ISSN:
1471-4159
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Medicine
Notes:
Abstract: Purified adrenomedullary plasma membranes contain two high-affinity binding sites for l25I-ω-conotoxin, with KD values of 7.4 and 364 pM and Bmax values of 237 and 1,222 fmol/mg of protein, respectively. Dissociation kinetics showed a biphasic component and a high stability of the toxin-receptor complex, with a t1/2 of 81.6 h for the slow dissociation component. Unlabeled ω-conotoxin inhibited the binding of the radioiodinated toxin, adjusting to a two-site model with Ki1 of 6.8 and Ki2 of 653 pM. Specific binding was not affected by Ca2+ channel blockers or activators, cho-linoceptor antagonists, adrenoceptor blockers, Na+ channel activators, dopaminoceptor blockers, or Na+/H+ antiport blockers, but divalent cations (Ca2+, Sr2+, and Ba2+) inhibited the toxin binding in a concentration-dependent manner. The binding of the dihydropyridine [3H]nitrendipine defined a single specific binding site with a KD of 490 pM and a Bmaxof 129 fmol/mg of protein. At 0.25 μM, co-conotoxin was notable to block depolarization-evoked Ca2+ uptake into cultured bovine adrenal chromaffin cells depolarized with 59 mMK+for 30 s, whereas under the same conditions, 1 μM nitrendipine inhibited uptake by ∼60%. When cells were hyper-polarized with 1.2 mM K+ for 5 min and then Ca2+ uptake was subsequently measured during additions of 59 mMK+, ω-conotoxin partially inhibited Ca2+ uptake in a concentration-dependent manner. These results suggest that two different types of Ca2+ channels might be present in chromaffin cells. However, the molecular identity of ω-conotoxin binding sites remains to be determined.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1471-4159.1989.tb07394.x
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