Cell biology of somatolactin.

Somatolactin (SL) is a novel pituitary protein, isolated for the first time from the Atlantic cod. The corresponding proteins have been identified in several toleost species, but not in other classes of vertebrates. Comparison of amino acid sequence has revealed SL molecules to be related to growth hormone (GH) and prolactin (PRL) in teleosts and other vertebrates, suggesting that SL is a new member of the GH/PRL family. Unlike GH and PRL, SL can exist in either glycosylated or nonglycosylated form, depending on the species; most teleosts possess glycosylated SLs, except for salmonids whose SLs are simple proteins. The SL-producing cells are located in the pars intermedia bordering the neurohypophysis and are distinct from melanocyte-stimulating hormone (MSH)-producing cells. The SL cells are PAS positive in most teleosts but chromophobic in salmonids, which may reflect the glycosylation status of SL. Its biochemical and molecular features have become increasingly clear, whereas its physiological significance is still poorly understood. Several possible roles for SL have been suggested, including roles in maturation, calcium regulation, stress response, acid-base regulation, fat metabolism, and background adaptation. Although direct evidence is lacking for any of the proposed functions, this involvement in acid-base regulation appears most probable, since other proposed biological events linked to SL should more or less affect the acid-base status in fish. More detailed studies are needed to define the function of SL.