Antagonism of synaptosomal calcium channels by subtypes of omega-agatoxins

• Published in: The Journal of biological chemistry 1992-02, Vol.267 (4), p.2610-2615
• Main Authors: Venema, V J, Swiderek, K M, Lee, T D, Hathaway, G M, Adams, M E
• Format: Article
• Language: English
• Subjects: Agatoxins; Amino Acid Sequence; Amino Acids - analysis; Animal poisons toxicology. Antivenoms; Animals; ARANEAE; Biological and medical sciences; brain; Brain - drug effects; Brain - metabolism; CALCIO; CALCIUM; Calcium Channel Blockers - isolation & purification; Calcium Channel Blockers - pharmacology; Calcium Channels - drug effects; CATION; CATIONES; CELLULE; CELULAS; CEREBRO; Chickens; Chromatography, Liquid; Electrophoresis, Polyacrylamide Gel; ENCEPHALE; INHIBICION; INHIBITION; ION; IONES; Mass Spectrometry; Medical sciences; MEMBRANAS CELULARES; MEMBRANE CELLULAIRE; Molecular Sequence Data; NERF; NERVIOS; omega -agatoxin; Peptide Mapping; POLLITO; POUSSIN; PURIFICACION; PURIFICATION; Spider Venoms - genetics; Spider Venoms - isolation & purification; Spider Venoms - pharmacology; synaptosomes; Synaptosomes - drug effects; Synaptosomes - metabolism; Toxicology; TOXINAS; TOXINE; VENENOS; VENIN
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)45925-X

Venom of the funnel web spider Agelenopsis aperta inhibits the binding of 125I-omega-conotoxin GVIA (omega-CgTx) to calcium channels in chick brain synaptosomal membranes. Fractionation of the venom by liquid chromatography shows that this inhibitory activity is associated primarily with a diverse class of peptide toxins called omega-agatoxins (omega-Aga). Using binding inhibition as an assay, we purified and identified the novel, 76-amino acid toxin, omega-Aga-IIIA. Inhibition of 125I-omega-CgTx binding to chick synaptosomal membranes by omega-Aga-IIIA and omega-Aga-IIA is correlated with block of potassium-stimulated 45Ca entry into synaptosomes; omega-Aga-IA neither inhibits 125I-omega-CgTx binding nor 45Ca entry under identical conditions. omega-Aga-IIA and omega-Aga-IIIA are 20-30-fold more potent than omega-CgTx as antagonists of synaptosomal calcium channels. However, whereas omega-CgTx completely blocks 45Ca entry into synaptosomes at saturating concentrations, the omega-agatoxins maximally block only 60-70% of 45Ca entry. Pretreatment of synaptosomes with omega-Aga-IIIA occludes block of 45Ca entry by omega-CgTx. The results indicate that, while the omega-agatoxins bind to the entire population of omega-CgTx-sensitive calcium channels in chick synaptosomal membranes, they exert only a partial block of 45Ca flux. Such block could occur via two distinct mechanisms. Toxin binding may alter the kinetics of a homogeneous population of channels, resulting in lower overall conductance upon depolarization. Alternatively, the omega-agatoxins may bind to two distinct channel subtypes, only one of which is blocked as a result of toxin occupation.