Differential antagonism of transmitter release by subtypes of omega-agatoxins

1. The omega-agatoxins from Agelenopsis aperta spider venom are a diverse group of voltage-sensitive calcium channel antagonists. Subtypes of omega-agatoxins are distinguished as type I (omega-Aga-IA, omega-Aga-IB, and omega-Aga-IC), type II (omega-Aga-IIA and omega-Aga-IIB), and type III (omega-Aga-IIIA and omega-Aga-IIIB). All except type III toxins block calcium channels in insect motor nerve terminals and in neuronal cell bodies at nanomolar concentrations. 2. The potency and maximum level of block of the excitatory junctional potential (EJP) by omega-agatoxins are dependent on the extracellular calcium concentration ([Ca]o). Saturating concentrations of type I or II omega-agatoxins block 97-99% of the evoked EJP in low [Ca]o (0.75 mM) saline. A remnant of the EJP (1-3%) that persists after toxin exposure suggests that a small amount of voltage-dependent calcium entry is toxin resistant. When [Ca]o is elevated to 5 mM, this resistant component increases dramatically, revealing differences between type I and type II block. Under these conditions, 60-70% of the EJP is resistant to type I toxins and approximately 20% is resistant to type II toxins. 3. Sequential application of type I and II toxins in high [Ca]o leads to enhanced block of the EJP, suggesting that type I and II omega-agatoxins may block calcium channels by different mechanisms. 4. Type I and type II omega-agatoxins also block calcium channels in the somata of locust dorsal unpaired median (DUM) neurons. In agreement with studies on neuromuscular transmission, block of barium action potentials is incomplete after either type I or type II toxin exposure and combined application of the toxins results in enhanced block. 5. Partial calcium channel antagonism by type I and type II toxins could be explained either by altered kinetics of toxin-modified channels or by selectivity for different subtypes of presynaptic calcium channels.