FIGURE 9.

Simulated effect of calcium on MT channel and hair-bundle mechanics. (A) Probability of MT channel opening, p_O (lower) and hair-bundle displacements, X (middle), for force steps (upper) at four different calcium concentrations: 3, 10, 32, and 100 μM at the fast-adaptation site (C_FA) with the MT channel open. These calcium concentrations are approximately equivalent to 20 μM to 1.2 mM externally. Note the change in the speed of adaptation and of the secondary component of bundle displacement as the calcium increases. The adaptation time constants for small changes in p_O for different values of C_FA were 0.12 ms at 100 μM; 0.25 ms at 30 μM; 0.6 ms at 10 μM; and >1 ms at 3 μM calcium. (B) Effects of calcium on p_O-X relationships for the four calcium concentrations. All relationships were fitted with a first-order Boltzmann equation. (Left to right) 3 μM (solid circles), 10 μM (asterisks), 32 μM (crosses), and 100 μM (solid squares) intracellular calcium. Note that the 10–90% operating range increased from 47 to 200 nm as C_FA was increased from 3 to 100 μM. (C) F-X relationships for four different calcium concentrations (3, 10, 32, and 100 μM). Black lines are from the passive hair bundle with DHS (solid line) and BAPTA treatment (dashed line). With progressively increasing calcium concentration, the F-X relationship deviates further from the DHS curve. Simulations in this figure were performed only with the b₂ model.