Figure 3.

Flash responses from rods lacking rhodopsin kinase. (A) Saturating responses from RK+/+ and RK+/− rods (Inset), and an RK−/− rod to a flash that elicited about 300 photoisomerizations in each case. Unlike the +/+ and +/− responses (see Inset), the RK−/− response was greatly prolonged, and it recovered in a series of stepped transitions that represent the deactivation and occasional reactivation of individual rhodopsin molecules. The dark currents of the three cells were 15.0 pA (+/+), 16.7 pA (+/−), and 16.6 pA (−/−). Flashes were delivered at t = 0. (B) Form of the single photon response from rods of each of the three lines. The RK−/− trace is the average of 14 responses; the RK+/− trace is the average of 80 responses; and the RK+/+ trace is the average of 57 responses. The dark current of the three cells was 13.8, 18.9 and 11.3 pA, respectively. (C) Distribution of the durations of 198 RK−/− quantal events measured at half-maximal amplitude from three RK−/− cells. The smooth curve is an exponential function with a time constant of 3.3 s. The Inset is an example of a quantal event from an RK−/− rod whose dark current was 7.0 pA. (D) Dependence of the rate of change of PDE activity on the number of flash-induced photoisomerizations in RK+/+ and −/− rods. Slope of the linear regression is 0.99.