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  • Title: Hydro-osmotic activity of "carba' analogues of oxytocin and [8-arginine] vasopressin on frog (Rana esculenta) bladder.
    Author: Barth T, Le Bars NC, Roy C, Jard S.
    Journal: Eur J Pharmacol; 1975; 32(02):214-20. PubMed ID: 1080112.
    Abstract:
    The biological potency of neurohypophysial hormone analogues in stimulating water transport across the frog bladder (Rana esculenta) was estimated from the pD2 (negative logarithm of the peptide concentration eliciting 50% of its maximal hydroosmotic effect) and from the maximal response. Most analogues elicited maximal responses similar to those of the natural hormones. Both replacement of sulfur by a methylene group in the disulfide bridge and omission of the terminal amino group in oxytocin reduced hydroosmotic activity; carba substitution in oxytocin led to a greater drop in pD2 than the same substitution in deamino-oxytocin. On the contrary, the effects of substituting phenylalanine for tyrosine and of omitting the amino group were almost additive. Elimination of the carboxyl-terminal tripeptide sequence considerably reduced hydroosmotic potency; pressinamide was inactive up to 3 mug/ml. The highly potent antidiuretic peptides, deamino-[D-Arg8]vasopressin and deamino-6-carba-]D-Arg8]vasopressin, were weakly active in the hydroosmotic assay.
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