Page 363 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
In darkness (! D, left), cGMP is bound to ducin. GAP (GTPase-activating protein) accel-
cation channels (Na , Ca ) in the outer seg- erates the regeneration of transducin. Phos-
+
2+
ment of the photosensor, thereby keeping ducin, another protein, is phosphorylated in
+
them open. Na and Ca 2+ can therefore enter the dark (! D6) and dephosphorylated in light
the cell and depolarize it to about – 40 mV (! D7). The latter form binds to the "γ subunit
(! D3, D4). This darkness-induced influx into (! D7, E3), thereby blocking the regeneration
the outer segment is associated with the efflux of transducin. This plays a role in light adapta-
tion (see below).
+
of K from the inner segment of the sensor. The
Central Nervous System and Senses exchanger (! p. 36), so the cytosolic Ca 2+ con- unit of cGMP phosphodiesterase (I PDE) is re-
2+
Ca
! Phosphodiesterase (PDE). In the course of
entering the outer segment is immedi-
+
ately transported out of the cell by a 3 Na /Ca
transducin regeneration, the inhibitory sub-
2+
centration [Ca ] i remains constant at ca. 350–
2+
leased again and PDE is thus inactivated.
+
2+
500 nmol/L in darkness (! D6). If the cytosolic
exchanger still
! cGMP. Since the 3 Na /Ca
cGMP concentration decreases in response to a
functions even after photostimulation-in-
duced closure of Ca
2+
light stimulus (! D2), cGMP dissociates from
2+
channels, the [Ca ] i
starts to decrease. When a threshold of ca.
the cation channels, allowing them to close.
2+
100 nmol/L is reached, the Ca -binding pro-
The photosensor then hyperpolarizes to ca.
tein GCAP (guanylyl cyclase-activating pro-
– 70 mV (sensor potential: ! D, right). This in-
2+
lyl cyclase, thereby accelerating cGMP synthe-
the sensor pedicle (! D5), which subsequently
causes changes in the membrane potential in
sis. Thus, the cGMP concentration rises, the
downstream retinal neurons (! p. 354).
cation channels re-open, and the sensor is
12 hibits the release of glutamate (transmitter) at tein) loses its 4 Ca ions and stimulates guany-
2+
ready to receive a new light stimulus. This Ca
Deactivation of Photic Reactions and cycle therefore mediates a negative feedback
Regeneration Cycles loop for cGMP production.
! Rhodopsin (! E2). Rhodopsin kinase (RK)
competes with transducin for bindings sites on Ca 2+ Ions and Adaptation (see also p. 352)
metarhodopsin II (MR-II); the concentration of In the dark, the [Ca ] i is high, and calmodulin-
2+
transducin is 100 times higher (! E2, right). bound Ca 2+ (! p. 36) stimulates the phospho-
Binding of RK to MR-II leads to phosphoryla- rylation of phosducin with the aid of cAMP and
tion of MR-II. As a result, its affinity to trans- phosphokinase A (! D6). In light, the [Ca ] i is
2+
ducin decreases while its affinity to another low; phosducin is dephosphorylated and rapid
protein, arrestin, rises. Arrestin blocks the regeneration of transducin is not possible
binding of further transducin molecules to (! D7, E3). Moreover, Ca 2+ accelerates the
MR-II. All-trans-retinal detaches from opsin, phosphorylation of MR-II in light with the aid
which is subsequently dephosphorylated and of another Ca 2+ binding protein, recoverin
2+
re-loaded with 11-cis-retinal. (! E2). Ca is therefore essential for the adap-
! All-trans-retinal (! E1) is transported out of tation of photosensors (! p. 352).
the photosensor and into the pigmented Although they contain similar enzymes and
epithelium, where it is reduced to all-trans- transmittes, the photosensitivity of the cones
retinol, esterified and ultimately restored to is about 100 times less than that of the rods.
11-cis-retinal. After returning into the photo- Thus, the cones are unable to detect a single
sensor, it binds to opsin again, thereby com- quantum of light, presumably because photic
pleting the cycle (! E2). reactions in the cones are deactivated too
! Note: Retinol is vitamin A 1. A chronic deficiency of quickly. Compared to rhodopsin in the retinal
vitamin A 1 or its precursors (carotinoids) leads to im- rods, the three visual pigments in the three
paired rhodopsin production and, ultimately, to types of cones (11-cis-retinal with different
night blindness (! p. 352). opsin fractions) only absorb light in a narrow
! Transducin (! E3). Since the GTPase activity wavelength range (! p. 357 E), which is a pre-
of α s-GTP breaks down GTP into GDP + P i, the requisite for color vision (! p. 356).
350 molecule deactivates itself. The α s-GTP
molecule and "γ subunit then reunite to trans-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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