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Changes of cytosolic free Ca2+ ( Ca2+i) in response to receptor activation were studied at the single cell level by using digital imaging fluorescence microscopy of fura-2-loaded primary cultured hepatocytes. In response to phenylephrine and vasopressin, individual hepatocytes displayed dose-dependent oscillations of Ca2+i similar to those observed in aequorin-injected hepatocytes by Woods et al. (Woods, N. M., Cuthbertson, K. S. R., and Cobbold, P. H. (1986) Nature 329, 719-721). With increasing agonist concentration, the frequency of oscillations increased and the latent period decreased. For a given cell, peak Ca2+i was independent of applied agonist concentration. However, there was considerable variation from cell to cell in the absolute value of peak Ca2+i. There was also marked intercellular heterogeneity in the latency, frequency, and overall pattern of the Ca2+ responses. Such asynchronous responses can be explained in part by the apparent differential agonist sensitivity of individual cells for latency and frequency. At high doses, phenylephrine maintained an oscillatory pattern, whereas vasopressin produced a complex mixture of spiking and sustained Ca2+i responses. Vasopressin and phenylephrine also displayed differently shaped Ca2+i oscillations at submaximal doses, due primarily to a slower rate of decay with vasopressin. Despite the large cell-cell variation in the patterns of Ca2+i oscillations, successive readditions of the same agonist elicited identical cell-specific patterns of oscillation. In the absence of extracellular Ca2+ the frequency but not the magnitude of Ca2+i oscillations was decreased. Buffering of Ca2+i by increasing the fura-2 load of single hepatocytes also decreased the frequency of oscillations without affecting the peak Ca2+ level. These data provide further support for the importance of frequency modulation in agonist-induced Ca2+ responses and suggest that Ca2+ itself plays an important role in regulating the frequency of Ca2+i oscillations. Furthermore, the data demonstrate a broad heterogeneity in hepatocyte Ca2+i oscillations which may underlie the nonoscillatory responses of cell populations.
Rooney et al. (Sun,) studied this question.