Abstract
Ca 2+ signalling events were analyzed in single myocytes from rat portal vein by using a laser confocal microscope combined with the patch-clamp technique. Increase in inositol 1,4,5-trisphosphate (InsP 3) concentration was obtained by photorelease from a caged precursor or intracellular dialysis of 3F-InsP 3. Low InsP3 concentrations activated either small elevations of [Ca 2+] i or localized Ca 2+ transients whereas high Ins P 3 concentrations activated either homogeneous Ca 2+ responses or propagated Ca 2+ waves. The InsP 3-evoked localized Ca 2+ transients had spatio-temporal properties characteristic of Ca 2+ sparks. In addition, compounds that blocked Ca 2+ sparks and Ca 2+ responses activated by Ca 2+ jumps reduced the global InsP 3-activated Ca 2+ responses and suppressed the Ca 2+ transients. In contrast, Ca 2+ responses evoked by flash-photolytic Ca 2+ jumps or caffeine were not affected by heparin (an InsP 3 receptor antagonist). These results suggest that the absence of elementary Ca 2+ events evoked by InsP 3 may be related to the lack of clustered InsP 3 receptor units in these cells, as confirmed by immunocytochemistry. Cooperativity between InsP 3 and ryanodine-sensitive Ca 2+ channels may represent a novel mechanism to amplify Ca 2+ release from the same intracellular store and give rise to propagated Ca 2+ waves.
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