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Isolated rat hepatocytes were loaded with the Ca2+ indicator fura-2 to measure cytosolic free Ca2+ concentrations (Ca2+i) in individual cells by digital ratio imaging microscopy. Stimulation with 0.1 nM vasopressin, 0.5 microM phenylephrine, or 0.5 microM ATP caused repetitive spikes of high Ca2+i in a high percentage of cells, in agreement with Woods et al. (Woods, N. M., Cuthbertson, K. S. R., and Cobbold, P. H. (1986) Nature 319, 600-602), but unlike the results of Monck et al. (Monck, J. R., Reynolds, E. E., Thomas, A. P., and Williamson, J. R. (1988) J. Biol. Chem. 263, 4569-4575). Reduction in extracellular Ca2+ decreased the frequency but not the amplitude of the spikes, suggesting that the spikes result from dumping of intracellular stores and that the entry of extracellular Ca2+ affects only the rate of replenishment of those stores. Membrane depolarization failed to elevate Ca2+i and had an effect similar to removal of extracellular Ca2+ in decreasing the frequency of agonist-evoked Ca2+i oscillations or inhibiting them altogether, arguing against any significant role for voltage-operated Ca2+ channels.
Kawanishi et al. (Tue,) studied this question.