Multiple intracellular signal transduction pathways mediating inward current produced by the neuropeptide, achatin-I

Abstract

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, applied locally onto the neurone tested, were examined under voltage clamp using two identifiable Achatina giant neurone types, v-RCDN (ventral-right cerebral distinct neurone) and PON (periodically oscillating neurone). H-89 ( N-[2-( p-bromocinnamylamino)-ethyl]-5-isoquinolinesulfonamide) (adenosine-3′,5′-cyclic monophosphate (cyclic AMP)-dependent protein kinase inhibitor) markedly suppressed the achatin-I-induced I in on PON, whereas this drug was ineffective on the I in of v-RCDN. Dose (pressure duration)-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. KT5823 ( N-methyl-(8 R∗,9 S∗,11 S∗)-(−)-9-methoxy-9-methoxycarbonyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1 H, 8 H, 11 H-2,7b,11a-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 noncompetitive and partly uncompetitive manners, but this drug had no effect on the I in of PON. W-7 ( N-(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) enhanced 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 intracellular signal transduction pathways mediating the achatin-I-induced I in: the I in of PON is via cyclic AMP-dependent and probably Ca 2+ calmodulin-dependent protein kinases, and that of v-RCDN via cyclic GMP-dependent protein kinase. Other signal transduction system inhibitors including calphostin C (2-[12-[2-(benzyloxy)-propyl]-3,10-dihydro-4,9-dihydroxy-2,6,7,11-tetramethoxy-3,10-dioxo-1-peryleny]-1-methylethyl carbonic acid 4-hydroxyphenyl ester) (protein kinase C inhibitor) did not significantly affect the I in of both v-RCDN and PON.