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The final non enzymic secretion of salivary glands and exocrine pancreas into GI tract may differ from primary secretions produced in the secretory unit of each gland. Discuss how this occurs and the reasons for any differences. Are there any similarities in process involved?
Secretion and fluid transport mechanisms in exocrine glands need an organised function of transporters and channel proteins in a specialised secretory unit. Salivary and exocrine portion of the pancreas have very similar secretory units and mechanism that aid in their final secretions. Saliva is slightly viscous liquid containing water, proteins and ions which is secreted by the salivary glands into the oral cavity. The amount of saliva produced on average is about 750-1000ml a day, and it functions as a protective fluid which has disinfectant role and pH buffering action. Saliva also cleanses the oral cavity, lubricates the mucosal structures within the buccal cavity, the pharynx and oesophagus plus begins the digestive process by breaking down carbohydrates.
The exocrine pancreas secretes roughly 1.5L of alkaline pancreatic fluid into the GI tract a day. It functions as a neutralising fluid which reduces the effect of the stomach acid in the duodenum protecting its digestive structures. The pancreatic juice contains many enzymes which cause the digestion of carbohydrates, proteins, lipids and nucleic acids in a hydrolysis reaction.
The Structure of Secretory Unit
The secretory structure for pancreas and salivary glands are very similar. Primary secretion occurs in acinar cells which cluster up to form an end piece called acinus. The lumen of the acinus drains its fluid into the proximal end of intercalated ducts. These ducts merge with many other intercalated ducts to form interlobular ducts and striated duct in the pancreas and salivary glands. The duct cells in both these structures modify the fluid content of primary secretion to form the final secretory fluid. The acinar cells of the salivary glands and the epithelial duct cells of pancreas are the main secretory cells (Ref to Fig. 1).
Fig.1. A diagram of the functional organisation of the salivary and pancreatic acinar and ductal cells in the acini.
Primary Secretions produced by Acinar Cells
The acinar cell of salivary glands produces an isotonic solution rich in NaCl and secretes in into the acini lumen. Transporter proteins found in the apical and basolateral side of acinar cells transport ions in the secretory direction, setting up an osmotic gradient allowing the passage of water. The activation of these transporters is due the rise of intracellular calcium concentration in acinar cells which therefore control the amount of ions released (Ref to Fig.2).
Fig.2. A diagram to show the transport of ion and water is salivary and pancreatic acinar cells.
The basolateral membrane contains pumps which help regulate ion concentrations. The Na+/K+ ATPase in the pumps maintain an inward sodium electrochemical gradient. This movement is used by the calcium activated Na+/K+/2Cl- cotransporter to increase the intracellular concentration of ions. Also the influx of ions is maintained by Cl- /HCO3- and Na+ / H+ antiporters causing an exchange of Na+ and Cl- into acinar cell.
The calcium gated K+ on the basolateral membrane causes efflux of K+ into interstitial space, this removal balances the influx of K+ produced by the Na+/K+/2Cl-. It also hyperpolarises the membrane which increases the electrochemical gradient for Cl- to move into the lumen. The luminal space then becomes very negative which increases the electrochemical gradient for Na+ ions to move passively. The overall effect of these channels and transporter proteins is to increase the concentration of Cl- and Na+ in the lumen. An accumulation of NaCl produces an osmotic gradient to set up across the acinar cells causing the water to follow the passive movement of ions. Water can move trancellulary through aquaporins expressed in the apical membrane or paracellulary where water leaks through the tight junctions in-between cells. The pancreas also uses this mechanism to produce small amounts of isotonic fluid rich in NaCl.
The Modification of Primary Secretion by Duct cells
The striated ductal epithelial cells of salivary glands have both an excretory and absorptive function which helps reabsorb NaCl and secrete KHCO3. It is also impermeable to water so the resulting fluid produces as the final secretion is hypotonic. The reabsorption of Na+ ions is due to the functioning of Na+/K+ ATPase situated in the basolateral membrane coupled with the activity of Na+ / H+ and Na+ channels expressed in the apical membrane. Due to the inward movement of positive ions the charge balances needs to be maintained by the transport of Cl- ions across apical membrane via Cl- channels. This is also aided by and Cl- /HCO3- antiporters which pumps HCO3- into saliva increasing the salivaâ€™s bicarbonate levels. The high levels of K+ inside the cell, due to Na+/K+ ATPase, is used by K+ /H+ and K+/ HCO3- cotransporter in apical membrane to pump ions into the lumen. (Ref to Fig. 3).
Saliva is made hypotonic to reduce the effect of high salt concentration (Na+) on the taste buds that is normally found in plasma. It is also made hypotonic to hydrate the mucosal lining of the oral cavity and expand the mucins strengthening the biofilm coating. Saliva is made alkaline to neutralise the acidic components of food stuffs.
The pancreatic duct cells secrete more than they absorb, secreting high amount of alkaline fluid. The fluid is plentiful in HCO3- needed to help neutralise the acid chyme that comes in from the stomach. The Cl- /HCO3- antiporters in the luminal surface of ductal cells secrete HCO3- into the lumen while reabsorbing the Cl-. Calcium dependent Cl- channels and CFTR channels in the apical membrane aid the Cl- /HCO3- antiporters by increasing luminal Cl- concentrations. If a disease such as Cystic Fibrosis prevents these channels from working sufficiently not enough bicarbonate ions will be pumped out which will decrease the amount of fluid flow. The reduced fluid flow will be much more viscous and will cause enzymes to precipitate out and block the ducts. Carbonic Anhydrase is an enzyme in the ductal cells which increase the amount of HCO3- available, it catalyses the reaction of CO2 and H20 to form H2CO3. The H+ ion then dissociates from the H2CO3 to form the HCO3-. The excess H+ ions are pumped out of the ductal cells into the blood to neutralise the alkaline tide create by oxyntic cells found in the stomach (Ref to Fig.3).
Fig.3. A diagram to show the modification of primary secretion in ductal cells of salivary striated ducts and pancreatic intercalated ducts.
Salivary Ductal Cell
Pancreatic Ductal cell
The Composition of Secretion Due to Flow Fate
In salivary glands the larger the flow rate of saliva through ducts the greater its tonicity (less hypotonic), plus it becomes more basic (Ref to Fig 4). This is because the ion transporters proteins and channels cannot act on the primary secretions sufficiently as the saliva is moving through the duct lumen too quickly. When flow rate slows down more Na+ can be reabsorbed from the lumen.
Fig.4. Graphs to compare the average composition of saliva against flow rate and the composition of plasma.
In the pancreas the levels of Na+ and K+ remains the same as flow rate increases but the concentrations of Cl- ions decreases and HCO3- increases (Ref to Fig 5). This helps when there is a high consumption of food. When the amounts of food in the stomach increases, the mechanoreceptors detect the distension so there will be more gastrin secreted into the blood. Gastrin increases the amount of acid secreted in stomach as well as increasing the pancreatic secretions. Higher rate of pancreatic flow through acinus which will give the pancreatic juice an excessive amount of HCO3- secreted into it, facilitating the neutralisation of the large quantity of acid.
Fig.5. A graph to show the composition of ions in pancreatic juice against flow rate
In conclusion the structure of salivary unit and the transporter proteins it contains are well coordinated to produce a final secretion that is sufficient for its role in the body. The primary secretion produced by the salivary gland and the exocrine pancreas are very similar in that they are rich in NaCl and isotonic to plasma, but the modification of the primary secretion produced by ductal cells differ. The transporter proteins in the pancreas and salivary glands are virtually the same but the way they are arranged and due to the electrochemical gradients they function differently to produce a different final solution. Flow rate in both structures affect the final outcome of the product. In salivary glands the faster the flow rate the less hypotonic the solution and in pancreas the faster the flow rate the more bicarbonate ions it contains.