MULTIPLE INTRACELLULAR SIGNAL-TRANSDUCTION PATHWAYS MEDIATING INWARD CURRENT PRODUCED BY THE NEUROPEPTIDE, ACHATIN-I

The effects of intracellular signal transduction system inhibitors on the inward current (I-in) caused by achatin-I (Gly-D-Phe-Ala-Asp), an Achatina endogenous tetrapeptide having a D-phenylalanine residue, app lied locally onto the neurone tested, were examined under voltage clam p using two identifiable Achatina giant neurone types. v-RCDN (ventral -right cerebral distinct neurone) and PON (periodically oscillating ne urone). H-89 mocinnamylamino)-ethyl]-5-isoquinolinesulfonamide) (adeno sine-3',5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase inhibitor) markedly suppressed the achatin-I-induced I-in on PON, wher eas this drug was ineffective on the I-in of v-RCDN. Dose (pressure du ration)-response study of achatin-I on PON in a physiological solution and in the presence of H-89, and Lineweaver-Burk plot of these data. indicated that H-89 inhibited the I-in in a noncompetitive manner. KT5 823 -triazadibenzo[a,g]cycloocta[c,d,e]-trinden-1-one) (guanosine-3',5 '-cyclic monophosphate (cyclic GMP)-dependent protein kinase inhibitor ) suppressed the achatin-I-induced I-in of v-RCDN in mainly noncompeti tive and partly uncompetitive manners, but this drug had no effect on the I-in of PON. W-7 (6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide ) (calmodulin inhibitor) suppressed noncompetitively the I-in of PON, but this drug had no effect on the I-in of v-RCDN. IBMX (3-isobutyl-1- methylxanthine) (cyclic nucleotide phosphodiesterase inhibitor) enhanc ed the achatin-I-induced I-in of v-RCDN, but this drug was ineffective on the I-in of PON. However, IBMX might have effects on the achatin-I receptor sites on v-RCDN. These findings suggest multiple intracellul ar signal transduction pathways mediating the achatin-I-induced I-in: the I-in of PON is via cyclic AMP-dependent and probably Ca2+/calmodul in-dependent protein kinases, and that of v-RCDN via cyclic GMP-depend ent protein kinase. Other signal transduction system inhibitors includ ing calphostin C -tetramethoxy-3,10-dioxo-1-peryleny]-1-methylethyl ca rbonic acid 4-hydroxyphenyl ester) (protein kinase C inhibitor) did no t significantly affect the I-in of both v-RCDN and PON.