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Calcium channels as targets of “calcium blockers” and sympathetic neurotransmitter In the last lecture we saw that the action potentials in different regions of the heart show considerable diversity generic cefuroxime 500 mg without prescription. Unlike spikes in skeletal muscle trusted 500mg cefuroxime, cardiac modifying action potentials not only trigger contractions but also have contraction considerable influence on contractility In this lecture buy discount cefuroxime 500mg on line, we will deal with the ionic basis for cardiac action potentials. Not surprisingly in view of these different functions, the underlying ionic mechanisms in various parts of the heart are somewhat different. The upstroke of the cardiac action potential closely resembles the spike in other excitable tissues. In ventricular and atrial muscle and in Purkinje fibers (where rapid conduction is critical) the ionic basis is quite similar to nerve. The rapidly rising phase is generated by an increase in Na conductance (gNa) as in the squid giant axon. If most of the external Na ions are replaced by choline (or NaCl replaced by dextrose) the following changes are observed: 1. The action potential is briefer (the plateau configuration is much less pronounced). This suggests that some Na current may also contribute to maintaining the plateau. Similar changes take place when Purkinje fibers or working heart muscle are treated with tetrodotoxin, a specific sodium channel blocker. Although such changes do not occur within the clinical range of plasma sodium concentrations, changes in other ion concentrations (K+, Ca2+) have indirect effects on the impulse generating mechanism which prove to be very important. We have suggested that the Na channel mechanism in heart is similar to that in nerve or skeletal muscle. But heart differs from other excitable cells in one important sense: it has a much longer refractory period, lasting tenths of a second rather than a few milliseconds. During the refractory period the muscle is completely unresponsive to electrical stimuli. This behavior was noted as long ago as 1876 by the French physiologist Marey, who observed that stimuli falling within the early part of mechanical systole were unsuccessful in eliciting any additional contraction. More recently, electrical recording has shown that the refractoriness is associated with the absence of a propagated action potential. There is an intermediate situation between full refractoriness and no refractoriness. A stronger-than-usual stimulus is needed to produce this response, and once it is generated, it propagates very slowly. Such poorly-propagated responses arise in cases of defects in heart rhythm (arrhythmias). An erratic pacemaker, or the existence of more than one pacemaker can give rise to premature "stimuli". The poorly- propagating response will cause excitation of some regions, but not of others; these regions will therefore show varying excitability for the next pacemaker signal, and eventually, complete asynchrony may result. The above figure shows that the ability to propagate a second action potential is restored rapidly once repolarization has proceeded to a sufficiently negative level (about 10-20 mV positive to the usual resting potential). In fact, agents which shorten or prolong the action potential also produce similar changes in the duration of refractoriness. This point may be puzzling, since the plateau potential exceeds (is positive to) the threshold for eliciting an action potential in the first place. In heart, as in nerve, gNa is switched on by a sudden depolarization from the resting potential. The membrane potential must be returned to a level near the resting potential in order to reverse the inactivation process. As the inactivation is removed, the sodium channels become available once again for rapid opening in response to a depolarization beyond threshold, as shown in the figure on the following page. Weidmann was the first to show how membrane potential influenced the availability of sodium channels in heart. He used the rate-of-rise of action potentials as a means of measuring sodium current, and studied the effect of changes in the steady potential preceding sudden stimuli: H. The availability of sodium channels is strongly dependent on membrane potential over the range between -90 mV and -60 mV. This explains the gradual recovery of excitability during the final repolarization phase of the action potential. One function of the plateau, then, is to postpone the recovery of excitability, thereby preventing premature excitation (why would re- excitation during systole be detrimental? The delayed repolarization in conducting system may serve to protect the ventricular muscle from the possibility of premature excitation during the so-called "vulnerable period" when the myocardium is partially refractory. During this period the ventricles are particularly susceptible to the initiation of arrhythmias by a single premature excitation. The longer Purkinje fiber plateau may ensure that impulses cannot reach the ventricles until they have fully repolarized. The relationship between membrane potential and sodium channel availability also explains a phenomenon called accommodation, where slow, subthreshold depolarizations decrease the conduction velocity and rate-of-rise of a subsequent stimulated action potential. This process occurs in axons and sensory receptors as well as heart muscle, and it is caused by inactivation of sodium channels. It is an important phenomenon in Purkinje fibers because drugs such as epinephrine or digitalis can cause accommodation by promoting slow diastolic depolarization. They can influence the membrane potential before the arrival of the impulse, or 2. Potassium ions act in the first way: The inactivation curve is unchanged, but membrane potential is altered. Clinically observed variations in plasma K concentration produced changes in membrane potential in the critical range between -90 mV and -60 mV (remember Nernst potential lecture). Quinidine, procainamide, lidocaine and certain other anti-arrhythmic drugs influence conduction by the second mechanism, by altering the relationship between membrane potential and inactivation. With these drugs present, a greater degree of repolarization must occur before the membrane recovers responsiveness. At the normal resting potential, the drug reduces the membrane responsiveness (and concomitantly decreases excitability and conduction velocity). This takes place because lidocaine and other agents in its class bind preferentially to inactivated channels. The reduction can be counteracted by hyperpolarizing the membrane, thereby pulling channels back into the resting state. If the membrane potential is made negative enough, the ability of inactivated channels to bind the drug will eventually be overcome. Many studies have shown that sodium channels are not absolutely necessary for conduction of the cardiac impulse. Calcium channels can also underlie propagated activity when the normal sodium channels are blocked or inactivated This calcium current (sometimes called “slow inward current") is relatively small compared to the fast sodium current. It underlies slowly-rising, sluggishly propagating impulses called slow responses when the fast sodium current is not effective. The Na+ channels and Ca2+ channels have been distinguished by voltage clamp experiments.
These peptides are not completely selective for each type of receptor since the opioid peptides show a degree of sequence homology cheap cefuroxime 500mg amex, although modified synthetic agonists are more selective cheap 500mg cefuroxime. Investigation of mu receptor-mediated controls has been hampered by the lack of an endogenous ligand for the receptor in many areas effective cefuroxime 500mg, and in particular, within the spinal cord. Very recently, two peptides (endomorphin-1 and -2) have been isolated with high affinity and selectivity for m-opioid receptors, making it likely that they are the natural endogenous ligands for the receptor for morphine itself. Morphine acts on the mu receptor, and so do most of the clinically used opioid drugs. The detailed structure of these receptors has been described and we now have a reasonable understanding of their relative roles in physiological functions and in different pain states. The best-understood sites of action of morphine are at spinal and brainstem/ midbrain loci, producing both the wanted and unwanted effects of the opioid. The spinal actions of opioids and their mechanisms of analgesia involve (1) reduced transmitter release from nociceptive C-fibres so that spinal neurons are less excited by incoming painful messages, and (2) postsynaptic inhibitions of neurons conveying information from the spinal cord to the brain. At supraspinal sites, morphine can act to alter descending pathways from the brain to the cord which involve noradrenaline and serotonin and these pathways then act to reduce spinal nociceptive activity. In addition, these sites form a link between emotions, depression and anxiety, and the level of pain and analgesia in a patient. An intriguing area of research on opioids has been the accumulating evidence for plasticity in opioid controls. The degree of effectiveness of morphine analgesia is subject to modulation by other transmitter systems in the spinal cord and by pathological changes induced by peripheral nerve injury. Thus in neuropathic states, pain after nerve injury, morphine analgesia can be reduced (but can still be effective) and tactics other than dose-escalation to circumvent this will be briefly discussed in Chapter 21. Finally, there is little or no clinical evidence that morphine causes psychological dependence or drug-seeking behaviour, tolerance or problematic respiratory depression in patients. The reason is likely to be that the actions of morphine and the context of its use in a person in pain are neurobiologically quite different from the effects of opioids in street use. This peptide has been extensively studied since it was the first major peptide to be extracted from brain but only now are useful antagonists becoming available. Two closely related peptides were then isolated from mammalian tissues and can be added to a number of other tachykinins, many of which are found in amphibians. Early indications suggest that the peptide and its receptor play important roles in pain (see Chapter 21), inflammation and possibly stress, emesis, anxiety, depression and reward. This appears to be restricted to neurons in spinal cord and possibly brainstem which are associated with analgesia since antagonists potentiate only morphine analgesia, not respiratory depression. Finally, the peptide can induce anxiety and panic in normal and anxious volunteers. The marked vasoconstrictor actions of the peptide in the periphery will mean that systemic therapy will require receptor subtype selective agents. Few antagonists exist at present but the distribution of the peptide with high levels in cortex, hippocampus, amyglada and spinal cord may give some clues to potential functions of the peptide. By contrast, the peptide appears to promote convulsions Ð here its role may be through disinhibition. A well-established central role in the control of growth hormone release has given rise to hopes of treatment of agromegaly and in other contexts, motor actions and increases in sleep times in animals suggest a number of roles of this peptide. The 13 amino-acid peptide has been implicated in analgesia, thermoregulation and interactions with dopamine function in the nigrostriatal and mesolimbic pathways. The peptide is excitatory but whether there is a single receptor or two remains a point of dispute. There is a lack of any antagonist and apart from a truncated version of galanin having some GalR2 selectivity, no means of separation of the three receptors. However, the consequences of receptor activation are clear in that the GalR1 and 3 receptors are inhibitory and the GalR2 excitatory, although some mixed effects have been reported with the latter. The distribution of the 1 and 2 receptors differs, with the former being enriched in hippocampus, spinal cord and peripheral nerves whereas GalR2 has a wider distribution. Most is known about galanin in the spinal cord and the normal almost undetectable levels of the peptide increase after nerve damage with gene induction occurring. In normal animals spinal application of galanin has mixed effects on both spinal neurons and peripheral nerve activity and these are likely to reflect GalR1 and 2 receptors located together. Their discovery stimulated great expectations but most of these remain to be resolved. It is to be hoped that the synthesis of appropriate agonists and antagonists will make it possible to study the actions of the peptides and possibly develop appropriate therapy, even if this turns out to be a secondary line of attack. The localisation of a particular peptide to a particular brain area and possibly associated with a particular transmitter (e. Animal studies in which the peptide has been injected into the appropriate brain area or tested on slices taken from the brain area have sometimes been taken to confirm such hypotheses. These approaches have lined up the peptides for a whole range of potential roles, some of which are listed in Table 12. Whether these predictions are realities will depend on the availability of chemical agents and their evaluation, not only in animals but also in humans. Structurally it consists of an adenine ring, a ribose element and a triphosphate chain (Fig. It is mostly synthesised by mitochondrial oxidative phosphorylation using glucose taken up by the nerve terminal. This has been shown in many peripheral tissues and organs with sympathetic and parasympathetic innervation as well as in brain slices, synaptosomes and from in vivo studies with microdialysis and the cortical cup. Unfortunately techniques do not exist for demonstrating purinergic nerves but purinergic receptors have been established. The former tend to be located presynaptically, are activated mainly by adenosine and have been reclassified accordingly as A1 and A2 (and now A3). Those linked to a fast ionotrophic effect are classified as P2x, with currently six subtypes and those with slow metabotropic effects as P2y with seven subtypes. That requires the development of more specific antagonists and methods of mapping its location. Its basal extracellular level is 2 mM but this can increase rapidly when neuronal firing increases and can rise some twentyfold during seizures. The two enzymes responsible for its breakdown are adenosine kinase (Km 2 mM) and adenosine deaminase (Km 50 mM). It will be clear that as more adenosine is released during seizures, it will quickly saturate the kinase and its concentration can therefore only be controlled by deaminase. In fact deaminase but not kinase inhibitors are anticonvulsant as is adenosine and its analogues, while its antagonist theophylline is proconvulsant and a central stimulant. Adenosine has also been considered to play a role in sleep induction (Chapter 22). Recently much interest has been shown in the possible neuroproctive effects of adenosine but the responses are complex. Thus A3 agonists can offer some protection given chronically before ischaemic challenge but given acutely post-challenge they can be neurotoxic (see Jacobsen 1998). The development of immunohistochemical methods for the visualisation of histamine, and its synthesising enzyme histidine decarboxylase, now show there to be definite histaminergic nerves (see Tohyama et al.
It simply requires the presence of conduction disparity between tissue sites whereby an area of slower conduction would initially block a premature impulse and later allow it to reenter discount 500mg cefuroxime overnight delivery, setting up a self-sustaining loop (Figure 5) generic cefuroxime 500 mg with mastercard. It should be noted that the conduction delay or block may not necessarily be created by a fixed purchase 500mg cefuroxime fast delivery, structural abnormality. A simple disparity of conduction due to physiologic or pathologic anisotropy is sufficient to cause reentry. More commonly, areas of slow conduction occur as a result of tissue pathology, such as heterogeneity in areas of healed myocardial infarction or cellular hypertrophy and fibrosis from cardiomyopathy. The latter conditions are usually present in patients with reduced left ventricular function and the arrhythmia is frequently fatal. It is estimated that sudden death from fatal ventricular tachycardia and/or fibrillation affects over 400,000 people in the U. Ventricular tachycardia and ventricular fibrillation are the most "malignant arrhythmias. To effectively treat reentry arrhythmia, the slow conduction should be corrected or the alternate pathway eliminated or both. This should really not surprise anyone because drugs that are aimed at blocking any of the ionic channels may actually make the situation worse. Beta blockers and calcium-channel blockers have limited effect on the atrial and ventricular myocardium. The Na+- channel blockers may further slow conduction (and not surprisingly, some are indeed "proarrhythmic"). K+-channel blockers offer some promise because by prolonging recovery, they may terminate reentry. For supraventricular tachycardias, the definitive therapy usually requires the elimination (ablation) of the altemate pathway or key elements of the reentry circuit. The most effective immediate therapy for life threatening reentrant ventricular tachycardias is electrical cardioversion. By using a large transthoracic voltage it is possible to depolarize the entire heart muscle and thereby abolishing discrepancies in conduction that are essential for reentry. Some of the commonly encountered reentry arrhythmias that constitute distinct clinical entities and will be discussed below. This is called the Sinus Rhythm, occurring at rates from 60 to 100 beats per minute. The rate of the sinus node impulse formation is influenced by sympathetic and parasympathetic regulation. Self Study Question #1 Normal sinus rhythm occurs at rates from _____ to _____ beats per minute. Abnormalities of Sinus Rhythm Two abnormalities of sinus rhythm are sinus bradycardia and sinus tachycardia. While described as an arrhythmia, sinus bradycardia frequently occurs normally during sleep, in young adults and in athletes. This may result in symptoms of lightheadedness and loss of consciousness, and may require a permanent pacemaker (see therapy section below). Sinus tachycardia occurs when the sinus rhythm is greater than 100 beats per minute. Therefore, causes for sinus tachycardia should be investigated, but by itself does not require treatment in most situations. Study Question #2 As sympathetic stimulation increases, sinus _____________ can occur. Atrial Fibrillation occurs when there is rapid chaotic disorganized electrical activity in the atria due to multiple wavefronts. This random electrical activity originates from within the atria only, not from other parts of the heart. The atrial activity occurs at rapid rates varying between 300 and 600 beats per minute. Since the A- V node does not play an obligate role in the perpetuation of the atrial fibrillation, even though vagal maneuvers or adenosine will block conduction via the A-V node transiently, they will not terminate the rhythm. In multiple locations in the atria, there are wavefronts that activate different parts of the atria. Only some of the impulses travel from the atria to the ventricles via the A-V node. Impulses bombard the A-V node at an irregular rate and the A-V node only permits some of these impulses to travel to the ventricles. Atrial fibrillation may occur in patients with enlargement of the atria associated with increased atrial pressures. Atrial fibrillation may be associated with hyperthyroidism, congestive heart failure, and increased age. Because of the multiple electrical wavefronts occurring during atrial fibrillation, the coordinated contraction of the atrium immediately preceding ventricular contraction is absent. Atrial contraction in sinus rhythm, sometimes called “an atrial kick” provides an additional blood Label1 volume to the ventricles and results in an increase in cardiac output of between 10-25%. The absence of atrial contraction may lead to “stagnation” of blood in the atria, potentially causing blood clots, which may embolize to the brain and other parts of the body. Atrial fibrillation is an important cause of stroke, particularly in patients with heart failure, hypertension, or increasing age. Reentry (see above) creates an electrical wavefront to move in a circular path through the atria so that each wave is identical to the next wave. The atrial rate is commonly 300 beats per minute usually from 250 to 350 beats per minute. As in atrial fibrillation, in atrial flutter, the A-V node does not play an obligate role in the perpetuation of the atrial rhythm. Thus, vagal stimuli or adenosine (that transiently blocks conduction through the A-V node) will not terminate atrial flutter. Atrial Flutter Atrial flutter is caused by a single reentrant circuit Atrial fibrillation is caused by multiple wavefronts Atrial flutter has a characteristic “sawtooth” pattern while atrial fibrillation has a non- repetitive “wavy” appearance The ventricular complexes in atrial flutter may be somewhat regular while they are usually quite irregular in atrial fibrillation Figure 9 Arrhythmias - Paul J. Raventricular Tachycardia is a term used to describe arrhythmias in which impulse conduction begins above the ventricles (hence supraventricular) and then travels via the A-V node through the rest of the conduction system. These tachycardias have atrial rates, which usually range from 150-250 beats per minute and have ventricular rates, which may be the same or less depending on the arrhythmia’s mechanism. Atrial Fibrillation (top), Atrial Flutter (middle), and Supraventricular Tachycardia (bottom). Atrial fibrillation is irregular due to multiple wavefronts in the atria, and is not due to a single reentrant circuit. Atrial flutter is due to a reentrant circuit in the atria, causing a repetitive saw toothed pattern.
It produces contraction of the unstriped muscular fibers purchase cefuroxime 500 mg with amex, slowly but permanently stimulating the removal of excess of growth cheap cefuroxime 500 mg mastercard. In parturition it is not so immediate or forceful as ergot buy cefuroxime 250 mg with mastercard, but acts mildly in the same manner. In uterine subinvolution, in menorrhagia or metrorrhagia from this cause, it is the best remedy we have. It is useful also in post-partum hemorrhage, but is rather slow in its action when immediate results are demanded. In the incipient stage of the development of tumors within the uterine structure, or fibroid growths, it is not excelled by ergotine. It may be used hypodermically in these cases, and its results are comparatively permanent. He uses the mother tincture in conjunction with conium, giving five drops at a dose three or four times a day, the hydrastis before, the conium after meals. He says: “Sometimes, I mix them and give ten drops of the mixture three times a day. Where the swelling is soft or undulated and painful on pressure, and pain extending into the axilla, we find phytolacca in the same doses better than either. Sometimes, all three remedies are good together, and none of them is valuable in the open cancer. The remedies must be continued a long time to make a decided impression, and their effect is even increased by the same remedies being applied externally in the form of a plaster. In all catarrhal conditions, especially if there be muscular relaxation and general enfeeblement, it is a useful remedy. It is used locally in solution and is of much value as an application wash, irrigating fluid or gargle in all such catarrhal, ulcerating, aphthous, indolent and otherwise unhealthy conditions of mucous surfaces. It is a most useful gargle in aphthous or ulcerated sore mouth, in conditions where the gums are spongy or loosened from the teeth or bleed easily. Ellingwood’s American Materia Medica, Therapeutics and Pharmacognosy - Page 256 Ten minims of a fluid preparation, to the ounce, may be used, or a solution of the hydrochlorate of hydrastine in nasal catarrh, in inflammation of the eyes and in gonorrhea One grain of the hydrochlorate in an ounce of rose water, with or without five grains of the sulphate of zinc, is of superior value in purulent conjunctivitis. The same preparation, diluted, is useful in gonorrhea Five drops of the solution in a dram of warm water is the proper strength. The colorless hydrastis in a solution with a small quantity of the potassium chlorate is sometimes superior in nasal catarrh. It is the best of washes in leucorrhea, whatever the cause, and it can be used freely without danger and in various strengths—from one dram to three, to the pint of hot water. It is of much service when the discharge is thick, yellow, and the membranes relaxed and feeble. It forms an excellent wash in eczema of the anus, with ulcers or fissures within the rectum. Its use may be followed with the application of a zinc ointment, with twenty-five per cent its weight of bismuth subnitrate. In mild solutions of the hydrochlorate of the alkaloid one-fourth grain to the ounce, it is serviceable in catarrh of the bladder, as an irrigating fluid. We find in addition to the tonic influence of this remedy, that it has been used in a number of cases of gall stone, with curative results. Professor Farnum claims to have cured a number of cases with the powdered hydrastis. The cases which he regards amenable to this treatment are, first, ordinary cases of cholelithiasis, where the symptoms are transient and not severe; second, acute inflammatory cases, usually attended with fever, and catarrhal conditions of the gall ducts; third, cases ordinarily called biliary colic. Those, of course, where organic change has not taken place, or where the stones are not impacted, in the gall bladder or in the ducts. He uses it as an efficient remedy in catarrhal jaundice where there is no pain to indicate the presence of gall stones in the ducts. This is in harmony with its influence on general catarrhal conditions of the gastro-intestinal tract. Goss claimed that it had direct catalytic power, and aided the digestion, while it corrected the biliary function. It restores the mucous lining of the gall duct, in the same manner that it influences other mucous surfaces. Ellingwood’s American Materia Medica, Therapeutics and Pharmacognosy - Page 257 Cuthberton gave hydrastis canadensis as a tonic to a pregnant woman who had a goitre of recent appearance. As a result of this observation, he treated twenty-five other cases of goitre at the time of puberty, or during the pregnant state. At times when interference with the function of the reproductive organs seemed to produce reflex irritation. One of the patients had become steadily worse under the use of iodine, the iodides, and thyroid extract. This patient began to improve as soon as hydrastis was given, and was promptly cured with this remedy alone. It has been reported as a remedy for mammary cancer, but its more satisfactory influence is upon painful fulness of the mammary gland, during the menstrual period, or for the treatment of local enlargements occurring more or less suddenly, of a benign character, either in maiden ladies or at the menopause. There is an abundance of authority for the use of hydrastis in conjunction with conium maculatum, in the treatment of non-malignant mammary tumors. Two minims each of these two remedies, in the specific form, was given by Webster before meals and at bed time, in these cases, with satisfactory results. The doctor reports in detail quite a number of cases which were relieved or cured by this treatment. The above named agent is a native of Ceylon where it is used as a remedy for all dropsical conditions. It is but little known in America, but may be received on the reliable authority of the native physicians of Ceylon, who use an infusion of the plant, two ounces to the pint, the entire quantity to be given within twenty-four hours. The remedy produces active diuresis, but the reports are as yet so meager that we have no way of knowing in what way the agent acts upon the organs to produce its desired effects. Physiological Action—Henbane is a narcotic and causes deranged vision, headache, giddiness, dilated pupils, dry throat, hoarseness, weakness of the lower limbs, spasms, cramps, paralysis, loss of speech, or loquacious delirium with hallucinations, followed by a dreamy sleep, according to the amount taken. A continuous use of the medicine causes an eruption of the skin of a red color, which is dry and itching. While power to temporarily increase nerve force—mild stimulant properties—is ascribed to hyoscyamus, that influence is much less marked than belladonna and stramonium, although its general effects are in many ways similar to these agents in medicinal doses. It is almost entirely devoid of irritant properties, but is soothing, calmative and sedative to a marked degree. Specific Symptomatology—It is specific in excitable mental conditions, and in the violent and noisy delirium of fevers and acute inflammations, to subdue the excitement and to induce sleep. In all conditions where there are busy delirium, hallucinations, weight in the front part of the brain, extreme activity of the mind, disturbed sleep with wild and frightful dreams, coma vigil, flushed face, wild, red and restless eyes, it is a sure remedy. In the restlessness, ceaseless agitation and insomnia of exhaustion, and in diseases of infants and of the extreme aged and feeble, it is especially applicable. Therapy—It is valuable in the pneumonitis of infants for its general Ellingwood’s American Materia Medica, Therapeutics and Pharmacognosy - Page 259 soothing influence, and for its sedative effect upon the cough and respiration, and also in bronchitis, with short, sharp cough. A dry cough, increased upon lying down and relieved upon getting up, is surely relieved by its use.
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