Neuropeptide Y (NPY) and peptide YY (PYY) effects in the epididymis of the guinea-pig: evidence of a pre-junctional PYY-selective receptor

Abstract

1. The effects of peptide YY (PYY), neuropeptide Y (NPY) and structurally related peptides upon field stimulation-induced and phenylephrine-mediated contractile responses in the cauda epididymis of the guinea-pig were investigated. 2. Preparations of cauda epididymis responded to field stimulation with contractions which were completely attenuated by both the neurotoxin, tetrodotoxin (500 nM), and also by the alpha-adrenoceptor antagonist, phentolamine (3 microM). PYY (n=7) and the truncated peptide analogue PYY(3-36) (n=5) inhibited field stimulation-induced contractions (pIC50+s.e.mean: 8.9+/-0.2 and 9.4+/-0.2, respectively). Pancreatic polypeptide (PP, up to 1 microM, n=6), NPY (up to 100 nM, n=6) and the NPY analogues [Leu31,Pro34]NPY (n=6) and NPY(13-36) (both up to 1 microM, n=5) had no significant effect. 3. The NPY Y1 receptor antagonist BIBP3226 ((R)-N2-(diphenylacetyl)-N[(4-hydroxyphenyl)-methyl]-argininami de) at 750 nM (n=6) and 7.5 microM (n=6) did not affect the PYY-mediated inhibition of field stimulation-induced contractions (pIC50 8.9+/-0.3 and 9.0+/-0.3, respectively). In the presence of BIBP3226 (7.5 microM), NPY (n=6) inhibited field stimulation-induced contractions (pIC50 8.0+/-0.2). 4. NPY, PYY and PYY(3-36) inhibited [3H]-noradrenaline release from preparations of epididymis (pIC50 values 7.9+/-0.7, 9.6+/-0.8 and 10.0+/-0.9, respectively, all n=6). The agonists PP and [Leu31,Pro34]PYY (both up to 100 nM) were without significant effect (both n=6). 5. In preparations of cauda epididymis, stimulated with threshold concentrations of the alpha1-adrenoceptor agonist, phenylephrine (1 microM), both NPY (n=6) and PYY (n=7) elicited concentration-dependent increases in contractile force (with pEC50 values of 8.9+/-0.2 and 8.6+/-0.1, respectively). The effects of both NPY (n=6) and PYY (n=6) were antagonized by preincubation with BIBP3226 (75 nM; apparent pK(B)+/-s.e. values 8.3+/-1.0 and 8.2+/-0.6, respectively). The peptide analogues NPY(13-36) (n=5), PYY(3-36) (n=7) and [Leu31,Pro34]NPY (n=5) did not significantly augment responses to threshold concentrations of phenylephrine. 6. These results are consistent with the proposal that distinct NPY receptors mediate the (prejunctional) inhibition of field stimulation-induced contractions and the (postjunctional) potentiation of responses to phenylephrine in the cauda epididymis of the guinea-pig. The rank order of agonist potency (NPY > or = PYY >> NPY(13-36), [Leu31,Pro34]NPY and PYY(3-36) and the high potency of BIBP3226 indicate that the postjunctional receptor may be Y1-like. The rank orders of agonist potency in inhibiting field stimulation-induced contractile responses and [3H]-noradrenaline release (PYY(3-36) > or = PYY > NPY >> PP, NPY(13-36), [Leu31,Pro34]NPY and PYY(3-36) > or = PYY > NPY >> PP, [Leu31,Pro34]PYY, respectively) are consistent with the action of these peptides at a PYY-preferring receptor subtype, which may be distinct from the presently characterized NPY receptor subtypes.