Terbium binding to axonal membrane vesicles from lobster (Homarus americanus) peripheral nerve. A probe of calcium binding sites.

Tb3+, a fluorescent trivalent cation with physicochemical properties similar to Ca2+, binds to peripheral nerve membrane vesicles prepared from the walking leg nerve bundle of the lobster (Homarus americanus). Saturable binding is measured for at least two classes of binding site. Bound Tb3+ can be displaced by other cations in the order: Ca2+ greater than Mg2+ = Zn2+ greater than NH4+. The binding of Tb3+ to the lower affinity site (KD(app) = 6.0 microM) is inhibitable by Na+, Mg2+ and Ca2+, whereas the higher affinity site (KD(app) = 2.2 microM) is only sensitive to Ca2+. Using this spectral probe the role of Ca2+ in peripheral nerve membrane function can be investigated.