Phosphatase inhibitors and premature chromosome condensation in human peripheral lymphocytes at different cell-cycle phases

The cytogenetical reaction of human peripheral lymphocytes to okadaic acid and calyculin A was examined. Calyculin A could induce PCC about 20 times m ore effectively than okadaic acid. Their mechanisms of PCC induction were j udged similar by their dose-dependent manner and chromosome morphology. Con trary to earlier studies suggesting that chemicals could not induce PCC in G(1) cells where little cyclin B is present, the present study showed that calyculin A could induce PCC in lymphocytes not only at S and G(2)/M but al so at the second G(1) phase after PHA stimulation in vitro. PCC was induced slightly in lymphocytes both at G(0) and the first in vitro G(1) phase eve n when the calyculin A concentration increased one hundredfold. It was foun d that calcium ionophore A23187 increased frequencies of G(0)-PCC induced b y calyculin A, although a further refinement is necessary to obtain a suita ble morphology of G(0)-PCC for cytogenetic studies.