title: Uptake of [3H]benzamil at different sodium concentrations. Inferences regarding the regulation of sodium permeability.
abstract: 1. The effect of benzamil on short-circuit current in frog skin was measured at different external sodium concentrations. A linear relationship exists between the concentration of benzamil reducing short-circuit current by 50% and the external sodium concentration, indicative of some form of competitive antagonism between sodium and benzamil. 2. Uptake of [3H]benzamil into isolated frog skin epithelium and whole skin (0.95 cm2 pieces) was measured at different external sodium concentrations. With a sodium concentration of 111 mM in the external medium the uptake of [3H]benzamil is linear with concentration. Uptake amounted to 8.8 f-mole nM-1, a value similar to the linear component of the uptake measured at low (1.1 mM) sodium concentration. 3. Using a variety of other conditions the maximal number of specific binding sites for [3H]benzamil was calculated from displaceable binding and the fractional occupancy, the latter being derived from the inhibition of short-circuit current. This approach gave similar binding site densities to those reported previously at low sodium concentrations. 4. The reduction in specific [3H]benzamil uptake at high sodium may result from two mechanisms, competition of sodium with the ligand for an external binding site and a reduction in the site density as the intracellular sodium concentration increases. 5. It is concluded that the saturation of sodium transport which occurs at high sodium concentration is likely a consequence of the reduced availability of entry sites, rather than saturation of the uptake process.
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000378:metabolism*; D000818:Animals; D001001:Anura; D001549:Benzamides / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D001692:Biological Transport / Q000187:drug effects; D002463:Cell Membrane Permeability; D004848:Epithelium / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D011719:Pyrazines / Q000378:metabolism*; D011896:Rana temporaria; D012867:Skin / Q000378:metabolism*; D012879:Skin Physiological Phenomena; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D001549:Benzamides; D011719:Pyrazines; C013407:benzamil; D000584:Amiloride; D012964:Sodium