What causes increased contractility of cardiac muscles?

Several factors can cause increased contractility of cardiac muscles, leading to stronger and more efficient contractions. Here are some key factors:

1. Increased Calcium Levels: Calcium ions play a crucial role in triggering muscle contractions. When calcium levels in the cardiac cells rise, it binds to troponin, leading to a conformational change that allows myosin to interact with actin and generate force. Increased calcium levels during cardiac excitation-contraction coupling enhance the contractility of the heart muscle.

2. Sympathetic Nervous System Activation: Stimulation of the sympathetic nervous system releases norepinephrine, which binds to beta-adrenergic receptors on cardiac muscle cells. This binding activates a cascade of intracellular events, including increased calcium release from the sarcoplasmic reticulum and enhanced contractile function.

3. Increased Circulating Catecholamines: Conditions or medications that elevate circulating catecholamines, such as adrenaline (epinephrine) and noradrenaline (norepinephrine), can also lead to increased cardiac contractility. Catecholamines bind to beta-adrenergic receptors, leading to the same effects as sympathetic nervous system activation.

4. Enhanced Myofilament Sensitivity to Calcium: Some substances, such as certain drugs (e.g., levosimendan) or conditions (e.g., hyperthyroidism), can increase the sensitivity of cardiac muscle fibers to calcium. As a result, even normal calcium levels may trigger more robust contractions.

5. Enhanced Sarcoplasmic Reticulum Calcium Handling: The sarcoplasmic reticulum (SR) is the primary intracellular calcium store in cardiac muscle cells. Factors that improve the SR's ability to store and release calcium, such as increased expression of calcium-handling proteins (e.g., SERCA2a pump), can enhance cardiac contractility.

6. Modulation of Ion Channels: Certain substances or conditions can influence the activity of ion channels involved in cardiac excitation-contraction coupling. For instance, blocking potassium channels or enhancing sodium-calcium exchange can prolong the cardiac action potential and increase calcium influx, leading to stronger contractions.

7. Structural Changes in Myofilaments: Alterations in the structure or composition of myofilaments, such as changes in myosin heavy chain isoforms or titin filaments, can affect the contractile properties of cardiac muscle. Some structural changes may enhance contractile force.

8. Hormonal Influences: Certain hormones, such as thyroid hormone and growth hormone, can influence cardiac contractility. Hyperthyroidism, for example, is associated with increased sympathetic activity and enhanced calcium handling, leading to increased contractility.

9. Myocardial Ischemia: In some cases, myocardial ischemia (reduced blood flow to the heart) can paradoxically cause increased contractility in the affected region. This phenomenon, known as the stunned myocardium, is attributed to alterations in calcium handling and energy metabolism.

It's important to note that excessive or sustained increases in cardiac contractility can have adverse effects on heart function and may contribute to the development of heart failure if not properly regulated.