Cardiophysiology Exam I Flashcards
How does contractile tone depend on membrane potential?
Depolarization and vasoconstriction are at an increased tone
- normal basal tone around -50 to -60 mV
Properties of Inward Rectifying (Kir)
- expressed in arterioles
- open at basal membrane potential
- open state increased by K+
- blocked by Ba2+
Roles of Inward Rectifying (Kir)
- supplies part of outward current for basal membrane potential
- mediates vasodilation by interstital K+ in exercising muscle, myocardium, and brain
Properties of ATP-depnedent (KATP)
- opened by low ATP or raised ADP, GDP, adenosine A1 receptors and [H+]
- inhibited by alpha-2 adrenoceptors
- blocked (contraction) by glibenclamide
- activated (dilated) by diazoxide, pinacidil, cromakalim, nicorandil, CGRP, and VIP
Roles of ATP-dependent (KATP)
- links vascular tone to metabolic state in exercise and hypoxia
- low, basal open state due to basal PKA activity
- open state raised in cAMP-PKA-mediated vasodilation
Properties of Voltage-Dependent Kv
- Opens slowly on depolarization beyond -30mV
- blocked by 4-aminopyridine (4-AP)
Roles of Voltage-Dependent (Kv)
- part of outward current for basal potential in resistance vessels
- action potential repolarization
Properties of Calcium-Activated (KCa)
(or BK)
- open state promoted by Ca2+ and depolarization
- strongly expressed in large artery of VSM
- blocked by tetraethyl ammonium (TEA), iberiotoxin, charybdotoxin, and ethanol
Roles of Calcium-Activated (KCa)
- contributes to basal membrane potential and repolarization
- if abundantly expressed, BK suppress action potentials
- provides a “brake” on myogenic contraction
- implicated in action of NO
Properties of Voltage-Sensitive Ca2+ (VSCC)
- mainly L-type
- large conductance and long opening
- abundant in resistance vessels
- blocked by dihydropyridines
- Ex: nifedipine
Roles of Voltage-Sensitive Ca2+ (VSCC)
supplies inward current for action potentials, graded electromechanical coupling and Bayliss myogenic response
Properties of Receptor-Operated channel (ROC)
- poorly selective between Ca2+, Na+, and K+
- activated by diacylglycerol when alpha receptors and other G-coupled protein receptors are activated
- insensitive to Nifedipine
Roles of Receptor Operated channel (ROC)
- mediates pharmacomechanical coupling by NAd, angiotensin, vasopressin, 5HT, and histamine
- related channel contributes depolarizing current icat of slow excitatory junction potential (EJP)
Activation of Store-Operated cation channel (cat-SOC)
When IP3 discharges the SR Ca2+ store
Role of Store-Operated Cation Channel (cat-SOC)
conducts extracellular Ca2+ into VSM when Ca2+ store released from SR
Properties of Stretch-Activated cation channel (SAC)
- activated by stretch
- inward Na+ and Ca2+ currents cause depolarization and VSCC activation
Roles of Stretch-Activated cation channel (SAC)
- contractile response of VSM to stretch
- myogenic response
- autoregulation of blood flow
Properties of Calcium-Activated Chloride Channel (ClCa)
Open state promoted by Ca2+ at >200 uM
Roles of Calcium-Activated chloride channel (ClCa)
- Contributes ‘inward’ current iCl for slow EJP
- depolarizes membrane (more positive)
- contributes to vasomotion
VSM Contraction Properties
- Sacromere-like unit
- longer actin filaments allow for greater shortening
- no striations
- myosin activation causes contraction
- force determined by [Ca2+] and its sensitivity
- long contraction
Homocellular Gap Junctions
connexons between cells
- ion-permeable and electrically conductive
- connects vascular myocytes
Heterocellular Gap Junctions
(myoendothelial gap junction)
endothelial and smooth muscle junction
- between innermost myocytes of the tunica media and endothelial cells
- transmit hyperpolarizing signals
Sarcoplasmic Reticulum in VSM
- 2 Types of Calcium release channels
- IP3-Ca
- Ryanodine
- small Ca2+ store
- CCB (nifedipine) are good resistance vessel dilators
IP3-Ca2+ Release Channel
releases the SR calcium store
- raises cytosolic Ca2+ ‘globally’ and increases vascular tone
Ryanodine Receptor (RyR)
release spontaneous bursts of Calcium “sparks”
- activates nearby Ca2+-dependent K+ channels which hyperpolarize the cell
- does not cause contraction
Caveolae
invaginations within the cell wall that increase SA by 75%
- thought to be signal pathways
- contain lots of B-receptors, G proteins, Calcium channels, etc
Depolarizing the cell induces _____
vasoconstriction
Effect of Epinephrine on alpha receptors
vasoconstriction
Effect on Epinephrine on Beta-2
vasodilation
Effect of Histamine on H1 receptors
vasoconstriction
Myosin Light Chain Kinase (MLCK)
- phosphorylates the myosin light chains in the presence of ATP
- cross bridge formation
- smooth muscle contraction
“Latch State” of VSM
slow crossbridge cycling
- maintains vascular tension
- consumes less energy
Ion Movement in VSM
[Ca2+] cytosolic range in VSM
100-350 nM
Ca2+ Sequestration
Ca2+-ATPase pump in the SR that removes calcium out of the cytoplasm
Ca2+ Expulsion
Ca2+-ATPase pump in the sarcolemma that removes Calcium from the cytoplasm
anything that causes VSCC, ROC, or other cation channels to open results in _____
vasoconstriction
Explain hypoxia and the role of K channels
Hypoxia increases the fraction of open K channels, leading to hyperpolarization
- closes voltage-sensitive Ca channels
- reduces calcium infux
- contributes to hypoxic vascular relaxation
What ion is unusually high in VSM?
chloride
54 intra – 134 extra
Which Potassium channel senses ischemia and contributes to hypoxic vasodilation?
ATP-dependent K+
Which potassium channel senses extracellular potassium and contributes to vasodilation in exercising muscle, myocardium, and brain?
Inward-Rectifier K (Kir)
KATP-blocker drug
Glibenclamide
- causes partial depolarization and vasoconstriction
KATP-activating drug
Nicorandil
- causes vasodilation
- nitrodilator used to treat angina
Which potassium channels contribute to resting potential and prevent vasospasm?
Voltage-dependent (Kv) and Ca-dependent (BK)
Which Calcium channel mediates depolarization-dependent contraction?
Voltage-Sensitive Ca2+ (VSCC)
Which calcium channel mediates depolarization-dependent contraction and contributes to agonist-induced electrical excitation?
Ca2+-conducting TRP
When potassium exits the VSM cell, the membrane potential becomes negative and as a result _____
less calcium enters the cell
Anything that opens a VSM potassium channel results _____
vasodilation
Agonism of Beta1 receptors result in _____
chronotropy, inotropy, lucitropy, and dromotropy.
Alpha-1 receptor agonist results in
vasoconstriction
Beta2 receptor agonism results in _____
vasodilation
Vasoconstrictor agonists examples
angiotensin II, vasopressin, serotonin, thromboxane, and endothelin
Alpha-1
- numerous in systemic blood vessels
- NE > Epi
- activation leads to depolarization and vasoconstriction
Alpha-2
- numerous in cutaneous blood vessels
- Epi > NE
- Decreases KATP conductance
- depolarization and vasoconstriction
Beta-1
- found in cardiac pacemakers and myocardium
- NE > Epi
- increase in heart rate and contractility
- chronotropy and inotropy
- cAMP pathway
Beta-2
- found in arterial vessels of myocardium, skeletal muscle, and liver
- Epi > NE
- coupled to G-protein: cAMP
- vasdilation
SR distribution
- If scanty and close to sarcolemma
- activates Ca-activated chloride channels
- depolarizing current contributes to slow-rising excitatory junction potential
- If extensive
- global rise in cytosolic calcium
- contraction
Fast Excitatory Junctional Potential (fast EJP)
- initial rapid depolarization
- ATP mediated
- binds to purinergic receptor (P2x)
- conducts Na or Ca
Slow EJP
- triggered by norepinephrine
- may or may not cause action potentials
e - membrane potential
t - contractile tension
- slow EJP was blocked by prazosin
- depolarization-independent contraction
- Fast EJPs will always have an action potential, slow EJPs depend on intensity
Initial Phase of Contraction
rapid increase in cytosolic calicum leading to contraction
- occurs synchronously in all myocytes
- rise in tension
- large arteries
- calcium comes from SR via IP3
- small arteries
- influx through VSCC after icat and iCl(Ca)
Phase 2 of Contraction
tonic phase
- decrease in cytosolic [Ca2+] leading to partial depolarization
- vasoconstriction maintained through calcium sensitization
Calcium Sensitization
mediated by rhoA kinase
- inhibits MLC phosphatase
- favors phosphorylation and contraction
- also influenced by protein kinase C-alpha
- same mechanism as rhoaA
- increase/maintain contraction with decreased cytosolic levels of calcium
Vasomotion
rhythmic contractions that help reduce the net capillary filtration rate
Four mechanisms of Vasodilation
- hyperpolarization
- cAMP PKA
- cGMP PKG
- desensitization to calcium
Hyperpolarization mediated vasodilation
decreases opening of VSCCC leading to fall in free calcium and vasodilation
- Examples
- skeletal muscle contraction
- sensory nerve neuropeptides
- KATP-activating drugs
Vasodilation via Nitric Oxide
increases cGMP which activates protein kinase G
- phosphorylation of phospholamban
- decrease calcium sensitivity
Calcium Channel Blockers
bind to L-type calcium channels
- smooth muscle relaxation
- negative inotropy
- affects phase 0 of pacemaker current
- negative dromotropy
Therapeutic uses for CCB
- hypertension
- decrease SVR
- angina
- vasodilator and cardiodepressant
- decrease afterload and oxygen demand
- dilate coronary arteries and prevent vascular spasm
- vasodilator and cardiodepressant
- arrhythmias
- decrease conduction velocity and prolong repolarization
(3) types of CCB
dihydropyridines, phenylalkylamine, and benzothiazepine
Dihydropyridines
vascular smooth muscle selective CCB
- decrease SVR
- powerful systemic vasodilators
- “-pine”
Phenylalkylamine
CCB selective for myocardium
- decreases myocardial oxygen demand
- reverses coronary vasospasm
- decreases HR
- Verapamil
Benzothiazepine
CCB with intermediate selectivity
- decrease inotropy
- vasodilator
- Diltiazem
vascular ‘tone’
tension exerted by vascular smooth muscle
- Determines
- local blood flow
- capillary recruitment and capillary pressure
- arterial pressure
- central venous pressure
Basal Tone
vascular tone of arterial vessels when the tonic sympathetic vasoconstrictor nerve activity is blocked
Extrinsic mechanisms for vascular control
controls the needs of entire organism
- vasomotor nerves
- circulating hormones
- Epi, NE, angiotensin, vasopressin, insulin
Intrinsic regulatory mechanisms
- Bayliss myogenic response
- endothelial secretions
- vasoactive metabolites
- autocoids
- temperature
Responses mediated through Intrinsic mechanisms
- flow autoregulation
- hyperaemia
- inflammatory vasodilation
- arterial vasospasm
Vascular Control Hierarchy
- 1st Tier (least)
- myogenic response
- autoregulation
- myogenic response
- 2nd Tier
- intrinsic regulatory chemicals
- vasodilators of metabolic hyperemia
- intrinsic regulatory chemicals
- 3rd Tier (most)
- extrinsic regulation
- vasomotor nerves and hormones
- extrinsic regulation
Bayliss Myogenic response
the contraction of a blood vessel that occurs when intravascular pressure is elevated and, conversely, the vasodilation that follows a reduction in pressure
- contributes to basal tone
- stabilizes local tissue blood flow and capillary filtration pressure
- autoregulation