Time-dependent cGMP-activated conductance of detached patches of ROS plasma membrane

On the activation of phosphodiesterase by a 26 kDa protein

The effects of a 26 kDa protein isolated from vertebrate retina rod outer segments (ROS) and its reconstituted analog on the phosphodiesterase (PDE) activity and cGMP-dependent conductance have been studied [Nature 313 (1985) 310-313]. Using the patch-clamp technique it was shown that the 26 kDa protein in concentrations up to 1 microM accelerates hydrolysis of cGMP by near-membrane PDE by 1-2 orders of magnitude. This process is suggested to be mediated by some intracellular agent. At the same concentrations the 26 kDa protein was shown to inhibit cGMP-dependent conductance of the photoreceptor membrane. A possible role of these effects in the processes of phototransduction and adaptation is discussed.

Photosensitivity of 8BrcGMP-induced conductance in ROS-excised patches

Inside-out patches from ROS plasma membranes contain the basic enzymes of the phototransduction cascade. Similar to a native photoreceptor cell, such patches are capable of responding to light, the effect of which suppresses the cGMP-activated current. Photoresponses are observed only in the presence of GTP, whereas ATP essentially accelerates the current recovery to a dark level. Photoresponses are also observed in the presence of 8BrcGMP. Phosphodiesterase (PDE) hydrolyzes 8BrcGMP two orders of magnitude slower than cGMP, so the light inhibition of the 8BrcGMP-induced current cannot be accounted for by PDE activation. It seems that activity of cGMP-gated channels depends not only on cGMP concentration, but is additionally controlled by some other regulatory mechanisms.

Inactivation of cGMP-dependent conductance of rod outer segment plasma membrane induced by cGMP

Integral cGMP-dependent currents as well as activity of single cGMP-activated channels in plasma membrane of rod outer segment (ROS) were recorded using the patch-clamp method. The dependence of integral currents on cGMP concentration is shown to be bell-shaped. Decrease in cGMP-dependent conductance at high cGMP concentration results from a decrease in channel opening probability. Thus, cGMP in high concentrations inactivates cGMP-dependent conductance.