Rapid increases in inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and cytosolic free Ca2+ in agonist-stimulated pancreatic acini of the rat. Effect of carbachol, caerulein and secretin.

We compared the time course of increases in isomers of inositol trisphosphate [Ins(1,4,5)P3] and Ins(1,3,4)P3] and the tetrakisphosphate [Ins(1,3,4,5)P4] with changes in cytosolic free Ca2+ [( Ca2+]i) in dispersed pancreatic acini of the rat. There were rapid (5s) increases in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in response to carbachol, caerulein and secretin, whereas Ins(1,3,4)P3 increased more slowly. All three secretagogues induced increases in [Ca2+]i, which reached a peak at 15-20 s. Our results indicate that the very rapid formation of Ins(1,4,5)P3 is compatible with its second-messenger role in the initial elevation of [Ca2+]i.