SYNERGISM BETWEEN TOXIN-GAMMA FROM BRAZILIAN SCORPION TITYUS-SERRULATUS AND VERATRIDINE IN CHROMAFFIN CELLS

Toxin-gamma (T gamma) from the Brazilian scorpion Tityus serrulatus ve nom caused a concentration-and time-dependent increase in the release of norepinephrine and epinephrine from bovine adrenal medullary chroma ffin cells. T gamma was similar to 200-fold more potent than veratridi ne judged from EC50 values, although the maximal secretory efficacy of veratridine was 10-fold greater than that of T gamma (1.2 vs. 12 mu g /ml of catecholamine release). The combination of both toxins produced a synergistic effect that was particularly drastic at 5 mM extracellu lar Ca2+ concentration ([Ca2+](o)), when 30 mu M veratridine plus 0.45 mu M T gamma were used. T gamma (0.45 mu M) doubled the basal uptake of Ca-45(2+), whereas veratridine (100 mu M) tripled it. Again, a dras tic synergism in enhancing Ca2+ entry was seen when T gamma and veratr idine 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 lev els. In contrast, T gamma elevated basal [Ca2+](i) levels, causing onl y small oscillations. When added together, T gamma and veratridine ele vated the basal levels of [Ca2+](i) without causing large oscillations . T gamma shifted the current-voltage (I-V) curve for Na+ channel curr ent to the left. The combination of T gamma with veratridine increased the shift of the I-V curve to the left, resulting in a greater recrui tment 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 Ca 2+ entry and secretion.