Time course of ceramide production and acidic pH, Mg²⁺-independent and neutral pH, Mg²⁺-dependent sphingomyelinase (ASMase and NSMase, respectively) activation in total populations of Molt-4 cells treated with 8 nmol/L GW1843. Log-phase Molt-4 cells were treated with 8 nmol/L GW1843, aliquots taken at the indicated times, and assayed for ASMase activity (•), NSMase activity (▪), and ceramide levels (▴) as described in Materials and Methods. The graph represents the average of four separate experiments (mean ± SEM).