The signal transduction cascade that regulates fluid secretion may be briefly summarised as: Binding of ACh to muscarinic ACh receptors increases intracellular IP3 levels. IP3 causes Ca2+ release from intracellular stores and the increase in intracellular [Ca2+] triggers fluid secretion. The remaining question is how? How does an increase in [Ca2+] trigger fluid secretion.
Fluid secretion is driven by electrolyte secretion, so the trick is to secrete the electrolytes. Acinar cells concentrate Cl- within themselves which they release across the apical membrane in response to an increase in [Ca2+]. The secretion of Cl- leads to Na+ secretion and together NaCl secretion drags water across the cells by osmosis.
The Na+/K+ ATPase makes direct use of ATP to pump Na+ out of the cell and create an inwardly directed Na+ gradient. This energises the Na+/K/2Cl- cotransport system (1) which in turn concentrates Cl- above its electrochemical potential (2). Increased [Ca2+]i opens the Ca2+-dependent K+ and Cl- channels and Cl- crosses the apical membrane into the lument of the acinus (2). Na+ follows Cl- across the cell to maintain electroneutrality and the resultant osmotic gradient moves water (3)
Fluid secretion is inevitably a process with multiple steps because biological systems cannot actively transport fluid as such. The only way of moving fluid rapidly across a tissue is by osmosis.
Fluid secreting tissues, including salivary acinar cells, concentrate electrolytes by active transport and the concentration gradient forces water to move.
Acinar cells use active transport to increase concentrate Cl- concentration inside the cell so that activation of an apical membrane Cl- channel allows Cl- to leave down its electrochemical gradient into the lumen of the acinus. Na+ crosses the acinar cells to maintain electroneutrality and the movement of Na+ and Cl- create the osmotic gradient across the tissue and water follows.
The pivotal step, the single step that determines whether or not a cell is secreting is activation of the apical membrane Cl- channel. This step is regulated by increased [Ca2+]i. A cell wide increase in [Ca2+]i will also activate the basolateral K+ channel which keeps the membrane potential at a high negative value and thus preserves the driving force for Cl- efflux.
Electrolyte led fluid transport movement is always isotonic. Once isotonicity is reached, there is no additional driving force for water movement.
Try the PowerPoint animation to see the whole stimulus-secretion signal transduction cascade.
An osmotic driving force cannot drive water secretion unless the water can cross the epithelium.
There are two possible routes for water to take across the cell, either through the tight junctions between the cells (paracellular) or across both the apical and basolateral membranes (transcellular).
There has been much discussion as to which is the dominant route and little evidence to distinguish absolutely between them.
The intrinsic water permeability of the plasma membrane is very low and both apical and basolateral membranes must therefore contain water channels to facilitate transcellular water transport. Water channels in salivary acinar cells are members of the aquaporin (AQP) family. Aquaporins are membrane proteins composed of 4 subunits, each of which has 6 membrane spanning domains that form a water permeable pore. Aquaporins come in two types, one of which transports only water and another which is also permeable to glycerol. Neither type conducts ions.
There are at least 10 mammalian aquaporin isoforms and AQP5 has been localised to the apical membrane of salivary gland acinar cells. AQP5 knockout mice show a 60% reduction in stimulated flow in airway mucosal glands which would suggest that at least this proportion of water flow is transcellular.
The ability of salivary glands to generate an hypotonic saliva lies with the striated ducts. Striated duct cells pump electrolytes from the primary saliva by active transport. At first sight, it might seem that will simply reverse the secretory process, but the striated ducts are impermeable to water, so there can be no osmotically driven water reabsorption.
The basic outline of this secretory process was identified as the 'two stage hypothesis' by Thaysen et al in 1954. The fluid secretory process in the acinar cells has a much greater capacity than the electrolyte reabsorptive process in the ducts. This is why the composition of saliva changes with flow rate. At low, resting, flow rates, saliva moves slowly through the ducts and the striated ducts are able to substantially modify the composition of the saliva. At high, stimulated, flow rates, the saliva passes rapidly through the ducts with little alteration. The composition of saliva at high flow rates more closely resembles that of the primary saliva produced by the acinar cells.