Contraction of Heart and Blood Vessels Flashcards
(14 cards)
Describe the ionic currents involved in the ventricular cardiomyocyte action potential.
✅ Answer:
Phase 0 (Depolarization): Voltage-gated Na⁺ channels open → rapid influx.
Phase 1 (Partial repolarization): K⁺ channels open briefly.
Phase 2 (Plateau): Ca²⁺ influx via L-type calcium channels, balanced by K⁺ efflux.
Phase 3 (Repolarization): Delayed rectifier K⁺ channels restore resting potential.
Phase 4 (Resting): Set by inward rectifier K⁺ channels (Kir), ~ -80 to -90 mV.
Cardiac action potentials raise intracellular calcium only partially. Where does most of the calcium for contraction come from?
✅ Answer: Approximately 75–90% of calcium comes from calcium-induced calcium release (CICR) via ryanodine receptors (RyR2) on the sarcoplasmic reticulum.
❓Q3: How is calcium removed from cardiomyocytes after contraction?
✅ Answer:
75% by ATPase-driven Ca²⁺ pumps (SERCA) on the SR membrane.
25% by Na⁺/Ca²⁺ exchanger (NCX) on the sarcolemma.
Explain the steps of cross-bridge cycling in cardiac muscle.
✅ Answer:
Ca²⁺ binds troponin, displacing tropomyosin.
Myosin heads bind to actin.
Power stroke: Myosin heads flex, pulling actin filaments (ATP-dependent).
ATP binds myosin, detaches from actin.
ATP hydrolysis resets the myosin head.
What is the significance of mitochondria in cardiomyocytes?
✅ Answer: They provide ATP for:
Cross-bridge cycling
Ion pumps (SERCA, Na⁺/K⁺ ATPase) Cardiomyocytes are highly aerobic and mitochondria occupy ~30–35% of cell volume.
Define preload and describe its effect on cardiac contractility.
✅ Answer: Preload is the initial stretch of the ventricle (end-diastolic volume). Increased preload enhances contraction via the Frank-Starling mechanism by:
Improving actin-myosin overlap.
Increasing Ca²⁺ sensitivity of the sarcomere.
Provide two physiological and two pathological conditions that affect preload.
✅ Answer:
Physiological: Exercise, pregnancy
Pathological: Hypertension, valvular disease (e.g., aortic regurgitation)
How does adrenaline enhance cardiac contraction?
✅ Answer:
Binds β₁-adrenergic receptors → activates Gs protein.
Increases cAMP → activates PKA.
PKA phosphorylates:
L-type Ca²⁺ channels → ↑ Ca²⁺ influx.
Phospholamban → ↑ SERCA activity.
Potassium channels → speeds repolarization.
RyR2 → enhances Ca²⁺ release from SR.
Describe the action of Endothelin-1 on the heart.
✅ Answer:
Binds ET-A receptor, activates Gq protein.
Gq → PLC activation → generates IP₃ and DAG.
IP₃ releases Ca²⁺ from intracellular stores.
DAG activates PKC → further modulates Ca²⁺ entry and sensitivity.
What are two pharmacological agents that modulate cardiac contractility and how?
✅ Answer:
Caffeine: At high doses, stimulates RyR2, increasing SR Ca²⁺ release. At low doses, inhibits PDE, raising cAMP.
Ouabain: Inhibits Na⁺/K⁺ ATPase, increasing intracellular Na⁺ → decreases Ca²⁺ efflux via NCX → ↑ intracellular Ca²⁺.
How does vascular smooth muscle contraction differ from cardiac contraction?
✅ Answer:
No troponin in VSMCs.
Contraction regulated by Ca²⁺–calmodulin complex → activates MLCK → phosphorylates myosin light chains.
VSMCs rely on long-lasting L-type Ca²⁺ channels, enabling sustained contraction.
What is the myogenic response and its significance?
✅ Answer: The myogenic response is a pressure-induced constriction of small arteries, crucial for blood pressure regulation and tissue perfusion. Triggered by stretch-sensitive mechanisms → opens L-type Ca²⁺ channels → contraction.Explain calcium sensitization in vascular smooth muscle.
Explain calcium sensitization in vascular smooth muscle.
✅ Answer: Besides increasing Ca²⁺, contraction is enhanced by inhibition of myosin light chain phosphatase, which increases sensitivity to Ca²⁺, sustaining contraction without additional Ca²⁺.
What type of ion channels dominate in:
(a) Cardiac myocytes?
(b) Vascular smooth muscle cells?
✅ Answer:
(a) Fast Na⁺, L-type Ca²⁺, K⁺ channels (for rhythmic depolarization/repolarization).
(b) L-type Ca²⁺ channels, K⁺ channels (for tone regulation and slow contraction).
