Mass spectrometry analysis of endogenous phosphoinositide and IP₃ production in murine platelets. (A) Endogenous levels of PtdInsP(PIP) in platelets lacking a single isoform of PITPα, PITPβ, or both. The total PIP response include PtdIns(3)P, PtdIns(4)P, and PtdIns(5)P in all fractions, including C38:4, C38:3, and C36:2. (B) PtdInsP2 production in platelets with the deletion of either a single isoform or both PITPs. The total PtdInsP2 response include PtdIns(3,4)P2, PtdIns(3,5)P2, and PtdIns(4,5)P2 in all the fractions. The assay was repeated 3 times for each group. The data represent endogenous phosphoinositide levels in 5 × 10⁶ cells and normalized by adding known amounts of phosphoinositide as an internal control (mean ± s.d.). IP₃ production was impaired in thrombin-stimulated (1 U/mL for 1 minute) PITPβ-null (C, red trace) platelets and PITPα/β-null (D, teal trace) platelets compared with wild-type littermate controls.