THE AMILORIDE-SENSITIVE EPITHELIAL NA- BINDING-SITES AND CHANNEL DENSITIES( CHANNEL )

The amiloride-sensitive Na+ channel found in many transporting epithel ia plays a key role in regulating salt and water homeostasis. Both bio chemical and biophysical approaches have been used to identify, charac terize, and quantitate this important channel. Among biophysical metho ds, there is agreement as to the single-channel conductance and gating kinetics of the highly selective Na+ channel found in native epitheli a. Amiloride and its analogs inhibit transport through the channel by binding to high affinity ligand-binding sites. This characteristic of high-affinity binding has been used biochemically to quantitate channe l densities and to isolate presumptive channel proteins. Although the goals of biophysical and biochemical experiments are the same in eluci dating mechanisms underlying regulation of Na+ transport, our review h ighlights a major quantitative discrepancy between methods in estimati on of channel densities involved in transport. Because the density of binding sites measured biochemically is three to four orders of magnit ude in excess of channel densities measured biophysically, it is unlik ely that high-affinity ligand binding can be used physiologically to q uantitate channel densities and characterize the channel proteins.