Synergism between toxin-γ from Brazilian scorpion Tityus serrulatus and veratridine in chromaffin cells

Abstract

Toxin-γ (Tγ) from the Brazilian scorpion Tityus serrulatus venom caused a concentration- and time-dependent increase in the release of norepinephrine and epinephrine from bovine adrenal medullary chromaffin cells. Tγ was ∼200-fold more potent than veratridine judged from EC50 values, although the maximal secretory efficacy of veratridine was 10-fold greater than that of Tγ (1.2 vs. 12 μg/ml of catecholamine release). The combination of both toxins produced a synergistic effect that was particularly drastic at 5 mM extracellular Ca2+concentration ([Ca2+]o), when 30 μM veratridine plus 0.45 μM Tγ were used. Tγ (0.45 μM) doubled the basal uptake of45Ca2+, whereas veratridine (100 μM) tripled it. Again, a drastic synergism in enhancing Ca2+ entry was seen when Tγ and veratridine were combined; this was particularly pronounced at 5 mM [Ca2+]o. Veratridine induced oscillations of cytosolic Ca2+ concentration ([Ca2+]i) in single fura 2-loaded cells without elevation of basal levels. In contrast, Tγ elevated basal [Ca2+]ilevels, causing only small oscillations. When added together, Tγ and veratridine elevated the basal levels of [Ca2+]iwithout causing large oscillations. Tγ shifted the current-voltage ( I-V) curve for Na+ channel current to the left. The combination of Tγ with veratridine increased the shift of the I-V curve to the left, resulting in a greater recruitment of Na+channels at more hyperpolarizing potentials. This led to enhanced and more rapid accumulation of Na+ in the cell, causing cell depolarization, the opening of voltage-dependent Ca2+ channels, and Ca2+ entry and secretion.