Figure 4.

α₇₂₈βγ is less sensitive to proteolytic activation than α₆₆₉βγ and activation of both isoforms is [Na⁺] dependent. (A) Example demonstrating similar time course of activation with trypsin to that previously observed with α₆₆₉βγ (not shown). “A” indicates 10 µM amiloride. (B) Activation required the presence of all 3 subunits similar to that required for α₆₆₉ (not shown) as activity of α₇₂₈ alone and α₇₂₈γ were not appreciably activated by proteases. (C and D) Activation by subtilisin and trypsin (E and F) in high and low Na⁺. In both cases α₇₂₈ was less sensitive to activation by proteases and in both isoforms a larger activation was observed in high Na⁺. Note similar profile but difference in scale between subtilisin and trypsin. n = 6-20 for B, 17 for C and D, 14 for E and F. (P < 0.05 for all comparisons between isoforms except panel B)