Delayed "all-or-none" activation of inositol 1,4,5-trisphosphate-dependent calcium signaling in single rat hepatocytes.

When single rat hepatocytes were stimulated with the phospholipase C-activating hormone, vasopressin (from 300 pM to 1 microM), the [Ca2+]i signals were always "all-or-none" responses. At low concentrations of vasopressin, Ca2+ release was maximal because liberation of additional inositol 1,4,5-trisphosphate (IP3) by photolysis of its caged precursor at the top of the [Ca2+]i spike failed to increase [Ca2+]i further. However, if IP3 was generated by photolysis of caged IP3 in previously unstimulated cells, [Ca2+]i increased immediately, and the magnitude of the response was a graded function of the quantity of IP3 released. We also analyzed the kinetics of activation of intracellular IP3 receptor/Ca2+ channels by monitoring the quench of sequestered dye by the entry of cytoplasmic Mn2+ into fura-2-loaded intracellular IP3-sensitive organelles. This Mn(2+)-induced quench was precipitous and always preceded by a delay inversely related to the vasopressin concentration. In hepatocytes stimulated with 10 nM vasopressin, IP3 increased slowly, and the half-time of the IP3 rise was comparable with the latency for the release of intracellular calcium. The slow rise in IP3 would be predicted to produce accelerating Ca2+ release. This is consistent with the results of the Mn2+ quench experiments, which revealed accelerating activation of intracellular IP3-regulated calcium channels. We conclude that this accelerating release of Ca2+, which does not occur with instantaneous increases in IP3 due to flash photolysis, is likely to be important for generating the all-or-none Ca2+ mobilization that initiates the processes of intracellular [Ca2+]i oscillations.