Inositol 1,2-cyclic 4,5-trisphosphate concentration relative to inositol 1,4,5-trisphosphate in pancreatic minilobules on stimulation with carbamylcholine in the absence of lithium. Possible role as a second messenger in long- but not short-term responses.

A method for the extraction of cyclic inositol phosphates in the absence of acid after short incubation times is described. A modified high pressure liquid chromatography method is also described which separates inositol 1,2-cyclic 4,5-trisphosphate (IcP3), inositol 1,4,5-trisphosphate (I(1,4,5)P3), and inositol 1,3,4-trisphosphate (I(1,3,4)P3). Mouse pancreatic minilobules were preincubated with [3H]inositol for 1 h in the absence of lithium, washed, and incubated without and with carbamylcholine without lithium for various times. On adding carbamylcholine, I(1,4,5)P3 peaked at 10 s, followed by a fall to a steady-state level which was two-thirds the peak value. This level was maintained for 20 min. IcP3, on the other hand, rose very slowly; at 10 s, it was only 5% of I(1,4,5)P3. It continued to rise until it equaled the steady-state level of I(1,4,5)P3 at 20 min. I(1,3,4)P3 rose gradually but at a faster rate than IcP3, peaking at 40 s at the same level as that achieved by I(1,4,5)P3 at that time and then falling in parallel with I(1,4,5)P3. Assuming equal potencies of IcP3 and I(1,4,5)P3 in mobilizing intracellular stores of Ca2+ in pancreatic minilobules, as seen in Limulus photoreceptor cells, platelets, and 3T3 cells, IcP3 would appear to play no messenger role at very early times. Thus, I(1,4,5)P3 should be responsible for almost all of Ca2+ release at very early times (10 s), while at later times (20 min) IcP3 and I(1,4,5)P3 should contribute equally to Ca2+ release. The significance of these results is discussed.