Ole H. Petersen
University of Liverpool

Local and Global Cytosolic Ca²⁺ and Calmodulin Oscillations

Pancreatic acinar cells produce a variety of digestive enzymes and these are stored in granules (zymogen granules). Secretion, by exocytosis, occurs in response to stimulation with either the neurotransmitter acetylcholine (ACh) or the hormone cholecystokinin (CCK). These agonists evoke release into the cytosol of Ca²⁺ stored in the endoplasmic reticulum and it is this rise in the Ca²⁺ concentration that initiates exocytotic secretion. In this system agonist-selective Ca²⁺ signal patterns have been established. ACh, at low concentrations (25 – 50 nM) evokes repetitive short-lasting cytosolic Ca²⁺ spikes confined to the apical granule-containing part of the cells, whereas low (physiological) CCK concentrations (2-10 pM) evoke a mixture of the shortlasting apical Ca²⁺ spikes and much longer and larger global Ca²⁺ transients. At the slightly higher CCK concentration of 50 pM, repetitive global Ca²⁺ transients are observed. The inositol trisphosphate (IP₃) receptor is crucially involved in the Ca²⁺ oscillations, which are most likely due to Ca²⁺-dependent positive and negative feedback processes. However, recent work in our laboratory shows that other intracellular Ca²⁺ release channels are also involved. Electropharmacological investigations demonstrate that the common oscillator unit in the acinar cells depends on concerted activity of both IP₃ and ryanodine receptors. The route to this common unit is different for the pathways initiated by ACh and CCK, respectively. Intracellular receptors for nicotinic acid adenine dinucleotide phosphate (NAADP) constitute the trigger for internal Ca²⁺ release in the CCK pathway. This trigger Ca²⁺ signal is amplified by cyclic ADP-ribose (cADPR) – sensitive ryanodine receptors. In the case of ACh stimulation there is no involvement of NAADP or cADPR receptors, but activation of IP₃ receptors is the triggering event. These two different messenger pathways converge on the common oscillator units. The role of calmodulin (CaM) is important, since this protein is essential for the Ca²⁺ mediated negative feedback on the IP₃ receptor, but also essential for cADPR-stimulation of ryanodine receptors. The Ca²⁺-CaM reaction depends on both the Ca²⁺ and the CaM concentrations and it is interesting that Ca²⁺ can itself induce intracellular translocation of CaM. We have been able to demonstrate different types of oscillations of the CaM concentration in different parts of the cell in response to 50 pM CCK. The global and relatively uniform cytosolic Ca²⁺ oscillations observed in this situation are associated with synchronous CaM oscillations in the apical granular part, but with mirror-like changes in the CaM level in the basal non-nuclear area. In the nucleus, step-like increments in the CaM concentration were observed in the same period. Thus relatively uniform global Ca²⁺ oscillations can give rise to three different patterns of CaM oscillations in three different parts of the cell.

Reviews: 

Petersen, O.H., Petersen, C.C.H. & Kasai, H. Annu Rev Physiol 56, 297-319, 1994

Petersen, O.H., Burdakov, D. & Tepikin, A.V. BioEssays 21, 851-860, 1999

Petersen, O.H. & Cancela, J.M. Trends Neurosci  22, 488-494, 1999