Involvement of prejunctional alpha 2-adrenoceptor in bovine ciliary muscle movement

The effects of adrenergic agents on bovine ciliary muscle were studied in vitro. Norepinephrine, phenylephrine, or isoproterenol did not affect the muscle tone. High concentrations of guanethidine (10(-5) M) partly inhibited the nerve-mediated contractions of the muscle. The nerve-mediated contractions also were inhibited in the following order: norepinephrine > epinephrine >> isoproterenol. The inhibition was blocked by yohimbine or phentolamine, but not by optimum concentrations of prazosin, moxisylyte, propranolol, and carteolol. Usual concentrations of phosphodiesterase inhibitors markedly inhibited the nerve-mediated and carbachol-induced contractions. These results suggest that an adrenergic alpha 2 receptor located at the nerve terminals inhibits the contraction of cholinergically innervated ciliary muscle. Adrenergic innervation seems to play an inhibitory role in the muscle tissue when the muscle tone is elevated.