Understanding salivary fluid and protein secretion

Abstract

Mammalian salivary glands are mainly composed of two epithelial cell types, the acinar cells that secrete the salivary fluid as well as most of the salivary proteins, and the ductal cells that secrete some protein and modify the ionic composition of the saliva as they convey it to the mouth. For a variety of reasons, including their physiological interest, their relative abundance and ease of preparation, and their robust stimulation-secretion responses, salivary acinar cells been intensively studied and many of their properties are well, albeit certainly not completely, understood. Although salivary ducts have also received considerable experimental attention, particularly over the past 10 years, less is known about their behavior. Because of this and because of the central role of the acinar cells in salivary fluid and protein secretion we will concentrate almost exclusively on them in this article. For more information on salivary ducts the interested reader is referred to several recent reviews and the references therein (7; 62; 6). It is also worth mentioning at this point that most of the information presented below has been obtained from the salivary glands of experimental animals (mainly rats and rabbits). Except for morphologic studies and several non-invasive procedures, such as the collection of saliva after the application of various stimuli, experiments using human salivary tissues are rare. However, in those cases where the functional properties of human salivary acini have been investigated they appear to conform well to the results and conclusions derived from animal studies. The easiest way to understand the proposed mechanisms of salivary fluid secretion is to consider a specific model. Figure 1 shows a schematic representation of a salivary acinar cell. The cell contains four ion transporters, the Na+/K+ adenosine triphosphatase (ATPase), a Na+–K+–2Cl− cotransporter and a Ca2+-activated K+ channel, all located in the basolateral membrane, and a Ca2+ activated Cl− channel located in the apical membrane. Fluid secretion is thought to arise from the concerted actions of these four transporters as follows. The Na+/K+ ATPase maintains intracellular Na+ concentration low and intracellular K+ concentration high relative to the interstitium by exchanging 3Na+ for 2K+ at the expense of cellular adenosine triphosphate (ATP). The Na+–K+–2 Cl− cotransporter (also known as NKCC1) is a secondary active transport system that transports 1Na+, 1K+ and 2Cl− into the cell in a tightly coupled fashion. Because of the extracellular to intracellular Na+ gradient maintained by the Na+/K+ ATPase, Cl− is likewise concentrated in the acinar cytoplasm above electrochemical equilibrium by NKCC1. In the resting (unstimulated) state intracellular Ca2+ concentration is low and the Ca2+ activated K+ and Cl− channels are therefore closed. But when the cell is stimulated by secretagogues (in situ, typically the muscarinic agonist acetylcholine) intracellular Ca2+ concentration rises and the K+ and Cl− channels open. These Ca2+ associated changes in K+ and Cl− conductance allow KCl to flow out of the cell resulting in the accumulation of Cl− ions and their associated negative electrical charge in the acinar lumen. Na+ is then thought to follow Cl− by leaking from the interstitium through the tight junctions between the cells to preserve electroneutrality, and the resulting osmotic gradient for NaCl causes a transepithelial movement of water from interstitium to lumen. In the continued presence of the secretagogue a net transepithelial Cl− flux and a concomitant secretion of fluid is sustained as a result of Cl− entry via NKCC1 and exit via the apical Cl− channel. When the stimulus is removed the intracellular Ca2+ concentration falls to resting levels, the K+ and Cl− channels close, and the cell returns to its resting state. Model for salivary fluid secretion based on active transepithelial Cl– transport and the osmotic coupling of salt and water fluxes (see text for details) As discussed in more detail below, considerable experimental evidence indicates that the mechanism presented in Figure 1 can account for most of the salivary secretion from rat, rabbit and presumably human, major salivary glands. However, there is also evidence that two alternate mechanisms based on the same osmotic coupling principle outlined in Figure 1 (viz., transepithelial anion transport driving the secretion of salt followed by osmotically obliged water) can also make significant contributions to salivary fluid secretion. The first of these differs in the Cl− entry step. Here, NKCC1 is replaced by a Cl−/HCO3− exchanger and a Na+/H+ exchanger (Figure 2a). In this model the decrease in intracellular Cl− concentration resulting from secretagogue-induced KCl loss leads to increased Cl− entry via the Cl−/HCO3− exchanger. The HCO3− is then replaced by the diffusion of CO2 into the cell and its conversion into HCO3− plus H+ by carbonic anhydrase (CA).