. Author manuscript; available in PMC: 2006 Jan 20.

Published in final edited form as: Invest Ophthalmol Vis Sci. 2005 Sep;46(9):3060–3066. doi: 10.1167/iovs.05-0257

Table 2.

Comparison of the effects of IBMX and vardenafil on frog rod PDE6 catalytic activity and on cGMP levels of intact frog ROS.

	frog PDE6 K_I (μM)^a		Intact ROS (μM)^b
Inhibitor	activated	nonactivated	observed EC₅₀	predicted IC₅₀
IBMX	4.3 ± 0.5	14 ± 1.3	1000	73
vardenafil	0.0019 ± 0.0004	0.022 ± 0.004	50	0.14

The K_I values were determined as described in Table 1, using nucleotide-depleted frog ROS homogenates to measure nonactivated (10 μM cGMP, 4.0 nM PDE6) or activated (1.0 μM cGMP, 20 pM PDE6) PDE6 (see Methods).

Intact frog ROS (4.8 μM rhodopsin) were prepared as described in Methods. The observed EC₅₀ is an estimate from the results of Fig. 3. The predicted IC₅₀ was calculated with the equation: IC₅₀ = K_I*(1 + [cGMP]/K_M) where the nonactivated K_I value is used, the cGMP concentration is the dark-adapted value for intact ROS (50 μM), and the KM (60 μM) refers to the value for nonactivated frog PDE6 with cGMP as the substrate 31.