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.

Amino Acid Sequence↗

PubMedDate not supplied

Hormone-mediated inositol lipid breakdown in hepatocytes and WRK1 cells: relationship to receptor function.

All hormones and neurotransmitters which provoke their intracellular effects by increasing the cytosolic concentration of Ca2+ in their target cells also stimulate the breakdown of inositol phospholipids. Much evidence suggests that this breakdown is intimately involved in the mechanism which couples cell-surface receptor activation to intracellular Ca2+ mobilization. Recent results indicate that the primary, receptor-mediated event in stimulated cells is a phosphodiesteric hydrolysis of phosphatidylinositol 4,5-bisphosphate to yield inositol trisphosphate and diacylglycerol. It is likely that both products of this reaction fulfill 'second messenger' roles within stimulated cells.

Acetylcholine↗

PubMedDate not supplied

Novel approach to the design of synthetic radioiodinated linear V1A receptor antagonists of vasopressin.

We report the solid phase synthesis of six analogs of the potent and selective linear AVP vasopressor (V1a receptor) antagonist: Phaa1-D-Tyr(Et)2-Phe3-Gln4-Asn5-Lys6-Pro7-Arg-NH(8)2(A) (where Phaa = phenylacetyl) in which the Phaa1 residue is replaced by hydroxyphenylacetyl (HO-Phaa), hydroxyphenylpropionyl (HO-Phpa) and phenylpropionyl (Phpa) and the D-Tyr(Et)2 and Lys6 residues by D-Tyr(Me)2 and Arg6 substituents. The phenolic-containing peptides were synthesized to test the feasibility of using this approach for the design of high affinity selective ligands for AVP V1a receptors. The following analogs of A were synthesized: 11 [(HO)Phaa1]; 2. [(HO)Phaa1,D-Tyr(Me)2]; 3. [(HO)Phaa1,D-Tyr(Me)2, Arg6]; 4. [(HO)Phaa1,Arg6]; 5. [Phpa1]; 6. [(HO)Phpa1]. All six peptides were examined for agonistic and antagonistic potencies in vasopressor (V1a-receptor) and antidiuretic (V2-receptor) and in vitro oxytocic assays in rats. The affinities of the phenolic-containing peptides for hepatic V1a and uterine receptors were also determined. The phenolic-containing peptides all exhibit potent V1a antagonism. Their anti-V1a pA2 values range from 8.23 to 8.63 (the anti-V1a pA2 value of A = 8.69). Their inhibition constants (Ki in nM) range 0.4 to 1.0. They are weak antidiuretic agonists with activities ranging from 0.022 U/mg to 0.13 U/mg (A = 0.033 U/mg). They all exhibit OT antagonism in vitro. Their anti-OT pA2 values range from 7.28 to 7.71 (A = 7.62). All five phenolic compounds were iodinated using iodine chloride and tested in the same in vivo and in vitro assay system.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMedDate not supplied

Functional architecture of vasopressin/oxytocin receptors.

Three-dimensional models of G protein-coupled receptors (GPCR) have been defined using most experimental data available and protein modeling techniques. The endogenous ligand binding sites have been qualitatively described and putative receptor activation mechanisms have been proposed. The model has been recently refined to take into account recent crystallographic data. Most experimental results published are in excellent qualitative agreement with the initial model. We have undertaken to study more systematically by site directed mutagenesis the vasopressin/oxytocin receptor binding domain as a prototype of neuropeptide receptors. The experimental results are in very good agreement with the models. The residues responsible for the neuropeptide binding have been identified and confirm the predicted localization of the neuromediator in the transmembrane domain of the receptors. The side chain of the 8th residue of vasopressin interacts with a non-conserved receptor residue located in the first extracellular loop. As predicted from the model, this interaction is completely responsible for the selectivity of the ligand-receptor interaction. Finally, aromatic residues which allow the modulation of the efficacy of agonists have been identified.

Affinity Labels↗