Figure 7. Comparrson of the Voltage Ranges over Which I_Kx and I_h Gate and the Simulated Effect of I_Kx on the Photovoltage.

Recordings were made from cells bathed in Ringer solution containing 5 mM Cs⁺ and 0.1 mM Cd²⁺.

(A) Activation and τ curves for I_Kx and I_h as used in our computer reconstruction of the light response. The reversal potentials for I_Kx and I_h were –74 and –32 mV, respectively, and the rod's dark resting potential is marked by the vertical broken line. The data for salamander rod I_h come from Hestrin (1987).

(B, top) The photocurrent in response to a dim flash measured from published data for salamander rods (Baylor et al., 1984, their Figure 2A; response to 0.98 photons mm^–2) with the points (closed circles) fitted by eye using the Independence equatron (I = (15.5(exp(–1.6t)))(1 – exp(–1.6t))³) from Baylor et al. (1974), in which I is the photocurrent (pA) and t is time (s). Zero on the vertical axis corresponds to an inward dark current of 40 pA. This current trajectory was applied to a computer model circuit with the following current components: g_Kxn_Kx(E – E_Kx) and g_L(E – E_L), in which E is membrane potential, E_Kx and E_L are reversal potentials with values equal to –74 and –77 mV, respectively, g_Kx is the maximum conductance underlying I_Kx (1.04 ± 0.23 nS, mean ± SD, n = 12), and n_Kx is a value between 0 and 1 that represents the proportion of channels in the open state. A voltage-independent leak conductance, g_L, of 0.35 nS and a membrane capacitance of 21 pF (Baylor et al., 1984) were assumed. (B, middle) Two calculated voltage responses to the photocurrent are shown: (a) the continuous line, when n_Kx was allowed to vary according to the differential equation: dn_Kx/dt = α(1 – n_Kx) – β(n_Kx) (α and β from Figure 3C); (b) the broken line, when the n_Kx term was held constant at the resting value of 0.719 (i.e., I_Kx with no voltage dependence). Zero on the vertical axis corresponds to a resting potential of –38.2 mV. The vertical arrows point to the peaks of the voltage responses. (B, bottom) Cating of the channels underlying I_Kx (described by n_Kx) during the photocurrent, correspondrng to the two voltage responses described above.