A population of sensory nerve fibres in the rabbit iris is known to contain calcitonin generelated peptide and tachykinins, such as substance P and neurokinin A. In the presence of atropine and guanethidine, the isolated iris sphincter responded to electrical stimulation with a contraction that could be abolished by tachykinin antagonists. Capsaicin, known to release tachykinins from sensory fibres, evoked a long-lasting tachykinin-mediated contraction of the iris sphincter. Repeated application of capsaicin led to tachyphylaxis, possibly reflecting depletion of releasable neuronal stores of tachykinins. At this stage, electrical stimulation failed to elicit contraction. The capacity of capsaicin to release neuropeptides from sensory fibres was confirmed by determination of substance P- and calcitonin gene-related peptide-like immunoreactivity in the incubation medium and in the iris tissue. The concentrations of substance P and calcitonin gene-related peptide in the iris after capsaicin exposure were reduced by about 25%. Like capsaicin, bradykinin evoked a tachykinin-mediated contraction and tachyphylaxis. However, after development of tachyphylaxis to bradykinin, electrical stimulation or exposure to capsaicin still evoked tachykinin-mediated contraction, albeit a reduced one compared with the response before bradykinin. Hence, capsaicin completely depletes tachykinin stores releasable by prolonged electrical stimulation, whereas bradykinin exhausts only a sequestered pool. The possibility that tachykinins occur in several releasable pools in sensory nerves was investigated in yet another way: the iris sphincter muscle was stimulated electrically once every 2.5 min over several hours. The contractile response diminished gradually. At the stage when the contractile response was almost lost the concentrations of substance P and calcitonin gene-related peptide in the iris had been reduced by about 70%; there was no further reduction in response to prolonging the stimulation for 12h. When the contraction amplitudes were plotted graphically the slope of the curve was consistent with two elimination processes, one with an elimination constant of 12% and another with an elimination constant of 3%. The slow-release pool was approximately two times larger than the rapid-release pool. In one series of experiments stimulation was interrupted at a stage when the rapid-release phase was over. After allowing the iris preparation to rest for l h before stimulation was resumed, the rapid-release phase returned. Hence, the rapid-release pool was refilled from the slow-release pool. When the iris sphincter muscle had been exposed to bradykinin to the point of tachyphylaxis before electrical stimulation, the elimination curve was monophasic with an elimination constant of 3%. Conceivably therefore, the rapid-release pool is identical with the bradykininsensitive pool. The slow-release pool on the other hand is sensitive to capsaicin but resistant to bradykinin.
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