Bioelectric properties of proximal bronchiolar epithelium.

Distal airway epithelium is widely believed to secrete ions and liquid into the airspace, although this process has never been demonstrated in intact small airways. To determine the characteristics of active ion transport in distal airway epithelium, the effects of selective inhibitors of active Na+ absorption and Cl- secretion on the bioelectric properties of intact proximal bronchiolar epithelium were evaluated. Large bronchioles (450-1,200 microns outside diameter, 1.5-5.0 mm length) were excised from 4- to 8-wk-old pigs, cannulated with glass microcannulas, and perfused. Transepithelial potential difference (PD), short-circuit current (Isc), and resistance (Rm) were measured by cable analysis. In 14 tissues, resting PD, Isc, and Rm were -3.4 +/- 0.4 mV (lumen negative), 19.6 +/- 4.7 microA/cm2, and 255 +/- 50 omega.m2, respectively. The conductive Na+ channel blocker amiloride (10 microM) significantly (P < 0.05) reduced PD and Isc by 37 and 41% and significantly increased Rm by 23% in seven tissues. Subsequent bumetanide (10 microM), a blocker of active Cl- secretion through inhibition of Na(+)-K(+)-2Cl- cotransport, significantly reduced the amiloride-insensitive PD and Isc by 41 and 50%, whereas Rm significantly increased 15%. Because amiloride is known to induce Cl- secretion, the order of drug addition was reversed to determine the fractional contribution of active Cl- secretion to the resting PD, Isc, and Rm. In seven different bronchioles, bumetanide did not significantly affect PD, Isc, or Rm.(ABSTRACT TRUNCATED AT 250 WORDS)