-----start----- Class of 2000 Pharmacology 1/22/98 11-12 Lecturer: Kotlikoff Transcriber: hillary (no mike, no slides today! no one opened the closet) Today's Topic: Ion Channel Antagonists and Agonists We're going to talk about drugs that block ion channels. The outline of the lecture is on page one of the handout. We'll review the structure of ion channels and their classifications. We'll discuss how they open and inactivate, and that is very important, because he'll present us a scheme by which drugs interact with these channels. We can understand how cardiac sodium channel blockers work, the difference between how class 1b and 1a antiarrhythmics work, how anticonvulsants work, all by understanding interaction of drug with ion channel in different ways. We'll discuss sodium channel antagonists which are anticonvulsants, calcium channel blockers which are now used in veterinary clinics fairly frequently, and then a bit more about potassium channel blockers which are used somewhat clinically. A bit of introduction: Ion channels make up one of the two members of what are called "excitability proteins" which are transmembrane spanning proteins that are great drug targets. Most of the drugs we talk about either interact with a g protein coupled or tyrosine kinase receptor, or an ion channel. We know that it turns out that evolution has worked to make molecules that work at these sites as well. Some of the most potent naturally occuring toxins interact with ion channels. Recall the sodium channel - has an internal 4fold symmetry - wraps around a central core, has + charges consistently placed in each of the 4 areas, that sense voltage across membrane, and changes structurally when voltage changes. Calcium and potassium channels are also voltage dependent ion channels. Ca+= channels have the 4fold symmetry too, and K+ channels are the evolutionary precursor to those - more simple, made as tetramers, four subunits of the same type around a central core. They're glycosylated on the outside, have alpha, beta (Na+) and other subunits (ca++). They can be modified, phosphorylated - so cell can control excitability remember, there are also ligand gated ionotropic (nicotinic) receptors, and there are metabotropic ion channels like the ones in the eye, where light changes concentration of cyclic nucleotide, opening the channel from the inside. Voltage dependent ion channels are called such because they go from closed state to open state and that is enhanced, made more probable, by a depolarization. C = closed state - available to open. When membrane depolarizes, probability of channel opening is greatly enhanced, and the channels open, enter the O state. Now, if the membrane stays depolarized, the channel goes to a state in which the pore is closed, but the channel can't open again = I = the inactive state. The channel is closed in C and in I but in I it is not available to open. It turns out that as the channels depolarize, it moves quickly from O to I. So membrane depolarizes, channels open, and then rapidly move to I, and sodium current comes down, the turn off is the inactivation of the channels - this occurs before the membrane repolarizes. Once we go to I, we will not transition from I to C fully until we repolarize the membrane, because when you are depolarized, I is favored, and repolarization takes us back to the closed state. there are probably also other transition states. In a general way, though, you can say you go from C, to O, to I and back to C. It turns out that many drugs have affinity for O state or I state but not the C state. so what would happen if a drug boun din the O state? if you put a drug on a nerve and nerve didn't fire, nothing would happen. Until the channels open, there is no effect. Then, every time the channels open, drug would bind, and stop it from going back to C. So you can capture channels this way, and progressively block more and more. This is "use dependent blockade." Now, R ready = C closed. In C state, no ions get through. In O, there is ion permeability, in I, no ions get through and you aren't ready to open again yet. As we said, inactivation - the movement from open to inactivated state increases as you depolarize the membrane. Looking at membrane potential and number of channels in I state - as you depolarize, you get less current, as more channels move to I state, and less are in C or R state as you give the same signal. If you give one impulse and look at sodium current, it shows channel opens rapidly and inactivate over time. Rate of inactivation depends on length of depolarization. Now you put a drug there. Repeated stimulations then show decreasing responses, because every time a channel opens, some drug binds, and if it comes off slowly, the next time you try to open channel, it isn't ready. This is true for drugs that havea higher affinity for O channels than C channels. They produce use dependent blockade. Also pay attention to the rate the drug comes off once it binds. If you bind the O with drug but then the drug comes off right away before the next depolarization, then there is no use dependent blockade. The main difference in antiarrhythmics is this rate at which they unbind. Modulated receptor hypothesis - drugs have different affinities for different states of the channel - can have high affinity for open or inactive state and lower for closed state. for local anesthetics like lidocaine, this is the hypothesis - drug binds inactivated state and keeps it inactivated. but there is a rate of unblock to consider. p 4 of handout: Sodium channel blockers: these drugs block nerve conduction by blocking the Na+ channels and preventing nerve depolarization. It looks like they bind either to an internal surface of the channel (lipid soluble drugs) once channel opens, or they may just get in through the pore, and block the channel from extracellular side, depending. These drugs all show strong use dependent blockade. The resting nerve isn't susceptible to block, but when it fires, it gets blocked. It blocks by binding to O or I state, preventing movement back into R or C state. 1. lidocaine - most widely used local anesthetic. for nerve blocks, and also an antiarrhythmic. it blocks and unblocks very quickly. because the rate of firing of nerve cells is fairly high, it will effectively block the or set a maximum rate of depolarization on the nerve. if you think about it - if time between depolarizations is too fast, channel won't be ready to open yet. if it is slower, channel will be ready b/c drug will be off by then. Blocks pain and sensation to touch but not motor function. You can do a nerve block and jog an animal and look to see if animal improves once you have removed the sensation of pain. Lidocaine is also an antiarrhythmic - class 1 B. If you take the cardiac antiarrhythmics, they are divided as such: Class 1: Na+ channel blockers a. quinidine, procainamide b. lidocaine, phenytoin c. flecainamide Class 2: B blockers Class 3: Prolong Repolarization Class 4: Ca++ channel blockers we're going to talk about class 1 and 4 with Dr. Kotlikoff. Class 1 is subdivided by rate of removal. Class 1 b drugs don't generally affect normal heart rate, because normal firing rate of SA node is such that between one opening of Na+ channels and the next, the drug has already come off the channel. So 1b drugs are mainly used for ventricular arrhythmias or ventricular tachycardia, and they effectively limit the maximum rate of firing. But when heart function is driven by SA depolarization, these drugs have little effect on HR. when an area of abnormal tissue drives high frequency events, these drugs are effective at limiting HR. They will limit ability of abnormal tissue to generate electrical events by blocking their sodium channels preferentially (because more of them are in O or I state instead of C state.) Lidocaine is class 1 b antiarrhythmic and a nerve blocker. little effect on normal cells and HR or on supraventricular cells/purkinje fibers - they repolarize normally because lidocaine unbinds rapidly. Lidocaine blocks abnormally rapid events. 2. tetrodotoxin and saxitoxin - tetrodotoxin is the puffer fish toxin, small amounts will kill you. Saxitoxin is found in small sea organisms. these toxins bind Na+ channels, esp neuronal Na+ channels which are sl different from the cardiac ones. Class 1a antiarrhythmics: quinidine, procainamide: these unblock more slowly than the 1b. So, these drugs are more useful in supraventricular arrhythmias, or arrhythmias that occur at slower rates than ventricular arrhythmias. They are Na+ blockers, they interact with I or O and prevent return to C state. They slow HR at sl. elevated or normal rates. These drugs are more likely to affect normal HR than 1b drugs, because they unblock more slowly. Because they block I or O state channels, they preferentially, electrically remove functionality in areas of ectopic foci, of damaged heart tissue, as do the 1b drugs. Procainamide also is a more negative ionotrope, and quinidine has vagolytic side effects. 3. Anticonvulants - phenytoin, dilantin. This is also a class 1 b antiarrhythmic, but more often used as an anticonvulsant. carbamazine is another similar drug. For an anticonvulsant, you want a drug with no effect on brain or cardiac function (normal brain/cardiac function, that is), you want it to be relatively uncharged and able to get into brain across BBB, and that will block irregular brain activity. When you have rapid electrical discharges in the brain occuring quickly, phenytoin will block them - prevents initiating events and sustained abnormal activity by essentially slowing the maximum rate of firing of the nerve -while having minimal effects on normal brain function. Calcium channel blockers: A. calcium channels in the heart for example, a main way of turning on these channels is beta adrenergic receptor stimulation, increased cAMP production, kinase activity, etc. The structure of the Ca++ channel is a bit different in heart/smooth muscle vs neuronal Ca++ cannels. Neuronal Ca++ channels have a different structre, and can be blocked selectively with little/no effect on the cardiac/muscle Ca++ blockers. we're going to talk about blocking the heart/sm muscle L type Ca++ channel. This is the Long type Ca++ channel. three major classes of antagonists: dihydropyridines, phenylalkylamines, and benzothiazepines diltiazem (a benzo), verapamil (a phenyl), nifedipine (a dyhydro) are used clinically. these all show use dependence and voltage dependence - bind more strongly when channel is open and tissue is depolarized. the dihydros are more effective as vasodilators than the others. diltiazem has more cardiac selective effects w/o the hypotensive effects. these are - have two modes of action. they can be antiarrhythmics (class 4) and are esp good in that regard with supraventricular arrhythmias like SA arrhythmia - recall that ventricular AP and nodal AP look different, and nodal is more heavily influenced by Ca++ channels. In addition, they vasodilate the coronary artery and so that removes some of the angina or ischemia of the heart in the failing heart. these blockers are used in vet med most commonly in the case of feline cardiac hypertrophy (hypertrophic cardiomyopathy), where the - you have a thickened wall and a decreased ventricular volume and ejection fraction, and these drugs relax the heart a bit, allowing it to fill more. This is sometimes confusing b/c you think the heart is already failing, why relax it? it allows the heart to fill more. Dr. Spear will talk more about these drugs later. In terms of antiarrhythmic effects - these drugs can electrically remove areas of ectopic depolarization. Potassium channel agents: there are many kinds of K+ channels. one interesting one is the ATP dependent or ATP sensitive one. These open when ATP levels drop. It turns out that drugs that have been used as agents - antidiabetic agents - are blockers of Katp, this ATP sensitve channel. These are the sulfonylurea drugs that were used in people and diabetic dogs (at first we thought it wouldn't work in dogs, now using it more). Idea is in pancreatic beta cell, which has these channels, because insulin is low, ATP falls within the cell even though glucose is high outside cell, so channels open, hyperpolarizing cells, preventing insulin release. so pancreatic beta cell is hyperpolarized during diabetes, limiting or preventing insulin release by the cells that can release insulin. the drugs like glyburide, tolbutamide, etc block these channels without blocking other K+ channels elsewhere. these drugs are moderately effective in treating hyperglycemia - not as good as insulin of course, but are still useful in people with type II diabetes and some animals. There are also some Katp agonists - drugs that cause hair growth in people are Katp agonists - looks like the hair follicle cells like to have these channels stimulated by minoxidil and other drugs, and in return they will grow some hair for you :) Summary: I. modulated receptor hypothesis Assumes that receptor, because it undergoes structural changes, has various affinities for drugs in different states. Most drugs we're interested in bind to O and I preferentially, and prevent movement from I to C, trapping channels in inactive state. Important thing to remember is that a big difference between the drugs is the rate the drug dissociates from the receptor after it binds the receptor. drugs that come off rapidly are 1b antiarrhythmics like lidocaine, and phenytoin, good for high frequency blocks, nerve blocks, epilepsy, ventricular tachycardias. 1b. Fast (un)blockers - lidocaine, phenytoin(dilantin) 1a. intermediate - quinidine, procainimide. Dr Spear will discuss these more. these come off more slowly and are more effective for slower, atrial arrhythmias. nerve blocks, antiarrhythmics, anticonvulants. these are all sodium channel blockers. -- calcium channel blockers...remember diltiazem, verapamil, and if you remember dihydropirine and you see nifedapine you'll be ok. the antagonists are antiarrhythmics, and smooth muscle relaxants, esp in those cells where Ca++ channel is important for depolarization or contraction, like SA node. -- K+ channel agonists - more important in the future? not many useful now. K+ Antagonists - sulfonylurea, which targets Katp channel and closes it, promoting insulin secretion by beta cells. ---end----