Venom antimicrobial peptide-6

We investigated, in murine colon circular muscle, the role of adenosine 5'-triphosphate (ATP) and pituitary adenylate cyclase activating peptide (PACAP) as inhibitory neurotransmitters of the fast component of nerve-evoked inhibitory junction potential (fast IJP). Fast IJP was antagonised by apamin or suramin, abolished by desensitisation with the P2Y receptor agonist, adenosine 5'-O-2-thiodiphosphate (ADPbetaS), unaffected by desensitisation with P2X receptor agonist, alpha,beta-methylene ATP (alpha,beta-meATP), and reduced by PACAP-(6-38), a PACAP receptor antagonist. ATP induced membrane hyperpolarization resistant to tetrodotoxin, N(omega)-nitro-L-arginine methyl ester (L-NAME) or PACAP-(6-38), but antagonised by apamin, suramin, P2X and P2Y receptor desensitisation. PACAP-(1-27) caused membrane hyperpolarization antagonised by PACAP-(6-38), apamin and P2Y receptor desensitisation, reduced by tetrodotoxin, but not affected by L-NAME and by P2X receptor desensitisation. Therefore, in murine colon circular muscle, an ATP-like endogenous P2Y purinoceptor ligand is the final nonadrenergic, noncholinergic (NANC) inhibitory mediator involved in the generation of fast IJP. A neuromodulator role of PACAP in the inhibitory pathway is supposed.

1. This study examined the activity and mechanisms of action of urodilatin in bovine bronchi. For comparison, the ability of urodilatin to evoke bronchodilatation or protect against subsequent challenge was compared to that of the closely related peptide alpha-human atrial natriuretic peptide (ANP). 2. Urodilatin reversed methacholine-evoked contraction in a concentration-dependent manner in bovine bronchi. In the absence of any attempt to prevent degradation by neutral endopeptidases, urodilatin was more potent than ANP in this tissue. 3. The bronchodilator properties of urodilatin were significantly augmented by the neutral endopeptidase inhibitor, phosphoramidon (3.68 x 10(-5) M). This provides evidence for at least partial degradation of urodilatin by neutral endopeptidases. With phosphoramidon present, urodilatin and ANP were equipotent. 4. In the presence of phosphoramidon (3.68 x 10(-5) M), pre-incubation with urodilatin (10(-6) M) had a protective effect against subsequent methacholine-induced contraction. This action of urodilatin was quantitatively similar to that of ANP in the presence of this endopeptidase inhibitor. 5. The actions of urodilatin appear to involve ATP-sensitive K+ channels since tolbutamide (10(-6) - 10(-5) M) significantly attenuated the relaxations induced by this peptide. 6. Small conductance Ca(2+)-activated K+ channels seem likewise to be implicated in the actions of urodilatin since blockade of these channels with apamin (10(-7) - 10(-6) M) resulted in a marked attenuation of urodilatin-evoked responses. 7. The presence of charybdotoxin (10-9 M-10-M) had no significant effect on subsequent responses tourodilatin suggesting that large conductance Ca2+-activated K+ channels are not involved in the relaxations evoked by this peptide.8. In the presence of phosphoramidon (3.68 x 10-5 M), urodilatin (10-6 M) evoked elevation of cyclic GMP levels within bovine bronchial tissue. Levels of cyclic GMP increased significantly within 5-10 s in response to this peptide and preceded the initiation of relaxant responses. Maximum increases in cyclic GMP levels were reached within 5 min; the time required for maximal relaxation evoked by this peptide.9. In conclusion, urodilatin, like ANP reversed and protected against, subsequent methacholine-induced bronchoconstriction; an action enhanced by the presence of phosphoramidon (3.68 x 1O-5 M).Associated with these actions of urodilatin was a rise in cyclic GMP levels as well as the opening of ATP-sensitive K+ and small conductance Ca2+-activated K+ channels.