01/15/16 Flashcards
General Classification of Alpha-Adrenergic Blockers
- Having considered alpha and beta-adrenergic agonists, we will not focus on the actions of antagonists, starting with alpha-adrenergic antagonists. These are agents which bind to alpha-adrenergic receptors and block their function.
- There are several classes of alpha-adrenergic blockers, the first we will consider are the haloalkylamines which include dibenamine and phenoxybenzamine.
Mechanism Phenoxybenzamine
- Dibenamine and phenoxybenzamine can both form ethylene immonium ions and alkylate the alpha receptors. They form a covalent complex with the alpha receptors which gives persistent, non-reversible inactivation. You essentially have to wait for the degradation of the receptor and its re-synthesis for the actions of these drugs to be reversed. There actions can persist for at least a day.
- The pharmacological actions of these drugs are what you would expect for blockade of alpha-adrenergic receptors:
- You get a decrease in peripheral resistance and increase in cardiac output via reflex tachycardia. There is a relatively small effect on BP when the patient is recumbent and but larger effect when the patient is standing to postural hypotension.
- By blocking alpha receptors in other target organs they can cause miosis and enter CNS where they can cause nausea and sedation.
****epinephrine reversal by phenoxybenzamine ****
- Looking at the top panel, this is an experiment looking at BP when you treat with a high dose of E. Remember at low doses of E you act primarily through beta-2 receptors in the VSM and cause vasodilation, but at high doses of E you activate both Beta-2 an Alpha-1, the activation of alpha-1 predominates and you get vasoconstriction and a BP increase.
- The lower plot shows what happens when you treat with phenoxybenzamine (POB) and block the alpha-1 receptors. This leaves only the beta-2 response, giving vasodilation and a decrease in BP. By blocking the alpha-1 receptors we have converted this pressor response to a depressor response.
- The ability of alpha-antagonists to change the response to high levels of E is often referred to as E reversal.
E reversal but not NE reversal by phenoxybenzamine
- This slide shows a similar experiment comparing the ability of phenoxybenzamine to reverse the effects of E or NE on BP and HR:
- In both cases we get an increase in BP due to vasoconstriction through alpha-1 receptors.
- If we now pre-treat with phenoxybenzamine, by blocking alpha-1 receptors again we see the E reversal. E is able to work through beta-2
- Pharmacologically applied NE is not able to work through beta-2 receptors and you see no effect on BP after blockade of alpha receptors.
- Why the change in heart rate? When you block the alpha-receptors you no longer get an NE pressor effect., you see direct action of NE on beta-1.
- With E, when you block the alpha receptors and you no longer get any vasoconstriction, and a reflex increase in HR due to the large decrease in BP.
- Although phenoxybenzamine isn’t used extensively clinically, it is used occasionally for the treatment of pheochromocytoma. This is a tumor of the adrenal medulla which results in excessive levels of circulating catecholamines causing hypertension. Phenoxybenzamine is used preoperatively before the tumor is removed to counteract any effects of catecholamines that might be released during surgery.
general alpha-adrenergic antagonists
There is another class of alpha-adrenergic blockers that interact reversibly, noncovalently, with alpha- adrenergic receptors. This includes Phentolamine and Tolazoline. These are reversible antagonists of alpha- adrenergic receptors. They are short-acting and readily reversible.
Phentolamine / Epinephrine Reversal
Like phenoxybenzamine we would expect phentolamine to cause epinephrine reversal for the reasons we just discussed, i.e. blocking the alpha-1 receptors and leaving the beta-2 receptors for E to stimulate. E reversal by phentolamine is illustrated here.
Comparison of Competitive vs. “Non-equilibrium” Blockade
- This data shows the distinction between a competitive, reversible alpha-1 antagonists such as phentolamine and a nonreversible covalent antagonist such as phenoxybenzamine.
- The dose response curve for NE stimulation of VSM is shifted by phentolamine, but you can overcome the competitive inhibition and reach the same effect by going to higher levels of NE.
- In contrast, with an irreversible antagonist like phenoxybenzamine you cannot overcome the effect with higher levels of NE because you have effectively removed alpha-1 receptors covalently. This non-reversible blockade that you get with the alkylating antagonists is sometimes referred to as a non-equilibrium blockade.
- Phentolamine has been used in the past as a diagnostic test for pheochromocytoma. If there is hypertension due to high levels of catecholamines, by blocking with a short-acting reversible alpha-blocker you should convert it to a hypotensive response. Now it is more common to just measure the levels of degradation products of cathecholamines in the urine.
- The effects of phentolamine are generally what you would expect for blocking alpha receptors. This includes postural hypotension, a reflex tachycardia due to the vasodilation.
- The increase in heart rate comes from several factors including: A reflex increase in sympathetic activity and a decrease in parasympathetic activity.
Presynaptic Receptors Inhibit NE Release From Terminals
Some of this NE can act back onto the presynaptic alpha-2 receptors which decreases the NE release. These presynaptic alpha-2 receptors are coupled to inhibition of adenylyl cyclase, lowers cAMP and decreases the release of NE.
As you get the reflex increase in sympathetic activity due to the decrease in BP, you get an increase in NE release which activates Beta receptors in the heart to speed up HR. The presynaptic alpha-2 receptor is normally a feedback mechanism which prevents too much NE release. When you block the alpha-2 receptors with phentolamine or phenoxybenzamine, you inhibit the normal feedback mechanism. Consequently, you get more NE released than would occur than if you did not have the alpha-adrenergic blocker present, i.e. more tachycardia than would occur in the absence of the alpha-blockers.
comparison of phenoxybenzamine and prazosin in blocking presynaptic alpha-2 receptors
- You would like a drug to lower blood pressure that just works on alpha-1 receptors in vascular smooth muscle without blocking presynaptic alpha-2 receptors. Drugs such as phenoxybenzamine are relatively nonspecific for alpha receptors and interact with alpha-1 and alpha-2. Is there any way we could just block the alpha-1 receptors without blocking the alpha-2 in order to get vasodilation?
- There is a group of drugs that are relatively specific for alpha-1 receptors with low activity for alpha-2 receptors. These includes prazosin, which is a reversible alpha-1 antagonist that has very low alpha-2 activity. This drug is relatively specific for alpha-1 receptors and used as an antihypertensive. You get less tachycardia than you would get with phenoxybenzamine because it does not stimulate the release of NE by blocking presynaptic alpha-2 receptors. Other related analogues include terazosin and doxazosine which are longer- acting parazosin analogues.
long-term use of prazosin to treat hypertension
When patients are treated with prazosin for long periods of time, it causes a long-lasting decrease in BP.
Yohimbine
There is an alpha-2 specific antagonist, Yohimbinee. It enters the CNS and produces a number of effects including increased BP, HR. It is effective to treat male impotence and erectile dysfunction. Blockade of pre-synaptic α2 receptors facilitates the release of several neurotransmitters in the central and peripheral nervous system. In the corpus cavernosum (part of the erectile tissue in the penis) it stimulates release of nitric oxide which in the corpus cavernosum is the major vasodilator contributing to the erectile process.
Beta Blockers classes
- The beta-blockers are a class of drugs that have a number of therapeutic applications.
- Several of the beta-adrenergic blockers including propranolol block both beta-1 and beta-2 receptors. Essential the same affinity for beta-1 and beta-2 receptors.
- Other beta-antagonists that are relatively nonselective for beta-1 and beta-2 are pindolol, and timolol.
- They are all competitive antagonists of beta-1 and beta-2 receptors.
Effect of propranolol on IP stimulation of heart and BP
- What happens if you inject propranolol into an animal? If you first administer IP alone you see the expected increase in HR and contractile force associated with activation of beta-1 receptors in heart. You also see a decrease in BP because of the activation of vasodilatory beta-2 receptors in VSM.
- If you pretreat with propranolol, it is able to block both the beta-1 and the beta-2 responses.
Comparison of the effects of phentolamine (alpha blocker) and propranolol on IP stimulated- decrease in BP.
If you compare the effects of phentolamine and propranolol on the BP decrease caused by IP (i.e. activation of beta-2 receptors in VSM), pretreatment with phentolamine is without effect on the IP response while propranolol inhibits the response to IP.
NE causes a BP increase which is reversed by phentolamine….. which is reversed by propranolol.
When you give a high dose of NE you see a BP increase due to the dominant effect on alpha-1 receptors in VSM and vasoconstriction.
When you add phentolamine you block the alpha-1 vasoconstriction. Propranolol has no effect.
Therapeutic usage of propranolol
- Most of the therapeutic uses of propranolol are based on its ability to block beta-adrenergic receptors, especially the cardiovascular applications. Go to slide.
- Propranolol and related drugs have an additional action which can affect the rhythm of the heart, propranolol is sometimes said to have a “membrane-stabilizing” effect. Somewhat like a local anesthetic and it decreases excitability-antiarryhthmic activity. This can contribute to propranolol’s anti-arryhtmetic activity. However, at the doses that are normally used therapeutically, these effects are relatively minor.
- Beta blockers can also be used in the treatment of glaucoma, specifically timodol. Beta-blockers decrease intraoclular pressure by decreasing the production of aqueous humor. Probably does this by acting on beta- receptors on the ciliary “processes” of the ciliary epithelium. Adrenergic agonists such as E are also used with the same response. The E is thought to work by causing desensitization of beta-receptors in the ciliary epithelium that normally control accumulation of aqueous humor.
antihypertensive mechanism for beta-blockers
- There are several reasons beta-blockers have anti-hypertensive activity”
- They block beta-receptors in the heart and decrease cardiac output and decrease BP.
- It is also though that another contributing factor is that there are beta-receptors which are involved in renin release. B blocking beta-receptors you block the amount of renin released. This leads to a decrease in angiotension II/ aldosterone system, which normally increases BP.
- There may also be effects in the CNS to affect sensitivity of the baroreceptor reflexes or effects on sympathetic outflow. There are probably a variety of reasons why beta-blockers reduce BP, the most obvious are due to direct effects on beta receptors in heart.
survival rate improved with propranolol
It has been shown that administration of propranolol or timolol will decrease the incidence of second heart attacks with cardiac patients.
Adverse effects of beta-blockers
- As you might expect a class of drugs that have so many pharmacological effects will also have a number of adverse effects.
- Because of it activity on the heart it can cause bradycardia and a variety of other unwanted cardiovascular effects. Obviously it can cause hypotension. By blocking metabolic pathways it can cause hypoglycemia. The CNS effects are relatively rare.
- Claudication-limping
- Paresthesias-prickly feeling, like being stuck with a pin
Abrupt withdrawal of Inderal
- One additional important consideration when taking beta-blockers is that there is a very dangerous reaction to the abrupt withdrawal of beta blockers such as Inderal for patients who have been taking the drug for some time. In particular, they can experience angina, tachycardia, etc.
- The reason for this is not too hard to understand. We have seen several cases where surgical innervation often causes supersensitivity of the target organs to the transmitter. When you have persistent treatment with a beta- blocker you can see that supersensitivity in the heart to the transmitter.
Beta-receptors in heart increases when you treat with rats with propranolol
- There have been animal studies in which rats were chronically treated with propranolol that the number of beta-adrenergic receptors in heart increases.
- When patients are treated with propranolol those up-regulated beta receptors are blocked but if you rapidly stop treatment they are exposed and this leads to supersensitivity to NE…. give the unpleasant effects.
- Consequently, drugs such as Inderal are withdrawn progressively. Since patients with graded withdrawal do not show supersensitivity responses, it implies that there is return of beta-receptors in heart to normal levels when the drug is slowly withdrawn.
timolol, FEV, and asthmatic patients
- There are draw backs in using the non-selective beta-blockers. This slide shows data with patients treated opthalmogically with timolol.
- FEV (Forced Expiratory volume) is being monitored with normal (control patients and asthma patients. So you can see that just applying a little timolol to the asthmatic patient causes a very significant decrease in FEV. Remember that beta-2 receptors can mediate bronchial smooth muscle relaxation.
Beta-1 Specific Blockers
- To try to get over some of the problems associated with the non-selective beta-blockers, beta-1 specific antagonists have been developed:
- Atenol
- Esmolol
- Metoprolol
- Acebutolol
- Betaxol
- Practolol
- These drugs these are relatively specific for blocking beta-1 receptors but not absolutely specific.
Comparison of propranolol and practolol in blocking various beta-1 and beta-2 receptor mediated responses.
- Propranolol has about the same affinity for beta-1 and beta-2 receptors. When you compare its dose response for effecting beta-1 responses (e.g. Iso or sympathetic nerve stimulation of Heart output) with blockade of beta- 2 responses (Iso stimulated vasodilation or Iso-stimulated bronchodilation, the curves for propranolol are all about the same.
- However, with practolol which is a beta-1 selective antagonist, it is much more effective in blocking the beta-1 mediated responses in heart than it is in blocking beta-2 mediated increases in vasodilation or bronchodilation. These data also illustrate that practolol and other beta-1 antagonists are selective but not absolutely specific.
- The beta-1 selective antagonists are used to inhibit cardiac sympathetic activity.
Beta-2 Specific Blocker
Butoxamine is a beta-2 selective antagonist. It is more effective in blocking beta-2 receptor mediated vasodilation than in blocking beta-1 mediated effects on heart output. I know of known therapeutic uses of butoxamine.
Labetalol
- The final adrenergic blocker that I want to mention is labetalol which is an alpha- and beta- adrenergic antagonist, with higher affinity for beta receptors. At low concentrations it produces mostly beta-adrenergic blockade. It used as an anti-hypertensive agent. Labetalol is a mixture of four stereoisomers. It is a non- selective beta blocker and a relatively specific for alpha-1.
- Adrenergic Neuron Blockers
- Most of the adrenergic drugs that we have been talking about so far work directly on adrenergic receptors. There is another class of drugs that work on the adrenergic nervous system, the adrenergic neuron blockers. These drugs act by:
- Blocking the release of neurotransmitter
- Depleting terminals of neurotransmitter
general adrenergic transmission model
- Reserpine is a drug that blocks the uptake system of the transmitter from the cytoplasm into the vesicles. This transport system is different than the one that transports catecholamines through the cytoplasmic membrane, the one that transports catecholamines from out side the cell inside. This interferes with adrenergic transmission by several different mechanisms.
- First, to make NE from DA you have to transport DA into the vesicle. Inside the vesicle DA converted into NE by dopamine-beta- hydroxylase. Essentially block the syntheses of NE.
- Also, the NE that is normally released gets transported back into the cytoplasm, but when reserpine is present, the NE doesn’t get transported back into the vesicle and it gets degraded by MAO which is present at very high amounts at nerve terminals.
- In addition there is normally some leakage of NE form the vesicle which gets transported back rapidly into the vesicle, it gets degraded. Reserpine causes depletion of catecholamines from adrenergic neurons, from the adrenal medulla, and from the brain.
depletion of catecholamines in innervated VSM by reserpine
- This slide shows using the histochemical technique that we talked about earlier to visualize catecholamine containing nerves. This is a vascular smooth muscle preparation where you can see the extensive innervation by catecholamine containing nerve terminal. This panel shows the depletion of catecholamines cause by 24 hr pretreatment with reserpine.
- The effects of reserpine are what you expect for a drug that depletes catecholamines this leads to decrease in HR and BP, and an increase in GI tone and motility.
reserpine effects on BP effects of tyramine and NE
- Tyramine is an indirect acting sympathiomimetic. You may remember that tyramine causes the release of transmitter at the nerve terminal:
- When you treat an animal with tyramine, there is release of NE at the nerve terminals, there is vasoconstriction and an increase in BP.
- When you treat with NE it causes a similar effect by acting directly on the alpha-1 receptors.
- If you now pre-treat the animal with reserpine before treatment with tyramine there is a depletion of NE in the vesicles so that tyramine’s effect on BP is greatly diminished. However, NE still acts directly on the nerve terminal and is unaffected by prior treatment with reserpine. The receptor is not blocked and NE can still activate alpha-1 receptors and increase BP. Reserpine blocks the transport of transmitter from the cytoplasm into the vesicle but does not affect transport from the cytoplasm into the neuron, like cocaine does.
- Also, reserpine causes depletion of drugs from the adrenal medulla.
- Reserpine can also cause depletion of catecholamines from adrenergic neurons and in the CNS, consequently it can cause sedation and depression.
- The central effects of reserpine are much like the phenothiazines, sedation and tranquilizers effects. This is due to lowering brain catecholamine levels.
Summary of reserpine effects
Reserpine is active against sympathetic neurons, the adenrenal medulla and the CNS. It has no cocaine-like effects.
Drugs acting at Adrenergic terminals, bretylium
- Bretylium is transported into the nerve terminal by the catecholamine uptake system and in the nerve terminal it inhibits the release of NE, release that is brought about by nerve stimulation.
- In addition bretylium also blocks the cytoplasmic membrane catecholamine uptake system and has “cocaine like activity”. Bretylium has no effects in the CNS or on the adrenal medulla.
Response of smooth muscle to hexamethonium or bretylium
- Here were are looking at the contraction of smooth muscle in response to direct administration of NE to the sympathetic ganglia or we are stimulating the sympathetic ganglia pre-or post-ganglionically… stimulating the ganglionic fiber that goes directly to the smooth muscle or preganglionically.
- In the controls direct application of NE, pre-ganglionic stimulation and post-ganglionic stimulation all lead to smooth muscle contraction.
- If you pretreat with hexamethonium, which blocks the transmission across the sympathetic ganglia, you no longer get preganglionic stimulation but still get SM contraction when you stimulate post-ganglionically or when you stimulate with NE.
- If you pretreat with bretylium, smooth muscle contraction cause by either pre or post-ganglionic stimulation is lost because bretylium inhibits the release of NE at the nerve terminal. The effect of direct application of NE, is however, not affected.
Comparison of the effects of phenoxybenzamine, bretylium and reserpine pretreatment on smooth muscle contraction
- This is an analogous experiment where we are looking at the contraction of smooth muscle in response to direct administration of NE at the terminal or we are stimulating the sympathetic ganglia presynaptically.
- Since phenoxybenzamine is an alpha-antagonist and works directly on the alpha receptors in the muscle it blocks all three forms of stimulation.
- Since Beryllium and reserpine pretreatment inhibit the release of NE or deplete NE they block responses generated by ganglionic stimulation, but not the direct effect of NE.
summary table
Guanethidine and guanadrel, a related drug, combine the actions of reserpine and bretylium. They block the release of NE from nerve terminals and displace NE from stored in vesicles. They are transported into the nerve terminal they displace NE stored in vesicles and also blocks the release of NE leading to depletion.
pretreatment with guanethidine cause NE depletion and blocks electrical stimulation of vasoconstriction and NE release.
- This slide shows the effect of guanethidine on VSM constriction and NE release from a VSM preparation. NE release is measured on the top. The preparation is subjected to electrical stimulation of contraction by stimulation of the vasoconstrictor nerves.
- Initially, you can see when the nerve is stimulated that there is release of NE and contraction of VSM. However, after treatment with guanethidine, NE release is blocked and muscle contraction is blocked.
Effect of guanethidine on the pressor responses to NE, tyramine or amphetamine
- You can see that prior to treatment with guanethidine, that NE and two indirect acting sympathomimetics, amphetamine or tyramine, all stimulate a BP increase.
- When you pretreat with guanethidine, NE still causes the BP increase but the indirect acting agents tyramine and amphetamine no longer cause a BP increase. If anything, it looks like the effect of NE may be somewhat higher after guanethidine treatment.
- Guanethidine and guanadrel have been used in the treatment of hypertension.
Chart showing that dihydroalprenolol binding sites increase when animals are treated with guanethidine
Long term treatment with guanethidine can lead to supersensitivity, at least in animal models. This table shows that the apparent number of beta-receptors in the heart goes up with long-term treatment of rats with guanethidine.
effects of clonidine on BP
- There are some other drugs that interfere with the function of the sympathetic nerve system and have antihypertension activity by virtue of their effects on the sympathetic nerve system.
- The first is clonidine. Clonidine is an alpha-adrenergic agonist. It was discovered that clonidine had some unexpected properties. This slide shows the effect of clonidine on BP when it is injected IV or into the cerebrospinal fluid (ICV).
- When injected IV there is a transient increase in BP which you might expect for an alpha-adrenergic agonist, but this was followed by a decrease in BP.
- When you administer the drug to the cerebrospinal fluid, you only see a decrease in BP which is more pronounced and longer-lasting. This is thought to be due to an effect in of clonidine in the CNS. Clonidine can interact with alpha-2 receptors to decrease sympathetic output.
effect of clonidine of splanchnic nerve activity and BP
This slide shows the effect of clonidine on splanchnic nerve activity and BP. The splanchnic nerve is a measure of sympathetic nerve activity. When you give clonidine BP starts to drop and at the same time electrical activity in the splanchnic nerve decreases. This provides good evidence that clonidine is activating centrally rather than peripherally. Clonidine is sued to treat hypertension.
correlation between clonidine’s decrease in heart rate and BP.
This data shows that there is a good correlation between the decrease in BP caused by clonidine and the decrease in HR.
Synthesis of NE and alpha-methyl-NE.
- There is a drug, alpha-methyl dopa which is an analogue of dopa. it gets taken up into the nerve terminals and is concerted to alpha-methyl NE and stored in granules. It is released upon neuronal stimulation and functions as a “false neurotransmitter”. It lowers BP and is used in the treatment of hypertension.
- Don’t confuse alpha-methyl dopa with alpha-methyl tyrosine. Alpha-methyl-tyrosine is an inhibitor of tyrosine hyroxylase, the enzyme that is in the first biosynthetic. step for catecholamines. Alpha-methyl tyrosine is used in combination with phenoxybenzamine for the treatment of pheochromocytoma
