Conductive cation transport in apical membrane vesicles prepared from fetal lung.
In order to characterise the apically-located conductive cation pathway of the type II pneumocyte, apical plasma membranes were prepared from mature fetal guinea pig lung. The protocol yielded purified apical membranes that enriched 19-fold with the brush border enzyme marker alkaline phosphatase; there was no significant contamination with other cellular membranes. A technique for imposing an outwardly-directed electrochemical Na+ gradient was used to amplify conductive 22Na+ uptake into vesicles. Uptake of 22Na+ was time-dependent, proportional to the magnitude of the Na+ gradient, specific and sensitive to the amiloride analogues phenamil and EIPA (apparent minimum Ki values of 50 nM and 10 microM, respectively, with maximum uptake inhibition of 42% and 39% at 100 microM). Uptake experiments in which the outwardly-directed Na+ gradient was replaced by outwardly-directed gradients of small monovalent cations and molecular cations were performed. The Na+/K+ permeability ratio was 1.2:1, and over the extended range of small monovalent cations, a permeability sequence of Na+ > K+ > Li+ > Rb+ > Cs+ was observed, indicating the presence of fixed negative charge in or spatially close to the pore. The molecular cation permeability sequence of NH4+ > methylamine+ > dimethylamine+ > choline+ > N-methyl-D-glucamine+ > tetraethylammonium+ > tetramethylammonium+, after transformation, gives an estimate of 8 A for the conducting pore diameter. These data are consistent with the presence in the apical membrane of fetal type II pneumocytes of a cation specific channel with low Na+ selectivity and amiloride sensitivity.
Fyfe
G K
GK
Department of Child Health, Centre for Research into Human Development, Ninewells Hospital and Medical School, University of Dundee, UK.
Kemp
P J
PJ
Cragoe
E J
EJ
Olver
R E
RE
D000818:Animals; D002412:Cations / Q000378:metabolism*; D005333:Fetus / Q000378:metabolism; D006168:Guinea Pigs; D015640:Ion Channel Gating; D017136:Ion Transport; D008168:Lung / Q000196:embryology / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism
D002412:Cations; D012964:Sodium