1 In the presence of 10 mM Li+ the histamine-stimulated accumulation of [H-3]-inositol monophosphates ([H-3]-IP1) in HeLa cells prelabelled with [H-3]-inositol increased over 10-20 min to a plateau level, which was normally maintained up to 60 min. Levels of [H-3]-inositol bis- and trisphosphates ([H-3]-IP2 and [H-3]-IP3) initially increased rapidly but declined to near basal levels by 20 min. 2 The same pattern of histamine-induced [H-3]-IP1 accumulation was observed in cells in which [H-3]-inositol was present 30 min before and during the incubation with histamine. Concentration-response curves for histamine measured in the presence of 10 mM Li+ were closely similar in cells prelabelled for 24 h with [H-3]-inositol and in cells exposed to [H-3])-inositol for only 30 min before addition of histamine, without removing the [H-3]-inositol. The EC50 for histamine was 1.6 +/- 0.2-mu-M. 3 [H-3]-IP1 accumulation induced by a sub-maximal concentration of histamine, 1-mu-M, also reached a plateau, but at a lower level than with 1 mM histamine. 4 Addition of 10 mM NaF at the plateau phase of [H-3]-IP1 accumulation induced by 1 mM histamine resulted in a further increase in the level of [H-3]-IP1. The level of [H-3]-IP1 in the presence of histamine + NaF was 1.4 +/- 0.2 fold of that of the sum of the responses to histamine and NaF acting alone (basal levels subtracted). 5 Addition of 1-mu-M mepyramine at the plateau phase of [H-3]-IP1 accumulation induced by 1 mM histamine in the presence of 10 mM Li+ resulted in a decline in the level of [H-3]-IP1, implying that the metabolism of [H-3]-IP1 is not completely blocked by 10 mM Li+ in HeLa cells. 6 Omission of the 15 min preincubation period with 10 mM Li+ before stimulation with 100-mu-M histamine for 15 min resulted in an approximate halving of the level of [H-3]-IP1, without any significant change in basal accumulation. Periods of preincubation with 10 mM Li+ longer than 15 min did not produce any further increase in the level of [H-3]-IP1 induced by histamine. 7 Basal and histamine-induced levels of [H-3]-IP1 increased as the concentration of Li+ was increased from 0 to 60 mM, but the effect on the histamine-induced response was greater than on the basal level. Increasing the concentration of Li+ from 0 to 60 mM had only a small and mostly statistically insignificant effects on the levels of [H-3]-IP2 and [H-3]-IP3. The EC50 for histamine-induced [H-3]-IP1 accumulation in the presence of 30 mM Li+ was 2.4 +/- 0.4-mu-M. 8 The results indicate that IP1 metabolism in HeLa cells is much less sensitive to Li+ than in mammalian brain. The plateau phase of histamine-induced [H-3]-IP1 accumulation in the presence of 10 mM Li+ thus represents a steady-state level. On a simple model the plateau level will be proportional to the rate of [H-3]-IP1 formation at that time. The lower the concentration of Li+ present, the better the approximation will be. The information obtained is thus not the same as when [H-3]-IP1 metabolism is completely blocked, in which case [H-3]-IP1 accumulated can be used to calculate a mean rate of formation over the whole of the incubation period.
