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K. Yamaji, T. Yoshitomi, S. Usui; Effect of Somatostatin and Galanin on Isolated Rabbit Iris Sphincter and Dilator Muscles . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2243.
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Purpose: Somatostatin and galanin have been localized in the iris. Because a variety of biologically active peptides modulate iris muscle contraction, we examined the effect of somatostatin and galanin on rabbit iris smooth muscles. Methods: Rabbit iris muscle preparations were mounted in an organ bath, and tension changes were recorded by an isometric transducer. Electric field stimulation (100 Hz, 0.3 msec in duration, 10V in strength) was applied through a pair of platinum plate electrodes. Results: The sphincter muscle response consisted of two components: a fast cholinergic component and slow substance P-ergic component. Both somatostatin and galanin attenuated the cholinergic component in a dose-dependent manner (from 0.3nM to 0.1µM) but had no effect on substance-P ergic responses. Galanin was more effective (attenuation of 41% at 0.1µM) compared with somatostatin (attenuation of 17% at 0.1µ M) in reducing the cholinergic response. Neither peptide affected the contraction induced by acetylcholine (1 mM). The dilator muscle response to field stimulation was not changed by either peptide, even at the highest concentrations we examined. Conclusions: Both somatostatin and galanin inhibited iris sphincter smooth muscle contraction due to electrical field stimulation but not direct acetylcholine stimulation. Therefore both peptides inhibited cholinergic transmission in the sphincter muscle, although the degree of inhibition by each was different. In contrast, neither peptide affected contraction of the dilator muscle. We conclude that somatostatin and/or galanin may induce mydriasis by attenuating cholinergic neurotransmitter release.
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