THE LIFETIME OF INOSITOL 1,4,5-TRISPHOSPHATE IN SINGLE CELLS

In many eukaryotic cell types, receptor activation leads to the format ion of inositol 1,4,5-trisphosphate (IP3) which causes calcium ions (C a) to be released from internal stores. Ca release was observed in res ponse to the muscarinic agonist carbachol by fura-2 imaging of N1E-115 neuroblastoma cells. Ca release followed receptor activation after a latency of 0.4 to 20 s. Latency was not caused by Ca feedback on IP3 r eceptors, but rather by IP3 accumulation to a threshold for release. T he dependence of latency on carbachol dose was fitted to a model in wh ich IP3 synthesis and degradation compete, resulting in gradual accumu lation to a threshold level at which Ca release becomes regenerative. This analysis gave degradation rate constants of IP3 in single cells r anging from 0 to 0.284 s(-1) (0.058 +/- 0.067 s(-1) SD, 53 cells) and a mean IP3 lifetime of 9.2 +/- 2.2 s. IP3 degradation was also measure d directly with biochemical methods. This gave a half life of 9 +/- 2 s. The rate of IP3 degradation sets the time frame over which IP3 accu mulations are integrated as input signals. IP3 levels are also filtere d over time, and on average, large-amplitude oscillations in IP3 in th ese cells cannot occur with period <10 s.