SUIMMARY1. Fluid transport rate and oxygen consumption (Qo2) were studied in rabbit gall-bladder preparations in vitro exposed on both sides to identical Ringer solutions with NaCl concentrations (and osmolarities) varying from 70 to 140 m-equiv Na+/l. (and 173-313 m-osmole/l.).2. The time sequence of acute effects on transport rate resulting from sudden changes in the NaCl concentration of the bathing solutions indicated that, (a) as a primary effect, fluid volume transfer rate remained unaffected whereas Na transport rate changed abruptly in direct proportion to the Na concentration of the bathing media; (b) a secondary, delayed and partly reversible depression of fluid transfer rate following elevation of the NaCl concentration was observed only when the rate of transport was relatively high initially. solutions, respectively. 5. Removal of K from the bathing solutions was followed by a gradual and partly reversible depression of fluid transport rate to a minimum level (about 100 x 10 4 #1 H20. min' . mg-') independent of the initial transport rate.6. It is concluded that the range of absorption rates of isosmotic fluid * Graduate student research scholar.0. FREDERIKSEN AND PAUL P. LE YSSAC from the gall-bladder lumen represents a range of energy requiring capacities for transfer of fluid volume units; the data suggest that the intracellular (cytoplasmic) ion composition, depending on the presence of external K, as well as hormonal action may influence the capacity of the transcellular fluid transport mechanism. 7. A model (a 'mechanical volume pump') for transcellular transfer of fluid volume units, allowing for flexible specificity with regard to the actively transported solutes, and requiring the presence of Na+ and Cl-, is proposed.

In the present study, we describe a novel three-dimensional airway epithelial explant preparation and demonstrate its use for ion transport studies by electrophysiological technique. Suspension cultures of sheets of epithelial cells released by protease treatment from cystic fibrosis (CF) and non-CF nasal polyps developed free-floating, monolayered epithelial spheres, with the apical, ciliated cell membrane facing the bath and the basolateral cell membrane pointing toward a fluid-filled lumen. Microelectrode impalement of both non-CF and CF spheroids revealed lumen-positive transepithelial electrical potential differences (PDs) that were inhibited by amiloride, indicating that the spheroids were inflated due to amiloride-sensitive Na+absorption followed by water. Transformation to a Cl− secretory state was achieved by addition of ATP to the bath, leading to the development of a diphenylamine-2-carboxylate-sensitive PD. A cAMP-induced increase in PD was seen in non-CF spheroids only. In response to hydrocortisone treatment, Na+ transport reflected by amiloride-sensitive PD increased and more so in CF than in non-CF spheres. We concluded that this preparation is a useful model for the airway surface epithelium and is suitable for studies of transport mechanisms and regulation.

Net ion transports in rabbit nasal airway epithelium (RNAE) were estimated from unidirectional fluxes of 22Na+, 36Cl-, and 86Rb+ (K+ tracer), short-circuit current (Isc), and epithelial conductance (Gt) under short-circuit conditions in excised parallel RNAE from the two sides of the nasal septum mounted in Ussing chambers at 37 degrees C. Net Na+ absorption (JNa; 76 nmol.min-1.cm-2) was nearly equal to the net charge flux (Jnet), equivalent to the control Isc of 137 microA/cm2 (Jnet = Isc/zF = 85 neq.min-1.cm-2). Secretions of Cl- (9 nmol.min-1.cm-2) and K+ (Rb+) (1.2 nmol.min-1.cm-2) were small. Intra-animal variations between right- and left-side Isc values were small compared with large interanimal variations, suggesting long-term regulation of JNa. Serosal ouabain (10(-4) M) abolished Isc. Mucosal amiloride (10(-4) M) maximally inhibited Isc by 68% and JNa by 78%, abolished K+ (Rb+) secretion, increased Cl- secretion slightly (to 16 nmol.min-1.cm-2), and decreased control Gt (13.4 mS/cm2) by 26%. Amiloride-insensitive JNa was inhibited approximately 50% by mucosal hydrochlorothiazide (10(-4) M) but not by mucosal bumetanide, phloridzine, or ethoxzolamide. The Cl- secretion was abolished by serosal bumetanide (10(-4) M) and also by mucosal diphenylamine-2-carboxylate (2.5.10(-4) M) or bumetanide (10(-4) M). Serosal Ba2+ (2 mM) inhibited JNa and increased K+ (Rb+) secretion. The latter was blocked by mucosal Ba2+. Passive Cl- (but not Na+) fluxes varied proportionally with Gt and were approximately four times higher than passive Na+ fluxes, suggesting 1) significant anion selectivity of a low-resistance paracellular pathway and 2) separate routes for paracellular Cl- and Na+ fluxes. We conclude that RNAE is a suitable model organ for studies of regulation of JNa in native human airway epithelia.

4. The juxtaglomerular cells were four to five times more sensitive to changes in osmolarity through sucrose than sodium chloride concentration. Changes in potassium chloride concentration (7-57 mM) had little effect.5. Sodium chloride had no direct ionic effect on renin release outside its osmotic properties.6. The findings support a previous proposal that the rate of renin release in vitro relates directly to the volume of the juxtaglomerular cell. The hypothesis is developed that a similar mechanism may underlie renin secretion in vivo.

We investigated purinergic receptors involved in ion transport regulation in the intact rabbit nasal airway epithelium. Stimulation of apical membrane P2Y receptors with ATP or UTP (200 microM) induced transient increases in short-circuit current (Isc) of 13 and 6% followed by sustained inhibitions to 8 and 17% below control level, respectively. Serosal application of nucleotides had no effect. The ATP-induced response appeared to involve additional activation of apical adenosine (P1) and P2X receptors. The inhibitory effect of ATP and UTP on Isc was eliminated by pretreatment with amiloride (100 microM), while the stimulatory effect was potentiated, indicating that ATP and UTP inhibit Na+ and stimulate Cl- current. Ionomycin (1 microM) induced responses similar to UTP and ATP and desensitized the epithelium to the nucleotides, indicating involvement of intracellular Ca2+ (Ca2+ i. Furthermore, ATP, UTP and ionomycin induced 21, 24, and 21% decreases, respectively, in transepithelial conductance. Measurements of unidirectional isotope fluxes showed a 39% decrease in the dominant net Na+ absorption in response to ATP, while the smaller net Cl- secretion increased only insignificantly and unidirectional Cl- fluxes decreased significantly. The results suggest that nucleotides released to the airway surface liquid exert an autocrine regulation of epithelial NaCl absorption mainly by inhibiting the amiloride-sensitive epithelial Na+ channel (ENaC) and paracellular anion conductance via a P2Y receptor-dependent increase in Ca2+ i, while stimulation of Cl- secretion is of minor importance.

Airway epithelium explants from cystic fibrosis (CF) patients and non-CF subjects formed monolayered spheres, with the apical ciliated cell membrane facing the bath and the basolateral cell membrane pointing toward a fluid-filled lumen. With the use of two microelectrodes, transepithelial potential difference and changes in potential difference in response to passage of current pulses were recorded, and epithelial resistance and the equivalent short-circuit current were calculated. Non-CF control potential difference and short-circuit current values were significantly lower than the CF values, and amiloride inhibited both values. Fluid transport rates were calculated from repeated measurements of spheroid diameters. The results showed that 1) non-CF and CF spheroids absorbed fluid at identical rates (4.4 microl x cm(-2) x h(-1)), 2) amiloride inhibited fluid absorption to a lower residual level in non-CF than in CF spheroids, 3) Cl(-)-channel inhibitors increased fluid absorption in amiloride-treated non-CF spheroids to a level equal to that of amiloride-treated CF spheroids, 4) hydrochlorothiazide reduced the amiloride-insensitive fluid absorption in both non-CF and CF spheroids, and 5) osmotic water permeabilities were equal in non-CF and CF spheroids ( approximately 27 x 10(-7) cm x s(-1) x atm(-1)).

Osmotic water permeability (P(f )) was measured in spheroid-shaped human nasal airway epithelial explants pre-exposed to increasing levels of hyperosmotic stress. The fluid-filled spheroids, derived from nasal polyps, were lined by a single cell layer with the ciliated apical cell membrane facing the outside. The P(f ) was determined from diameter changes of the spheroids in response to changes in bathing medium osmolarity forth and back between 300 and 225 mOsm x l(-1). Continuous diameter measurements also allowed determination of spontaneous fluid absorption. Hyperosmotic pretreatment (increase from 300 up to 600 mOsm x l(-1)) caused a time- and osmolarity-dependent increase (up to approximately 1.5 times) in epithelial P(f ) which was of similar magnitude in cystic fibrosis (CF) and non-CF spheroids. The effect saturated at approximately 450 mOsm x l(-1) and at approximately 24 h. Expression of aquaporin-5 (AQP5), studied by immunofluorescence and confocal microscopy, showed an increase in parallel with the increase in P(f ) following hyperosmotic stress. The AQP5 was localized both in cytoplasmic vesicles and in apical cell membranes. Spontaneous fluid absorption rates were equal in CF and non-CF spheroids and were not significantly influenced by hyperosmotic stress. The results suggest that hyperosmotic stress is an important activator of AQP-5 in human airway epithelium, leading to significantly increased transepithelial water permeability.

Osmotic water permeability (P(f)) was studied in spheroid-shaped human airway epithelia explants derived from nasal polyps by the use of a new improved tissue collection and isolation procedure. The fluid-filled spheroids were lined with a single cell layer with the ciliated apical cell membrane facing the outside. They were capable of surviving hours of experiment involving continuous superfusion of the bathing medium and changes of osmolarity. A new image analysis technique was developed for measuring the spheroid diameters, giving high time and measurement resolutions. The transepithelial P(f), determined by the changes of the apical solution osmolarity, was not influenced by the presence of glucose, Na(+), or Na(+)/glucose-cotransport inhibitors in the bath, but was sensitive to the aquaporin (AQP) inhibitor HgCl(2). The measured P(f) levels and the values of activation energy were in the range of those seen in AQP-associated water transport. Together, these results indicate the presence of an AQP in the apical membrane of the spheroids. Notably, identical values for P(f) were found in CF and non-CF airway preparations, as was the case also for the calculated spontaneous fluid absorption rates.