Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Abstract

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Fig. 1. The acinar and ductal segments of secretory glands and fluid and electrolyte secretory functions

The figure illustrates the relationships between the acinar and ductal portion of secretory glands. The acini secrete isotonic fluid with NaCl as the major salt. The fluid passes through the centroacinar cells to the duct. The pancreatic duct absorbs the Cl⁻ and secretes HCO₃⁻ and most of the water in the pancreatic juice. Active HCO₃⁻ secretion drives fluid secretion. The salivary duct absorbs both the Cl⁻ and the Na⁺ and secretes HCO₃⁻ and K⁺. ENaC is expressed in the salivary glands, but not in the pancreatic duct, and is the main pathway for Na⁺ absorption by the salivary glands duct.

Fig. 2. A model depicting the mechanism of acinar cells fluid and electrolyte secretion

Shown are the major transporters in the basolateral and luminal membranes of acinar cells and their regulation. The major Cl⁻ loading transporter at the basolateral membrane is NKCC1, with part of the Cl⁻ loading (about 30%) provided by the parallel functioning of NHE1 and AE2. The membrane potential is determined by two Ca²⁺-activated K⁺ channels, the MaxiK and mIK1 channels. TMAM16a/Ano1 is the major Ca²⁺-activated Cl⁻ channel at the luminal membrane that also expresses the water channel AQP5. Fluid and electrolyte secretion by acinar cells is regulated by Ca²⁺-mobilizing receptors and is a Cl⁻ secretion-driven process. The receptor-evoked [Ca²⁺]_i increase initiates at the apical pole where the Ca²⁺ signaling complexes are located to activate TMEM16a/Ano1. The Ca²⁺ signal then propagates to the basal pole to activate the K⁺ channels. The Ca²⁺-mediated channels activation results in luminal Cl⁻ efflux and basolateral K⁺ efflux. Na⁺ then flows through the tight junction to the luminal space. The secretion of NaCl leads to water efflux through AQP5 and cell shrinkage. Cell shrinkage reduces [Ca²⁺]_i to inhibit the Cl⁻ and K⁺ channels and at the same time activates the volume-sensitive NKCC1 (and NHE1 and AE2) to restore cell Cl⁻ and K⁺. The cycle repeats itself during each spike of Ca²⁺ oscillations.

Fig. 3. A model depicting the mechanism of ductal fluid and HCO₃⁻ secretion

Shown are the major transporters in the basolateral and luminal membranes of duct cells. In the pancreas ductal secretion is driven by HCO₃⁻ secretion. The major HCO₃⁻ loading mechanism is the basolateral Na⁺-HCO₃⁻ cotransporter NBCe1-B. Luminal HCO₃⁻ secretion is mediated by the CFTR-Slc26a6 complex. The duct also expresses the HCO₃⁻ salvage mechanisms NHE3 and NBCn1-A. The salivary glands, but not the pancreatic, duct also expresses ENaC at the luminal membrane. The functioning of the transporters in ductal fluid and HCO₃⁻ secretion is illustrated in Fig. 6.

Fig. 4. Ductal proteins with PDZ ligands that participate in fluid and HCO₃⁻ secretion

All receptors that stimulate ductal secretion and key transporters that mediate ductal fluid and HCO₃⁻ secretion have PDZ ligands, highlighting the key role of PDZ scaffolds in ductal function. See text for details.

Fig. 5. The IRBIT/PP1 and WNK/SPAK pathways in ductal function

IRBIT is a key regulator of ductal fluid and HCO₃⁻ secretion that regulates both the resting and stimulated states of ductal secretion. PZD scaffolds assemble a basolateral membrane complex composed of NBCe1-B, the WNK/SPAK kinases and IRBIT (which can recruit the phosphatase PP1 to the complex). Similar complex exists in the luminal membrane with CFTR and perhaps Slc26a6 as the major transporters. In the resting state the WNK/SPAK kinases phosphorylate all transporters to reduce their surface expression and thus activity. Part of IRBIT is sequestered by IP₃Rs and part is bound to NHE3 and NBCn1-A to activate them and thus affect HCO₃⁻ salvage. Upon cell stimulation IRBIT recruits PP1 to the complexes, which overrides the function of the WNK/SPAK pathway and dephosphorylates the HCO₃⁻ secreting transporters to increase their surface expression. IRBIT then binds and neutralizes the effect of the HCO₃⁻ secreting transporters inhibitory domains. The combined effects stabilize the secretory state of the duct. The mutual stimulation of CFTR and Slc26a6 in the complex further augments ductal secretion.

Fig. 6. A model of pancreatic duct fluid and HCO₃⁻ secretion

Ductal fluid and HCO₃⁻ secretion is a two stage process. In the proximal duct IRBIT antagonizes the effect of the WNK/SPAK pathway to stimulate ductal secretion. HCO₃⁻ accumulates in the duct cytosol by NBCe1-B and exits into the lumen mostly by Slc26a6, which mediates 1Cl⁻/2HCO₃⁻ exchange, with CFTR recycling the Cl⁻. This result with osmotic secretion of HCO₃⁻ and together with transcellular Na⁺ fluxes through the paracellular pathway drives fluid secretion. The water is secreted by AQP1. The proximal duct thus absorbs part of the Cl⁻ and secretes as much as 100 mM HCO₃⁻ to secret large fraction of the fluid in the pancreatic juice. As the fluid arrives the more distal portions of the duct, the reduced luminal Cl⁻ and activated CFTR results in intracellular Cl⁻ concentration ([Cl⁻]_i) of less than 10 mM. The low [Cl⁻]_i activates WNK1 that phosphorylates SPAK/OSP1, which, in turn, acts on CFTR to change its Cl⁻/HCO₃⁻ selectivity, converting it primarily a HCO₃⁻ channel. At the same time the WNK/SPAK pathway inhibits the function of Slc26a6 to prevent HCO₃⁻ re-absorption. HCO₃⁻ efflux by CFTR thus determines the final HCO₃⁻ concentration in the secreted fluid.