Cytoplasmic free calcium concentration ((Ca2+]i) was evaluated by dual-wavelength microspectrofluorometry of fura-2-loaded individual rat pancreatic acinar cells. Resting [Ca2+]i in unstimulated acini was 94.1 +/- 4.1 nM. Stimulation with high concentrations of cholecystokinin (CCK, 100 pM to 1 nM) led to an immediate rise in [Ca2+]i to 400-1,000 nM followed by a fall within 2-5 min to a plateau only slightly above the prestimulation level. Lower and more physiological concentrations of CCK (1-30 pM), after a latent period of 60-90 s, induced a smaller sustained increase in [Ca2+]i (30-40 nM) with superimposed repetitive transient [Ca2+]i spikes. These oscillations averaged 120-150 nM in amplitude, occurred at a frequency which averaged 1.5 times/min, and were maintained as long as the stimulus was applied. Similar [Ca2+]i oscillations were observed when acini were stimulated with submaximal concentrations of carbamylcholine (0.1-1 microM) and neuromedin C (0.1-1 nM). Intracellular Ca2+ stores were not depleted during [Ca2+] oscillations, since a subsequent increase to 1 nM CCK led to an immediate rise in [Ca2+]i indistinguishable from the response of cells initially stimulated at this concentration. Although extracellular Ca2+ was required for maintenance of frequency of the spikes, the major source of Ca2+ utilized for oscillations was intracellular, since elimination of medium Ca2+ or Ca2+ entry blockade with lanthanum failed to inhibit oscillations. Vasoactive intestinal polypeptide (10 nM) and high K+ (50 mM) did not affect [Ca2+]i oscillations. Antimycin (10 microM), which depletes cytoplasmic ATP, increased basal [Ca2+]i and inhibited the oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)