CHARACTERIZATION OF STIMULATION OF MAMMALIAN EXTRAOCULAR MUSCLES BY CHOLINOMIMETICS

¹ Department of Pharmacology, University of Illinois College of Medicine, Chicago, Illinois

The effects of acetylcholine, succinylcholine, 1 , 1-dimethyl-4-phenylpiperazinium, nicotine and muscarine on the superior oblique muscle of the cat were compared. All of these cholinomimetics evoked a sustained contraction of superior oblique muscle, in vitro and in vivo. Small doses of the cholinomimetics evoked repetitive activation of motor units of twitch and/or of diffusely, multiply innervated fibers as assessed electromyographically. After large doses, the repetitively responding motor unit potentials were reduced or absent. It is suggested that the sustained contraction produced by succinyicholine is the consequence of a contracture of some of the fibers in addition to repetitive activation of certain motor units. The twitch responses of extraocular muscles in situ to nerve stimulation were less sensitive to depression by i.v. administered tubocurarine than the titch responses of the triceps surae muscle. Muscarine and epinephrine were capable of inducing weak contractile responses of extraocular muscle. The stimulant action of muscarine was antagonized by atropine, in vitro and in vivo, whereas that induced by epinephrine was not. Although atropine antagonized the action of muscarine, it potentiated the extraocular contractile response to succinylcholine or acetylcholine in vivo.