Structural changes associated with fluid absorption by dog tracheal epithelium

Species differences in the physical and transport properties of airway secretions

The clearance of airway secretions is vital in protecting the mammalian lung from pollution and infection. Diverse animal models have been used to study lung diseases associated with impaired secretion clearance. The extrapolation of data from animal models to humans is based on the assumption that there are structural and functional similarities in the airway epithelium and secretions. This manuscript reviews regulation of mucus secretion as well as the physical and transport properties of respiratory mucus. As tracheal size increases, the rigidity of airway secretions decreases, and rigidity is inversely correlated with mucociliary transportability. These differences are placed in the context of previously reported species and regional differences in transepithelial potential difference and the tracheobronchial epithelial cell population. Tracheal mucus transport velocity varies with the species studied and has been shown to positively correlate with tracheal surface area. A progressive increase in the rate of mucus transport from the small to the large airways has also been reported. The reduction in mucus rigidity from small to large airways could be one of the mechanisms responsible for velocity gradients, which facilitate mucociliary transport. Because airway dimensions, rather than anatomic level of the airway, may better predict epithelial secretory response, studies to assess the physiologic responses in human airways require the use of an animal model with a similar-sized airway.

Second-messenger regulation of sodium transport in mammalian airway epithelia

1. Sodium absorption is the dominant ion transport process in conducting airways and is a major factor regulating the composition of airway surface liquid. However, little is known about the control of airway sodium transport by intracellular regulatory pathways. 2. In sheep tracheae and human bronchi mounted in Ussing chambers under short circuit conditions, the sodium current can be isolated by pretreating tissues with acetazolamide (100 microM) to inhibit bicarbonate secretion, bumetanide (100 microM) to inhibit chloride secretion and phloridzin (200 microM) to inhibit sodium-glucose cotransport. This sodium current consists of amiloride-sensitive (57%) and amiloride-insensitive (43%) components. 3. The regulation of the isolated sodium current by three second messenger pathways was studied using the calcium ionophore A23187 to elevate intracellular calcium, a combination of forskolin and the phosphodiesterase inhibitor zardaverine to elevate intracellular cyclic AMP, and the phorbol ester 12,13-phorbol dibutyrate (PDB) to stimulate protein kinase C. 4. In sheep trachea, A23187 produces a dose-related inhibition of the sodium current with maximal effect (38% of ISC) at 10 microM and IC50 1 microM. This response affects both the amiloride-sensitive and insensitive components of the sodium current and is not altered by prior stimulation of protein kinase C or elevation of intracellular cyclic AMP. In human bronchi, A23187 (10 microM) produced a significantly greater inhibition of ISC (68%), a response which was unaffected by prior treatment with PDB or forskolin-zardaverine. 5. In sheep trachea, stimulation of protein kinase C with PDB produced a dose-related inhibition of ISC maximal (56% of ISC) at 50 nM (IC50 7 nM). This response was abolished by amiloride (100 microM) pretreatment suggesting a selective effect on the amiloride-sensitive component of the sodium current. The response was not altered by prior elevation of intracellular calcium or cyclic AMP. PDB (10 nM) caused a similar inhibition of ISC in human bronchi (43%). The effect of PKC stimulation following pretreatment with A23187 was diminished in human bronchi. Elevating intracellular cyclic AMP did not alter this response. 6. Addition of forskolin (1 microM) together with the phosphodiesterase inhibitor zardaverine (100 microM) produced a mean 35-fold increase in intracellular cyclic AMP in sheep trachea. This was associated with a small, but significant, 6% transient increase in ISC followed by a significant 4% fall. Neither effect could be abolished by amiloride pretreatment. In human bronchi, a small decrease in ISC which could not be distinguished from that occurring in controls was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in cellular membrane and paracellular conductances by amphotericin B in the epithelium of the bullfrog cornea

In the isolated bullfrog cornea, measurements of DC electrical parameters in conjunction with AC impedance and ultrastructural analyses were used to determine the effects of 10(-5) M amphotericin B on epithelial cellular membrane and paracellular conductances. In NaCl Ringers, amphotericin B elicited a 3.5-fold increase in the specific apical membrane conductance (Ga/Ca); where Ga and Ca are the apical membrane conductance and capacitance, respectively. The basolateral membrane conductance (Gb) and the basolateral membrane capacitance (Cb) fell by 57% and 50%, respectively. In the paracellular pathway, the tight junctional complex (Gj) was unchanged whereas the lateral intercellular space resistance (Rp) decreased by 55%. The declines in Gb and Cb were suggestive of cell volume shrinkage because these changes were consistent with a previously described decline in intracellular K+ content and reduction in exposed basolateral membrane area to current flow. Ultrastructural analysis validated that amphotericin B caused cell volume shrinkage because there was: (1) increased folding of the basolateral membrane and waviness of the basal aspects- of the plasma membrane; (2) dilatation of the lateral intercellular spaces. This agreement suggests that intracellular activity decreased following exposure to amphotericin B which resulted in cell volume shrinkage and an impairment of Cl- uptake across the basolateral membrane.