Effects of capsaicin, tachykinins, calcitonin gene-related peptide and bradykinin in the pig iris sphincter muscle

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Abstract
Capsaicin-sensitive sensory neurons of the rabbit iris, by releasing tachykinins, exert a major role in the control of pupil motility in response to various noxious stimuli. However, the contribution of sensory innervation to the regulation of iris smooth muscle tone in other mammals species is not known. We have studied the effects produced by electrical field stimulation, capsaicin, substance P, neurokinin A, calcitonin gene-related peptide (CGRP), and bradykinin in the isolated iris sphincter muscle of the pig. Capsaicin (10 μM): a) contracted the isolated sphincter muscle and; b) released immunoreactivity for substance P (SP-LI) and CGRP (CGRP-LI) from this preparation. These two effects were no longer observed at the second exposure to the drug. Electrical field stimulation (10 Hz, 60 V, 0.5 ms for 5 s) produced a biphasic contractile response. The rapid component was inhibited by atropine (1 μM), while the delayed response was blocked by previous exposure to capsaicin (10 μM). Substance P and neurokinin A consistently produced contraction of the pig iris sphincter muscle, substance P being more potent than neurokinin A. CGRP induced a contractile response in more than 50% of the preparations. The tachykinin antagonist [D-Argl, D-Trp7,9, Leu11-substance P (3 μM) blocked: a) the effect of substance P (1 nM); b) the delayed response to electrical field stimulation and; c) reduced by more than 50% response to capsaicin. Bradykinin (10 μM) failed to release either SP-LI or CGRP-LI. The contractile response evoked by bradykinin was unaffected by in vitro pretreatment with capsaicin (10 μM). The existence in the pig iris of capsaicin-sensitive sensory fibres releasing neuropeptides and thus regulating sphincter muscle tone is proposed.