Re-evaluating the Na(+) conductance of adult rat alveolar type II pneumocytes: evidence for the involvement of cGMP-activated cation channels.
1. Alveolar epithelial type II pneumocytes were isolated and purified from adult rat lung by elastase digestion and differential adhesion, and cultured in serum-free medium for approximately 2 days on glass coverslips for subsequent patch-clamp studies employing symmetrical sodium isethionate solutions. 2. Whole-cell Na(+) currents exhibited essentially linear current-voltage relationships which were mildly inhibited (by approximately 25 %) by 10 microM amiloride. In contrast, 1 mM Zn(2+) inhibited the currents by approximately 55 % with an IC(50) of approximately 134 microM and maximal blockade achieved between 5 and 10 mM. The effects of Zn(2+) and amiloride were additive, and independent of the order of blocker addition. 3. Gd(2+), Zn(2+) and La(3+) at 10 mM were all effective at rapidly, reversibly and significantly blocking the amiloride-insensitive currents by approximately 60%. in contrast, Ni(2+) was a very weak inhibitor (30 % inhibition at 10 mM). 4. Pimozide (10 microM) caused inhibition of whole-cell cation conductance by approximately 55 %. The inhibitory effect of pimozide was concentration dependent with an IC(50) of approximately 1 microM and was maximally effective between 10 and 30 microM. Sequential addition of Zn(2+) and pimozide, in either order, revealed no overlapping inhibitory effect on the amiloride-insensitive conductance, and supported the notion that the Zn(2+)- and pimozide-sensitive currents are identical. 5. The amiloride-insensitive, Zn(2+)-blockable conductance was characterised by a Na(+)/K(+) permeability ratio (P(Na)/P(K)) of 0.73 +/- 0.02. 6. 8Br-cGMP (100 microM), a membrane-permeable analogue of cGMP, evoked a robust activation of whole-cell cation conductance to 220 % of control. This activation was apparent in either the absence or the presence of 10 microM amiloride, but was completely abolished in the presence of Zn(2+). 7. These data support the in vivo and in situ observations of a substantial amiloride-resistant Na(+) conductance, demonstrate directly that cyclic nucleotide-gated non-selective cation channels are functionally expressed in alveolar epithelial type II cells, and suggest that these channels may contribute to the fluid-reabsorptive driving force in adult lung.
Kemp
P J
PJ
School of Biomedical Sciences, Worsely Medical and Dental Building, University of Leeds, Leeds LS2 9JT, UK. p.z.kemp@leed.ac.uk
Kim
K J
KJ
Borok
Z
Z
Crandall
E D
ED
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002412:Cations / Q000378:metabolism; D002469:Cell Separation; D006152:Cyclic GMP / Q000502:physiology*; D004232:Diuretics / Q000494:pharmacology; D004558:Electric Stimulation; D007473:Ion Channels / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018408:Patch-Clamp Techniques; D010868:Pimozide / Q000494:pharmacology; D011650:Pulmonary Alveoli / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000187:drug effects*; D015032:Zinc / Q000494:pharmacology
D002412:Cations; D004232:Diuretics; D007473:Ion Channels; D015222:Sodium Channels; D010868:Pimozide; D000584:Amiloride; D006152:Cyclic GMP; D015032:Zinc