The name is absent

26

are indistinguishable from those in light-adapted conditions, suggesting that Ih channels are
not modulated by steady background light. Because HCN channels are also gated by cyclic
nucleotides in addition to hyperpolarizing voltages, we attempted to measure the sensitivity
of rod I_h to cAMP. Although other researchers have reported a small modulatory effect of
cAMP on the 7⅛ activation curve in rabbit rods [31], we were unable to produce a change
in activation curve or I-V curve with bath perfusion of 8-Br-cAMP. This is consistent with
reported evidence that HCN1 channels, which we believe are the primary isoform, are the
least sensitive of the four HCN isoforms [15]. The steady state IV curves for rods and cones,
generated by plotting the end steady state current at each holding potential, are both seen to
be a hyperpolarization activated inward current with an amplitude of around -150 pA at-134
mV (figure 3.1 A2 and B2). This similarity in magnitude and rectification provides evidence
that rods and cones have similar numbers of channels carrying the I_h current, assuming the
identity of the channels is the same.

The steady-state activation curves, shown in figure 3.1 A3 and B3, generated by plotting
the normalized amplitude of the tail currents at -4 mV vs. the command pulse amplitude,
are also similar. The half activation potential for rods, -77.8 ± 3.7 mV (mean ± std dev), is
slightly more negative than for cones, -73.8 ±1.2 mV. The activation rate constants are also
similar (10.5 ± 3.42 mV for rods and 7.1 ± .86 mV for cones), although the cone activation is
slightly steeper. These activation curves for both cell types agree well with results published
previously for rods [31, 75, 78, 54]. Also similar are the rod and cone activation time con-
stants (figure 3.1 A4 and B4), which were generated by fitting a single exponential decay to

<<   32   33   34   35   36   37   38   >>

More intriguing information