Physiological and pathological intracellular calcium release in human and murine pancreatic acinar cells

Sustained, toxic elevations of pancreatic acinar cell cytosolic free calcium ion concentration ([Ca2+]C), such as those observed with supramaximal secretagogue stimulation (CCK) are implicated in acute pancreatitis. However, Cholecystokinin (CCK) has been thought to act only indirectly on human pancreatic acinar cells via vagal nerve stimulation, rather than by direct CCK receptor activation as observed in rodent pancreatic acinar cells. However, in the series of experiments presented here using human pancreatic acinar cells, CCK at physiological concentrations (1-20 pM) elicited rapid, robust, oscillatory rises of the cytosolic Ca2+ ion concentration ([Ca2+]C), showing apical to basal progression in acinar cells, in the presence of atropine and tetrodotoxin. The [Ca2+]C rises were followed by increases in mitochondrial ATP production and secretion, concluding that CCK acts directly on acinar cells in the human pancreas. The earliest pathological mechanisms, such as sustained, toxic elevations of the acinar cytosolic free calcium ion concentration ([Ca2+]C), incriminated in experimental pancreatitis have been previously demonstrated by non-oxidative metabolites of ethanol (FAEE’s), as well as bile salts, at supramaximal concentrations. However, in the clinical situation such hyperstimulation is unlikely to occur. To simulate a more clinically relevant stimulus, pancreatic acinar cells were stimulated with lower doses of FAEE’s and/or bile salts in combination with physiological doses of secretagogues - a process which may precipitate pancreatitis clinically. Illustrated here, the toxic transformation of secretagogue induced physiological Ca2+ signalling occurs with the perfusion of low doses of TLCS and POAEE resulting in cell injury. The intracellular second messengers implicated are IP3, cADPR and NAADP with the IP3 receptor channel pivotal with both toxins. However, as previously demonstrated with supramaximal concentrations of POAEE, if supplementary ATP is added to the intracellular milieu, cellular injury is avoided with continued extrusion of large quantities of Ca2+ from the cytosol indicating functional Ca2+ ATPase pumps. This is not observed in cells which do not receive supplementary ATP. The toxic sustained Ca2+ elevation is also be prevented by the removal of external Ca2+ or blockade of IP3 receptor using caffeine and cell injury is again avoided. Therefore, it may be concluded, that it is the large, sustained toxic [Ca2+]c load which impairs mitochondrial function and ATP production leading to Ca2+ATPase pump failure and ultimately cell death. Lowering sustained intracellular [Ca2+]c by blockade of IP3 receptor channels may reduce cell injury in clinical acute pancreatitis