FIGURE 7.

The ΔΔβ-toxins do not compete with brevetoxin on binding to site-5. A, locust neuronal membranes were incubated with 0.1 nm ¹²⁵I-LqhαIT as in Fig. 4. Saturating concentration of PbTx-2 (1 μm; Ref. 19) increased ¹²⁵I-LqhαIT binding by 30 ± 8%. 0.1 μm ΔΔBj-xtrIT increased ¹²⁵I-LqhαIT binding (maximal enhancement, Fig. 4) by 172 ± 11%, and in the presence of a mixture of both (1 μm PbTx-2 + 0.1 μm ΔΔBj-xtrIT), the increase in ¹²⁵I-LqhαIT binding was comparable (168 ± 14%). B, [³H]PbTx-3 binding in the presence of the ΔΔβ-toxins. [³H]PbTx-3 (10 nm) was incubated with rat brain synaptosomes (70 μg) for 3 h at room temperature in the absence (Total binding) or presence of 2 μm PbTx-2 (nonspecific binding) or the indicated concentrations of ΔΔβ-toxins. Identical results were obtained when the ΔΔβ-toxins were preincubated with the membranes for 1 h prior to the addition of [³H]PbTx-3.