1. Mechanisms regulating the movements of motile iridophores of the paradise goby, Rhinogobius giurinus, were examined. 2. A high K+ solution and electrical stimulation each induced dispersion of the reflecting platelets within the cells. Both stimuli were ineffective in inducing the dispersion response of iridophores in chemically denervated preparations. 3. The actions of K+ ions and electrical stimulation were inhibited by an alpha-adrenergic antagonist, but not by a beta-adrenergic antagonist. 4. Norepinephrine acted to disperse the platelets within the iridophores. Its action was selectively antagonized by alpha-adrenergic blockers. 5. The platelet-dispersing effects of alpha2-adrenergic agonists were more than 300-fold greater than those of alpha1-adrenergic agonists. 6. The inhibitory effect of yohimbine on the dispersion response to norepinephrine was about 1000-fold greater than that of corynanthine. 7. Forskolin and 8-Br-cAMP induced aggregation of the platelets. 8. Melatonin was effective in causing aggregation of the platelets. 9. The conclusions from these results are as follows. 1)The motile iridophores are regulated by both nervous and endocrine systems. 2) Nervous information, which is adrenergic, brings about the dispersion of platelets via alpha-adrenoceptors that are alpha2 in nature. 3) Decreases in levels of intracellular cAMP induce dispersion of the platelets, while increases cause their aggregation.
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