Beta-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

Distal lung epithelial cells isolated from fetal rats were cultured (48 h) on permeable supports so that transepithelial ion transport could be quantified electrometrically. Unstimulated cells generated a short-circuit current (I(sc)) that was inhibited (~80%) by apical amiloride. The current is thus due, predominantly, to the absorption of Na(+) from the apical solution. Isoprenaline increased the amiloride-sensitive I(sc) about twofold. Experiments in which apical membrane Na(+) currents were monitored in basolaterally permeabilized cells showed that this was accompanied by a rise in apical Na(+) conductance (G(Na(+))). Isoprenaline also increased apical Cl- conductance (G(Cl-)) by activating an anion channel species sensitive to glibenclamide but unaffected by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). The isoprenaline-evoked changes in G(Na(+)) and G(Cl(minus sign)) could account for the changes in I(sc) observed in intact cells. Glibenclamide had no effect upon the isoprenaline-evoked stimulation of I(sc) or G(Na(+)) demonstrating that the rise in G(Cl-) is not essential to the stimulation of Na(+) transport.

Oxygen-evoked Na+ transport in rat fetal distal lung epithelial cells

Monolayer cultures of rat fetal distal lung epithelial (FDLE) cells generated larger spontaneous short circuit currents (ISC) when maintained (48 h) at neonatal alveolar PO2 (100 mmHg) than at fetal PO2 (23 mmHg). When cells were shifted between these atmospheres in order to impose a rise in PO2 equivalent to that seen at birth, no rise in ISC was seen after 6 h but the response was fully established by 24 h. Studies of basolaterally permeabilised cells revealed a small rise in apical Na+ conductance (GNa) 6 h after PO2 was raised but no further change had occurred by 24 h. A substantial rise was, however, seen after 48 h. Reporter gene assays showed that no activation of the -ENaC (epithelial Na+ channel -subunit) promoter was discernible 24 h after PO2 was raised but increased transcriptional activity was seen at 48 h. Studies of apically permeabilised cells showed that a small rise in Na+ pump capacity was evident 6 h after PO2 was raised and, in common with the rise in ISC, this effect was fully established by 24 h. The rise in ISC thus develops 6-24 h after PO2 is raised and is due, primarily, to increased Na+ pump capacity. The increase in GNa thus coincides with activation of the -ENaC promoter but these effects occur after the rise in ISC is fully established and so cannot underlie this physiological response. The increased transcription may be an adaptation to increased Na+ transport and not its cause.

NF-kappaB blockade reduces the O2-evoked rise in Na+ conductance in fetal alveolar cells

Electrophoretic mobility shift assays revealed minimal levels of NF-kappaB activity in rat distal lung epithelial cells cultured at fetal (23 mmHg) or adult alveolar (100 mmHg) P(O2), but revealed significant activation of this transcription factor in cells exposed to a rise in P(O2) mimicking that experienced at birth. This response was entirely abolished by pretreating cells with 5 mM sulfasalazine (SSA). This shift in P(O2) also evoked a rise in apical Na+ conductance (G(Na+)) that may underlie the O2-evoked stimulation of Na+ transport seen in these cells. Pretreatment with SSA had no effect upon G(Na+) in cells cultured continually at adult or fetal P(O2) but did inhibit the increase in G(Na+) seen in cells that had experienced the rise in P(O2). O2-evoked activation of NF-kappaB may thus mediate the increased Na+ transport that occurs when the distal lung epithelial cells are exposed to a physiologically-relevant increase in P(O2).

Detection of Cl- flux in the apical microenvironment of cultured foetal distal lung epithelial cells

A self-referencing Cl--selective microelectrode (Cl- SrE) was developed and used to detect changes in the direction and magnitude of the Cl- flux (J(Cl)) from the apical region of cultured foetal distal lung epithelial cells (FDLEs) as a function of external Cl- concentration ([Cl-]e) and in response to pharmacological challenges. The technique, which is similar to that developed for other ion-selective microelectrodes, centres on the oscillation of a Cl--selective microelectrode between known points, micrometres apart, orthogonal to the plasma membrane. Application of the Fick principle to the differential voltage obtained per excursion amplitude (the referenced signal) yields the Cl- flux (pmol × cm(−2) × s(−1)). A Cl- effusion gradient was used to confirm that empirical measurements of J(Cl) using the Cl- SrE were statistically similar to predicted flux values calculated from the fall in [Cl-] with distance from the tip of the effusion source. Apical J(Cl) was then measured as a function of [Cl-]e from polarised FDLE cultures grown on permeable supports. At [Cl-]e<50 mmol × l(−1), an apical-to-basolateral (inward) flux, maximal at 400 pmol × cm(−2) × s(−1), was observed; this reverted to a continuous basolateral-to-apical (outward) flux of 203 pmol × cm(−2) × s(−1) at [Cl]e>100 mmol × l(−1). At [Cl-]e>100 mmol × l(−1), isoproterenol (basolaterally applied, 10 micromol × l(−1)) activated a Cl- influx of 561 pmol × cm(−2) × s(−1), whereas UTP (apically applied, 100 micromol × l(−1)) stimulated a Cl- efflux of 300 pmol × cm(−2) × s(−1). In all cases, 50–70 % of J(Cl) was abolished by Cl- channel blockade using 10 micromol × l(−1) diphenylamine-2-carboxylic acid (DPC) or 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). We conclude that the Cl- SrE resolves a Cl- gradient in the microenvironment of the apical region of lung epithelia that varies in both direction and magnitude as a function of external [Cl-]e and in response to Cl- channel blockade and to beta2 adrenoreceptor and P2Y receptor agonists.

Multiple P2Y receptor subtypes in the apical membranes of polarized epithelial cells

Apical ATP, ATP, UTP and UDP evoked transient increases in short circuit current (I(SC), a direct measure of transepithelial ion transport) in confluent Caco-2 cells grown on permeable supports. These responses were mediated by a population of at least three pharmacologically distinct receptors. Experiments using cells grown on glass coverslips showed that ATP and UTP consistently increased intracellular free calcium ([Ca(2+)](i)) whilst sensitivity to UDP was variable. Cross desensitization experiments suggested that the responses to UTP and ATP were mediated by a common receptor population. Messenger RNA transcripts corresponding to the P2Y(2), P2Y(4) and P2Y(6) receptors genes were detected in cells grown on Transwell membranes by the reverse transcriptase - polymerase chain reaction. Identical results were obtained for cells grown on glass. Experiments in which I(SC) and [Ca(2+)](i) were monitored simultaneously in cells on Transwell membranes, confirmed that apical ATP and UTP increased both parameters and showed that the UDP-evoked increase in I(SC) was accompanied by a [Ca(2+)](i)-signal. Ionomycin consistently increased [Ca(2+)](i) in such polarized cells but caused no discernible change in I(SC). However, subsequent application of apical ATP or UTP evoked a small rise in I(SC) but no rise in [Ca(2+)](i). UDP evoked no such response. As well as evoking increases in [Ca(2+)](i), the ATP/UTP-sensitive receptors present in Caco-2 cells thus allow direct control over ion channels in the apical membrane. The UDP-sensitive receptors, however, appear to simply evoke a rise in [Ca(2+)](i).

Effect of luminal nucleotides on Cl- secretion and Na+ absorption in distal bronchi

P2Y receptor agonists stimulate Cl- secretion across both normal and cystic fibrosis (CF) airway epithelia, and therefore have potential for use in the treatment of CF. Although CF pathology is manifest primarily in the distal airways, most studies of P2Y-receptor-mediated airway epithelial ion transport have used cells cultured from proximal regions. Here we report the results of studies of P2Y-receptor-mediated ion transport in distal bronchi isolated from porcine lungs, cannulated and perfused. A luminal microelectrode was used to record transepithelial potential difference (PD) and cable analysis was applied to determine resistance (Rt) and equivalent short-circuit current (I(SC)). Luminal UTP (100 micromol/l) transiently hyperpolarized PD (from -5.8+/-0.3 to -6.5+/-0.4 mV) and increased I(SC) (from 47+/-6 to 55+/-8 microA cm(-2)) before inhibiting PD to below the pre-UTP level (-5.0+/-0.4 mV). The decline was attenuated by pretreatment with amiloride, and additional treatment with bumetanide inhibited the initial hyperpolarization, suggesting that UTP stimulates Cl- secretion and inhibits Na+ absorption across distal bronchi. Luminal addition of P2Y1 [ADP, 2-methylthio-ATP (2MeSATP)] and P2Y6 (UDP) receptor agonists had no effect on ion transport. Pretreatment with thapsigargin (0.3 micromol/l) did not prevent the UTP-induced increase in PD and I(SC) but attenuated the secondary fall in PD. Pretreatment with BAPTA/AM (50 micromol/l), however, had no effect on the response to UTP. Additional studies of isolated bronchial epithelial cells using Fura-2 showed that UTP increases [Ca2+]in, and this increase is blocked by pretreatment with thapsigargin. These results suggest that in intact distal bronchi luminal UTP stimulates Cl- secretion by a Ca2+-independent mechanism and inhibits Na+ absorption by a Ca2+-dependent mechanism. Both effects are likely to favour increased hydration of the airway surface, and may therefore be beneficial in CF.

P2Y2 receptor-mediated inhibition of ion transport in distal lung epithelial cells

1 Rat foetal distal lung epithelial cells were plated onto permeable supports where they became integrated into epithelial sheets that spontaneously generated short circuit current (ISC). 2 Apical ATP (100 microM) evoked a transient fall in ISC that was followed by a rise to a clear peak which, in turn, was succeeded by a slowly developing decline to a value below control. Apical UTP evoked an essentially identical response. 3 UDP and ADP were ineffective whilst ATP had no effect when added to the basolateral solution. These effects thus appear to be mediated by apical P2Y2 receptors. 4 The rising phase of the responses to ATP/UTP was selectively inhibited by anion transport inhibitors but persisted in the presence of amiloride, which abolished the inhibitory effects of both nucleotides. Thus, apical nucleotides appear to evoke a transient stimulation of anion secretion and sustained inhibition of Na+ absorption. 5 Basolateral isoprenaline (10 microM) elicited a rise in ISC but subsequent addition of apical ATP reversed this effect. Conversely, isoprenaline restored ISC to its basal level following stimulation with ATP. Apical P2Y2 receptors and basolateral beta-adrenoceptors thus allow their respective agonists to exert mutually opposing effects on ISC.

Modulation of the permeability of H2 receptor antagonists cimetidine and ranitidine by P-glycoprotein in rat intestine and the human colonic cell line Caco-2

The influence of secretory transporters on intestinal permeability characteristics of the H2 receptor antagonists ranitidine and cimetidine was studied in Caco-2 monolayers and rat intestinal mucosa mounted in Ussing chambers. Both drugs exhibited vectorial transport across rat ileum with significantly greater (2-4-fold) permeability in the serosal-to-mucosal than the mucosal-to-serosal direction, indicative of net mucosal secretion. Mucosal ranitidine secretion was also observed in rat distal colon, although to a lesser degree. Ileal ranitidine secretion was concentration dependent and significantly reduced by the P-glycoprotein (P-gp) substrates verapamil and cyclosporin. In contrast, probenicid, an inhibitor of the multidrug-related protein, had no effect on ranitidine permeability. The paracellular marker mannitol showed no evidence of asymmetric permeability or sensitivity to P-gp inhibitors. Significant expression of P-gp protein in rat intestinal epithelial cells was confirmed by immunoblotting. Caco-2 monolayers, which overexpress P-gp, also showed asymmetric permeability of ranitidine and cimetidine. In this model, ranitidine permeability in the mucosal-to-serosal direction decreased by approximately 95% as monolayer resistance increased from 150 to 500 Omega/cm2, indicating a primarily paracellular route of transport. However, serosal-to-mucosal permeability was insensitive to resistance changes, consistent with a primarily transcellular route in this direction. These data indicate that ranitidine and cimetidine can act as substrates for intestinal P-gp and suggest that the balance between absorptive and secretory mechanisms as a factor in determining intestinal absorption needs to be a routine consideration even for compounds expected to have a predominantly paracellular route of absorption.

Culture substrate-specific expression of P2Y2 receptors in distal lung epithelial cells isolated from foetal rats

ATP and UTP did not evoke [Ca2+]i signals in rat foetal lung epithelial cells grown on glass but elicited clear responses in cells grown into functionally polarised epithelia on permeable supports. Moreover, P2Y2 receptor mRNA could not be detected in cells on glass by the polymerase chain reaction but this mRNA species was clearly expressed by polarised cells. P2Y2 receptor expression thus appears to be a feature of the polarised phenotype.

Influence of morphometric factors on quantitation of paracellular permeability of intestinal epithelia in vitro

PURPOSE

The relative contribution of the small and large intestine to paracellular absorption is a subject of some controversy. Direct comparison of paracellular permeability in different epithelia is complicated by variations in junctional density and/or the absorptive surface area.

METHODS

This study used a combination of morphometric analyses and in vitro absorption studies to define permeability characteristics in relation to the amount of paracellular pathway present in rat ileum, colon and the model epithelium, Caco-2.

RESULTS

Mucosal to serosal amplification was higher in ileum (3.9) than colon (1.9) or Caco-2 (1). Tight junctional density (lp) of ileal crypts was approximately 3 fold greater (91 m/cm2) than that measured in ileal villi, colonic surface and crypt cells or Caco-2 monolayers (34-37 m/cm2). However, when the relative contributions of the crypts and villi was taken into account there was no significant difference in the mean lp per mucosal area for the three epithelia studied. Using these data to correct for morphometric differences the permeabilities of a range of small hydrophilic molecules (atenolol, D-PheAsp and PEG oligomers MW 282-634) was measured. Permeability of rat ileum and colon were virtually identical for all compounds studied. In contrast, Caco-2 monolayers showed a significantly lower permeability than intestinal tissues with the difference increasing markedly with molecular size.

CONCLUSIONS

These studies suggest the importance of accounting for morphological variation when comparing the permeability characteristics of different epithelial systems.

Oral absorption of D-oligopeptides in rats via the paracellular route

PURPOSE

This study was undertaken to examine the structural determinants of oral bioavailability in the rat of a set of oligopeptides comprising D-amino acids, which were taken to be absorbed paracellularly based on a pronounced sensitivity of permeability to electrical resistance in Caco-2 cell monolayers.

METHODS

The study series comprised eleven D-oligopeptides, designed not to be recognised by peptidases or transport proteins, and to have molecular weights between 222 and 406 daltons with different net electrical charges and composition of D-amino acids. All the peptides were [3H]-radiolabelled and analyzed by HPLC with radiometric detection. Bioavailability was estimated based on 24-hr urinary excretion of unchanged peptide after oral and intravenous administrations.

RESULTS

As expected, the series proved metabolically stable. Bioavailability was independent of oral dose when varied by a factor of 10,000, suggesting passive absorption. Whereas bioavailability decreased sharply from 30% to 1% with increasing molecular weight, net charge showed little, if any, effect on bioavailability.

CONCLUSIONS

This D-oligopeptide model series served as a useful probe for the structural requirements for paracellular absorption in vivo. A critical determinant of bioavailability is molecular size, expressed as molecular weight in this study; net charged appeared of much lesser importance.

Comparison of HT29-18-C1 and Caco-2 cell lines as models for studying intestinal paracellular drug absorption

PURPOSE

To compare the permeability characteristics of HT29-18-C1 colonic epithelial cell line with Caco-2, an established model of intestinal drug transport.

METHODS

Cell lines were grown as epithelial monolayers. Permeability was measured over a range of transepithelial electrical resistance (Rt) using a group of drug compounds.

RESULTS

HT29-18-C1 develop Rt slowly when grown in culture, allowing permeability to be measured over a wide range (80-600 Omega x cm2). In contrast, Caco-2 monolayers rapidly develop Rt of approximately equal 300 Omega x cm2 and require Ca2+ -chelation to generate Rt equivalent to human intestine (60-120 Omega x cm2). Permeability of atenolol, ranitidine, cimetidine, hydrochlorothiazide and mannitol across HT29-18-C1 decreased 4-5 fold as Rt developed from 100-300 Omega x cm2 indicating they permeate via the paracellular route. In contrast, ondansetron showed no difference in permeability with changing Rt consistent with transcellular permeation. Permeability profiles across low Rt HT29-18C1 and pulse EGTA-treated Caco-2 monolayers were the same for all 5 paracellular drugs suggesting that transient Ca2+ removal does not alter selectivity of the tight junctions. Permeabilities of cimetidine, hydrochlorothiazide and atenolol across 100 Omega x cm2 HT29-18-C1 monolayers reflect more closely those reported for the human ileum in vivo than did mature Caco-2 monolayers.

CONCLUSIONS

HT29-18-C1 monolayers can be used to study drug permeability at Rt values similar to human intestine without the need for Ca2+ chelation. As such, they offer a useful alternative to Caco-2 for modelling intestinal drug absorption.

The Rocky Mountain-Samuel Bulkley Travelling Fellowship

Rocky Mountain Orthodontics and The Australian Society of Orthodontists Foundation for Research and Education provide a Fellowship to enable a new orthodontic graduate the opportunity to spend time overseas engaged in research. The present discussion deals not so much with the details of the research (which are to be published), but rather provides an overview of the Fellowship activities, and aims to encourage present postgraduate students to apply for the award.