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

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.

cDNA cloning of somatolactin, a pituitary protein related to growth hormone and prolactin

From a flounder pituitary cDNA library, cDNA clones encoding a 28-kDa glycoprotein produced by the pars intermedia of the pituitary were isolated and characterized. Nucleotide sequencing demonstrated a precursor of the 28-kDa protein, which consisted of 231 amino acid residues, to be cleaved into a signal peptide (24 amino acids) and a mature protein (207 amino acids) containing one N-glycosylation site. By comparison of amino acid sequences, the 28-kDa protein was found to be distantly and similarly related to growth hormone and prolactin. Consequently, it was named somatolactin. Somatolactin mRNAs were specifically expressed as 1.2 and 1.8 kb poly(A)+ RNAs in flounder pituitary.

Immunocytochemical demonstration of pituitary cell types in the teleost Poecilia latipinna, by light and electron microscopy

Using the unlabelled antibody method at the light microscope level, and the immunogold method at the electron microscope level, the distribution of the different adenohypophysial cells was demonstrated in the teleost Poecilia latipinna, by means of antisera to both teleostean and mammalian pituitary hormones and their subunits. Anti-salmon prolactin, but not anti-rat or -ovine prolactin, gave a specific staining of the acidophils of the rostral pars distalis (RPD), while anti-trout growth hormone (GH), but not anti-rat GH, stained similar but always separate cells in the proximal pars distalis (PPD). Antisera to the whole molecules of mammalian glycoprotein hormones stained the entire population of basophils in the PPD, but separate populations of gonadotrophs and thyrotrophs could be discriminated using anti-salmon gonadotrophin and anti-human thyrotrophin beta subunit. Antisera to ACTH (1-24) and (11-24) sequences, as well as beta-endorphin and met-enkephalin, stained the lead haematoxylin-positive cells of the RPD and pars intermedia (PI), whereas anti-alpha-MSH stained only the PI cells. Ultrastructural examination showed that these immunoreactivities were present in the same secretory granules, and were always greater in pale granules rather than electron dense granules. In the RPD, blebs of ACTH-immunoreactive cytoplasm were found to protrude through the gaps in the basement membrane into the neurohypophysis. The second "PAS-positive" cell type of the PI showed a strong cross-reaction with anti-salmon gonadotrophin, suggesting that it may produce a glycoprotein chemically related to the gonadotrophin(s).

Environmental control of prolactin synthesis in the teleost fish Oreochromis (formerly Sarotherodon) mossambicus

In the cichlid fish Oreochromis mossambicus prolactin cell activity is inversely related to the osmolarity and the Ca2+ concentration of the ambient water. Prolactin cell activity was estimated, at the end of a 3-week experimental period, by determination of the rate of prolactin synthesis during incubation of the rostral parts of the pituitary gland, in the presence of [3H]lysine. Since the secretory activity of isolated prolactin cells is known to be inversely related to the osmolarity of the incubation medium, the possibility was investigated that the effects of changes in ionic composition of the ambient water on prolactin secretion in vivo are mediated by changes in osmolarity of the blood plasma. No support was found for this hypothesis. In fish exposed to high water osmolarities prolactin cell activity was reduced, while plasma osmolarity increased. In contrast, at high Ca2+ concentrations of the water, when prolactin secretion was inhibited to a similar extent, plasma osmolarity was significantly reduced. Although direct effects of plasma osmolarity on prolactin cells cannot be excluded completely, it is unlikely that plasma osmolarity is the predominant factor in the control of prolactin cell activity in situ. The physiological significance of the capacity of isolated prolactin cells to respond to changes in osmolarity of the ambient medium--a capacity shared with some other endocrine cell types--is therefore unclear.

Responses of the PAS-positive pars intermedia cells in the cichlid fish Sarotherodon mossambicus to ambient calcium and background adaptation

The function of the PAS-positive pars intermedia cells in fish has been connected with control of background adaptation and of plasma calcium levels. Since background reflectivity and illumination influence calcium metabolism in S. mossambicus, we have tested the hypothesis that the effects of background reflectivity on the PAS-positive cells are mediated by changes in plasma calcium. However, total plasma calcium does not correlate with the activity of the PAS-positive cells as estimated by morphometrical criteria. Transfer of S. mossambicus to a white background leads to a drop in plasma calcium, and a marked reduction of the activity of the PAS-positive cells. Adaptation to low-calcium freshwater, on a neutral background, has the same effect on plasma calcium but has no effect on the PAS-positive cells. The characteristic structural features of highly active PAS-positive cells in fish from a black background are not due to the slight hypocalcemia that occurs in these fish, since addition of calcium to the water in concentrations that fully prevent the drop in plasma calcium does not suppress the PAS-positive cells. These findings make it very unlikely that these cells produce a hypercalcemic factor in S. mossambicus.