ISSN:
1432-2013
Keywords:
Ciliary epithelium
;
Tissue culture
;
Na+/H+ exchange
;
Amiloride
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract Uptake studies with22Na were performed in cultured bovine pigmented ciliary epithelial cells, in order to characterize mechanisms of Na+ transport. A large part of Na+ uptake was sensitive to amiloride, quinidine and harmaline. Na+ uptake was stimulated by intracellular acidification (using the NH 4 + prepulse technique), and was inhibited with increasing extracellular proton concentration. Decreasing extracellular pH from 7.5 to 7.0 increased the apparentK M for Na+ from 38 to 86 mM without considerable changes inV max. In the presence of 5 mM Na+ half maximal inhibition of amiloride sensitive Na+ uptake by extracellular protons was observed at a hydrogen concentration of 50 nM. In the presence of 50 mM Na+ the proton concentration necessary for 50% inhibition was 139 nM. Thus, the mode of inhibition of extracellular H+ seemed to be competitive with aK i of 20–40 nM. 10 μM amiloride increased the apparentK M for Na+ from 33 mM to 107 mM, whileV max remained nearly unchanged. IC50 for amiloride was 6 μM at 5 mM Na+ and 36 μM in the presence of 150 mM Na+. Thus, amiloride behaves as a competitive inhibitor with aK i of about 5 μM. The affinities of Na+ to the transport site (K M≈16 mM), to the inhibitory site for protons (K M≈21 mM), and to the inhibitory site for amiloride (K M≈26 mM) were in the same order of magnitude. In summary, we have presented evidence for the presence of a Na+/H+ exchanger in cultured bovine pigmented ciliary epithelial cells. The kinetic data suggest the presence of only one common extracellular binding site for Na+, H+ and amiloride.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00583734
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