FIGURE 5.

Representative single-channel currents and outward current block by spermine. Stoichiometry of mutant/wild-type (160D/171A:N160/T171) subunits was determined by the distribution of open probability with and without 100-uM spermine at different mutant to wild-type cRNA ratios. Note that sensitivity to outward current block by intracellular spermine increases with the number of mutant subunits, due to the presence of the 160D mutation. In the presence of spermine, sensitivities were found that correlated with experiments using a known 0:4 subunit ratio (only mutant cRNA injected, completely blocked) and known 4:0 subunit ratio (only wild-type cRNA injected, little or no block). Three major sensitivities between these extremes were also found, consistent with their being from channels 1:3, 2:2, and 3:1 subunit ratios. Only the 0:4, 2:2, and 4:0 subunit single-channel currents are shown.