Lecture 3: Direct Acting Vasodilators Flashcards
What is preload?
- The passive stretching of muscle fibers in the ventricles.
What causes the stretching in Preload?
- The stretching results from blood volume in the ventricles at the end of diastole
- The more the heart muscles stretch during diastole, the more forcefully they contract during systole
What is contractility
- Refers to the inherent ability of the myocardium to contract normally
What influences contractility?
Preload, the greater the stretch the more forceful the contraction
What is afterload?
(Resistance) Refers to the pressure that the ventricular muscles must generate to overcome the higher pressure in the aorta to get the blood out of the heart
What is the narrow range that Arterial blood pressure is regulated?
120/80
Why is arterial BP regulated within a narrow range?
to provide adequate perfusion of the tissues without causing damage to the vascular system, particularly the arterial intima
Arterial BP is directly proportional to _ _ and _ _ _
cardiac output and peripheral vascular resistance
What two overlapping mechanisms control cardiac output and peripheral resistance? ⭐️
- Baroreflexes (symp nervous system)
- Renin-angiotensin-aldosterone system (RAAS)
What are the effect of most antihypertensive drugs?
Antihypertensive: decreasing BP
Reducing cardiac output and/or decreasing peripheral resistance
What is the speed of the BP response mediated by baroreflexes (SNS)?
FAST= rapid, moment to moment
What is the speed of BP response mediated by the renin-angiotensin-aldosterone system (RAAS)?
SLOW= Long-term
What are the locations and nerves used for baroreceptors?
- Aortic arch receptors via the vagus nerve (CN X)
- Carotid sinus receptors via carotid sinus nerve to nerve IX (glossopharyngeal)
Fill in the blanks for the factors that affect cardiac output
- Heart rate
- Contractility
- Filling pressure (blood volume & venous tone)
Explain the pathway of the baroreflexes (mediated by SNS) response to a decrease in BP (SHORT TERM)
Decrease in BP→ ↑ Sympathetic activity via baroreceptors → Activates ⍺1 on the heart (↑ venous return and ↑ resistance) & β1 on smooth muscle (↑ CO, contractility, releases renin)→Increases BP
Explain the pathway of the Renin-angiotension-aldosterone systen (RAAS) response to a decrease in BP (LONG-TERM)
Decrease in BP→↓ in renal blood flow→Release renin (&↓ glomerular filtration)→↑ Angiotensin 2→↑ Aldosterone→ ↑ water/Na+ retention→ ↑ blood volume→ ↑CO→Increases BP
What are the two ways in which renin is released?
- Activation of β1 receptors (short-term)
- Low renal blood flow (long-term)
Explain the renin-angiotensin-aldosterone system
Angiotensinogen w/ release of RENIN→Angiotensin I w/ ACE →Angiotensin II →( increases SNS activity, tubular reabsorption (Na+,Cl-,H2O), aldosterone, vasconstriction and ADH)→ALL leads to increase in BP
ACE= Angiotensin Converting Enzyme
ADH=Antidiuretic hormone
What is the negative feedback in the renin-angiotensin-aldosterone sytem
Kidney decreases the release of RENIN
From the RAAS pathway, what organ secretes Angiotensinogen?
Liver
From the RAAS pathway, what organ secretes renin?
Kidney
From the RAAS pathway, what organs secretes Angiotensin Converting Enzyme (ACE)?
Lungs and kidneys (surface of pulmonary and renal endothelium)
From the RAAS pathway, what organ secretes Aldosterone?
Adrenal gland cortex
From the RAAS pathway, what organ secretes ADH?
Pituitary gland (posterior lobe)
Explain the pathway for cardiac myocyte contraction & relaxation
Symph. activation→releases NE→binds to β1 receptors→Gs increases cAMP→Increase Pk-a→Increases extracellular Ca2+ release→Intracellular Ca2+ release from SR→Binds to Troponin→Actin binds to Myosin→Contraction
What type of contractions does vascular smooth muscle undergo?
Slow, sustained, tonic contractions
List the 3 ways contractions in VSM (vascular smooth muscle) are initiated?
- Mechanical stimuli
- Electrical stimuli
- Chemical stimuli
Explain the mechanical stimuli that cause contraction in vascular smooth muscle (VSM).
- Passive stretching of VSM can cause contraction.
- Termed a Myogenic response
Explain the electricial stimuli that causes contractions in the (VSM).
Electrical depolarization of the VSM by opening voltage dependent Ca2+ channels, causing an increase in the intracellular concentraction of calcium
Explain the chemical stimuli that causes contractions in vascular smooth muslce (VSM).
A number of chemical stimuli such as norepinephrine, angiotensin II, vasopressin, endothelin-1, and thromboxane A2 can cause contraction.
Explain vascular smooth muscle (VSM) contraction
Contraction: An increase in free intracellular Ca2+ (through Ca2+ channels or by release from internal stores (SR))
- The free Ca2+ binds to calmodulin (CM)
- Calcium-calmodulin activates myosin light chain kinase (MLCK) an enzyme that phosphorylates myosin light chains (MLC) in the presence of ATP
- MLC phosphorylation leads to cross-bridge formation b/w the myosin head and actin filaments→VSM contraction
Explain vascular smooth muscle (VSM) relaxation
Relaxation: Reduced phosphorylation of MLC. This can result from:
- Reduced relase of Ca2+ by the SR or reduced Ca2+ entry into the cell
- Inhibition of MLCK by increased intracellular conc. of cAMP (Gs-R pathway)
- MLC dephosphorylation (nitric oxide (NO)→cGMP pathway)
Gs-R: Gs-linked vascular receptor
What type of drugs target MLC dephosphorylation?
Nitrate drugs
What is the pharmacodynamics of direct acting vasodilators?
Affects venous side with preload (capacitance) and affects resistance with afterload
List the different drug classes of direct-acting vasodilators.
- Nitrates
- Hydralazine
- Phosphodiesterase V inhibitors
- Calcium Channel Blockers (non-dihydropyridine)
- Calcium Channel Blockers (dihydropyridine)
What drugs are Nitrates (nitric oxide donors)? (3)
- Isosorbide dinitrate
- Nitroglycerine
- Nitroprusside
What drugs are Hydralazine? (1)
Hydralazine
What drugs are Phosphodiesterase V inhibitors? (1)
Sildenafil
What drugs are non-DHP calcium channel blockers (CCBs)? (2)
- Diltiazem
- Verapamil
What drugs are DHP calcium channel blockers (CCBs)? (2)
- Amlodipine
- Nifedipine
What is the mechanism of action of nitrates (nitric oxide donors)?
Drugs: Isosorbide dinitrate, Nitroglycerine, Nitroprusside
- Release NO when metabolized
- Relax smooth muscle: Vascular, corpora cavernosa, short-lived in others (e.g. bronchial, GI)
- Inhibit platelet aggregation