Synergism between toxin-gamma from Brazilian scorpion Tityus serrulatus and veratridine in chromaffin cells.
Toxin-gamma (Tgamma) 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. Tgamma was approximately 200-fold more potent than veratridine judged from EC50 values, although the maximal secretory efficacy of veratridine was 10-fold greater than that of Tgamma (1.2 vs. 12 microg/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 microM veratridine plus 0.45 microM Tgamma were used. Tgamma (0.45 microM) doubled the basal uptake of 45Ca2+, whereas veratridine (100 microM) tripled it. Again, a drastic synergism in enhancing Ca2+ entry was seen when Tgamma 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, Tgamma elevated basal [Ca2+]i levels, causing only small oscillations. When added together, Tgamma and veratridine elevated the basal levels of [Ca2+]i without causing large oscillations. Tgamma shifted the current-voltage (I-V) curve for Na+ channel current to the left. The combination of Tgamma 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.
Conceicao
I M
IM
Laboratorio de Farmacologia, Instituto Butantan, São Paulo, Brazil.
Lebrun
I
I
Cano-Abad
M
M
Gandia
L
L
Hernandez-Guijo
J M
JM
Lopez
M G
MG
Villarroya
M
M
Jurkiewicz
A
A
Garcia
A G
AG
D000313:Adrenal Medulla / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D002132:Calcium Radioisotopes; D002417:Cattle; D002838:Chromaffin System / Q000187:drug effects / Q000502:physiology*; D004357:Drug Synergism; D004553:Electric Conductivity; D007700:Kinetics; D009498:Neurotoxins / Q000494:pharmacology*; D009638:Norepinephrine / Q000378:metabolism; D012604:Scorpion Venoms / Q000494:pharmacology*; D015222:Sodium Channels / Q000502:physiology; D014701:Veratridine / Q000494:pharmacology*
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002132:Calcium Radioisotopes; D009498:Neurotoxins; D012604:Scorpion Venoms; D015222:Sodium Channels; C003609:tityustoxin; D014701:Veratridine; D002118:Calcium; D009638:Norepinephrine