Effect of 23A11 on cAMP generation and in vitro megakaryopoiesis. (A) Measurements of cAMP levels in Caco2 (left panel) and MEG01 (right panel) cells under basal conditions and after stimulation with PACAP, VIP, and both agonists in the absence and presence of PP1A4 or 23A11 after 12 hours (light-gray bars), 1 hour (■), and 10 minutes () incubation time. Bars represent the means plus or minus SD of 3 experiments. *P < .05; **P < .01; #P < .001 by ANOVA. (B) Immunoblot analysis of VPAC1 expression in Caco2 and MEG01 cells after loading equal amounts of total protein (50 μg). (C) Megakaryocyte colonies on day 12 from bone marrow–derived Sca1⁺ cells incubated with a control antibody or 23A11 (left panel). Sca1⁺ murine bone marrow cells incubated with 23A11 resulted in increased numbers of CFU-MKs after 12 days (right panel). Bars represent the means plus or minus SD (P < .01).