HEMISPHERIC-ASYMMETRY OF MACROSCOPIC AND ELEMENTARY CALCIUM SIGNALS MEDIATED BY INSP(3) IN XENOPUS OOCYTES

1. The mechanisms underlying hemispheric asymmetry of the inositol 1,4 ,5-trisphosphate (InsP(3))-calcium signalling pathway in Xenopus oocyt es were examined by fluorescence imaging of calcium signals and record ing calcium-activated Cl- currents (I-Cl,I-Ca) evoked by intracellular calcium injections and photorelease of InsP(3). 2. The maximal I-Cl,I -Ca evoked by strong photorelease of InsP(3) was 8 times greater in th e animal than the vegetal hemisphere, but the average threshold amount s of InsP(3) required to evoke detectable currents were similar in eac h hemisphere. 3. Currents evoked by injections of calcium were about 2 .5 times greater near the animal pole than near the vegetal pole, wher eas fluorescence signals evoked by injections were similar in each hem isphere. 4. Calcium waves were evoked by photolysis flashes of similar strengths in both hemispheres of albino oocytes, but peak calcium lev els evoked by supramaximal stimuli were 70% greater in the animal hemi sphere. 5. Elementary calcium release events (puffs) in the animal hem isphere had amplitudes about double that in the vegetal hemisphere, an d more often involved coupled release from adjacent sites. Calcium rel ease sites were more closely packed in the animal hemisphere, with a m ean spacing of about 1.5 mu m compared with 2.25 mu m in the vegetal h emisphere. 6. The larger amplitude of currents mediated by InsP(3) in the animal hemisphere, therefore, involves an increased flux of calciu m at individual release units, a more dense packing of release units a nd a higher density of Cl- channels.