Sodium channels site density

The importance of these studies lies in the determination of site density. If site density is known with accuracy, together with a knowledge of the maximal sodium conductance, then the conductance of individual ionic channels can be estimated. The first part of this review was concerned with the alternatives of carrier-mediated, or pore-mediated transfer of ions across artificial membranes. Direct estimates of channel conductances in biological membranes allow tentative conclusions to be drawn concerning mechanisms in biological membranea. 

In the squid axon, channel density has been calculated from the rate of onset and offset of the effect of TTX on sodium conductance. A value of 522 sites/ jum was obtained which is almost identical to that derived from TTX binding [120]. An alternative approach has been to calculate sodium channel density from the maximal size of the gating currents in squid nerve. The result, 483/ jum, is again close to that obtained by direct measurement [121]. Of further interest are the calculations given by Hodgkin [122] who found that the optimal density of sodium channels in an umnyelinated nerve which are required to give a maximal conductance velocity was around 500/jum. ... 

For example, when tissues were deprived of sodium for several days the site density increased 3-fold from around 250//im, but more importantly there was a proportional increase in the level of transport which was maintained by the epithelium [197] (Figure 1.15). This effect seems an important homeostatic device possessed by the cell to deal with exposures to divergent sodium concentrations. Furthermore, the result indicates that the entry step, rather than the exit step from the epithelium which requires sodium pumping, is the rate determinant of the level of transport. The time course of the effect of sodium deprivation may mean that it is dependent on the de novo synthesis of new membrane permeases. However, increasing the membrane potential across the mucosal face of the cells causes an immediate appearance of new channels, while reducing the potential does the converse [198], indicating that there are binding sites (and... 

The complexity of the changes of amiloride-binding site density, and hence presumably of sodium channels, which can take place in these epithelia are only just beginning to be understood. It is not surprising, therefore, that the ideal diuretic has not been found, for any perturbation at one part of the nephron is likely to affect the ambient sodium concentration at a distal part. The ensuing adaptive changes, plus the influence of hormones, are such as to produce complex patterns of excretion. 

---