An intracellular ATP-activated, calcium-permeable conductance on the basolateral membrane of single renal proximal tubule cells isolated from Rana temporaria
L. Robson and M. Hunter *
Department of Biomedical Science, University of Sheffield, Western Bank, Sheffield S10 2TN and * Department of Biomedical Science, University of Leeds, Leeds LS2 9NQ, UK
The following study describes the properties of a non-selective cation channel, which has a unit conductance below the resolving power of the single channel technique, located on the basolateral membrane of single proximal tubule cells isolated from frog kidney. The conductance was examined using cell-attached, inside-out and outside-out patches. Due to the small single channel magnitude, macroscopic patch currents were measured.
Addition of 2 mM ATP to the intracellular surface of excised patches activated an outwardly rectifying conductance (MCANS): outward (Gout) and inward (Gin) conductances increased by 46·8 ± 6·7 and 11·6 ± 2·1 pS, respectively (n = 29). MCANS was more selective for cations than anions, with a cation:anion selectivity ratio of 10·1 ± 1·7 (n = 7), but did not discriminate between Na+ and K+. It was more selective for Ca2+ over Na+, with a Ca2+:Na+ selectivity ratio of 4·49 ± 0·69 (n = 7).
In cell-attached patches addition of 100 µM strophanthidin to the bath increased both Gout and Gin. However this increase in conductance was absent in the presence of Gd3+, which inhibits MCANS.
These data suggest that single proximal tubule cells isolated from frog kidney contain an ATP-activated, non-selective cation conductance. The conductance does not discriminate between Na+ and K+, but is more selective for Ca2+ over Na+. Considering the prevailing electrochemical gradients for these ions, functional activation of the conductance would be expected to lead to a rise in intracellular Ca2+. MCANS is linked to the activity of the Na+,K+-ATPase and may therefore provide a link between the ATPase and K+ channel activity in the basolateral membrane and form an integral part of the pump-leak mechanism in transporting epithelia.
