Ion transport mechanisms in native rabbit nasal airway epithelium

Abstract

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.

Nucleotide regulation of paracellular Cl− permeability in natural rabbit airway epithelium

“…Such data are in contrast to our previous observations from natural rabbit nasal airway epithelium where amiloride was demonstrated to induce a large (26%) decrease in G t simultaneous with a similar (25%) decrease in passive m-to-s Cl − fluxes [33]. Furthermore, our recent study on cellular effects of nucleotides strongly suggested that stimulation of apical purinergic P2Y 2 receptors caused a rapid inhibition of the conductance of the paracellular pathway [30].…”

“…Under short-circuit conditions, unidirectional fluxes of Cl − in native rabbit nasal airway epithelium are large and approximately fourfold greater than passive Na + fluxes, while net Cl − transport is close to zero or slightly secretory [33,34]. Furthermore, unidirectional Cl − fluxes vary proportionally with the epithelial conductance (G t ) (while this does not apply to passive Na + or non-electrolyte fluxes [5,33]).…”

“…The dominating cellular active transport mechanism is Na + absorption via apical, amiloride-sensitive Na + channels (ENaC) and basolateral Na + /K + pumps [33]. Chloride absorption is electrically coupled to Na + absorption and utilizes either a paracellular pathway through tight junctions or a cellular pathway through membrane Cl − channels.…”

“…In preparations with the highest resistances, Cl − absorption is probably transcellular due to a favourable apical electrochemical gradient [41]. In contrast, natural airway epithelia typically have a transepithelial resistance below 100 Ω cm 2 [3,18,19,21,24,33], suggesting the presence of a high paracellular conductance.…”

“…Furthermore, unidirectional Cl − fluxes vary proportionally with the epithelial conductance (G t ) (while this does not apply to passive Na + or non-electrolyte fluxes [5,33]). These data suggest that (1) the natural airway epithelium contains an anion-selective high-conductance tight junction pathway; (2) mucosa-to-serosa (m-to-s) Cl − fluxes represent passive diffusion through this paracellular pathway; (3) under in vivo conditions, a blood-side positive potential difference of the airway epithelium created by active cellular Na + absorption drags Cl − passively through the paracellular pathway; and (4) the tight junction pathway may be regarded as a heterogeneous structure containing numerous narrow anion-selective channels that restrict according to electrical charge and determine the high G t , in parallel with much fewer larger pores (permeable to Na + and non-electrolytes) that restrict according to molecular size [14,33].…”