The epithelial cells of the choroid plexuses secrete cerebrospinal fluid (CSF), by a process which involves the transport of Na + , Cl -and HCO 3 -from the blood to the ventricles of the brain. The unidirectional transport of ions is achieved due to the polarity of the epithelium, i.e. the ion transport proteins in the blood-facing (basolateral) membrane are different to those in the ventricular (apical) membrane. The movement of ions creates an osmotic gradient which drives the secretion of H 2 O. A variety of methods (e.g. isotope flux studies, electrophysiological, RT-PCR, in situ hybridization and immunocytochemistry) have been used to determine the expression of ion transporters and channels in the choroid plexus epithelium. Most of these transporters have now been localized to specific membranes. For example, Na + -K + ATPase, K + channels and Na + -2Cl --K + cotransporters are expressed in the apical membrane. By contrast the basolateral membrane contains Cl --HCO 3 exchangers, a variety of Na + coupled HCO 3 -transporters and K + -Cl -cotransporters. Aquaporin 1 mediates water transport at the apical membrane, but the route across the basolateral membrane is unknown. A model of CSF secretion by the mammalian choroid plexus is proposed which accommodates these proteins. The model also explains the mechanisms by which K + is transported from the CSF to the blood. Keywordschoroid plexus; blood-cerebrospinal fluid barrier; epithelial cells; ion transport; ion channels; Na + -K + ATPaseThe cerebrospinal fluid (CSF) is a major part of the extracellular fluid of the CNS. The CSF fills the ventricles of the brain, the spinal canal and the subarachnoid space ( Fig. 1), and in humans has a total volume of approximately 140 ml. The CSF is separated from neuronal tissue by the ependyma (which lines the ventricles and canals), and the pia (which covers the external surface of the brain). The composition of the CSF does, however, influence neuronal activity, notably in the central chemoreceptors of the medulla oblongata which control respiration by responding to changes in CSF pH.The CSF has a number of important functions. It helps provide mechanical support for the brain, i.e. the brain "floats" in the CSF reducing its effective weight by more than 60% (Segal, 1993). CSF also acts as a drainage pathway for the brain, by providing a "sink" into which products of metabolism or synaptic activity are diluted and subsequently removed * Corresponding author. Tel: +44-161-275-5463; fax: +44-161-275-5600. E-mail address: peter.d.brown@man.ac.uk (P. D. Brown).. Abbreviations:CFTRcystic fibrosis transmembrane conductance regulator CSF cerebrospinal fluid. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts (Segal, 1993). The CSF may also be an important route by which some nutrients reach the CNS . A final putative role for the CSF is that it acts as a route of communication within the CNS, i.e. it carries hormones and transmitters between different areas of the brain ....

The role of aquaporins in cerebrospinal fluid (CSF) secretion was investigated in this study. Western analysis and immunocytochemistry were used to examine the expression of aquaporin 1 (AQP1) and aquaporin 4 (AQP4) in the rat choroid plexus epithelium. Western analyses were performed on a membrane fraction that was enriched in Na(+)/K(+)-ATPase and AE2, marker proteins for the apical and basolateral membranes of the choroid plexus epithelium, respectively. The AQP1 antibody detected peptides with molecular masses of 27 and 32 kDa in fourth and lateral ventricle choroid plexus. A single peptide of 29 kDa was identified by the AQP4 antibody in fourth and lateral ventricle choroid plexus. Immunocytochemistry demonstrated that AQP1 is expressed in the apical membrane of both lateral and fourth ventricle choroid plexus epithelial cells. The immunofluorescence signal with the AQP4 antibody was diffusely distributed throughout the cytoplasm, and there was no evidence for AQP4 expression in either the apical or basolateral membrane of the epithelial cells. The data suggest that AQP1 contributes to water transport across the apical membrane of the choroid plexus epithelium during CSF secretion. The route by which water crosses the basolateral membrane, however, remains to be determined.

The epithelial cells of the choroid plexus secrete cerebrospinal fluid (CSF), by a process that involves the movement of Na+, Cl− and HCO3− from the blood to the ventricles of the brain. This creates the osmotic gradient, which drives the secretion of H2O. The unidirectional movement of the ions is achieved due to the polarity of the epithelium, i.e., the ion transport proteins in the blood‐facing (basolateral) are different to those in the ventricular (apical) membranes. Saito and Wright (1983) proposed a model for secretion by the amphibian choroid plexus, in which secretion was dependent on activity of HCO3− channels in the apical membrane. The patch clamp method has now been used to study the ion channels expressed in rat choroid plexus. Two potassium channels have been observed that have a role in maintaining the membrane potential of the epithelial cell, and in regulating the transport of K+ across the epithelium. An inward‐rectifying anion channel has also been identified, which is closely related to ClC‐2 channels, and has a significant HCO3− permeability. This channel is expressed in the apical membrane of the epithelium where it may play an important role in CSF secretion. A model of CSF secretion by the mammalian choroid plexus is proposed that accommodates these channels and other data on the expression of transport proteins in the choroid plexus. Microsc. Res. Tech. 52:49–59, 2001. © 2001 Wiley‐Liss, Inc.

We have investigated the interactions between intracellular pH (pHi) and the intracellular free calcium concentration ([Ca2+]i) in isolated rat pancreatic acinar cells. The fluorescent dyes fura‐2 and BCECF were used to measure [Ca2+]i and pHi, respectively. Sodium acetate and ammonium chloride (NH4Cl) were used to acidify and alkalinize pHi, respectively. Cytosolic acidification had no effect on [Ca2+]i in resting pancreatic acinar cells, whereas cytosolic alkalinization released Ca2+ from intracellular stores. Cytosolic acidification using either acetate or a CO2‐HCO3−‐buffered medium enhanced Ca2+ signals evoked by acetylcholine (ACh) and cholecystokinin (CCK). In contrast, both NH4Cl and trimethylamine (TMA) inhibited Ca2+ signals during stimulation with either ACh or CCK. This inhibitory effect was also observed in the absence of extracellular Ca2+, and was therefore not due to changes in Ca2+ entry. Calcium oscillations evoked by physiological concentrations of CCK were enhanced by cytosolic acidification and inhibited by cytosolic alkalinization. In order to determine the effects of pHi upon Ca2+ handling by intracellular Ca2+ stores, intraorganellar [Ca2+] was monitored using the low affinity Ca2+ indicator mag‐fura‐2 in permeabilized cells. Addition of NH4Cl, which is expected to alkalinize intraorganellar pH, did not alter intraorganellar [Ca2+] in permeabilized cells, suggesting that changing intraorganellar pH does not release Ca2+ from intracellular stores. Addition of NH4Cl or acetate also did not affect the rate of Ca2+ release induced by inositol 1,4,5‐trisphosphate (InsP3). Modification of extraorganellar (‘cytosolic’) pH did not affect the rate of ATP‐dependent Ca2+ uptake into stores, but did modify the rate of Ca2+ release evoked by submaximal concentrations of InsP3. The rate of Ca2+ release was increased at more alkaline extraorganellar pHs. These results would suggest that manipulation of intraorganellar pH does not affect Ca2+ handling by the intracellular stores. In contrast, extraorganellar (‘cytosolic’) pH does affect InsP3‐induced Ca2+ release from the stores. In conclusion, changes in intracellular pH in pancreatic acinar cells can profoundly alter cytosolic [Ca2+]. This may shed light on earlier observations whereby cell‐permeant weak acids and bases can modulate fluid secretion in epithelia.

1. We have investigated interactions between intracellular pH (pHJ) and the intracellular free calcium concentration ([Ca2+]i) in collagenase-isolated rat lacrimal acinar cells. The fluorescent dyes fura-2 and 2',7'-bis(carboxyethyl)-5-carboxyfluorescein (BCECF) were used to measure [Ca2+] Ca2+ from the intracellular agonist-sensitive Ca2+ pool. However, releasing stored Ca2+ via alkalinization does not appear to trigger significant Ca2+ entry, perhaps because intracellular alkalinization inhibits either the Ca2P entry pathway or the mechanism which couples the entry pathway to store depletion.The primary signal controlling fluid and electrolyte Nae and with changes in intracellular pH (see Nauntofte, secretion in exocrine acinar cells is an increase in the intra-1992, for review). The interactions between these changes in cellular free calcium concentration ([Ca2P]