Role of acidic stores in secretory epithelia

• Acidic calcium store (AS) plays an important role in pancreatic acinar cells.
• Interactions of the AS with the ER in the apical area.
• Sensitivity of the AS to InsP3, cADPR and NAADP.
• The crucial role of the AS in the pathophysiology of pancreatitis.

There is growing evidence that intracellular calcium plays a primary role in the pathophysiology of the pancreas in addition to its crucial importance in major physiological functions. Pancreatic acinar cells have a remarkably large amount of Ca2+ stored in both the endoplasmic reticulum (ER) and the acidic stores. The vast majority of the classical ER Ca2+ store is located in the basal part of the acinar cells with extensions protruding into the apical area, however, the acidic stores are exclusively located in the secretory granular area of the cells. Both types of Ca2+ store respond to all three intracellular Ca2+ messengers – inositol trisphosphate (InsP3), cyclic-ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). The two stores interact with each other via calcium-induced calcium release; however, they can be separated using pharmacological tools. The ER relies on sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) that can be blocked by the specific inhibitor thapsigargin. The acidic store requires a low pH that can be modified by blocking vacuolar H+-ATPase.The acidic store is particularly important for pathological processes in the pancreas. Acute pancreatitis is initiated as a result of calcium overload in the apical pole, which leads to trypsinogen activation; two major causes are gall bladder stones and excessive alcohol consumption. Excessive Ca2+ release from the acidic stores plays a major role in both scenarios; however NAADP-induced calcium release from acidic stores is particularly important for bile-induced pancreatitis. Cell-permeable calmodulin (CaM) activators such as CALP3 boost the natural protective effect of CaM by inhibiting excessive calcium release from the internal stores through inositol trisphosphate (InsP3R) and ryanodine receptors (RyR). Alternatively calcium overload can be dramatically reduced by inhibiting Ca2+-release-activated Ca2+ (CRAC) currents that are required to reload the internal stores and therefore provide effective protection against the major triggers of acute pancreatitis.

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