Ins(1,4,5)P3-induced Ca2+ release from platelet membrane vesicles was blocked by apamin, a selective inhibitor of low-conductance Ca(2+)-activated K+ channels, and by tetrapentylammonium ion, and was weakly inhibited by tetraethylammonium ion. Other K(+)-channel blockers, i.e. charybdotoxin, 4-aminopyridine and glybenclamide were ineffective. A monoclonal antibody (mAb 213-21) obtained by immunizing mice with the InsP3-sensitive membrane fraction from platelets also blocked Ca2+ release by InsP3 from membrane vesicles obtained from platelets, cerebellum, aortic smooth muscle, HEL cells and sea-urchin eggs. ATP-dependent Ca2+ uptake and binding of [3H]InsP3 to platelet membranes was unaffected by either K(+)-channel blockers or mAb 213-21. Blockade of Ca2+ release by apamin, tetrapentylammonium and mAb 213-21 was not affected by the Na+/H+ carrier monensin or the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), but could be completely reversed by the K+/H+ ionophore nigericin and partially reversed by the K+ carrier valinomycin. The antibody-binding protein (ABP) solubilized from platelets, cerebellum, and smooth muscle chromatographed identically on gel filtration, anion-exchange and heparin-TSK h.p.l.c. ABP was purified to apparent homogeneity from platelets and aortic smooth muscle as a 63 kDa protein by immunoaffinity chromatography on mAb 213-21-agarose. These results suggest that optimal Ca2+ release by InsP3 from platelet membrane vesicles may require the tandem function of a K+ channel. A counterflow of K+ ions could prevent the build-up of a membrane potential (inside negative) that would tend to oppose Ca2+ release. The 63 kDa protein may function to regulate K+ permeability that is coupled to the Ca2+ efflux via the InsP3 receptor.

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