EVIDENCE FOR SYMPATHETIC, PURINERGIC TRANSMISSION IN THE IRIS DILATORMUSCLE OF THE RABBIT

Electrical transmural stimulation of isolated iris dilator muscle of t he rabbit produced a transient contraction that consisted of adrenergi c and nonadrenergic components. In contrast to the adrenergic componen t, the nonadrenergic component was resistant to prazosin and other adr enoceptor antagonists. However, both components were completely blocke d by guanethidine or tetrodotoxin. Among some tested compounds includi ng neuropeptide Y, both ATP and 2-methylthio ATP produced a transient contraction in the dilator muscle and the sustained treatment with eac h markedly attenuated the nonadrenergic responses to electrical stimul ation and to ATP. Suramin had no effect on and alpha,beta-methylene AT P potentiated the responses to electrical stimulation and to ATP. Thes e results suggest that the nonadrenergic contraction induced by electr ical transmural stimulation is a sympathetic purinergic response that may be mediated through unique purinoceptors.