Cytoplasmic Ca+ signals evoked by activation of cholecystokinin receptors: Ca2+-Dependent current recording in internally perfused pancreatic acinar cells

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Abstract
The effects on the cytosolic Ca2+ concentration of activating cholecystokinin receptors on single mouse pancreatic acinar cells have been investigated using patch-clamp whole-cell recording of Ca2+-dependent Cl− current. We used the nonsulphated octapeptide of cholecystokinin (CCK8-NS) since the effects of even high concentrations were rapidly reversible which was not the case for the sulphated octapeptide. A submaximal concentration of CCK8-NS (10nm) evoked a current response consisting of short-lasting (a few seconds) spikes, and some of these spikes were seen to trigger larger and longer (about half a minute) current pulses. At a higher concentration (100nm) CCK8-NS evoked smooth and sustained responses. The effect of CCK8-NS was almost abolished when the internal perfusion solution contained a high concentration of the Ca2+ chelator EGTA (5mm). The responses evoked by CCK8-NS were independent of the presence of Ca2+ in the external solution at least for the first 5 min of stimulation. Internal perfusion with GTP-γ-S markedly potentiated the effect of CCK8-NS or at a higher concentration itself induced responses very similar to those normally evoked by CCK8-NS. Caffeine added to the external solution at a low concentration (0.2–1mm) enhanced weak CCK8-NS responses, whereas high caffeine concentrations always inhibited the CCK8-NS-evoked responses. These inhibitory caffeine effects were quickly reversible. Forskolin evoked a similar inhibitory effect. Intracellular heparin (200 μg/ml) infusion markedly inhibited the response to CCK8-NS stimulation. We conclude that the primary effect of activating CCK receptors is to induced inositoltrisphosphate (IP3) production. IP3 evokes a small and steady Ca2+ release, and this in turn evokes pulsatile release of a larger magnitude from a caffeine-sensitive Ca2+ pool. The action of CCK is thus very similar to that previously established for muscarinic receptor activation in the same cells. Nevertheless, the pattern of the cytosolic Ca2+ fluctuations are different, and the basic process of Ca2+-induced Ca2+ release and Ca2+ signal spreading must therefore be modulated by a messenger yet unknown.