Mechanism of down regulation of luteinizing hormone receptors and steroidogenesis in corpora lutea

The mechanism of ‘down regulation’ of luteinizing hormone receptors was investigated in pseudopregnant rats using a modified radioimmunoassay capable of measuring endogenous tissue-bound hormone. Treatment of pseudopregnant animals with a desensitizing dose (desensitization treatment) of human chorionic gonadotropin resulted in a decrease in receptor concentration. This decrease was prevented if the animals were treated prior to the desensitization treatment with indomethacin, an inhibitor of prostaglandin biosynthesis, suggesting a role for prostaglandins in down regulation. The desensitization treatment resulted in a time-dependent decrease in subsequent responsiveness of the tissue to luteinizing hormone. Basal progesterone production rate was also decreased following desensitization. Total tissue cholesterol was found to be decreased following desensitization treatment, without any change in the ratio of free to esterified cholesterol. Mitochondrial cholesterol was significantly reduced and pregnenolone production by the mitochondria of desensitized corpora lutea was also markedly reduced. However, when cholesterol was added to the mitochondria of desensitized corpora lutea, pregnenolone production was increased, reaching values almost equal to that shown by the control mitochondria. These results show that decrease in the responsiveness following desensitization treatment is due to, besides receptor loss, decrease in tissue cholesterol, in particular mitochondrial cholesterol. The cholesterol side chain cleavage activity, although low, appears to be functionally intact; the low activity could be attributed to low levels of mitochondrial cholesterol.     

Growth and reproduction of the Nile perch,Lates niloticus,an introduced predator,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis Length-frequency data suggest Nile perch, Lates niloticus, from the Nyanza Gulf grew to a total length of 9 cm by age 118 days and 23 cm by age 287 days. A modified von Bertalanffy growth curve _t = 1.35·L(1-e^–K(t-t _o)) with the parameters L = 93.1, K = 0.272 and t_o = 0.046, is suggested to describe growth up to 5 years of age and the relationship _t = 1.35·(31.96 + 7.681t) for fish aged 6 years and above. Length-weight relationships were = 0.0234·-gt^2.74 for fish between 7 and 15.9 cm total length, = 0.0151·^2.94 for fish between 16 and 45.9 cm total length, and = 0.0023·^3.44 for fish between 46 and 120 cm total length. Male Nile perch first matured between 50 and 55 cm total length when they were probably 2 years old; female Nile perch first matured between 80 and 85 cm total length when they were probably 4 years old. Small males were common, large males were rare, with the reverse holding for females. Sex change, from male to female, is a possible explanation for this size dimorphism.     

Studies on the biosynthesis of tropane alkaloids in Datura stramonium L. transformed root cultures

The relative contributions made by the l-arginine/agmatine/N-carbamoylputrescine/putrescine and the l-ornithine/putrescine pathways to hyoscyamine formation have been investigated in a transformed root culture of Datura stramonium. The activity of either arginine decarboxylase (EC 4.1.1.19) or ornithine decarboxylase (EC 4.1.1.17) was suppressed in vivo by using the specific irreversible inhibitors of these activities, dl--difluoromethylarginine or dl--difluoromethylornithine, respectively. It was found that suppression of arginine decarboxylase resulted in a severe decrease in free and conjugated putrescine and in the putrescine-derived intermediates of hyoscyamine biosynthesis. In contrast, the suppression of ornithine decarboxylase activity stimulated an elevation of arginine decarboxylase and minimal loss of metabolites from the amine and alkaloid pools. The stimulation of arginine decarboxylase was not, however, sufficient to maintain the same potential rate of putrescine biosynthesis as in control tissue. It is concluded that (i) in Datura the two routes by which putrescine may be formed do not act in isolation from one another, (ii) arginine decarboxylase is the more important activity for hyoscyamine formation, and (iii) the formation of polyamines is favoured over the biosynthesis of tropane alkaloids. An interaction between putrescine metabolism and other amines is also indicated from a stimulation of tyramine accumulation seen at high levels of dl--difluoromethylornithine.Abbreviations ADC arginine decarboxylase - DFMA dl--dif-luoromethylarginine - DFMO dl--difluoromethylornithine - MPO N-methylputrescine oxidase - ODC ornithine decarboxylase - PMT putrescine N-methyltransferase We are indebted to Dr. E.W.H. Bohme of Merrell Dow Research Laboratories (Cincinnati, Ohio, USA) for kind gifts of DFMO and DFMA and to Dr. M.J.C. Rhodes for helpful advice and discussion.     

Alternatively spliced RNAs encode several isoforms of CD46 (MCP), a regulator of complement activation

Five alternative cDNA clones were isolated for CD46, also known as the membrane cofactor protein (MCP) for the factor I-mediated cleavage of the complement convertases. One of these cDNA clones (a) was identical to an earlier MCP clone. The other four CD46 clones 3ontained the four NH₂-terminanl short consensus repeat (SCR) units of MCP, but differed at the region encoding the carboxyl-terminal of the protein which includes an extracellular segment rich in Ser, Thr, and Pro residues, a hydrophobic membrane-spanning domain, and a 33 amino acid cytoplasmic tail. The different CD46 cDNAs have variously: (b) inserted a 93 base pair (bp) exon resulting in a new cytoplasmic tail of 26 amino acids; (c) deleted a 42 bp exon from the extracellular Ser/Thr rich region; (d) used a cryptic splice acceptor sequence to delete 37 bp from an exon encoding transmembrane sequence; or (e) failed to splice the intron after the four SCR units. These were shown by northern blot and polymerase chain reaction to arise by alternative splicing of CD46 RNA. Forms (a), (b), and (c) of CD46 RNA are common in placental RNA, but (d) was rare, and (e) was incompletely processed and therefore aberrant. The polymerase chain reaction (PCR) was used to map the sites of the intron/exon junctions and demonstrate further possible splice variants of CD46. The alternative RNAs for CD46 may correlate to the different isoforms of CD46 found in different tissues, tumors, and in serum.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession number M58050. Address correspondence and offprint requests to: D. F. J. Purcell.     

Cellular distribution ofl-glutamate decarboxylase (GAD) andγ-aminobutyric acidA (GABAA) receptor mRNAs in the retina

1. Gamma-aminobutryic acid (GABA), a major inhibitory transmitter of the vertebrate retina, is synthesized from glutamate by L-glutamate decarboxylase (GAD) and mediates neuronal inhibition at GABAA receptors. GAD consists of two distinct molecular forms, GAD65 and GAD67, which have similar distribution patterns in the nervous system (Feldblum et al., 1990; Erlander and Tobin, 1991). GABAA receptors are composed of several distinct polypeptide subunits, of which the GABAA alpha 1 variant has a particularly extensive and widespread distribution in the nervous system. The aim of this study was to determine the cellular localization patterns of GAD and GABAA alpha 1 receptor mRNAs to define GABA- and GABAA receptor-synthesizing neurons in the rat retina. 2. GAD and GABAA alpha 1 mRNAs were localized in retinal neurons by in situ hybridization histochemistry with 35S-labeled antisense RNA probes complementary to GAD67 and GABAA alpha 1 mRNAs. 3. The majority of neurons expressing GAD67 mRNA is located in the proximal inner nuclear layer (INL) and ganglion cell layer (GCL). Occasional GAD67 mRNA-containing neurons are present in the inner plexiform layer. Labeled neurons are not found in the distal INL or in the outer nuclear layer (ONL). 4. GABAA alpha 1 mRNA is expressed by neurons distributed to all regions of the INL. Some discretely labeled cells are present in the GCL. Labeled cells are not observed in the ONL. 5. The distribution of GAD67 mRNA demonstrates that numerous amacrine cells (conventional, interstitial, and displaced) and perhaps interplexiform cells synthesize GABA. These cells are likely to employ GABA as a neurotransmitter. 6. The distribution of GABAA alpha 1 mRNA indicates that bipolar, amacrine, and perhaps ganglion cells express GABAA receptors having an alpha 1 polypeptide subunit, suggesting that GABA acts directly upon these cells.