Concurrent absorption and secretion of airway surface liquids and bicarbonate secretion in human bronchioles

Abstract

Characterization of epithelial fluid and electrolyte transport in native small airway epithelia has been largely neglected due to the complex tissue architecture as well as by its inaccessibility and lack of applicable assays, especially in human tissues. To better understand how airway surface liquid (ASL) is maintained as a balance between absorbing and secreting functions, we measure ion transport properties of bronchioles with diameters of 1–2 mm isolated from freshly resected specimens of human lungs with a “mini”‐Ussing chamber (area≈1 mm2). We determined the effects of inhibiting absorption by applying amiloride to inhibit Na+ conductance through ENaC and secretion by applying bumetanide to inhibit the Na+‐K+‐2Cl− cotransporter to block Cldependent secretion demonstrated spontaneous absorptive as well as secretory short circuit currents in bilateral 150 mM NaCl‐Ringer solution. We also tested for the presence of the Clchannels, CFTR stimulating with Fsk/IBMX and CaCC with UTP. Both agonists independently stimulated additive equivalent short circuit current in the same specimen of small airway tissue, but evoked distinctly opposite effects on the transepithelial potential (Vt). While Fsk/IBMX (cAMP) activation depolarized Vt, UTP (Ca2+) stimulation hyperpolarized the Vt. The depolarization of Vt induced by cAMP suggests that activating CFTR may stimulate fluid absorption, while hyperpolarization of Vt by Ca2+ (UTP) suggest that activating CaCC may stimulate fluid secretion. Independent regulation of these fluid movements may be critical to regulation of ASL volume during conditions of stress. Human small airways constitutively and concurrently secrete and absorb fluid, structurally separated and independent epithelial cells for liquid secretion and absorption. Our results present new evidence for how the delicate layer of fluid lining the small airways is physiologically maintained in a steady state and appropriate airway hydration. Support or Funding Information The Nancy Olmsted Trust and the Cystic Fibrosis Research Inc. (CFRI). This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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