Specificity of antibodies and presence of^1,4,5IP₃R in platelets.

(A) Approximately 18 ng immunopurified type I, mainly type II, and mainly type III ^1,4,5IP₃R (see “Results” and ref. 24) were run in the lanes indicated and probed with antibodies R26, R45, and CT3 using Western blotting. Arrow indicates position of IP₃R. R45 and CT3 are highly specific for the type III receptor and R26 for the type I receptor. Cross- reactivity by R26 in lanes for the types II and III receptors reflects co-immunopurification of the type I receptor from AR4-2J and RINm5F cells. (B) Western blot analysis of platelet-mixed membranes with the antibodies CT1 (to type I), CT2 (to type II), and CT3 (to type III receptor). Arrows reflect position of IP₃ receptors. (C) Equal amounts of platelet lysates were immunoprecipitated with equal amounts of either CT1, CT2, or R45 antibodies. Immunoprecipitates were reprobed with the respective antibodies on Western blots. Each blot is typical of 3 separate experiments.