Heart agonist actions

Amphetamine s [am FET a meen] marked central stimulatory action is often mistaken by drug abusers as its only action. However, the drug can increase blood pressure significantly by a-agonist action on the vasculature as well as p- stimulatory effects on the heart. Its... 

Dopamine. Dopamine is u.scd in the treatment of shock. It is ineffective orally, in large part because it is a substrate for both MAO and COMT. Thus, it is used intravenously. In contrast with the catecholamines NE and epinephrine, dopamine increases blood flow to the kidney in doses that have no chronotropic effect on the heart or that cause no increa.se in blood pressure, lire increased blood How to the kidneys enhances glomerular filtration rate, Na excretion, and. in turn, urinary output. The dilation of renal blood ve.s-.sels produced by dopamine is the result of its agonist action on the D -dopaminc receptor. 

Drug X causes slowing of heart rate, but this is converted into tachycardia by hexamethonium and atropine—ie, the bradycardia is caused by reflex vagal discharge. Phenoxybenzamine also reverses the bradycardia to tachycardia, suggesting that alpha receptors are needed to induce the reflex bradycardia and that X has direct beta-agonist actions. The choices that evoke a vagal reflex bradycardia but can also cause direct tachycardia are limited the answer is (E). 

Isoproterenol increases heart rate due to nonselective pi/p2-a0renoceptor agonist actions, thereby shortening the QT interval and the effective refractory period. While there are no randomized controlled trials of isoproterenol use for torsades in humans (it has prevented quinine-induced torsades in a dog model), occasional case reports suggest benefit (Omar et al. 2014). It is probably particularly useful as a bridge to temporary pacing in patients unresponsive to magnesium sulfate. 

Catecholamines are also intimately involved in cardiac function, with 3-sympathetic agonists having a generally stimulant action on the heart. Some effort has thus been devoted to the synthesis of agents that would act selectively on the heart. (Very roughly speaking, 3 -adrenergic... 

Dichloro-isoprenaline was the first [3-adrenoceptor antagonist to he described (6). It was discovered in a search for specific (3-stimulants as bronchodilators. This compound is a partial agonist, i.e. it can antagonise the action of isoprenaline hut itself is a (3-stimulant. (The stimulant action of dichloro-isoprenaline is readily demonstrated in an animal depleted of natural catecholamines ( with say reserpine). In this type of preparation it will stimulate say heart rate to a maximum of 70% of that produced hy isoprenaline itself). 

Things turn out to be a bit more complex than I have suggested thus far. Acetylcholine is also known to cause contraction of skeletal muscle and to slow the rate of heartbeat. However, muscarine does neither of these things nor are these actions of acetylcholine blocked by atropine. Another plant-derived molecule, nicotine from tobacco, proved to be an acetylchohne agonist at skeletal muscle and heart. 

WARNING Long-acting p2-agonists may t risk of asthma-related death Uses COPD maint Action LA p2-agonist, relaxes airway smooth muscles Dose 15 meg neb bid, 30 meg/d max Caution [C, ] w/ CV Dz, X Contra Not for acute asthma component hyp sensitivity peds w/ phenothiazines Disp Meg neb SE Chest/back pain, D, sinusitis, leg cramps, dyspnea, rash, flu-synd, t BP, arrhythmias, heart block J-K EMS Monitor ECG for arrhythmias, heart block, and hypokalemia (flattened T waves) t risk of acute asthma attack, treat w/ shortacting p-agonist OD May cause CP, palpitations, muscle tremors and cramps, and syncope symptomatic and supportive... 

Bromocriptine (Parl el) [Antiparkinsonian Agent/Dopamine Receptor Agonist] Uses Parkin on Dz, hyperprolactinemia, acromegaly, pituitary tumors Action Direct-acting on the striatal dopamine receptors X prolactin secretion Dose Initial, 1.25 mg PO bid titrate to effect, w/ food Caution [B, ] Contra Severe ischemic heart Dz or PVD Disp Tabs, caps SE X BP, Raynaud phenomenon (vasospastic disorder resulting in discoloration of the fmgers/toes), dizziness, N, hallucinations Interactions T Effects W/ erythromycin, fluvoxamine, nefazodone, sympathomimetics, antihypertensives X effects W/ phenothiazines, antipsychotics EMS Monitor BP may cause intolerance to EtOH OD May cause NA, severe hypotension give IV fluids symptomatic and supportive... 

Terbutaline and albuterol are relatively selective Pj-adrenoceptor agonists. Both have a longer duration of action than isoproterenol because they are not metabolized by COMT. Like isoproterenol, they are not metabolized by MAO and are not transported into adrenergic neurons. Terbutaline and albuterol are effectively administered either orally or subcutaneously. Because of their selectivity for Pj-adrenoceptors, they produce less cardiac stimulation than does isoproterenol but are not completely without effects on the heart. 

The most important actions of the (3-blocking drugs are on the cardiovascular system. -Blockers decrease heart rate, myocardial contractility, cardiac output, and conduction velocity within the heart. These effects are most pronounced when sympathetic activity is high or when the heart is stimulated by circulating agonists. 

Mechanism of Action A rapid-acting vasodilator. An agonist for Dj -like dopamine receptors also produces vasodilation in coronary, renal, mesenteric, and peripheral arteries, Therapeutic Effect Reduces systolic and diastolic BP and increases heart rate. Pharmacokinetics After IV administration, metabolized in the liver. Primarily excreted in urine. Unknown if removed by hemodialysis, Half-Hfe Approximately 5 min. 

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