What is the organ distribution of the muscarinic receptors? Recall the intracellular effects of the stimulation of these receptors. What are the effects of stimulation of these receptors on the tissues?
Odd-numbered muscarinic receptors: increase intracellular Ca2+ through IP3 system
Even-numbered muscarinic receptors: decrease cAMP through inhibitory G-protein
M1: gastric parietal cells. As are adrenergic receptors, M1 receptors are found in other parts of the NS outside of the ANS-CNS and peripheral neurons
M2 receptors: heart (atria, SA and AV nodes), presynapatic terminals of peripheral and central neurons
M3: smooth muscle, exocrine glands (sweat glands), and endothelial cells
M4 and M5 have recently been discovered by cloning.
True or false: The same receptors found in the ANS are also present within the somatic and CNS. The rpnicples for modes of action of receptors within the ANS also apply to the CNS.
Smooth muscle of bronchi contain both alpha 1 and B2 adrenergic receptors. What is the dominant sympathetic repsonse of bronchiole smooth muscle? How may an asthmatic benefit from using a B2 agonist?
Dilation caused by activation of B2 receptors is the dominant sympathetic response. Activation of the B2 receptors in mast cells inhibits histamine release-benefits an asthmatic by improving ventilation (due to inhibition of inflammatory response)
True or false: The precise distribution of receptors has pharmacologic importance. For example, B1 receptors are located in the heart and B2 receptors are found in bronhci.
Which alpha adrenergic receptor is predominant in smooth muscle?
For vascular smooth muscle, where in the body are the largest effects observed due to alpha adrenergic stimulation?
What are the effects of the stimulation of alpha adrenergic receptors on smooth muscle? (intracellularly as well as big picture-effects on arteries, veins, arterioles).
Which alpha receptor is mainly responsible for the response of smooth muscle to adrenergic stimulation?
alpha adrenergic receptors in smooth muscle are mainly alpha 1, although there are alpha 2 receptors in vascular smooth muscle. In general, smooth muscle contracts in response to alpha adrenergic stimulation. Alpha 1 adrenergic activation causes contraction by release of calcium via the IP3 pathway. Alpha 2 receptors are diffuse throught the surface of smooth muscle fibers and their stimulation causes contraction by lowering levels of cAMP which results in decreased PKA activity.
For vascular smooth muscle, the vasoconstriction is particularly strong in the skin and splanchnic beds. Constriction of large arteries, veins, and arterioles results in decreased compliance, increased CVP, and increased peripheral resistance. This leads to an increase in systolic and diastolic arterial pressure. This increased pressure activates the baroreceptor reflexes which cause reflex bradycardia.
Alpha 1 receptors are close to the sites of pre-synaptic release and are therefore mainly responsible for neurally controlled vasoconstriction.
How are M3 receptors stimulated by sympathetic innervation in vascular smooth muscle? What are the effects of the stimulation of these receptors?
ACh released by the sympathetic NS causes vasodilation of vascular beds through M3 receptors in endothelial cells. Remember that the parasympathetic NS does not innervate vascular smooth muscle. Sympathetic cholinergic stimulation of blood vessels does not occur via direct stimulation of smooth muscle cells but rather through endothelial cells. Endothelial cells line the lumen of blood vessels. Stimulation of M3 receptors leads to an increase in intracellular Ca2+ which acivates nitric oxide synthase (NOS) which leads to nitric oxide (NO) production in the lining endothelial cells. Bc NO is a gas, it easily diffuses through to the smooth muscle. Once there, NO stimulates guanylate cyclase which produces cGMP from GTP. cGMP activates PKG. Through activation of PKG, uptake of Ca2+ by the SR Ca2+ pump is increased (through PKG phosphorylation of phospholamban) and phosphorylation of PLC (inhibiting Ca2+ release). PKG also increases K+ permeabilty through opening K+ channel(s) which hyperpolarizes the membrane and further promotes vasodilation (Ca2+ channels cannot be opened as easily due to lower Vm). Also, cGMP activates MLC phosphatase which removes the phosphate from myosin and therefore inactivates it and promotes vasodilation.
When does stimulation of alpha 1 adrenergic receptors not cause smooth muscle contraction? In this cause, how does it lead to relaxation?
Alpha 1 adrenergic stimulation casues relaxation of the smooth muscle in the GI tract. This occurs via increases in K+ permeability which causes hyperpolarization.
B adrenergic stimulation causes what response in smooth muscle?
Which B receptor causes these effects in bronchiolar smooth muscle and through what pathway?
What is the other (lesser) pathways through which this receptor can exert its effects?
What is the effect of B adrenergic stimulation of GI smooth muscle?
B adrenergic stimulation usually causes relaxation of smooth muscle. In bronchiolar smooth muscle, activation of B2 receptors is responsible for bronchodilation. Activation of these receptors leads to increases in cAMP which activates PKA which phosphorylates and inactivates MLCK which leads to relaxation.
Above is the main pathway for relaxation but B stimulation can also cause extrusion of Ca2+ from smooth muscle cells which augments relaxation. In bronchiolar smooth muscle, PKA also phosphorylates a K+ channel which hyperpolarizses the cell membrane and thereby further promotes relaxation.
B adrenergic stimulation leads to relaxation of GI smooth muscle and dilation of blood vessels.
Through what receptors does the parasympathetic NS exert its effects on smooth muscle? Discuss its effects on the GI tract and bladder.
What intracellular cascades result due to stimulation of these receptors?
The PS nervous system generally innervates smooth muscle other than those of blood vessels. These smooth muscle cells, as a rule, contain M3-receptors and parasympathetic stimulation leads to muscle contraction. This follows from M3 stimulation, through IP3, causing a rise in intracellular Ca2+, directly causing contraction. Thus tone and amplitude of contraction, as well as peristalsis, is increased in the G.I. (stomach and intestines) system. In the case of smooth muscle of the bladder, it appears that M1 receptors are more important than M3 receptors. M1-stimulation causes a rise in intracellular Ca2+, through IP3, in exactly the same way as does M3 stimulation. Consequently, independent of whether M1 or M3 receptors dominate, PS stimulation results in the contraction of the bladder smooth muscle that leads to bladder emptying. The secretory activity of the G.I. system is also enhanced, through M3 receptors. In the case of gastric acid secretion, secretion is stimulated through activation of M1 receptors.
Through what receptors does the sympathetic NS exert its effects on the pacemaker cells of the heart? What inctracellular cascade results?
The sympathetic post-ganglionic neurons innervate all tissues of the heart. The adrenergic receptors of the myocardium are predominantly B1. B1-adrenergic activation leads to a rise in cAMP, which stimulates PKA. Stimulation of PKA leads to phosphorylation of Ca2+ channels, allowing them to open at lower voltages and increasing the time these channels are open. This causes a depolarization within the SA and AV nodes and therefore a faster heart rate (positive chronotropic effect). The pacemaker current, If, is also affected by B1-adrenergic stimulation. But cAMP appears to directly interact with the pacemaker channels rather than through stimulation of PKA. Rises in cAMP levels shifts the activation curve of If to more positive potentials. As a consequence, the inward current is greater for every membrane potential.
Through what receptors does the parasympathetic NS exert its effects on the pacemaker cells of the heart? What inctracellular cascade results?
Parasympathetic activation leads to cardiac slowing (decreased heart rate–negative chronotropic effect-- and decreased conduction velocity in the SA and AV nodes) and somewhat decreased cardiac output (negative inotropic effect in atria). Reduced cardiac output is mainly due to decreased force of contraction in the atria: ventricles have sparse PS innervation. The innervation is through M2-receptors (abundant in nodal and atrial regions, sparse in the ventricle), which in general causes a reduction in cAMP. This is directly opposite to sympathetic B1-stimulation accounting for reciprocal effects caused by the S and PS systems. M2-receptors also open K+ channels, via direct coupling through G-proteins (probably through beta and gamma subunits). The consequent hyperpolarization directly leads to decreased heart rate.
What effects does the sympathetic NS have on cardiac myocytes? Through what receptor does it exert its effects?
B1 receptor stimulation leads to a rise in cAMP and activation of PKA. PKA phosphorylates L-type Ca2+ channels which leads to increased Ca2+ influx and therefore increased calcium induced Ca2+ release-greater force of contraction-positive inotropic effect. PKA also phosphorylates the inhibitory phospholamban of the SR Ca2+pump leading to increased activity of the SR Ca2+ pump. This helps with faster relaxation as well as increasing the amount of Ca2+ available for release for the next cycle of contraction. Enhanced rate of relaxation is also promoted by phosphorylation of troponin which decreases the affinity of TnC for Ca2+. PKA also phosphorylates delayed recitifier K+ channels which depolarize the membrane faster and result in higher HR. Additionally, PKA phosphorylates and stimulates Na+/K+ ATPase.
What effects does the parasympathetic NS have on cardiac myocytes? Through what receptor does it exert its effects?
M2 stimulation has dual effects. It hyperpolarizes the membrane through opening of K+ channels through coupling to Bgamma subunits of the Gi protein which inhibits opening of L-type Ca+ channels. More importantly for the contraction of myocytes, binding of the inhibitory alpha subunit to AC results in dc cAMP production wihc leads to less PKA. This negative inotropic effect is directly opposite to positive intropy produced by sympathetic stimulation.
The S and PS systems, as we know, exhibit reciprocal effects on the level of the myocardium. How do the S and PS have reciprocal effects at the level of NT release? What other level do they have reciprocal effects on one another? (hint: reflex)?
The S and PS systems not only exhibit reciprocal effects on the level of myocardium (e.g., through affecting cAMP concentrations), but their synapses are found in close proximity. There are reports that release of NE inhibits release of ACh and vice versa. For example, activation of M2 muscarinic receptors on the nerve terminals of NE releasing cells would lead to a decrease in cAMP levels which would reduce Ca2+ influx and in turn reduce NE release. Also remember, that through the baroreceptor reflex, stimulation of S depresses activity of PS, and vice versa.
What are the overall general effects of sympathetic stimulation on metabolism? (discuss what tissues are involved, what metabolic substances are regulated, etc.)
Sympathetic stimulation directly converts energy stored as glycogen and fat to forms that are readily utilizable -- glucose and free fatty acids. Glycogen breakdown occurs in liver and skeletal muscle. In liver, the breakdown of glycogen leads directly to an increase in blood glucose. In contrast to liver, skeletal muscle glycogen is not broken down to free glucose, but to lactate which passes into the blood. Blood lactate can be used for liver gluconeogenesis. Adrenergic stimulation also indirectly raises blood glucose levels via altering glucagon and insulin release from the pancreas.
How is glycogen/glucose regulated through sympathetic innervation? (what receptor, what proteins are activated and inactivated)
B1 adrenergic stimulation leads to phosphorylation and activation of phosphorylase kinase (see attached pic). Phosphorylase kinase activates glycogen phosphorylase resulting in the breakdown of glycogen to glucose. B1 adrenergic stimulation also leads to phosphorylatoion of a phosphatase inhibitor. When phosphorylated, this inhibitor protein binds to the phosphatase and inhibits it-cannot undo effects of kinases. Thus, by causing activation of phosphorylase kinase and inhibiton of the phosphatase, B adrenergic stimulation has a larger effect on glycogen metabolism than it would be if it effected only one enzyme or the other.
The cAMP and Ca2+ signalling pathways interact at several levels. For example, the phosphorylase kinase of skeletal muscle is a multisubunit enzyme (pg 95 of notes). One subunit is catalytic, the other three regulatory. Of the three regulatory subunits, one is calmodulin and confers Ca2+-dependence to the enzyme. As a result, the Ca2+ that initiates muscle contraction also insures that there is sufficient glucose to sustain contraction. Ca2+ binding to calmodulin increases kinase activity which leads to increased breakdown of glycogen to glucose. The other two regulatory subunits of phosphorylase kinase are regulated by cAMP: they can be phosphorylated by PKA. Therefore, the rise of epinephrine that results from stimulation of the sympathetic nervous leads to PKA activation. This leads to increased glycogen metabolism to glucose, allowing for greater muscle activity. In short, the sympathetic response provides an anticipatory response for skeletal muscle activity; increased intracellular Ca2+ not only promotes contraction, but also metabolically allows for sustained use.
Through what receptors on the pancreas does the sympathetic NS have an effect on insulin release? What is the dominant effect on insulin release (increased or decreased)?
Insulin secretion is inhibited via alpha 2-receptors, but enhanced by B2-receptors. The dominant effect for epinephrine is inhibition of insulin secretion. This is through a net decrease in cAMP levels, which results in a smaller influx of Ca2+ through pancreatic beta cells. Lower intracellular Ca2+ leads to less exocytotic release of insulin.
Through what receptors does sympathetic innervation effect adipocytes? What is the effect on adipocytes?
Simulation of B3 receptors on adipocytes leads to activation of triglyceride lipase through cAMP. This causes breakdown of triglycerides which raises the concentration of free FA in the blood.
True or false: The parasympathetic effect on metabolism is relatively unimportant.
True or false: The movement of water is always a consequence movement of solutes: in order to account for secretions, solute (i.e., ion) movement is key. In essence, secretion occur via changes in ion permeabilities: movement of salt changes tonicity; water flow restores tonicity. Therefore, in order to understand how the ANS controlssecretion, it is necessary to appreciate how ANS stimulation affects ion permeability across membranes and through tight junctions.
True or false: Through both alpha 1 and B receptors, secretion is directly enhanced in salivary and lacrimal glands. The enhancements are not as significant as produced by the PS. However, stimulation of the sympathetic nervous system can somewhat inhibit net secretion through reduced blood flow caused by vasoconstriction.
What are the effects of the parasympathetic NS on exocrine gland secretion? What receptor mediates this response? What intracellular changes occur as a result? Use secretion from acinar cells as examples.
Innervation is through M3-receptors. Stimulation produces mainly excitatory effects such as increased glandular secretions from salivary glands.(Activation of M3 receptors also stimulates sweating. But note that secretion through sweat glands is a cholinergic, sympathetic response.) M3 activation causes increases intracellular calcium which in turn opens Ca2+-activated K+ channels and Ca2+-activated Cl- channels. Both cations and anions (i.e. salt) move through channels, and water movement follows. The rise in intracellular Ca2+ also causes protein secretion via exocytotic release.
For example, in acinar cells (cells of exocrine glands that secrete material by exocytosis into an acinus--a small cavity that leads into ducts), muscarinic activation of M3-muscarinic receptors (in the basolateral membrane) leads to K+ and Cl- movement through the apical membrane into the lumen with water following (attached figure). In addition, upon stimulation with ACh, tight junctions open (by unknown mechanisms) so that NaCl passes from the basolateral side to the lumenal (apical) side, with water following. Thus secretion is via paracellular as well as cellular routes.
How may the ANS play a role in cancer? (use prostate CA as an example)
It has recently become clear that sympathetic and parasympathetic innervations can play important roles in the development and progression of cancer. The prostate gland receives both S and PS inputs. Consider prostate cancer as a concrete example:
It is well known that transformed cells release factors that induce the proliferation of blood vessels supplying the tumor, neoangiogensis,. In an analogous manner, cancer cells secrete substances (e.g., transforming growth factor B1) that induce neurite outgrowth and axoneogenesis into the tumor. For the prostate gland, sympathetic adrenergic outgrowth is critical in promoting early phases of CA development. The release of ACh by post-ganglionic parasympathetic neurons on stromal cells (notalbly fibroblasts and smooth muscle cells) stimulates invasion, migration, and metastasis of prostate cancer cells.
True or false: Norepinephrine (NE) and acetylcholine (ACh) are the classical neurotransmitters of the autonomic nervous system. But many substances coexist in adrenergic and cholinergic nerves and are thus secreted along with NE and ACh. Some have important post-synaptic effects. Some of these substances are neurotransmitters, defined as directly causing short term effects; others are neuromodulators, defined as substances that modify neurotransmission. In addition, long-term (trophic) interactions between autonomic nerves and effector cells can influence development and regeneration.
What NTs can ATP be released with? What is the name of the class of receptors that ATP stimulates? Where in the body are these receptors located?
In the sympathetic nervous system, NE and ATP are generally packaged within the same granules and act as cotransmitters. In the parasympathetic nervous system, ACh and ATP can also act as cotransmitters, but this duality is less ubiquitous than for the sympathetic nervous system. Receptors that are activated by ATP (or adenosine) are referred to as “purinergic.” These receptors are widely distributed throughout the brain, as well as most peripheral tissues, such as
the heart and kidney.
What specific purinergic receptors do ATP and ADP act on? What are the effects?
Secretion of ATP acts post-synaptically on P2 purinergic receptors. (ADP also activates P2 purinoceptors.) Without being hydrolyzed, ATP activates P2 purinoceptors, which can lead to significant effects. Whether or not the secreted ATP acts as a neurotransmitter depends on whether or not the target cells contain P2 purinoceptors.
a. P2X purinoceptors mediate fast responses via ligand gated channels. In some arterial smooth muscle cells, for example, P2 stimulation causes vasoconstriction either by directly activating a Ca2+ permeable channel or opening a non-selective cation channel. In either case, second messengers are not generated. The released NE also genrally causes vasoconstriction through activation of alpha 1 adrenergic receptors. ATP and NE thus act synergistically.
b. P2Y purinoceptors mediate slower responses via G proteins.
What specific purinergic receptors do AMP and adenosin act on? What are the effects?
Adenosine and AMP preferentially act on P1 purinoceptors. Activated P1 purinoceptors acts through adenylate cyclase, causing changes in levels of
A large number of peptides are secreted from various autonomic nerves and modulate post-synaptic events. These peptides include substance P, vasoactive intestinal peptide (VIP), somatostatin, and neuropeptide Y (NPY). These peptides are known to have endocrine or neuroendocrine functions and are thought, in most cases, to be neuromodulators rather than neurotransmitters.
What are the effects of substance P on sympathetic ganglia?
What are the effects of NPY on arterial smooth muscle?