A link to the top 200 prescription drugs (by number of US prescriptions) for 2001 can be found at this link http://www.rxlist.com/top200.htm. Of this number, 20 products (10 % of the total) contain drugs to be discussed in these 3 lectures. Therefore, a clear understanding of these drugs and the receptors that they act upon is of obvious importance and relevance.

Learning Objectives, Lecture I

1. Integrate pharmacodynamic principles to aid in the understanding of adrenergic receptors and the actions of drugs on these receptors.
   
2. Understand the criteria upon which alpha and beta receptors are defined.
   
3. Understand the second messenger systems utilized by alpha and beta receptors and how activation of these receptors leads to a change in physiologic function.
   
4. Understand the effects of alpha and beta receptor activation on the heart and blood vessels.
   
5. Understand the effects of isoproterenol, epinephrine and norepinephrine on the cardiovascular system.

Isoproterenol - Isuprel

Epinephrine - Adrenalin

Norepinephrine- Levophed

The adrenergic receptors which subserve the responses of the sympathetic nervous system have been divided into two discrete subtypes: alpha adrenergic receptors (alpha receptors) and beta adrenergic receptors (beta receptors). The classification of these receptors, and indeed receptors in general, is based on the interaction of agonists and antagonists with the receptors.

	PUTATIVE STRUCTURE OF ADRENERGIC RECEPTORS
PROPOSED BINDING OF NE TO THE BETA RECEPTOR

Beta receptors have been further subdivided into beta₁ and beta₂ receptors. It should be pointed out that beta₃ and beta₄ receptors have recently been isolated, cloned and characterized. The beta₃ receptor may be involved in regulating the metabolism of fatty acids. This receptor could be the site of antiobesity drugs in the future. The functions of the beta₄ receptor remains to be discovered. For the purposes of this material we will focus on the beta₁ and beta₂ receptors only. The classification of beta receptors is based on the interaction of a series of drugs with these receptors. The ability of epinephrine, norepinephrine and isoproterenol to increase the force of myocardial contraction was examined and the dose-response curves shown below were obtained. Equilibrium dissociation constants for these ligands were ISO, 80 nm, E, 800 nM, and NE, 1000 nM. Thus, the rank order of affinities for the beta receptor in the heart is ISO>E>NE. A beta receptor with these characteristics is referred to as a beta₁ receptor. The equilibrium dissociation constant is often used as a "finger print" to identify a receptor. Regardless of its location, the receptor will interact in the same manner with ligands and have the same dissociation constants for agonists and antagonists. The logic above is a standard approach to differentiate receptors. The interaction of agonists and antagonists with receptor systems often indicates heterogeneity within the main receptor population. These different receptors are referred to as receptor subtypes. Receptor subtypes are routinely exploited in drug development to make ligands that interact selectively with one subtype in preference to another. Specific examples of this principle are presented in these lectures and throughout the course.

The ability of the same compounds to produce bronchodilation was examined and a different set of dose response curves and equilibrium dissociation constants were obtained. The dissociation constants were ISO, 80 nm, E, 800 nM, and NE, 10,000 nM. Notice how the ability to active the beta receptors is dependent on the structure of the drugs under study. Clearly then the receptor in the lung is different from that in the heart and is referred to as a beta₂ receptor.

Most tissues express multiple receptors. However, the receptor mainly utilized by the sympathetic nervous system to affect myocardial function in the heart is the beta₁ receptor; while in vascular and nonvascular smooth muscle it is the beta₂ receptor.

Effect of Beta Receptor Activation on the Heart:

Activation of the beta₁ receptor leads to increases in contractile force and heart rate. The increase is myocardial contraction is a result of activation of those beta receptors associated with the atria and ventricle (especially the ventricles) while the increases in rate of contraction are due to activation of those receptors associated with the SA and AV nodes as well as the His-Purkinjie system.

1. Increase slope of phase 4 spontaneous depolarization.
   

2. Increase in maximal rate of phase 0 depolarization.
   

3. Increase conduction velocity.
   

4. Decrease refractory period.

These electrophysiologic factors contribute to the orderly, rhythmic electrical activity that assures the efficient contractile activity of the heart. In response to beta receptor activation, these parameters increase and the heart beats at a faster rate. However, excess stimulation of the beta receptor by catecholamines can enhance these variables to such an extent that arrhythmias can occur. Rhythm disturbances are a major concern with drugs that activate the beta₁ receptor. Drugs to be covered that have a tendency to generate arrhythmias include epinephrine, isoproterenol, norepinephrine, dopamine and dobutamine.

Effect of Beta Receptor Activation on Smooth Muscle

The beta₂ receptor associated with smooth muscle also utilizes the cAMP signaling system. However, the results of receptor mediated increases in cAMP levels in smooth muscle are different than those occurring in cardiac muscle. Therefore, steps 1-5 in the diagram would be the same and PKA would be activated. However, the consequences of PKA phosphorylation of key structures in smooth muscle lead to relaxation.

Results of PKA Phosphorylation in Smooth Muscle

1. sarcolemma - Decrease Ca²⁺ influx, increase Ca²⁺ efflux -step 7 in diagram.
   

2. sarcoplasmic reticulum - Enhance Ca²⁺ uptake -step 8 in diagram.
   

3. decrease actin-myosin interactions - muscle relaxation- step 9 in diagram

The net result of these activities is to inhibit calcium pathways in smooth muscle leading to relaxation.

ALPHA RECEPTORS SYSTEMS:

If the ability of isoproterenol, epinephrine and norepinephrine to produce constriction of vascular smooth muscle is studied, the following dose-response curves and equilibrium dissociation constants were obtain E, 5 uM, NE, 6, uM and ISO, 1000 uM. You should begin to understand the reasons why the receptor causing vasoconstriction MUST be different from that causing cardiac contraction or broncodilation.

The receptor that produces vasoconstriction is referred to as an alpha receptor (affinity rankings of E $ NE >>>ISO). Observe how the structure of each drug affects that ability of these ligands to activate the alpha receptor. The concentration of isoproterenol necessary to activate alpha receptors is so large that isoproterenol can be thought of as a pure beta receptor agonist.

Affinity receptors have also been subdivided into alpha₁ and alpha₂ receptors. Epinephrine and norepinephrine have equal affinity at both alpha₁ and alpha₂ receptors. However, other drugs were found to have higher affinity for one receptor over another. These differences in affinity were the evidence used to subclassify the receptors into alpha₁ and alpha₂. More recently, three subtypes of the alpha₁-receptor, the alpha_1A, alpha_1B and alpha_1D have been isolated, cloned and characterized. Similarly, 3 subtypes of the alpha₂-receptor, the alpha_2A, the alpha_2B and the alpha_2C have also been identified. There is little doubt that these receptor subtypes subserve different physiologic functions. Pharmacologic agents are being developed which have the potential to selectively activate or block one of these receptors in preference to another. However, for the purposes of this course, it is necessary to remember only the alpha₁ and alpha₂ receptors.

Alpha₁ and alpha₂ receptors exist postsynaptically. Like the beta receptor, these receptors are G-protein coupled receptors, thus they activate cellular signaling subsequent to interaction with a G-protein. Activation of these receptors on vascular smooth muscle leads to vasoconstriction. The mechanism linking the alpha₂ receptor to contraction is not well understood.

Mechanism of Alpha1 Receptor Activation of Smooth Muscle Contraction

In the case of the postjunctional alpha₁ receptor, the inositol phosphate/diacylglycerol signaling pathway is activated by receptor occupancy.

Alpha₂ receptors exist presynaptically. Activation of these receptors inhibits the release of norepinephrine. The mechanism of this regulatory involves the stimulation of a G-protein gated K⁺ channel leading to membrane hyperpolarization.

Norepinephrine acts at presynaptic alpha₂ receptors to inhibit its own release.

Effect of Catecholamines on Vascular Smooth Muscle Epinephrine

Associated with vascular smooth muscle are a large number of alpha₁ receptors relative to beta₂ receptors. However, epinephrine has a higher affinity for the beta₂ receptor relative to the alpha₁ receptor. Activation of the beta₂ receptor would produce vasodilation while activation of the alpha₁ receptor would result in vasoconstriction. Recall from the lectures on drug-receptor interactions the magnitude of effect is dependent on the degree of receptor occupancy. Therefore, the effect of epinephrine on smooth muscle is dependent on its relative affinity for alpha₁ and beta₂ receptors and its concentration. At low doses, epinephrine can selectively stimulate beta₂ receptors producing muscle relaxation and a decrease in peripheral resistance. However, once epinephrine concentrations are reached which bind to the alpha₁ receptor, vasoconstriction will occur. The two effects (smooth muscle relaxation and contraction) will oppose one another. However, as the concentration of epinephrine increases, the predominant effect will be vasoconstriction.

Recall that norepinephrine in physiologically relevant concentrations has little affinity for beta₂ receptors. Therefore, it will stimulate only alpha₁ receptors producing an increase in peripheral vascular resistance. In contrast, the lack of activity at the alpha₁-receptor means that isoproterenol will produce only a beta₂-receptor mediated vasodilation.

Alpha₁ receptors also exist on the myocardium. These receptors increase force without affecting rate. The role of these receptors in physiologic regulation of myocardial performance or as a site of drug action is unclear.

Applications to Therapeutics

Oral dosing of norepinephrine, epinephrine and isoproterenol is not possible due to rapid metabolism in gastrointestinal mucosa and liver. Therefore, these agents are given I.V., I.M., topically and in aerosol sprays. There is very limited clinical use of norepinephrine. Epinephrine is often used in combination with local anesthetic agents to prolong the duration of anesthetic action. This also reduces the toxicity of the local anesthetic by limiting its defusion away from the injection site. Epinephrine can also be used to in surgical procedures to reduce blood loss. A major concern with using pressors is the effect on systemic arterial pressure. Clinical studies have shown that epinephrine blood levels increase following its intraoral administration. There is also the risk that epinephrine could increase heart rate. The risk of these increases is dependent on characteristics of the patient. For example, hypertensive patients or those ischemic heart disease or patients taking other drugs that affect sympathetic nervous system function are at higher risk than patients without these conditions. Epinephrine is used in the treatment of various shock syndromes and in emergency situations related to bronchial asthma. Isoproterenol is also used in the acute management of airways dysfunction to produce broncodilation.

For the drugs listed below, indicate how the drugs would affect (increase, decrease, no changes) heart rate, contractile force, total peripheral resistance (TPR) and systemic arterial blood pressure. Recall the equations below. Remember also that the effectors in the cardiovascular system (brain, kidney, heart and blood vessels) are all involved in the integrated regulation of blood pressure.

Blood pressure = Cardiac Output X TPR
Cardiac output = Stroke Volume x Heart Rate
Blood pressure = (Stroke volume x Heart rate) X Total peripheral vascular resistance

Heart Rate Contractile Force TPR Blood Pressure

Be sure to make an attempt at answering the question BEFORE you click on the answer.

1. Understand the potential sites of action for sympathomimetics and sympatholytics.
   
2. Understand the pharmacologic actions and therapeutic effects of dopamine.
   
3. Understand how the pharmacodynamic actions of dopamine illustrate the properties of a drug that interacts with multiple receptors.
   
4. Know that there are beta₂ agonists, their mechanisms of action and therapeutic uses.
   
5. Know the mechanism of action and cardiovascular effects of amphetamine and cocaine.
   
6. Know the effects and therapeutic uses of drugs that can activate the alpha1 adrenergic receptors.

Key Drugs*

Amphetamine-Adderall

Albuterol - Ventolin - 13^th leading prescription drug in the US in 2003- source- rxlist.com

Cocaine

Dopamine - Intropin

Methylphenidate - Ritalin - 102nd leading prescription drug in the US in 2003- source- rxlist.com

Phenylephrine - Neosynephrine

Sympathomimetics: are synthetic analogs of naturally occurring catecholamines that bind to beta or alpha receptors and mimic the actions of the endogenous neurotransmitters. These agents can be divided into direct and indirect acting sympathomimetics.

Sympatholytics: are synthetic analogs which bind to beta or alpha receptors and block the actions of endogenous neurotransmitters or other sympathomimetics.

In addition to interacting with receptors, adrenergic agonists and antagonists can interact at sites on the nerve terminal to produce sympathomimetic or sympatholytic effects. These potential sites are indicated by the numbers. A majority of drugs are direct acting agonists or antagonists. Only a small number of drugs work through the other listed mechanisms.

1. Direct acting agonists or antagonists can act at postsynaptic receptors.
   

2. Indirect acting agonists release neurotransmitters from presynaptic nerve terminals to produce a sympathomimetic effect.
   

3. Drugs such as Guanethidine can inhibit the Ca²⁺-dependent release of norepinephrine and thus have a sympatholytic effect
   

4. Drugs such as Reserpine cause the destruction of storage granules, and as a result, depletion of the synaptic terminal of norepinephrine which is also a sympatholytic action.
   

5. Blockade of monoamine transporters by drugs such as cocaine and amphetamine produce a sympathomimetic effect. Transporters for norepinphrine as well as other monoamines are the site of action for tricyclic antidepressants and serotonin reuptake inhibitors.
   

6. Inhibition of monoamine oxidase by drugs such as Tranylcypromine.

The Consequences of Multiple Receptor Activation in the Treatment of Congestive Heart Failure

Dopamine can be used to treat congestive heart failure and cardiogenic shock. In congestive heart failure, the heart is not able to eject blood efficiently. As a result there is a decrease in cardiac output which triggers a series of compensatory actions. These include fluid retention, vasoconstriction, an increase in peripheral vascular resistance, an increase in the levels of circulating catecholamines and tissue hypoxia. Dopamine is used because it has the potential to improve these negative circulatory events. For example, by inducing renal vasodilation (via DA₁ receptors), blood flow to the kidney is improved and urine output increases. By increasing the force of myocardial contraction the cardiac output is increased. Dopamine can be used in home health care to improve congestive heart failure.

Similar to epinephrine and norepinephrine, dopamine has a short plasma half life. It can only be used intravenously in constant or intermittent infusions.

The effects of dobutamine on the cardiovascular system are summarized below

It is interesting to note that dobutamine used clinically is a racemic mixture of (+) and (-) isomers. These individual isomers have different pharmacologic properties:

1. (+) Dobutamine is a beta₁ and beta₂ agonist
   

2. (-) Dobutamine is an alpha₁ agonist.
   

3. The observed clinical profile is due to a combination of these pharmacological effects.

The use of isomers as a single drug product is very common. Most isomeric pairs have the same activity. Dobutamine is an unusual example of a pair of isomers that have distinctly different activities.

Dobutamine is used in similar situations as dopamine; namely, the short term treatment of cardiogenic shock and congestive heart failure. Its use is associated with a decrease in LV filling pressure. Like dopamine, dobutamine is not orally active and must be given intravenously and can be used in home health care.

These agents have a higher affinity for beta₂ receptors when compared to beta₁. These agents activate cellular processes by increasing cAMP levels as discussed in the lecture on Adrenergic Receptors.

Clinical Applications of Selective Beta₂ Agonists

These agents are used in situations that call for the relaxation of smooth muscle, specifically the smooth muscle associated with airways or uterus such as;

• Bronchial asthma

• Chronic bronchitis

• Emphysema

• Premature labor-tocolytics-Ritodrine

These are agents which directly active the alpha₁ -adrenergic receptor. They are less potent than the endogenous agonists epinephrine or norepinephrine. However, because of structural modifications they are orally active and have longer plasma half-lives. There are 2 structural classes of alpha₁ agonists the phenylethylamines which are close structural analogs of epinephrine and norepinephrine and the structurally unrelated imidazolines. The major action of these agents is to produce alpha₁-receptor mediated vasoconstriction.

1) Hemorrhage control - previously discussed for epinephrine

2) With local anesthetics - previously discussed for epinephrine. Levonordefrin (methylnorepinephrine) is also used for this purpose.

3) Hypotension - metaraminol, methoxamine

4) Ophthalmic preparations - to induce mydrasis (phenylephrine), to decrease intraocular pressure (apraclonidine, an alpha₂ agonist) and topically for symptomatic relief of irritation (many of the above agents).

5) Cough and cold preparations and nasal decongestants - Many of the above phenethylamines and imidazolines.

6) Alpha₁- adrenergic receptor agonists used to be used to slow heart rate in patients with atrial tachycardia - Can you reason why this would be so?

These agents require the presence of endogenous monoamine neurotransmitters (norepinephrine, epinephrine, dopamine, serotonin) to produce their effects. Indirect acting agonists work at the nerve terminal to promote the release and/or block the reuptake of endogenous neurotransmitters. These agents have little activity if these neurotransmitters are depleted. Cocaine and amphetamine interact with cell surface monoamine transporters for dopamine (DAT), serotonin (SERT) and norepipephrine (NET). These transporters are expressed peripherally and in specific brain loci and are the site of action of psychostimulants and antidepressant drugs.

Cocaine:  Blocks reuptake of monoamines into nerve endings. Cocaine also has local anesthetic activity.

Amphetamine: Promotes the release of monoamines from nerve endings from the terminal cytoplasm. There is only a limited amount of neurotransmitter in this pool. Amphetamine also blocks the reuptake of monoamines. Several structural analogs of amphetamine and "amphetamine like" agents are available for clinical use. These include:

Dexamphetamine (the resolved and more potent d-isomer of amphetamine)

Hydroxyamphetamine

Methamphetamine

Methylphenidate

An important site of action of these drugs is in the CNS. These agents produce a feeling of well being and euphoria. Cocaine and amphetamine have a significant abuse potential because of these mood enhancing effects. Tachyphylaxis or tolerance to the stimulating actions of these agents can develop. These agents produce an increase in systemic arterial blood pressure. Heart rate can either decrease or increase depending on the levels of the drug. Drug toxicity effects multiple organ systems and can result in arrhythmias, hypertension, psychosis and convulsions. The local anesthetic activity of cocaine can also contribute to rhythm disturbances.

Clinical Therapeutics of CNS Stimulants

1) Because of its local anesthetic activity, cocaine has some limited uses as a oral, nasal and ophthalmic local anesthetic.

2) Appetite suppression - amphetamine and analogs

3) Narcolepsy - methylphenidate, amphetamine analogs

4) Attention deficient disorder with hyperactivity (ADHD) - methylphenidate, amphetamine and analogs

Learning Objectives Lecture III

1. Understand how activation of α₂ receptors decreases sympathetic outflow and cause hypotension.
   
2. Understand the pharmacologic properties and therapeutic uses for prazosin-like drugs.
   
3. Understand the pharmacologic properties of propranolol and atenolol and the therapeutic uses of beta-adrenergic receptor blockers.

Key Drugs*

Atenolol - Tenormin and various trade names - 4^th leading prescription drug in the US in 2003- source- rxlist.com

Clonidine - Minipres, various trade names

Propranolol - Inderal - various trade names

Terazosin - Hytrin

Alpha₂ Agonists As Sympatholytics

• Clonidine

• Methyldopa

• Guanabenz

• Guanfacine

Prazosin and analogs(doxazosin, terazosin, trimazosin) - Selective, competitive antagonists

Tamsulosin- Selective, competitive antagonist

Phentolamine-Nonselective, competitive antagonist

Effects of Prazosin and Analogs on the Cardiovascular System:

1. Nonselective competitive "₁ and "₂ blocker.

2. Used to treat pheochromocytoma.

1. Irreversible "₁ and "₂ receptor antagonist.

2. Used to treat pheochromocytoma.

SELECTIVE AND NONSELECTIVE BETA BLOCKERS

Cardiovascular Effects of the Beta Blockers

Cardiovascular Effects and Clinical Uses of the Beta Blockers

The beta1-adrenergic receptor associated with the heart increases the force and rate of myocardial contraction. Beta antagonists block the ability of the sympathetic nervous system to increase the contractile force and the rate of contraction. The release of renin from the kidney is also regulated by the beta1-receptor. By blocking renin secretion beta1 blockers reduce the formation and hence the biological activity of angiotensin II. Beta1-receptor antagonists decrease blood pressure. While the mechanisms underlying this effect are not completely understood, they certainly involve a decrease in cardiac output and heart rate as well as decreasing angiotensin II levels. This reduction in blood pressure makes the beta blockers useful in the treatment of hypertension. Beta blockers are also useful in treating ischemic heart disease. This is because two major determinants of myocardial oxygen consumption are the force and rate of myocardial contraction which are diminished by this class of drugs. Beta blockers are also given following a myocardial infarction to prevent reinfarction. As will be discussed in the lectures on heart failure, certain beta blockers, specifically, metoprolol, bisoprolol and carvedilol, can be used to treat congestive heart failure. Certain arrhythmias are due to excess stimulation of the beta1-receptors. Thus beta blockers are useful in treating supraventricular tachyarrhythmias. There are many indications for beta blockers unrelated to cardiovascular therapeutics.

1) Propranolol is a nonselective beta blocker

2) It was the first clinically approved beta blocker and the standard to which newer drugs have been compared.

A major disadvantage of nonselective beta blockers is the fact that they will block beta₂ receptors associated with airway or vascular smooth muscle. This unwanted action can exacerbate airway diseases (asthma, emphysema, chronic bronchitis) or peripheral vascular disease (Raynaud’s Disease). To overcome this disadvantage, "selective" beta₁ blockers have been developed. These agents have the ability to preferentially block beta₁ receptors. However, this selectivity is only relative and in higher doses selective antagonists will also block beta₂ receptors.

Certain beta blockers actually have a modest degree of agonist activity. In other words these agents are partial agonists with low intrinsic activity. This is referred to as intrinsic sympathomimetic activity or ISA. These drugs may have a lesser effect on resting heart rate or cardiac output than compounds without ISA.

This refers to the ability of some of the beta blockers to also block sodium channels. As a result nerve cells become less excitable, hence the term "membrane stabilizing." In the case of beta blockers this membrane stabilizing activity does not contribute to its therapeutic action. However, you will often find this term used to describe the beta blockers.

Beta blockers should be used with caution in patients with diabetes. In fact, nonselective beta blockers are contraindicated in diabetic patients. This is because catecholamines utilize the beta₂ receptor to promote glycogenolysis and mobilize glucose. This effect would be blocked by non-selective beta blockers. In addition all beta blockers mask the tachycardia associated with hypoglycemia. As a result the diabetic patient is deprived of one of the earliest physiologic responses to hypoglycemia.

Side Effects

The beta blockers have a variety of side effects. These include sedation, fatigue, and impairment of mental function. Hypotension and bradycardia can occur. These agents increase triglycerides and decrease HDL cholesterol. The effects on glucose metabolism has been discussed above. Nonselective beta blockers exacerbate peripheral vascular disease and airway dysfunction.

Labetalol, Carvedilol

These ligands block alpha₁ receptors as well as beta₁ and beta₂ receptors. Labetalol is used to treat hypertension. The side effect profile is what would be expected of a drug that blocks both alpha₁ and beta receptors. These include orthostatic hypotension, sedation, fatigue and other affects attributed to the blockade of beta receptors. In addition to treating hypertension, several recent clinical trials have shown carvedilol to be very effective in treating congestive heart failure. There are several proposed mechanisms underlying this effectiveness. Blockade of the beta₁ receptor appears to be more relevant than alpha₁ receptor blockade. This results in an improvement in left ventricular function. One pathophysiology of heart failure is that the heart increases dimensions. These increases result in a hypertrophied heart with decreased contractile performance. Carvedilol reverses these changes. Furthermore, carvedilol as antioxidant and antiproliferative activity. The extent to which these actions contribute to therapeutic efficacy in not clear.

These drugs are not widely prescribed.

Reserpine

Guanethidine

1. Blocks the Ca²⁺ dependent release of catecholamines from nerve endings.
   

2. Long term use of Guanethidine depletes catecholamines from nerve terminals.
   

3. Does not interfere with central neurotransmitter storage or function.
   

4. Produces hypotension and bradycardia.

Uses of Reserpine and Guanethidine

1. Hypertension

1. Inhibit monoamine oxidase.

2. Produce hypotension.

3. Can precipitate a hypertensive crisis.