Cardiac Contractility

P2 - p.68-70; p.48-49

Skeletal vs Cardiac Muscle (photo)

no relaxation → no pumping

Frank-Starling law (4)

• Strength of cardiac contraction is proportional to the initial length of cardiac muscle fibers

• Increased blood volume at the end of diastole stretches the heart muscle → stronger systolic contraction

• More blood entering the heart during diastole results in a stronger contraction during systole

photo: blood returning from veins - resp. system plays a roll in it

Cardiac T tubules (6)

• 5x larger diameter

• 25x larger volume

• larger quantities of Ca²⁺

• contain mucopolysaccharide

• opens up to extracellular space

compared to skeletal muscle’s T tubules

Cardiac muscle contraction depends on what? (2)

• Ca²⁺ stored in T tubules

• Extracellular Ca²⁺

Sympathetic stimulation to the heart tissues (4)

• Increases the strength of myocardial contraction

• Phosphorylation of calcium channels lengthens their opening time

• Increases calcium concentration in the sarcoplasmic reticulum (SR)

• Enhances actin-myosin interaction via kinase activation

Effect of Parasympathetic stimulation on Cardiac Contractility (2)

• Affects mostly the atria, sparing the ventricles

• Opposes the action of the sympathetic system

Inorganic ions in cardiac function (3)

• Potassium (K)

• Sodium (Na)

• Calcium (Ca)

Potassium (K) - Inorganic ions (3)

• Hypokalemia is most commonly a complication of diuretic therapy

• Moderate hypokalemia increases contractility by higher depolarization and more Ca²⁺ influx

• Severe hypokalemia causes heart rhythm problems and decreases effective contractility

Sodium (Na) - Inorganic ions (2)

• Hyperkalemia causes smaller depolarization and less calcium influx, decreasing contractility

• High sodium concentration depresses contractility by inhibiting calcium entry into mitochondria, lowering ATP availability

Calcium (Ca) - Inorganic ions (3)

• Mild increase results in an increase in contractility

• High increase can cause the heart to stop in systole

• Very low calcium can cause the heart to stop in diastole

Hypoxia (2)

• ↓ oxygen supply, weakening contraction

• ↓ metabolism

pH in cardiac (4)

• Acidosis inhibits myofibrillar responsiveness to Ca²⁺ by decreasing contractile protein sensitivity

  • by reducing Ca²⁺ binding to troponin C

• Alkalosis can weaken contractility

  • by reducing available ionized calcium

Positive inotropic drugs (6)

• Epinephrine (Adrenalin) and Norepinephrine (Noradrenalin) mimic the sympathetic system

• Dopamine and Dobutamine stimulate beta receptors

• Levosimendan sensitizes troponin C to calcium

• Milrinone increases intracellular ionized calcium

• Digoxin binds to and inhibits the sodium-potassium pump

  • increasing intracellular sodium and calcium content

Negative inotropic drugs (3)

• Acetylcholine mimics the parasympathetic system

• Beta blockers block the sympathetic system

• Barbiturates decrease inward flow current during depolarization