Figure 5.

IHCs of hypothyroid rats do not acquire the fast BK conductance in the third postnatal week as IHCs of control rats do. A, B, Families of IHC outward K⁺ current traces of a control rat (A) and a hypothyroid rat (B) at P2 in response to step depolarizations reveal similar slow activation kinetics within the first 5 ms. C, In a P19 control IHC, a very large and rapidly activating K⁺ current was evident. D, E, Expression of the rapidly activating conductance was missing in a hypothyroid IHC at P19 (D) and was still incomplete in another hypothyroid IHC at P32 (E). F, I–Vs of the K⁺ currents in A–E taken at 1.2 ms after the start of the depolarization (the time point is indicated by vertical dashed lines in A–E, respectively). IHCs of the P2 control rat and of either the P2 or P19 hypothyroid animals did not show any fast K⁺ outward current within the whole voltage range whereas the P19 control IHC displayed a large and fast K⁺ current. The P32 hypothyroid IHC showed some rapidly activating current. G, Incomplete block of the fast K⁺ current at 0 mV of a P19 control IHC (thick black trace) by the BK channel blocker iberiotoxin (100 nm, thin black trace) and complete block of the fast K⁺ current at 0 mV of another P19 control IHC (thick gray trace) by the BK channel blocker paxilline (10 μm, thin gray trace). H, I–Vs taken from the cells in G at 1.2 ms after depolarization (indicated by the dashed line in G) demonstrate that paxilline, but not iberiotoxin, blocked the fast K⁺ current completely. I, Monoexponential activation time constants of K⁺ currents as a function of voltage for the cells in G and H before and during iberiotoxin and paxilline application. Control IHCs without toxin treatment and the iberiotoxin-treated control IHC had time constants of <1 ms. The control IHC treated with paxilline revealed activation time constants of 8 ms (at ∼5 mV) that declined to 2 ms (at ∼35 mV). K, Voltage-dependent monoexponential activation time constants of K⁺ currents for typical control and hypothyroid IHCs of different ages. A P19 control IHC and a P31 hypothyroid IHC displayed fast-activation time constants of ∼1 ms or less, whereas P9 control, P9 hypothyroid, P19 hypothyroid IHCs, and a P38 hypothyroid IHC displayed slow activation time constants decreasing from 8 to 10 ms (at ∼0 mV) to 2 ms (at ∼25 mV). L, IHCs of the same organ of Corti differed regarding the expression of the fast BK conductance in hypothyroid animals aged P31–P50. Selected individual curves of IHC K⁺ current activation time constants as a function of membrane potential are shown for three IHCs of a P31 (unfilled symbols) and a P38 (filled symbols) hypothyroid organ of Corti, respectively.