Passage 5 (Questions 23 - 27)A key modulator of action potential firing is the hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channel. These channels are permeable to both Na+ and K+. The HCN current, Ih, is found in neurons under hyperpolarization of the cell membrane and is proportional to the frequency of firing of action potentials.Neurologists wished to determine if a new drug, Compound X (Figure 1), is able to block Ih and reduce neuropathic stimulation, providing significant analgesic effects to those with pain.Figure 1 Structure of lidocaine (left) and Compound X (right), a potential ion channel blockerCompound X’s effect on neuronal firing was first compared to lidocaine, a known sodium channel inhibitor. Ganglion cells were treated with 3 different concentrations of Compound X. Sodium current was isolated and then recorded using patch-clamping. Figure 2 shows the results of these recordings.Figure 2 Na+ current in ganglion neurons treated with Compound X; traces were recorded in the presence and absence (control) of different concentrations of Compound XTo test if Compound X also blocked HCN channel currents, the drug’s effect on HCN currents in ganglion cell neurons was recorded. The HCN channel whole-cell current in neurons was induced by delivering a membrane potential step from a holding potential of -60 mV, to -140 mV for 700 ms, and then back to -70 mV. Solutions of 5 mL each were prepared with differing amounts of Compound X. Figure 3 shows the effect of Compound X concentration on HCN currents in neurons.Figure 3 Inhibition of HCN currents by addition of Compound X Question 27What alteration to an HCN channel will favor the transport of Na+ over K+? A.Increasing the diameter of the channel by adding amino acidsB.Decreasing the diameter of the channel by removing amino acidsC.Replacing the serine groups in the channel with alanineD.Replacing the alanine groups in the channel with serine