TURNOVER OF INOSITOL PENTAKISPHOSPHATES, INOSITOL HEXAKISPHOSPHATE AND DIPHOSPHOINOSITOL POLYPHOSPHATES IN PRIMARY CULTURED-HEPATOCYTES

We have used a non-transformed cell model, the primary cultured hepatocyte, to explore the turnover of inositol hexakisphosphate, multiple isomers of inositol pentakisphosphate and two novel diphosphoinositol polyphosphates. All of these compounds gradually accumulated radioactivity throughout a 70 h period of labelling with [H-3]inositol. However, a rapid metabolic rate was revealed upon inhibition of diphosphoinositol polyphosphate biphosphatase(s) with 1 mM fluoride for 40 min: this treatment elevated levels of [H-3]diphosphoinositol polyphosphates up to 10-fold, indicating that their cellular pools were normally turning over at least 10 times every 40 min. This was accompanied by a turnover of about 10 % of the pool of inositol hexakisphosphate. Control experiments established that 200 nM vasopressin brought about a typical activation of phospholipase C in hepatocytes after 62 h of primary culture. This agonist treatment did not affect steady-state levels of [H-3]inositol pentakisphosphates, [H-3]inositol hexakisphosphate or [H-3]diphosphoinositol polyphosphates. However, prolonged treatment of hepatocytes with 2 muM thapsigargin reduced steady-state levels of [H-3]diphosphoinositol polyphosphates by 50-70 %. This effect of thapsigargin was also observed in the presence of fluoride, indicating that thapsigargin inhibited the rate of synthesis of diphosphoinositol polyphosphates.