Immunoblotting technique to study release of melanophore-stimulating hormone from individual melanotrope cells of the intermediate lobe of Xenopus laevis.

The melanotrope cells in the pars intermedia in the pituitary of Xenopus laevis synthesize and release the melanophore-stimulating hormone (alpha MSH), a small peptide that causes skin darkening during the process of background adaptation. Evidence has been found for a heterogeneity in biosynthetic activity of the melanotrope cells. In the present study two questions were addressed: (1) does the melanotrope cell population also show heterogeneous alpha MSH-release, and (2) can this heterogeneity be changed by extracellular messengers? Since dopamine is known to inhibit alpha MSH-release, this messenger is used to study the regulation of the heterogeneity. To quantify alpha MSH-release from individual cells, a cell blotting procedure has been developed for the binding and relative quantification of the small alpha MSH peptide. The immunoblotting procedure involves binding of the cells to a carrier slide and binding of released alpha MSH to a nitrocellulose filter. After immunostaining, the amount of alpha MSH per cell was quantitated by image analysis. Untreated melanotrope cells reveal a distinct variability in alpha MSH-release, some cells showing low secretory activity, whereas others are strongly secreting, indicating heterogeneity of alpha MSH-release. Dopamine treatment strongly inhibits alpha MSH-release from individual cells, resulting in a clearly less pronounced melanotrope cell heterogeneity. The effect of dopamine appears to be dose-dependent as a low dopamine concentration has only a moderate effect on the alpha MSH-release. It is proposed that dopamine is a physiological regulator of the degree of melanotrope cell heterogeneity in alpha MSH-release.