Pharmacology (Cardiovascular) Flashcards
What are the regulatory influences on the heart rate?
Balance of autonomic input, stretch, temp, hypoxia, blood pH, thyroid hormones.
What are phase 4, 0 and 3 of the action potential of a pacemaker cell?
4 - pacemaker potential (increases steadily to threshold). 0 - Rapid depolarisation. 3 - repolarisation.
What currents cause the pacemaker potential (phase 4)?
Background sodium current inward (Ib), funny current inward (If), transient calcium current inward (IcaT), delayed rectifier potassium current outward (Ik).
What current causes phase 0 and phase 3 of the action potential of a pacemaker cell?
0 - long calcium current inward (IcaL). 3 - delayed rectifier potassium current (outward).
How does noradrenaline/adrenaline binding to an adrenoceptor in nodal and myocardial cells increase the heart rate?
Adrenoceptor coupled to a Gs protein which activates adenylyl cyclase to convert ATP into cAMP which increases [cAMP]i. This causes an increase in heart rate (positive chronotropic effect).
What effect does sympathetic stimulation have on the action potential of pacemaker cells?
It increases the slope of the pacemaker potential and decreases the threshold for AP initiation.
Describe the 2 effects of sympathetic stimulation on the cardiac myocytes that causes an increase in contractility?
It increases phase 2 (plateau phase) of the cardiac action potential in atrial and ventricular myocytes by enhanced Ca2+ influx and sensitises contractile proteins to Ca2+.
What do they terms chronotropic, inotropic, dromotropic and lusitropic mean?
Chronotropic - affects heart rate. Inotropic - affects contractility. Dromotropic - affects AV node conduction velocity. Lusitropic - affects duration of systole.
List all of the effects of sympathetic stimulation in the heart.
Increases heart rate, increases contractility, increases conduction velocity in AV node, increases automaticity, decreases duration of systole, increases activity of Na/KATPase and increases mass of cardiac muscle.
How does sympathetic stimulation cause an increase in conduction velocity in the AV node?
Due to enhancement of funny current and calcium current.
What is automaticity?
The tendency for non-nodal regions to acquire spontaneous activity.
How does sympathetic stimulation decrease the duration of systole?
Due to increase uptake of calcium into the sarcoplasmic reticulum.
Describe on the molecular level how parasympathetic stimulation affects heart rate.
ACh binds to M2 receptor, activates Gi protein which both decreases the activity of adenylate cyclase and reduces [cAMP]i and opens potassium channels (GIRK) to cause hyperpolarisation of SA node (mediated by betagamma subunits).
Why does the parasympathetic nervous system decrease the heart rate (negative chronotropic effect)?
Due to decreased slope of pacemaker potential caused by reduced If and ICa, hyperpolarisation caused by the opening of GIRK channels and increase in threshold for AP initiation caused by reduced ICa.
What effects does parasympathetic stimulation have on the heart?
Decreases heart rate, decreases contractility (atria only), decreases conduction in the AV node, and may cause atrial arrhythmias.
How does parasympathetic stimulation decrease contractility and where does it do this?
Due to decrease in phase 2 of cardiac AP and decreased Ca2+ entry.
How does parasympathetic stimulation decrease conduction in the AV node?
Due to decreased activity of voltage-dependent Ca2+ channels and hyperpolarisation via opening of K+ channels.
What are HCN channels?
Hyperpolarisation-activated cyclic nucleotide gated channels.
What current are HCN channels involved in?
The funny current (depolarising current in pacemaker potential).
What are HCN channels in the SA node activated by?
Hyperpolarisation following the action potential.
What happens to HCN channels which decreases the slope of the pacemaker potential and reduces heart rate?
They are blocked.
Give an example of a selective blocker of HCN channels and what disease is it used in?
Ivabradine and angina.
What does calcium released from the sarcoplasmic reticulum bind to and how does this cause cardiac muscle contraction?
Troponin C, shifts tropomyosin out of actin cleft and causes cross bridge formation.
What are the 6 steps in cardiac muscle relaxation?
- Repolarisation in phase 3 to phase 4.
- Voltage-activated L-type Ca2+ channels close.
- Ca2+ influx ceases. Ca2+ efflux occurs by the Na+/Ca+ exchanger 1 (NCX1).
- Ca2+ release from the sarcoplasmic reticulum ceases. Active sequestration via Ca2+-ATPase (SERCA) of Ca2+ now dominates.
- Ca2+ dissociates from troponin C. 6. Cross bridges between actin and myosin break.
What does SERCA stand for and what does it do?
Sarco-endoplasmic reticulum Ca2+ ATPase, it transports calcium into the sarcoplasmic reticulum.
What enzyme does cAMP activate?
Protein kinase A.
What does protein kinase A do in cardiac muscle cells?
- It phosphorylates L-type voltage-activated Ca2+ channel which increases the Ca2+ influx.
- It increases muscle fibre sensitivity to Ca2+ which increases contractility. 3. Causes phosphorylation of phospholamban which increases pumping of Ca2+ and rate of relaxation.
What are the pharmacodynamic effects of B2 agonists on the heart?
- Increase force, rate (cardiac output) and O2 consumption. 2. Decreases cardiac efficiency (O2 consumption increase more than cardiac work).
Why is adrenaline used in cardiac arrests?
Has positive inotropic and chronotropic actions (B1), redistributes blood flow to the heart (constricts blood vessels in skin, mucosa and abdomen, a1), dilates the coronary arteries (B2).
What adrenoceptor is dobutamine selective for, how is it given and when is it used?
B2 adrenoceptors, given IV, acute but potentially reversible heart failure.
What do the physiological effects of B-blockers depend on?
The degree to which the sympathetic nervous system is activated.
Give an example of a non-selective beta-blocker and 3 examples of beta-1 selective ones.
Non-selective: propranolol. Selective: atenolol, bisoprolol, metoprolol.
Give an example of a beta-blocker that is non-selective and a partial agonist.
Alprenolol.
How do non-selective blockers reduce maximal exercise tolerance?
They reduce rate, force and CO significantly.
What effect do non-selective beta-blockers have on coronary vessel diameter and myocardial oxygen requirement?
Reduced coronary vessel diameter marginally, myocardial O2 requirement falls so better oxygenation of the myocardium.
How do beta-blockers help in atrial fibrillation and supraventricular tachycardia?
Delay conduction through AV node and help restore sinus rhythm.
What are B-blockers used to treat?
Arrhythmias, angina, heart failure (compensated, carvedilol has additional a1 antagonist activity causing vasodilation), hypertension (no longer first-line unless co-morbidities are present).
What are the adverse effects of beta-blockers?
Bronchospasm, aggravation of cardiac failure, bradycardia, hypoglycaemia (in patients with poorly controlled diabetes), fatigue
What adverse effects of beta-blockers are removed by using B1-selective agents?
Bronchospasm, hypoglycaemia (as B2 adrenoceptors release glucose from the liver).
Why may beta-blockers cause aggravation of cardiac failure?
Patients with heart disease may rely on sympathetic drive to maintain an adequate CO.
Why may beta-blockers cause bradycardia?
B-adrenoceptors facilitate nodal conduction.
Why may beta-blockers cause fatigue?
CO (B1) and skeletal muscle perfusion (B2) in exercise are regulated by adrenoceptors.
Give an example of a non-selective competitive muscarinic antagonist.
Atropine
What are the pharmacodynamic effect of atropine?
Increase in HR in normal subjects at all but low doses, more pronounced in highly trained athletes (due to increased vagal tone).
What is atropine a first line drug for?
Severe or symptomatic bradycardia, particularly post MI (in which vagal tone is elevated).
How is atropine given and why should you not give too little?
Low-dose atropine may transiently slow heart rate.
What is an alternative to atropine?
Glycopyrronium.
What is the effect on the ventricular function curve in heart failure?
It is depressed.
Give 2 examples of drugs that enhance contractility.
Digoxin and dobutamine.
What transporter does digoxin block?
Na+K+ATPase in the sarcolemma.
Describe how digoxin blocking the sodium-potassium pump leads to greater contractility.
Pump blocked -> Increased intracellular sodium and decreased membrane potential -> decreased activity of Na+/Ca+ exchanger so greater intracellular calcium -> greater storage of calcium in sarcoplasmic reticulum -> greater CICR and contractility.
Where does digoxin bind to on the Na+/K+ATPase?
The alpha subunit, in competition with K+.
What can dangerously enhance the effects of digoxin and why is this particularly important?
Hypokalaemia, digoxin has low therapeutic range.
What are the indirect actions of digoxin on heart electrical activity?
Increased vagal activity (slows SA node discharge, slows AV node conduction; increases refractory period).