Range of Messenger Action of Calcium Ion and Inositol 1,4,5-Trisphosphate

The range of messenger action of a point source of Ca2+ or inositol 1,4,5-trisphosphate (IP3) was determined from measurements of their diffusion coefficients in a cytosolic extract from Xenopus laevis oocytes. The diffusion coefficient (D) of [3H]IP3 injected into an extract was 283 μm2/s. D for Ca2+ increased from 13 to 65 μm2/s when the free calcium concentration was raised from about 90 nM to 1 μm. The slow diffusion of Ca2+ in the physiologic concentration range results from its binding to slowly mobile or immobile buffers. The calculated effective ranges of free Ca2+ before it is buffered, buffered Ca2+, and IP3 determined from their diffusion coefficients and lifetimes were 0.1 μm, 5 μm, and 24 μm, respectively. Thus, for a transient point source of messenger in cells smaller than 20 μm, IP3 is a global messenger, whereas Ca2+ acts in restricted domains.