Fig. 8.

Distinct functional properties ofI_h currents generated by mHCN1 and mHCN2.A, Currents generated in response to hyperpolarizing voltage steps in Xenopus oocytes expressing mHCN1 (A1) or mHCN2 (A2), respectively.A1, mHCN1 currents shown during 3-sec-long hyperpolarizing voltage steps. A2a,b, mHCN2 currents shown during initial 3 sec (A2a) and entire 30 sec time course (A2b) in response to hyperpolarizing voltage steps. For both A1 and A2, membrane held at −30 mV and stepped from −35 to −105 mV in 10 mV increments (selected voltages indicated to right of current traces). B, Two exponential components are required to adequately fit activation time course of mHCN currents. Time and current scales as in corresponding panels in A. ForB1 and B2, bottom tracesshow current during hyperpolarizing step to −105 mV with superimposed fit using two exponential components. The middle andtop traces show the residuals of difference between the recorded current and the fitted single (top trace) or double (middle trace) exponential functions. Zero current is indicated by the arrowhead (labeled 0). The residuals from the single exponential fits are displaced from zero for clarity; zero current for these traces is indicated by thedashed line. To facilitate comparison between mHCN1 and mHCN2, the first 3 sec of the mHCN2 activation time course and fits to this are shown on an expanded time scale in B2a.