Characterization of the Hormone-sensitive Phosphatidylinositol Pool in WRK-1 Cells

WRK-1 rat mammary tumor cells respond to vasopressin with an increase in the rate of phosphatidylinositol turnover. Evidence derived from a series of experiments performed under various prelabeling conditions suggests that the hormone-sensitive phosphatidylinositol resides in a distinct pool within the cell, accounting for approximately 17% (8-37%) of the total cellular phosphatidylinositol. The possibility that two distinct cell types might explain this finding is unlikely since neither newly cloned nor thymidine-blocked cells exhibit any alteration in the nature of their response. This hormone-sensitive phosphatidylinositol moiety has the following characteristics. 1) Under equilibrium labeling conditions, it is completely turned over within 5 min of hormone addition. 2) It is both synthesized and degraded even in the absence of hormone, although at a much slower rate. 3) Under the conditions employed, there does not appear to be transfer of phosphatidylinositol from the insensitive to the sensitive pool. A model of these events is outlined.