Characterization of [3H]brevetoxin binding to voltage-dependent sodium channels in adrenal medullary cells.
We have previously reported that in bovine adrenal chromaffin cells Ptychodiscus brevis toxin-3 (PbTx-3) does not alter the veratridine-induced 22Na influx when given alone, but increases the influx of 22Na when co-applied with either alpha- or beta-scorpion venom (Wada et al. 1992). In the present study, we characterized [3H]PbTx-3 binding in bovine adrenal chromaffin cells. [3H]PbTx-3 binding was saturable, reversible and of high-affinity with an equilibrium dissociation constant (Kd) of 32.0 +/- 4.9 nmol/l and a maximum binding capacity (Bmax) of 6.2 +/- 1.2 pmol/4 x 10(6) cells (4.5 +/- 0.9 pmol/mg cell protein). A Hill plot revealed the lack of cooperative interaction among the binding sites. Unlabelled PbTx-3 inhibited [3H]PbTx-3 binding with an IC50 of 31 nmol/l. However, tetrodotoxin, veratridine, alpha- and beta-scorpion venom, or veratridine in combination with either alpha- or beta-scorpion venom did not alter [3H]PbTx-3 binding. All these results suggest that PbTx-3 binds to a site (site 5) distinct from the previously known four toxin binding sites, which does not gate voltage-dependent Na channels by itself, but is specifically involved in the allosteric modulation of Na channels in adrenal medullary cells.
Yuhi
T
T
Department of Pharmacology, Miyazaki Medical College, Japan.
Wada
A
A
Yamamoto
R
R
Urabe
M
M
Niina
H
H
Izumi
F
F
Yanagita
T
T
D000313:Adrenal Medulla / Q000378:metabolism*; D000818:Animals; D001667:Binding, Competitive; D002417:Cattle; D002478:Cells, Cultured; D002837:Chromaffin Granules / Q000378:metabolism; D008387:Marine Toxins / Q000378:metabolism*; D009498:Neurotoxins / Q000378:metabolism*; D035964:Oxocins; D012604:Scorpion Venoms / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism*; D013779:Tetrodotoxin / Q000378:metabolism; D014701:Veratridine / Q000378:metabolism; D000095702:Polyether Toxins
D008387:Marine Toxins; D009498:Neurotoxins; D035964:Oxocins; D012604:Scorpion Venoms; D015222:Sodium Channels; D013779:Tetrodotoxin; D014701:Veratridine; C053342:brevetoxin; D000095702:Polyether Toxins