THE ROLE OF CAMP IN THE ACTIONS OF THE PEPTIDE AF3 IN THE PARASITIC NEMATODES ASCARIS-SUUM AND ASCARIDIA-GALLI

AF3 (AVPGVLRFamide) is an endogenous RFamide-like peptide isolated fro m the parasitic nematode Ascaris suum. It has a potent and long lastin g excitatory effect in A. suum and Ascaridia galli. This is mediated b y a mechanism independent of the nicotinic-like acetylcholine (ACh) re ceptor, which mediates excitatory transmission at the neuromuscular ju nction of both nematodes. In addition, AF3 has been found to sensitise A. suum muscle to the contractile effect of ACh. In this study, the i nvolvement of the second messenger cAMP in mediating the action of AF3 on the somatic musculature of A. suum and A. galli has been investiga ted. Two approaches have been used the effects of drugs which raise in tracellular cAMP levels on the contractile responses to AF3 have been examined and biochemical assays have been used to measure the effects of AF3 on cAMP levels. AF3 contractions were inhibited in A. suum by 1 0 mu M forskolin (by 22% of control; P < 0.05; n = 9) and by 500 mu M isobutylmethylxanthine (IBMX, by 27% of control; P < 0.001; n = 6). AF 3 decreased cAMP concentrations in A. suum somatic muscle (basal, 1721 +/- 134 pmol mg(-1) protein; with 1 mu M AF3, 1148 +/- 133 pmol mg(-1 ) protein; P < 0.05, n = 5). AF3 (1 mu M) also reduced the 10 mu M for skolin induced potentiation of cAMP concentrations in A. suum (forskol in 3242 +/- 471 pmol mg(-1) protein; forskolin and AF3, 1524 +/- 143 p mol mg(-1) protein; P < 0.001, n = 6) and A. galli (forskolin 291 +/- 32 pmol mg(-1) protein, forskolin + AF3, 185 +/- 12 pmol mg(-1) protei n; P < 0.05, n = 5). These data suggest that in both nematodes the con tractile effect of AF3 is, at least in part, regulated by cAMP. (C) 19 98 Elsevier Science B.V. All rights reserved.