α-Latrotoxin of black widow spider venom was found to bind with high affinity (KA = 1.8·109M-1) to specific sites present in discrete number (∼6300/cell, ∼12/aμn2) at the surface membrane of PC12 cells. This binding correlated with (and therefore, probably caused) the secretory response produced by the toxin. Binding was enhanced (∼ 2-fold) in the presence of mM concentrations of various divalent cations (Ca2+, Mn2+ and Co2+) while Ba2+ and Sr2+ had a smaller effect and Mg2+ was inactive. Hypertonicity, concanavalin A and trypsin pretreatment of the cells blocked the binding interaction. The α-latrotoxin-induced stimulation of 3H-dopamine release was massive and occurred very rapidly when cells were exposed to the toxin in a Ca2+-containing Krebs-Ringer medium, whereas it occurred at a much slower rate in a Ca2+-free, Mg2+-containing Ringer. Introduction of Ca2+ into the latter medium resulted in a shift of the release rate from slow to fast. In contrast, in divalent cation-free medium the response was abolished. The toxin-induced secretory response was unaffected by Na2+ and Ca2+ channel blockers (tetrodotoxin and D600) as well as by calmodulin inhibitors (calmidazolium and trifluoperazine). The effects of Ca2+ and Mg2+ were found to be concentration-dependent, with half maximal responses occurring at approximately 0.3 and l.5mM for the two divalent cations, respectively. Other divalent cations could substitute for Ca2+ and Mg2+, the relative efficacy being Sr2+ > Ca2+ ≥> Ba2+ > Mn2+ > Mg2+ > Co2+. Moreover, the response occurring at suboptimal concentration of Ca2+ (0.4 mM) was potentiated by the concomitant addition of either Mg2+, Mn2+ or Co2+. The effect(s) of divalent cations in supporting the α-latrotoxin-induced release response seem(s) to occur primarily at step(s) beyond toxin binding because 1. (a) the stimulatory effects of the various cations on release were not matched by parallel effects on binding 2. (b) Ca2+ maintained its ability to stimulate fast release even when toxin binding had occurred in a Ca2+-free medium. Delays in the release responses were observed when cells were exposed to αLTx in Na2+-free, glucosamine or methylamine-based media, or depolarized with high K2+ (in the presence of D600) before toxin treatment. Moreover, in these two conditions the ability of Mg2+ to support the αLTx response was considerably decreased. Taken together with previous data of the literature these results appear consistent with the hypothesis that αLTx acts by activating a 'non-conventional' cation channel of low ionic specificity in the plasma membrane. This would result in a stimulated influx of various divalent cations, not only Ca2+ but also Mg2+ and the others investigated, which would then be able to stimulate the release response, although with different efficacy.
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