The pH dependence of disobutamide-induced clear cytoplasmic vacuoles in cultured cells.

Cellular uptake of disobutamide (D), and clear cytoplasmic vacuoles (CCV) induction by D in cultured rat urinary bladder carcinoma cells were dependent on the culture medium pH. At pH 6.0-6.7, drug uptake was slow and no CCV formed in 24 hr. At pH 7.0-8.0, the rate of D uptake and early appearance of CCV were directly proportional to increased basicity. This was explained by the increasing fraction of un-ionized D molecules at increasing basicity of the culture medium. It is only these electrically neutral D molecules which can penetrate the lipoidal cell membrane to induce formation of CCV. Intracellular presence of D was demonstrated by mass spectrometry methods. The results indicate that D is incorporated intracellularly, that D and not its metabolite(s) is in cells, and suggest that CCV are a result of drug sequesteration.