Ca2+Oscillations Induced by Hormonal Stimulation of Individual Fura-2-loaded Hepatocytes

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 εscopy. Stimulation with 0.1 nMvasopressin, 0.5 εMphenylephrine, or 0.5 εMATP 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+]iand had an effect similar to removal of extracellular Ca2+in decreasing the frequency of agonist-evoked [Ca2+]ioscillations or inhibiting them altogether, arguing against any significant role for voltage-operated Ca2+ channels.