Activation of adenylate cyclase

Figure 13.3 G protein-mediated activation of adenylate cyclase by hormone binding. Hormone binding on the extracellular side of a receptor such as the P adrenergic receptor activates a G protein on the cytoplasmic ATP side. The activated form of the G protein...

Group II assays consist of those monitoring cellular second messengers. Thus, activation of receptors to cause Gs-protein activation of adenylate cyclase will lead to elevation of cytosolic or extracellularly secreted cyclic AMP. This second messenger phosphorylates numerous cyclic AMP-dependent protein kinases, which go on to phosphorylate metabolic enzymes and transport and regulatory proteins (see Chapter 2). Cyclic AMP can be detected either radiometrically or with fluorescent probe technology. 

Cholera toxin, heat labile coli toxins Gs proteins ADP-ribosylation Activation of adenylate cyclase (cholera, traveler -d iarrhea)... 

It is perhaps not surprising that DA produces such mixed effects. The Di receptor is primarily linked to the activation of adenylate cyclase and then protein kinase A. The response to its activation will therefore depend on the ion channels and other proteins modulated by the kinase which can vary from one neuron to another. Since the D2 receptor is not so closely associated with just one G-protein, this gives it the potential for even more effects (see Greenhoff and Johnson 1997). 

Histamine receptors were first divided into two subclasses Hi and H2 by Ash and Schild (1966) on the basis that the then known antihistamines did not inhibit histamine-induced gastric acid secretion. The justification for this subdivision was established some years later when Black (see Black et al. 1972) developed drugs, like cimetidine, that affected only the histamine stimulation of gastric acid secretion and had such a dramatic impact on the treatment of peptic ulcers. A recently developed H2 antagonist zolantidine is the first, however, to show significant brain penetration. A further H3 receptor has now been established. It is predominantly an autoreceptor on histamine nerves but is also found on the terminals of aminergic, cholinergic and peptide neurons. All three receptors are G-protein-coupled but little is known of the intracellular pathway linked to the H3 receptor and unlike Hi and H2 receptors it still remains to be cloned. Activation of Hi receptors stimulates IP3 formation while the H2 receptor is linked to activation of adenylate cyclase. 

Here, the agonist-receptor complex (AR) combines with a G-protein (G) to form a ternary complex (ARG ), which can initiate further cellular events, such as the activation of adenylate cyclase. However, this simple scheme (the ternary complex model) was not in keeping with what was already known about the importance of isomerization in receptor activation (see Sections 1.2.3 and 1.4.3), and it also failed to account for findings that were soon to come from studies of mutated receptors. In all current models of G-protein-coupled receptors, receptor activation by isomerization is assumed to occur so that the model becomes ... 

FIGURE 7.1 Martin Rodbell s conception of the role of the G-protein transducer in the activation of adenylate cyclase by glucagon. (From Birnbaumer, L., FASEB J., 4, 3178, 1990. With permission.)... 

The effect of stimulation of cardiac adrenoceptors is even more leisurely because several more steps follow activation of the Gs protein by the p-adrenoceptor. For example, to increase the force of cardiac contraction, we have (1) activation of adenylate cyclase by Gas-GTP, (2) formation of cAMP, (3) activation of protein kinase A by the cAMP, then (4) phosphorylation of the calcium channel protein by the kinase. As a result, it takes about 5 to 6 sec from the time the receptors are... 

Moss J, Vaughan M Activation of adenylate cyclase by choleragen. Annu Rev Biochem 1979 48 581-600. 

Mehorta and coworkers (1989) observed that isolated fractions of brain and heart cells from rats orally administered 0.5-10 mg endrin/kg showed significant inhibition of Ca+2 pump activity and decreased levels of calmodulin, indicating disruption of membrane Ca+2 transport mechanisms exogenous addition of calmodulin restored Ca+2-ATPase activity. In vitro exposure of rat brain synaptosomes and heart sarcoplasmic reticuli decreased total and calmodulin-stimulated calcium ATPase activity with greater inhibition in brain preparations (Mehorta et al. 1989). However, endrin showed no inhibitory effects on the calmodulin-sensitive calcium ATPase activity when incubated with human erythrocyte membranes (Janik and Wolf 1992). In vitro exposure of rat brain synaptosomes to endrin had no effect on the activities of adenylate cyclase or 3, 5 -cyclic phosphodiesterase, two enzymes associated with synaptic cyclic AMP metabolism (Kodavanti et al. 1988). 

Figure 6.1. Regulation of adenylate cyclase activity by G-proteins. Occupancy of receptors such as the /3-adrenergic receptor result in the activation (+) of adenylate cyclase via coupling through stimulatory G-proteins (Gs). Alternatively, occupancy of receptors such as the 2-adrenergic receptor inhibit (-) adenylate cyclase via coupling through inhibitory G-proteins (Gj).

The sequences of events that occur during activation of adenylate cyclase after receptor occupancy are shown in Figure 6.3. This scheme thus shows activation of a Gofc-type protein (i.e. a process that leads to the activation of adenylate cyclase), whereas similar processes will occur with a Ga protein, except that the interaction with adenylate cyclase will result in its inactivation. In the same way, activation of phospholipases by mobile Ga-type subunits will occur via similar mechanisms. In the unstimulated state, Gas (or Gcq) is bound to GDP. Binding of the receptor with its agonist induces a conformational change in the receptor that activates its G-protein. This stim-... 

All of these experimental approaches have been adopted in neutrophil studies to show that activation of several receptor-mediated functions occurs via the participation of heterotrimeric G-proteins. In many cases, the conventional Gai/Gas nomenclature is used to describe these G-proteins, even though the subunits may not be linked to either inhibition or activation of adenylate cyclase. The nomenclature used is based on structural and functional similarities to other Ga-subunits in other cell types, and also on their sensitivities to cholera and pertussis toxins. Several of these G-proteins... 

Metabotropic receptors, in contrast, create their effects by activating an intracellular G protein. The metabotropic receptors are monomers with seven transmembrane domains. The activated G protein, in turn, may activate an ion channel from an intracellular site. Alternately, G proteins work by activation or inhibition of enzymes that produce intracellular messengers. For example, activation of adenylate cyclase increases production of cyclic adenosine monophosphate (cAMP). Other effector mechanisms include activation of phospholipases, diacylglycerol, creation of inositol phosphates, and production of arachidonic acid products. Ultimately, these cascades can result in protein phosphorylation. 

Figure 6.34 Effects of glucagon and insulin on the cyclic AMP level. Glucagon increases the activity of adenylate cyclase, which increases the concentration of cyclic AMP whereas insulin activates the phosphodiesterase which hydrolyses cyclic AMP to form AMP. Cyclic AMP activates protein kinase A.

Figure 12.21 Effect of the hormone-receptor complex on activation of the G-protein and the resultant effect on the activation of adenyl cyclase. The hormone bind to the receptor to produce the hormone-receptor complex that activates to G-protein.

Cholera Vibrio cholerae cholera Results in activation of adenylate cyclase which elevates cAMP, leading to changes in intestinal epithelial cells that cause loss of water and Na ions from the cells to the lumen of the intestine... 

Relaxation of smooth mnscles is controlled by the concentration of cyclic GMP in the muscle. This is regulated by the activities of the enzyme that forms cyclic GMP (i.e. gnanyl cyclase) and the enzyme that degrades cyclic GMP, that is, cyclic GMP phosphodiesterase (see Box 12.2). This is analogons to the enzyme system that regulates the concentration of cyclic AMP, by the activities of adenyl cyclase and phosphodiesterase ... 

For example, the stimulation of )6-adrenoceptors by noradrenaline results in the activation of adenylate cyclase on the irmer side of the nerve membrane. This enz)nne catalyses the breakdown of ATP to the very labile, high-energy compound cyclic 3,5-adenosine monophosphate (cyclic AMP). Cyclic AMP then activates a protein kinase which, by phosphorylating specific membrane proteins, opens an ion charmel to cause an efflux of potassium and an influx of sodium ions. Such receptors are termed metabotropic receptors. 

Seamon KB, Padgett W, Daly JW. (1981) Forskolin Unique diterpene activator of adenylate cyclase in membranes and in intact cells. Proc Natl Acad Sci USA 78 3363-3367. 

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