Cell Calcium (1995) 17. 367-374 0 Pearson Professional Ltd 1995 [Ca2+]i oscillations in rat chromaffin cells: frequency and amplitude modulation by Ca2+ and lnsP3 P. D’ANDREA’ and F. GROHOVAZ2 ’ Department of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste, Trieste and ‘CNR Center of Cytopharmacology & B. Ceccarelli Center, D/BIT, S. Raftaele Institute, Milano, Ha/y Abstract - Rat chromaffin cells in primary culture exhibit oscillations of cytosolic Ca2+ concentration, sustained by the rhythmic discharge of Ca2+ stores. Each Ca2+ from specialized intracellular spike starts from a discrete region of the cell (pacemaker), and then propagates across the entire cytosol. Spike initiation and propagation, governing the oscil- lation frequency and amplitude respectively, appeared to be controlled by different mechan- isms. The pacemaker was found to be directly activated by increases of cytosolic Ca2+ concentration obtained by either K+ depolarization or nicotinic stimulation. On the other hand, muscarinic or BP stimulation was required for an efficient spreading to occur, thus suggesting a key role of InsPs in the signal propagation. The pacemaker displayed an au- tonomous activity, as documented by the presence of local Ca2+ discharges, which were not necessarily accompanied by spreading to the rest of the cell. This uncoupling could be stimulated by the selective increase of the pacemaker firing rate, due to the rise of the intracellular Ca2+ concentration. Modulation of Ca2+ spike amplitude by treatments affecting either the pacemaker or the spreading phase might be related to quanta1 Ca2+ release from functionally discrete stores. Periodic oscillations of the cytosolic calcium con- centration, [Ca2+]i, due to the rhythmic discharge of Ca2+ from specialized intracellular compartments, are known to occur in many cell types [l]. On a spatiotemporal basis, individual spikes can be re- solved into a sequence of phases: an initial rise re- stricted to a subcellular area (pacemaker); a second- ary phase which involves a global Ca2+ rise across the entire cytosol (spreading); and a decay phase (recovery), whose kinetics and spatial aspects appear to be most variable [ 1,2]. In rat chromaffin cells, where [Ca2+]i oscillations occur spontaneously in over 50% of the population, we have reported the activity of a pulsatile pace- maker both in silent and active cells [3]. In oscillat- ing cells this activity is increased by treatments moderately elevating [Ca2+]i or [InsP3] [l-3]. The resulting dose-dependent increases of the oscillation frequency led to the hypothesis that signal interpre- tation by the cells is digitally encoded [4]. However, we recently demonstrated that oscillations can undergo amplitude modulation as well. In fact, treat- 367