Ins(1,3,4,5)P4 is effective in mobilizing Ca2+ in mouse exocrine pancreatic acinar cells if phospholipase A2 is inhibited

In enzymically isolated mouse pancreatic acinar cells, under conditions of whole-cell patch-clamp current recording, the effect of phospholipase C-coupled agonists can be mimicked by internal perfusion of the intracellular second messenger Ins(1,4,5)P₃ (10 µM) or its analogue Ins(2,4,5)P₃ (10 µM). The inositol trisphosphates mimic receptor activation by releasing Ca²⁺ from intracellular stores and by promoting Ca²⁺ influx across the surface membrane. This Ca²⁺-mobilizing role of inositol polyphosphates seems to be confined to the inositol trisphosphates because internal perfusion of Ins(1,3,4,5)P₄ (10 µM) is not associated with any Ca²⁺-dependent current activation. In this study we investigate the effects of 4-bromophenacyl bromide (4BPB), a putative inhibitor of phospholipase A₂ and arachadonic acid production, on inositol polyphosphate-induced Ca²⁺ signalling. At 10 µM, 4BPB has no effect on unstimulated Ca²⁺-dependent membrane currents. However, if 4BPB is applied to cells internally perfused with 10 µM Ins(1,4,5)P₃ or Ins(2,4,5)P₃ then the current responses are rapidly potentiated. In cells internally perfused with 10 µM Ins(1,3,4,5)P₄, which has itself no effect on membrane currents, application of 4BPB resulted in the activation of Ca²⁺-dependent currents, seen either as repetitive spikes of current or as sustained current activations. The application of arachidonic acid blocks the current responses evoked by the inositol trisphosphates and by Ins(1,3,4,5)P₄/4BPB. These results suggest that in enzymically isolated pancreatic acinar cells phospholipase A₂ activity is exerting an inhibitory effect on inositol polyphosphate-mediated Ca²⁺ mobilization. 4BPB removes this inhibition and potentiates the responses to internally perfused inositol trisphosphates and, importantly, makes 10 µM Ins(1,3,4,5)P₄ as effective as 10 µM Ins(1,4,5)P₃ in mobilizing intracellular Ca²⁺ and in promoting Ca²⁺ influx.