Inositol 1,4,5-trisphosphate- and ryanodine-sensitive Ca2+ release channel-dependent Ca2+ signalling in rat portal vein myocytes.

Ca2+ 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 (InsP3) concentration was obtained by photorelease from a caged precursor or intracellular dialysis of 3F-InsP3. Low InsP3 concentrations activated either small elevations of [Ca2+]i or localized Ca2+ transients whereas high InsP3 concentrations activated either homogeneous Ca2+ responses or propagated Ca2+ waves. The InsP3-evoked localized Ca2+ transients had spatio-temporal properties characteristic of Ca2+ sparks. In addition, compounds that blocked Ca2+ sparks and Ca2+ responses activated by Ca2+ jumps reduced the global InsP3-activated Ca2+ responses and suppressed the Ca2+ transients. In contrast, Ca2+ responses evoked by flash-photolytic Ca2+ jumps or caffeine were not affected by heparin (an InsP3 receptor antagonist). These results suggest that the absence of elementary Ca2+ events evoked by InsP3 may be related to the lack of clustered InsP3 receptor units in these cells, as confirmed by immunocytochemistry. Cooperativity between InsP3- and ryanodine-sensitive Ca2+ channels may represent a novel mechanism to amplify Ca2+ release from the same intracellular store and give rise to propagated Ca2+ waves.