Pharmacology Flashcards
What are the 3 princpiple voltage and time dependent conductances in the heart?
• Voltage active sodium channels (depolarise) • Voltage activated calcium channels (depolarise) • Voltage activated potassium channels (repolarise)
What fundamentally differs nodal tissue for cardiac muscle tissue?
Pacemaker potential - diastolic potential/phase 4 slope (rather than horizontal line in cardiac muscle tissue at phase 4 - resting potential). Also atrial/ventricular cells have a plateau on the AP which nodal cells do not.
What is over-drive suppression?
SA node discharges APs at the highest rate and overrides the spontaneous discharge in the AV node and purkinje system and so is the dominant pacemaker
**What 4 conductances underlie the diastolic/phase 4 potential in pacemaker nodal cells?
1) Background sodium current (Ib) - leaky and present all of the time 2) Funny current (If) - hyperpolarisation mediated 3) Delayed rectifier potassium current (Ik) - switched off in phase 4, therefore contribute to depolarisation by inhibiting hyper polarisation) 4) Transient Voltage Activated Calcium Conductor (Icat) - kicks in at the end of phase 4 to give final kick to threshold => these all contribute to depolarisation and allow pacemaker cells to reach threshold
**Once threshold has been reached, which channel kicks in to bring the upstroke in nodal cells?
5) Long Calcium Channel (Ical) - inward movement of calcium underlies nodal AP (unlike in atrial/ventricular cells where it is sodium mediated)
**Which channels underlie repolarisation in nodal cells?
Delayed rectifier potassium current (Ik) which were previously switched off, now switch on and cause the outward flow of potassium
Why is there not a gradual depolarising drift in atrial/ventricular cells, like there is in nodal cells?
Because there is a constant trickling of potassium out of the cell by Ik1 - an inward rectifying conductor which helps maintain the resting membrane potential with slight hyper polarisation
What therefore causes an AP in atrial/ventricular myocytes, if not a gradual depolarising drift?
The arrival of depolarising influence from an adjacent cardiac muscle cell.
**What causes Phase 0: Depolarisation in atrial/ventricular myocytes?
Arrival of a depolarising AP from adjacent cardiac muscle cells causes voltage-activated sodium channels- INa channels to open and allow a large current of Na influx, activating rapidly.
**What causes Phase 1: Early repolarisation in atrial/ventricular myocytes?
Rapid opening/closing of transient outward K current – It0, which combined with INa channel closure results in a slight repolarisation. However, don’t get complete repolarisation, as long calcium currents – ICaL are now activated after a delay – this helps maintain the depolarisation
**What causes Phase 2: Plateau phase in atrial/ventricular myocytes?
Ca2+ channels have opened, ICaL. This maintains the plateau. At the same time there is a slowly developing delayed rectifying K efflux (outward) current (Ik). Fine balance between Ca influx and K efflux maintains the plateau. INaL channels (previous Na channels from Phase 0 that stayed open, and reverse §NCX1 channels contribute too)
**What causes Phase 3: Late repolarisation phase in atrial/ventricular myocytes?
Eventually the outward potassium (Ik) wins, and depolarisation occurs. This involves components IK channels which activate rapidly and IK1 channels which activate slowly
Which GPCRs do B1 adrenoreceptors preferably couple with?
Gs - which increases the conversion of ATP to cAMP, which mediates most of sympathetic activity of the heart
How does sympathetic innervation increase heart rate?
1) Increase the slope of diastolic phase 4 depolarisation (enhanced If and Ica activity) 2) Reduction of the threshold for AP initiation (enhanced Ica)
How does sympathetic innervation increase contractility?
1) increase in phase 2 plateau of the cardiac AP in atrial and ventricular myocytes, therefore enhanced Ca2+ influx 2) increased sensitisation of contractile proteins to Ca2+
What effect does sympathetic innervation have on the heart?
1) Increased HR 2) Increased contractility 3) Increased conduction velocity (smaller AV nodal delay to match HR) 4) Increased automaticity 5) Decreased duration of systole (to allow complete emptying of ventricles with new HR) 6) Decreased cardiac efficiency (oxygen consumption increases) 7) Increased activity of Na+/K+-ATPase 8) Increased mass of cardiac muscle
Which parasympathetic receptors lie in the heart and with GPCR do they coupe with?
M2 muscarinic choliceptors. Couple with Gi
What does parasympathetic stimulation do in the heart, and what effect does this have?
1) decreases activity of adenylate cyclase and reduces [cAMP]I 2) opens potassium channels This causes: - Decreased HR - Decreased contractility - Decreased conduction velocity in AV node
What are the 2 main vagal manoeuvres (which employ the parasympathetic system to suppress AV conduction)?
Valsalva manœuvre and massage of the bifurcation of the carotids
Which type of channels mediate the funny current?
Hyperpolarization-activated cyclic nucleotide gated (HCN) channels (HCN4 in heart).
What is ivabridine?
Blocker of the HCN (funny current) channel, and therefore decreases the pacemaker slope and reduces the HR. Used in angina in particular
How does contraction occur in excitation-contraction couple in cardiac muscle?
- In the ventricular AP, we get Ca2+ entry during phase 2, due to voltage-activated Ca2+ channels (L-type) which moves into the cytoplasm 2. This creates a modest increase in the intracellular Ca2+ concentration, though this signal is not enough to cause an AP in itself, so needs to be amplified. 3. It does this using Calcium Induced Calcium Release (CICR) - The Ca2+ that has come through the L-type channels, bind to ryanodine-type 3 channel in the membrane of the SR, which is the main calcium store in the myocyte, and allow the efflux of calcium from the SR into the cytoplasm 4. This amplification leads to Ca2+ binding to troponin-c, which undergoes a conformational change which allows it to move tropmyocin from the synaptic cleft 5. This then allows cross-bridge formation between actin and myosin, resulting in contraction.
How does relaxation occur in excitation-contraction couple in cardiac muscle?
- Relaxation is controlled by the repolarisation in phase 3 to phase 4 of the AP 2. Repolarisation causes the voltage-activated Ca2+ channels to close 3. Ca2+ influx and CICR ceases and is pumped back to the sarcoplasmic reticulum, to be re-stored, by action of Ca2+ ATPase in the membrane of the SR. 4. Simultaneously, Na+/Ca2+ exchanger (NCX) pushes 1Ca out and 3Na in. 5. Under these 2 mechanisms, Ca2+ dissociates from troponin-C; the cross-bridges break up; actin and myosin are pumped back into the synaptic cleft and we get relaxation.
What are examples of β-adrenoceptor agonists?
Dobutamine, adrenaline and noradrenaline (catecholamines)
What is the clinical use of adrenaline?
Used in cardiac arrest; emergency treatment of asthma; anaphylactic shock - has both α/β agonist properties so causes: • Positive inotropic and chronotropic actions (β1) • Redistribution of blood flow to the heart (constricts blood vessels in the skin, mucosa and abdomen (α1)) • Dilation of coronary arteries (β2)
What is the clinical use of dobutamine, and is it selective?
Selective for β-adrenoceptors. Used as an IV infusion in acute heart failure short term
Which β-adrenoceptors antagonists are selective and non-selective?
Non-selective: propanolol Selective: Atenolol, bisoprolol, metoprolol
What are the CVS clinical uses of β-adrenoceptors antagonists (beta-blockers)?
1) Dysrhythmias (delay AV conduction and reduce excess sympathetic drive) 2) Hypertension 3) Angina 4) Heart failure (start low , go slow) - Carvedilol used in particular as has additional α1 antagonist activity causing vasodilation
What are the main adverse effects of β-Blockers?
- Bronchospasm (esp. in asthmatics) - Aggravation of heart failure - Bradycardia - Hypoglycaemia (β2 cause release of glucose from liver) - Fatigue -Cold extremities
What are examples of non-selective muscarinic Ach receptor antagonists?
Atropine
What are the clinical uses of atropine (muscarinic Ach receptor antagonists)?
1) Bradycardia following MI 2) Anticholesterase poisoning
How does digoxin increase contractility?
Blocking the sarcolemma ATPase, causing an increase of calcium and therefore increased contraction
When is digoxin used?
Heart failure, particular with AF
What is the main cardiac glycoside?
Digoxin
What are the main effects of digoxin?
Indirectly: Cause increases release of Ach into SA node so decreases firing and slows AV conduction so increases refractory period Directly: Shortens the AP and refractory period
What is the main example of calcium sensitisers and how do they work?
Levosimendan. They increase the sensitivity of the contractile elements to calcium, therefore increasing contractility.
What is the main example of inodilators and how do they work?
Amrinone and milrinone. They inhibit phosphodiesterase (PDE) enzyme in cardiac and smooth muscle cells and hence increase intracellular [cAMP]i
What are the main groups of inotropes?
1) Cardiac glycoside eg. digoxin 2) Calcium sensitisers eg. levosimendan 3) Inodilators eg. amrinone and milrinone
**What causes contraction in smooth muscle?
Relies on an increase in calcium which then promotes calmodulin, and this complex then activates MLCK which then forms Myosin-LC phosphatase – which promotes contraction
**What causes relaxation in smooth muscle?
cGMP dephosphorylates the myosin –LC-phosphatase causing relaxation
True or False: Endothelium acts to suppress platelet aggregation
True, this is why damage to the endothelial is important in thrombus formation
What produces NO and when?
Endothelial cells, and in response to anything causing increases calcium levels
How does NO cause relaxation?
- NO enters the enzyme guanylate cyclase, which converts GTP to cGTP. This activates protein kinase G, causing relaxation - Simultaneously, NO also causes Ca dependent K channels to open and cause K movement out, hyperpolarising the cell and causing relaxation
What are the effects of organic nitrates?
Essentially act as equivalents to endothelium. They metabolise to NO and therefore cause: - Venorelaxation - Arteriolar dilation - Coronary vessel dilation (also they dilate collateral branches which sprout from healthy areas to ischaemic areas)
What are the 2 main examples of organic nitrates?
Glyceryltrinitrate (GTN) and isosorbide mononitrate (ISMN)
Why can’t GTN be taken orally?
Completely metabolised on first pass to liver
What is endothelin and what does it do?
Acts as a vasoconstrictor, released from endothelial cells in response to adrenaline, angiotensin etc
What produces renin?
Granular cells of the juxtaglomerular apparatus of the kidney
What does renin do?
Splits angiotensinogen to angiotensin I (which is inactive)
What does ACE do?
Activates angiotensin I to angiotensin II
What does angiotensin II do?
Acts on g protein coupled angiotensin receptors which cause vasoconstriction. Also causes the release of aldosterone.
What does aldosterone do?
Acts on the kidney on the nephron – increasing the reabsorption of sodium and accompanying water from the urine back into the circulation
What do all ACE inhibitors end in?
‘-ipril’
What do all AT1 receptor antagonists end in?
’- sartan’
What effects of ACE inhibitors have?
Cause venous dilatation (↓preload) and arteriolar dilatation (↓afterload and ↓TPR) decreasing arterial blood pressure and cardiac load
Why do ACE inhibitors cause a dry cough?
Inhibit the metabolism of bradykinin
What effect do α1 receptors have?
Constriction of vessels
What effect do β1 receptors have?
↑Rate; ↑ Force; ↑ AV node conduction velocity
What effect do β2 receptors have?
relax the vessels and also the bronchi
Why are beta-blockers of benefit in angina?
- Decrease myocardial O2 requirement ( because ↓HR & ↓SV = ↓work = ↓O2 requirement ) 2. Counter elevated sympathetic activity associated with ischaemic pain 3. Increase the amount of time spent in diastole (↓HR), improving perfusion of the left ventricle
What effects do calcium antagonists have?
Calcium antagonists prevent the opening of L-type channels in excitable tissues in response to depolarization and hence limit ↑[Ca2+]i , therefore reduce rate, AV conduction and force of contraction
What are the 3 main calcium antagonists?
Verapamil, Amlodipine and Diltiazem
What effects do potassium channel openers have?
Cause hyperpolarization which switches off L-type Ca2+ channels
What are the 2 main potassium channel openers?
Minoxidil and nicorandil
What effects do α1-Adrenoceptor Antagonists have?
Cause vasodilatation by blocking vascular α1-adrenoceptors
What are main examples of α1-Adrenoceptor Antagonists?
Prazosin and doxazosin