AMILORIDE-BLOCKABLE CA2-ACTIVATED NA+-PERMEANT CHANNELS IN THE FETAL DISTAL LUNG EPITHELIUM()

The Na+ transport function of alveolar epithelium represents an import ant mechanism for clearance of fluid in air space at birth. I observed the activity of two types of amiloride-blockable Na+-permeant cation channels in the apical membrane of fetal distal lung epithelium cultur ed on permeable filters for 2 days after harvesting of the cells from Wistar rats of 20 days' gestation (term = 22 days). One type was a non selective cation (NSC) channel and had a linear current/voltage (I/V) relationship with a single-channel conductance of 26.9 +/- 0.8 pS (n = 5). The other type was highly Na+ selective (i.e. Na+ channel) and ha d an inwardly rectifying I/V relationship with a single-channel conduc tance of 11.8 +/- 0.2 pS (n = 5) around resting membrane potential. Th e NSC channel was more frequently observed (1.37 +/- 0.15 per patch me mbrane; n = 73) than the Na+ channel (0.15 +/- 0.40 per patch membrane ; n = 73). However, the open probability of the NSC channel was smalle r than that of the Na+ channel. Both types of the channels were activa ted by cytosolic Ca2+, however the sensitivity to cytosolic Ca2+ was m uch higher in the Na+ channel than in the NSC channel. Furthermore, bo th types of the channels were blocked by amiloride or benzamil. The ha lf-maximal inhibitory concentration (IC50) of amiloride or benzamil of the Na+ channel was 1-2 mu M, while that of NSC channel was less than 1 mu M Both channels were activated by insulin.