The mechanisms underlying inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ liberation were studied in Xenopus oocytes by using scanning and stationary-point confocal fluorescence microscopy to record Ca2+ signals evoked by photorelease of InsP3 from a caged precursor. 2. Fluorescence measurements from confocal images showed that increasing [InsP3] evoked three distinct modes of Ca2+ liberation: a diffuse 'pacemaker' signal, localized transient puffs, and propagating waves. Peak free Ca2+ concentrations during waves and puffs (respectively, 2-5 microM and 100-200 nM) varied only slightly with [InsP3], whereas the pacemaker amplitude varied over a wider range (at least 1-30 nM Ca2+). 3. The improved resolution provided by confocal point recording revealed discontinuous Ca2+ 'blips' during pacemaker release. These events were resolved only at particular locations and had time courses similar to the puffs (rise, approximately 50 ms; decay, a few hundred milliseconds) but with amplitudes one-fifth or less of puff amplitudes. 4. We conclude that blips may arise through opening of single InsP3-gated channels, whereas puffs reflect the concerted opening of several clustered channels due to local regenerative feedback by Ca2+.
