Isolation of the biosynthetic products of the PAS positive pars intermedia cells in the cichlid teleost Sarotherodon mossambicus.

The pars intermedia of teleosts contains two types of granular cells with the predominant type being similar to the pars intermedia cells in other vertebrate groups and containing peptides derived from the pro-opiomelanocortin precursor molecule. The function and products of the second cell type, the PAS positive cells, are unknown. This study reports on the identification of biosynthetic products of the PAS positive cells of the cichlid teleost Sarotherodon mossambicus. The experimental regimen took advantage of earlier morphometric analyses which showed marked differences in metabolic activity of the PAS positive cells resulting from adaptation to different background colours and illumination. Autoradiography at the light microscopic level showed that both cell types of the pars intermedia incorporate labeled amino acids during in vitro incubation. To identify the products synthesized by the PAS positive cells, labeled products of the pars intermedia tissue were analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-pressure liquid chromatography. Comparison of pulse incubations of pars intermedia tissue of fish adapted to different backgrounds and conditions of illumination revealed that an increase in the number and metabolic activity of the PAS positive cells, as deduced from morphometric data, was paralleled by an increase of the amount of label incorporated into 27K and 25K molecules. Pulse--chase experiments with pars intermedia lobes of white and black background adapted fish showed that these two products, unlike the other newly synthesized products, were not involved in any precursor-product relationship. Our data, therefore, suggest that the 27K and 25K peptides were synthesized by the PAS positive cells.