Water and electrolyte secretion, and antibacterial properties of the pancreatic juice

The mechanisms of neural, hormonal and intracellular signal transduction in ductules and acinar pancreatic cells in the process of water and electrolyte secretion in various animal species are described. In the pancreas, secretin has been implicated as a stimulant of ductal bicarbonate and acinar zymogen secretion, islet hormone secretion, and as a regulator of blood flow. Vagal stimulation releases several neuropeptides, which are known to stimulate pancreatic secretion. These include cholecystokinin, vasoactive intestinal peptide and pituitary adenylate cyclase activating peptide [1,2]. These peptides stimulate ductal secretion via the cAMP pathway, which is considered to be the main intracellular messenger controlling electrolyte and fluid secretion from pancreatic duct cells [3,4,5]. Exocrine acinar cells are capable of regulating fluid secretion, which is rich in Na(+) and Cl(-) and has a plasma-like ionic composition. Efflux of cellular K(+) and Cl(-) is thought to comprise the initial ionic movement following stimulation of exocrine acinar cells. However, in order to obtain a net K(+) and Cl(-) loss from the acinar cells, it is essential that K(+) channels located in the basolateral membrane and Cl(-) channels situated in the luminal area be active at the same time. These channels are activated by the agonist-induced increase in intracellular Ca(2+) concentration. Due to the segregation of channels in the acinar plasma membrane, it is necessary for the [Ca(2+)](i) to be increased by the same amount, at the same time, throughout the cells in order to activate the channels simultaneously [4,6]. Pancreatic juice from mammalian species has been demonstrated to possess antimicrobial properties. The antibacterial activity of pancreatic juice increases with age in piglets. This change occurs soon after weaning, and might be a compensatory phenomenon in maintaining microbial homeostasis in the upper small bowel at a time when it is presented with new food-borne microorganisms and has not yet developed an effective system of defence against them [7].