| Research Abstract |
Various receptors for hormonal substances and neurotransmitters are present on the membrane of fish chromatophores. However, the linkage between the reception of the chemical messengers and the actual mechano-chemical events has still been poorly understood. Up to now, the intracellular role of cAMP as a second messenger and the involvement of G-protein in the activation or inactivation of the adenylate cyclase have been suggested. In the present study, it was shown that Ca^<2+> and Inositol 1,4,5-trisphosphate (IP_3) are also the second messengers. That is, when the melanophores were stimulated by the neurotransmitters, norepinephrine (ME), an intracellular Ca^<2+> level was increased, which triggers an aggregation of melanosomes. In addition, the application of IP_3 to Brij 58-permeabilized cells caused the aggregation of pigment granules. Probably, IP_3 may bind to its specific receptors on the membrane of intracellular store of Ca^<2+> to release the ions. Electron microscopic obse
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rvations indicated that many pinocytotic vesicles and smooth endoplasmic reticulum are present beneath the membrane and in the cytoplasm, respectively. These organisms may be the Ca^<2+>- storing compartments. Although the influx of Ca^<2+> into the chromatophores through the calcium channels is undeniable at the present time, it was certainly confirmed that the melanophores responded to NE by the pigment aggregation even in Ca^<2+>, Mg^<2+>-free saline. The increase in cytosolic Ca^<2+> concentration may elicit the activation of calmodulin, a representative Ca^<2+>-binding protein, leading to the subsequent activation of other enzymes such as phosphodiesterase. W-7 was found to antagonize NE-induced responses of the chromatophores largely, but the inhibitory action of the drug reached the uppermost limit at the concentration of 50 muM (Degree of the chromatophore response : about 40 %).
These results imply the simultaneous progress of both processes concerning the signal transduction in fish chromatophores, Ca^<2+>-calmodulin system (IP_3) and cAPM system. In fact, ^<alpha>1- and alpha_2-adrenoceptors are present on the cell membrane, although the latter subtype is found to be dominant (about 70%). Moreover, we found that the pigment aggregation requires energy, and that dynein-tubulin interaction may generate the motive force for the migration. Less
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