Effects of somatolactin on melanosome aggregation in the melanophores of red drum (Sciaenops ocellatus) scales.

The effects of purified red drum somatolactin on pigment movement in red drum scales were studied in vitro and in vivo. The integument became pale within 2 min following an intramuscular injection of somatolactin (1 nmol/g body weight) in fish held in a black-background aquarium, and gradually regained its black coloration during the subsequent 30 min. No melanosome aggregation was observed in fish injected with vehicle or somatolactin over the dose range of 10(-9)-10(2) pmol/g. Melanosomes in the melanophores of scales were completely aggregated within 10 min of incubation with 1 microM somatolactin in vitro. The effect of somatolactin on melanosome aggregation was dose-dependent. Somatolactin caused only partial aggregation at a concentration of 500 nM and 250 nM somatolactin had little or no effect. Somatolactin caused melanosome aggregation in both innervated and denervated melanophores. Aggregated melanosomes which had been preincubated with somatolactin dispersed within 30 min after rinsing with a physiological buffer. No melanosome aggregation was observed in scales incubated with 10 nM-1 microM of red drum prolactin (PRL), red drum growth hormone (GH), ovine PRL, or recombinant tuna GH. These results indicate that the action of somatolactin on melanosome movement is direct, specific, reversible, and is probably mediated by a specific somatolactin receptor on the melanophores. Melanin-concentrating hormone (MCH) and norepinephrine (NE) also induced melanosome aggregation in scales at a low concentration of 10 nM. Addition of 1 microM alpha-melanophore-stimulating hormone (alpha-MSH) following preincubation of scales with 1 microM somatolactin, 10 nM MCH, or 10 nM NE resulted in partial dispersion of the melanosomes. These results suggest that melanosome migration in red drum scales is under multiple hormonal control. Although a direct action of somatolactin on melanosome aggregation is demonstrated in this study, its physiological role in the regulation of this process remains unclear.