EFFECT OF CYTOPLASMIC CA2-ACTIVATED HEPATOCYTES( ON (1,4,5)IP3 FORMATION IN VASOPRESSIN)

The role of cytoplasmic calcium as a regulator of phospholipase C in v asopressin-activated hepatocytes was examined. According to models in which calcium spiking arises because of a positive feedback by calcium on phospholipase C, Ca2+ is seen as a positive modulator of phospholi pase C under conditions of submaximal receptor activation. However, in hepatocytes whose precursor lipids had been labeled by incubation in [H-3]-inositol, no increase in [H-3]-(1,4,5)IP3 was detected in respon se to thapsigargin, in either unstimulated cells, or in cells stimulat ed with 1 nM vasopressin. Addition of a maximal concentration of vasop ressin (1 mu M) caused a rapid and substantial increase in [H-3]-(1,4, 5)IP3. These results indicate that changes in cytoplasmic calcium do n ot influence phospholipase C activity in hepatocytes, even under condi tions of submaximal agonist activation. These findings also support mo dels that provide for calcium spiking at constant levels of (1,4,5)IP3 at least in the case of the rat hepatocyte.