Sensory Transduction in Vision, Olfaction, and Gustation:-The Visual Signal Is Quickly Terminated
As your eyes move across this line, the images of the first words disappear rapidly—before you see the next series of words. In that short interval, a great deal of biochemistry has taken place. Very shortly after illumination of the rod or cone cells stops, the photosensory system shuts off. The α subunit of transducin (with bound GTP) has intrinsic GTPase activity. Within milliseconds after the decrease in light intensity, GTP is hydrolyzed and Tα reassociates with Tβ γ.
The inhibitory subunit of PDE, which had been bound to Tα -GTP, is released and reassociates with PDE, strongly inhibiting that enzyme. To return [cGMP] to its “dark” level, the enzyme guanylyl cyclase converts GTP to cGMP (step 7 in Fig. 12–34) in a reaction that is inhibited by high [Ca2+] (100 nM). Calcium levels drop during illumination, because the steady-state [Ca2+] in the outer seg ment is the result of outward pumping of Ca+2 through the Na+-Ca2+ exchanger of the plasma membrane and inward movement of Ca2 through open cGMP-gated channels. In the dark, this produces a [Ca2+] of about 500 nM—enough to inhibit cGMP synthesis. After brief illumination, Ca2+ entry slows and [Ca2+] declines (step 6). The inhibition of guanylyl cyclase by Ca2 is relieved, and the cyclase converts GTP to cGMP to return the system to its prestimulus state (step 7).
FIGURE 12–34 Molecular consequences of photon absorption by rhodopsin in the rod outer segment. The top half of the figure (steps 1 to 5) describes excitation; the bottom (steps 6 to 9 ), recovery and adaptation after illumination.
