LIGHT-SENSITIVE RESPONSE IN MELANOPHORES OF XENOPUS-LAEVIS .1. SPECTRAL CHARACTERISTICS OF MELANOPHORE RESPONSE IN ISOLATED TAIL FIN OF XENOPUS TADPOLE

Melanophores in the isolated tail from the amphibian larvae Xenopus La evis, Hyla japonicus, Rana pirica, and Hynobius retardatus aggregated melanin granules in response to light and dispersed them when placed i n darkness. The spectral characteristics for the melanin-aggregation r esponse were examined by irradiating the Xenopus tail-fin locally (dia meter, 2.1 mm) with monochromatic light; (380-1,020 nm). The spectral region of wave length which induced melanosome aggregation depended on the light intensity but was limited to the visible spectrum. At low l ight intensity (1.59 mu W/cm(2), Delta lambda = 5 nm), the aggregation response occurred in the spectral region between 400 and 600 nm and t he maximum response was observed at 500 nm. This range is very close t o the absorption spectrum of rhodopsin in the visual rod cell. Hypoder mic injection of cGMP into isolated tail-fin induced a marked melanin- dispersion in spite-of light-stimuli. When the tail-fin was treated wi th isobutylmethylxanthine (IBMX; phosophodiesterase inhibitor) in dark ness and then was re-exposed to light, the aggregation response was in hibited. The photo sensitive melanin aggregation was independent of a requirement for Ca2+ ions but melanosome dispersion in darkness was Ca 2+-dependent. K+-rich Hanks' solution, ouabain (inhibitor of Na+-K+-AT Pase) or nonactin (cation ionophore), which induced a change of the me mbrane potential of melanophores, inhibited the aggregation response w hen the melanophores were reexposed to light after a period in darknes s. These results suggest that the molecular mechanism of photoreceptio n in melanophores of amphibian tadpoles is similar to that in visual c ells. (C) 1996 Wiley-Liss, Inc.