared the effects of acetylcholine and carbachol on iris sphincters. The muscle strips (mostly bovine tissues) were isolated and incubated in a 0.2ml organ bath. Results revealed an approximately ten-thousandfold difference in potency between carbachol and acetylcholine, in the bovine iris. Acetylcholine and/or endogenous chemical agents were spontaneously released from bovine tissue. Acetylcholinesterase activity in this tissue strongly inhibited the acetylcholine action, in order to protect the nerve terminals from random depolarization due to the released acetylcholine. Among several species (bovine, rabbit, hamster), Ach action was the least in the bovine iris sphincter muscle.