VPAC1 inhibition during myelosuppressive therapy. (A) Platelet count in mice (n = 5 in each group) following administration of PP1A4 (■) or PBS (●) on days 0, 3, and 7 and busulfan treatment on days 7 and 10 (left panel). The plasma TPO levels from the same mice treated with PP1A4 (■) or PBS (●) at the indicated days (right panel). Each point represents the mean TPO value plus or minus SD. (B) Platelet count in mice (n = 5 in each group) following administration of PP1A4 (■) or PBS (●) on days 0, 3, and 7 and sublethal whole-body irradiation (8 Gy) on day 6 (left panel). The plasma TPO levels from the same mice treated with PP1A4 (■) or PBS (●) at the indicated days (right panel). Each point represents the mean TPO value plus or minus SD. (C) Mortality of mice (n = 5 in each group) injected with PP1A4 (■) or PBS (●) on days 0, 3, and 7 and lethal whole-body irradiation (9 Gy) on day 6. (D) Platelet count in rabbits (n = 3 in each group) following administration of 23A11 (■) or PBS (●) on days 0, 3, 7, and 10 and busulfan treatment on days 7 and 10. Each point represents the mean platelet count plus or minus SD. *P < .05; **P < .01; #P < .001 by ANOVA.