Figure 15-44. Cyclic gmp.

Figure 15-45. A rod photoreceptor cell. There are about 1000 discs in the outer segment. The disc membranes are not connected to the plasma membrane.

Figure 15-46. The response of a rod photoreceptor cell to light. Rhodopsin molecules in the outer-segment discs absorb photons. Photon absorption leads to the closure of Na⁺ channels in the plasma membrane, which hyperpolarizes the membrane and reduces the rate of neurotransmitter release from the synaptic region. Because the neurotransmitter acts to inhibit many of the postsynaptic retinal neurons, illumination serves to free the neurons from inhibition and thus, in effect, excites them.

Figure 15-47. Amplification in the light-induced catalytic cascade in vertebrate rods. The divergent arrows indicate the steps where amplification occurs.

Figure 15-48. The roles of G-protein-linked receptor kinases (GRKs) and arrestins in receptor desensitization. The binding of an arrestin to the phosphorylated receptor prevents the receptor from binding to its G protein and can direct its endocytosis. Mice that are deficient in one form of arrestin fail to desensitize in response to morphine, for example, attesting to the importance of arrestins for desensitization.

Table 15-1. Some Hormone-induced Cell Responses Mediated by Cyclic amp

TARGET TISSUE	HORMONE	MAJOR RESPONSE
Thyroid gland	thyroid-stimulating hormone (TSH)	thyroid hormone synthesis and secretion
Adrenal cortex	adrenocorticotrophic hormone (ACTH)	cortisol secretion
Ovary	luteinizing hormone (LH)	progesterone secretion
Muscle	adrenaline	glycogen breakdown
Bone	parathormone	bone resorption
Heart	adrenaline	increase in heart rate and force of contraction
Liver	glucagon	glycogen breakdown
Kidney	vasopressin	water resorption
Fat	adrenaline, ACTH, glucagon, TSH	triglyceride breakdown

Table 15-2. Some Cell Responses in Which G-Protein-linked Receptors Activate the Inositol-Phospholipid Signaling Pathway

TARGET TISSUE	SIGNALING MOLECULE	MAJOR RESPONSE
Liver	vasopressin	glycogen breakdown
Pancreas	acetylcholine	amylase secretion
Smooth muscle	acetylcholine	contraction
Blood platelets	thrombin	aggregation

Table 15-3. Three Major Families of Trimeric G Proteins*

FAMILY	SOME FAMILY MEMBERS	ACTION MEDIATED BY	FUNCTIONS
I	Gs		activates adenylyl cyclase; activates Ca2+ channels
	Golf		activates adenylyl cyclase in olfactory sensory neurons
II	Gi		inhibits adenylyl cyclase
			activates K+ channels
	Go		activates K+ channels; inactivates Ca2+ channels
		 and 	activates phospholipase C-
	Gt (transducin)		activates cyclic GMP phosphodiesterase in vertebrate rod photoreceptors
III	Gq		activates phospholipase C-
* Families are determined by amino acid sequence relatedness of the  subunits. Only selected examples are shown. About 20  subunits and at least 4  subunits and 7  subunits have been described in mammals.