Response of isolated rat iris dilator to adrenergic and cholinergic agents and electrical stimulation

Responses of isolated rat iris dilator to some agents and to electrical stimulation were examined. Norepinephrine and epinephrine produced contraction, which was antagonized by 0.03 microM phentolamine. Acetylcholine produced relaxation at low concentrations (1 nM - 1 microM) as great as 80% of the resting tone while contraction at high concentrations (greater than or equal to 1 microM). Both responses were suppressed by 0.02 microM atropine and enhanced by 0.03 microM physostigmine. Electrical stimulation at low voltage or low frequency (up to 10 Hz) elicited relaxation while stimulation at high voltage or high frequency (30 Hz) produced contraction. Stimulation with intermediate strength elicited biphasic response. The contraction and relaxation induced by electrical stimulation were abolished by 3 microM phentolamine or by 0.05 microM atropine, respectively. Both phases were abolished by tetrodotoxin (0.3 microM). It is suggested that in the rat the cholinergic relaxation of the dilator may assist the cholinergic contraction of the sphincter (1). The pronounced cholinergic relaxation of nonvascular tissue is to be noted.