A monoclonal antibody (PL/IM 430), previously found to inhibit the uptake of Ca2+ into highly purified platelet intracellular membrane vesicles (Hack, N., Wilkinson, J. M. and Crawford, N. 1988, Biochem. J.250, 355–361) has been introduced into saponin-permeabilised platelets. At a saponin concentration (20–25 μg/ml) commensurate with total LDH release, sequestration of Ca2+ into intracellular non-mitochondrial stores is inhibited by the antibody (~50% inhibition at 20 μg/ml IgG). At higher saponin concentrations when intracellular binding of125I-labelled mAb is maximum, inhibition of Ca2+ sequestration approaches 70%. The inhibition is specific, control studies with non-platelet directed mouse IgG and mAbs which immunoblot platelet antigens other than the 100 kDa protein did not affect the Ca2+ sequestration.

No effect of the antibody were observed against IP3-induced release of prestored Ca2+, either in permeabilised platelets or with isolated intracellular membrane vesicles. The mAb PL/IM 430 appears to bind only to the Ca2+ translocating channel protein associated with the intracellular membrane (Ca2++Mg2+) ATPase and not to Ca2+ channels responsive to IP3.

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