Second messenger function of inositol 1,4,5-trisphosphate. Early changes in inositol phosphates, cytosolic Ca2+, and insulin release in carbamylcholine-stimulated RINm5F cells.

The second messenger function of inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) was investigated in carbamylcholine-stimulated RINm5F cells by analysis of the early changes in inositol phosphates, cytosolic free Ca2+ concentration ([Ca2+]i), and insulin secretion. After a lag of 2 s, [Ca2+]i rose to a peak at 13 +/- 2 s, a response which was due mainly to mobilization from intracellular stores since it persisted even in the absence of extracellular Ca2+. The Ca2+ response had already declined toward prestimulatory levels by the time insulin secretion reached its maximal rate (2-3 min). Although the rises in inositol trisphosphate preceded those of both inositol bisphosphate and monophosphate, all three attained maximal concentrations after 1 min and remained elevated for at least 10 min. The accumulation of inositol trisphosphate was truly Ca2+-independent since it persisted under conditions in which the rise in [Ca2+]i was abolished by prior depletion of intracellular Ca2+ pools. Further analysis by high performance liquid chromatography revealed the presence of the two isomers, Ins-1,4,5-P3 and Ins-1,3,4-P3 in stimulated cells. The latter was virtually absent under nonstimulatory conditions but started to accumulate after a 5-s lag and reached maximal levels after 30 s of stimulation. Ins-1,4,5-P3 doubled within 1 s of carbamylcholine addition, reached a peak after 5 s, and, although declining thereafter, remained slightly elevated for at least 3 min. Hence, both the onset and peak of the rise of Ins-1,4,5-P3 preceded that of [Ca2+]i, which in turn preceded the peak in insulin release. These results strongly suggest that Ins-1,4,5-P3 acts as the second messenger by which carbamylcholine mobilizes intracellular Ca2+ during the initiation of insulin release.