Feedback inhibition of Ca²⁺ release by Ca²⁺ is the underlying mechanism of agonist-evoked intracellular Ca²⁺ oscillations in pancreatic acinar cells

Oscillations of free intracellular Ca²⁺ concentration ([Ca²⁺](i)) are known to occur in many cell types during physiological cell signaling. To identify the basis for the oscillations, we measured both [Ca²⁺](i) and extracellular Ca²⁺ concentration ([Ca²⁺](o)) to follow the fate of Ca²⁺ during stimulation of [Ca²⁺](i) oscillations in pancreatic acinar cells. [Ca²⁺](i) oscillations were initiated by either t-butyloxycarbonyl- Tyr(SO₃)-Nle-Gly-Tyr-Nle-Asp-2-phenylethyl ester (CCK-J), which mobilized Ca²⁺ from the inositol 1,4,5-trisphosphate (IP₃)-insensitive pool, or low concentration of cholecystokinin octapeptide (CCK-OP), which mobilized Ca²⁺ from the IP₃-sensitive internal pool. Little Ca²⁺ efflux occurred during the oscillations triggered by CCK-J or CCK-OP in spite of a large average increase in [Ca²⁺](i). When internal store Ca²⁺ pumps were inhibited with thapsigargin (Tg) during [Ca²⁺](i) oscillations, a rapid Ca²⁺ efflux occurred similar to that measured in intensely stimulated, nonoscillatory cells. Tg also stimulated ⁴⁵Ca efflux from internal pools of cells stimulated with CCK-J or a low concentration of CCK-OP. Hence, a large fraction of the Ca²⁺ released during each spike is reincorporated by the internal store Ca²⁺ pumps. Surprisingly, when the increase in [Ca²⁺](i) during stimulation of oscillations was prevented by loading the cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, a persistent activation of Ca²⁺ release and Ca²⁺ efflux occurred. This was reflected as a persistent increase in [Ca²⁺](o) in cells suspended at low [Ca²⁺](o) or persistent efflux of ⁴⁵Ca from internal stores of cells maintained at high [Ca²⁺](o). Since agonist-stimulated Ca²⁺ release evidently remains activated when [Ca²⁺](i) is highly buffered, the primary mechanism determining Ca²⁺ oscillations must include an inhibition of Ca²⁺ release by [Ca²⁺](i). Loading the cells with 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid had no apparent effect on the levels or kinetics of IP₃ formation in agonist-stimulated cells. This suggests that [Ca²⁺](i) regulated the oscillation by inhibition of Ca²⁺ release independent of its possible effects on cellular levels of IP₃.