Functional innervation and contractile properties of the human iris sphincter muscle

To investigate the pattern of innervation and the contractile properties of human iris sphincter muscle, we have observed the effects of field stimulation, various drugs and excess-K+ solutions on isometric tension development in strips of the iris sphincter from five humans. Spontaneous mechanical responses were not observed in these tissues. Electrical field stimulation (50 V, 0.8 msec, 20 Hz) evoked a contraction which was abolished by 10(-7) M tetrodotoxin (TTX) or by 10(-6) M atropine, thereby indicating that the response was cholinergically mediated. In the presence of atropine, electrical field stimulation did not affect muscle tone, even when this was elevated by histamine. The effect of exogenously applied noradrenaline (NA) varied among the specimens, i.e. in three out of four cases a contraction was evoked while a relaxation occurred in one preparation. The contraction evoked by NA was blocked by phentolamine (10(-6) M) and the relaxation was blocked by timolol (10(-6) M). Excess-K+ solutions, dose dependently contracted the muscle strips and application of 10(-6) M atropine decreased the amplitude of these contractions. The results indicate that the human iris sphincter is innervated by excitatory cholinergic nerve fibres and that the adrenergic innervation is sparse. In addition, this muscle possesses both alpha and beta adrenoceptors. Activation of alpha receptors produces a contraction whilst activation of the beta adrenoceptors causes relaxation.