EFFECTS OF ACIDIC PH ON THE FORMATION OF VINBLASTINE-INDUCED PARACRYSTALS IN CHINESE-HAMSTER OVARY CELLS

The pH-related change in morphology of vinblastine (VLB)-induced parac rystals formed in Chinese hamster ovary (CHO) cells was examined immun ohistochemically in order to determine both the mechanism of tubulin c rystallization and the influence of acidic pHs on cytoskeletal microtu bules. Lowering the extracellular pH (pHe) rapidly reduced the intrace llular pH (pHi) in CHO cells. Lowering the pHi to near the neutral ran ge significantly accelerated the growth of VLB-induced paracrystals, c ompared to that of paracrystals formed at a physiological pHe. However , further cytoplasmic acidification caused by the addition of sodium a zide into the culture medium induced the disappearance of typical para crystals and the appearance of a highly organized meshwork of tubulin appearing as short, thick filaments at the light microscopic level. Tr eatments using different concentrations of VLB at different pHe's show ed that low pHi's (6.7 and 6.3) suppressed paracrystal-formation at lo wer concentrations of VLB (5x10(6) M and 10(-5) M). At higher concentr ations of VLB (5x10(-5) M and 10(-4) M), only short filaments were for med at pHi 6.3. Electron microscopy revealed that the filaments had a ladder-like structure probably consisting of a stacked series of fused rings. This indicates that paracrystals may be modified by extremely low pH. These results show that paracrystals are unstable in living ce lls and that their formation is regulated by environmental pH.