Myocardial contractility represents the innate ability of the heart muscle ( cardiac muscle or myocardium) to

contract A contract is an agreement that specifies certain legally enforceable rights and obligations pertaining to two or more parties. A contract typically involves consent to transfer of goods, services, money, or promise to transfer any of thos ...

. It is the maximum attainable value for the

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

of contraction of a given heart. The ability to produce changes in force during contraction result from incremental degrees of binding between different types of tissue, that is, between filaments of

myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

(thick) and

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

(thin) tissue. The degree of binding depends upon the concentration of calcium ions in the cell. Within an

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...

intact heart, the action/response of the sympathetic nervous system is driven by precisely timed releases of a catecholamine, which is a process that determines the concentration of calcium ions in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

of cardiac muscle cells. The factors causing an increase in contractility work by causing an increase in intracellular calcium ions (Ca⁺⁺) during contraction.

Mechanisms for altering contractility

Increasing contractility is done primarily through increasing the influx of calcium or maintaining higher calcium levels in the cytosol of cardiac myocytes during an

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

. This is done by a number of mechanisms: # Sympathetic activation. Increased circulating levels of catecholamines (which can bind to β-Adrenergic activation) as well as stimulation by sympathetic nerves (which can release norepinepherine that binds to β₁-adrenoceptors on myocytes) causes the G_s subunit of the receptor to render adenylate cyclase activated, resulting in increase of cAMP - which has a number of effects including phosphorylating phospholamban (via Protein kinase A). # Phosphorylating phospholamban. When phospholamban is not phosphorylated, it inhibits the calcium pumps that pump calcium back into the sarcoplasmic reticulum. When it's phosphorylated by PKA, levels of calcium stored in the sarcoplasmic reticulum are increased, allowing a higher rate of calcium being released at the next contraction. However, the increased rate of calcium sequestration also leads to an increase in lusitropy. # Sensitizing troponin-C to the effects of calcium. # Phosphorylating L-type calcium channels. This will increase their permeability to calcium, allowing more calcium into the myocyte cells, increasing contractility. # An abrupt increase in afterload enhances myocardial contractility and prolongs systolic ejection time through the Anrep effect. This response involves a two-phase recruitment of myosin from resting states to contraction-ready configurations, boosting the heart's contractile force. # An increase in heart rate also stimulates inotropy ( Bowditch effect; treppe; frequency-dependent inotropy). This is probably due to the inability of Na⁺/K⁺- ATPase to keep up with the sodium influx at the higher frequency of action potentials at elevated heart rates # Drugs. Drugs like

digitalis ''Digitalis'' ( or ) is a genus of about 20 species of herbaceous perennial plants, shrubs, and Biennial plant, biennials, commonly called foxgloves. ''Digitalis'' is native to Europe, Western Asia, and northwestern Africa. The flowers are ...

can act as a positive inotropic agent by inhibiting the Na⁺/K⁺ pump. High Na⁺ concentration gradient is necessary to pump out sarcoplasmic calcium via the Na⁺/Ca⁺⁺

antiporter An antiporter (also called exchanger or counter-transporter) is an integral membrane protein that uses secondary active transport to move two or more molecules in opposite directions across a phospholipid membrane. It is a type of cotransporte ...

. Inhibition of the Na⁺/K⁺ causes extra sodium to accumulate inside the cell. The buildup the Na⁺ concentration inside the cell will cause the gradient from inside the cell to the outside of the cell to decrease slightly. This action will make it more difficult for calcium to leave the cell via the Na⁺/Ca⁺⁺ antiporter. # Increase the amount of calcium in the sarcoplasm. More calcium available for

Troponin Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that are integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Measurements of cardiac-spe ...

to use will increase the force developed. Decreasing contractility is done primarily by decreasing the influx of calcium or maintaining lower calcium levels in the cytosol of cardiac myocytes during an action potential. This is done by a number of mechanisms: #

Parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

activation. # If the heart is experiencing anoxia,

hypercapnia Hypercapnia (from the Greek ''hyper'', "above" or "too much" and ''kapnos'', "smoke"), also known as hypercarbia and CO2 retention, is a condition of abnormally elevated carbon dioxide (CO2) levels in the blood. Carbon dioxide is a gaseous pro ...

(increased CO₂) or acidosis, the heart cells will enter a state of dysfunction and not work properly. Correct sarcomere crossbridges will not form the heart becomes less efficient (leading to myocardial failure). # Loss of parts of the myocardium.

Heart attack A myocardial infarction (MI), commonly known as a heart attack, occurs when Ischemia, blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) to the heart muscle. The most common symptom ...

can cause a section of the ventricular wall dies off, that portion cannot contract and there is less force developed during

systole Systole ( ) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. Its contrasting phase is diastole, the relaxed phase of the cardiac cycle when the chambers of the heart are refilling ...

Inotropy

A measurable relative increase in contractility is a property of the myocardium similar to the term "inotropy". Contractility may be iatrogenically altered by the administration of '' inotropic agents''. Drugs that positively render the effects of catecholamines such as

norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...

and epinephrine that enhance contractility are considered to have a positive inotropic effect. The ancient herbal remedy

appears to have both inotropic and

chronotropic Chronotropic effects (from ''chrono-'', meaning time, and ''tropos'', "a turn") are those that change the heart rate. Chronotropic drugs may change the heart rate and cardiac cycle, rhythm by affecting the electrical conduction system of the hea ...

properties that have been recorded encyclopedically for centuries and it remains advantageous today.

Model as a contributing factor

Under one existing model , the five factors of myocardial performance are considered to be *

Heart rate Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...

* Conduction velocity * Preload * Afterload * Contractility By this model, if myocardial performance changes while preload, afterload, heart rate, and conduction velocity are all held constant, then the change in performance must be due to a change in contractility. However, changes in contractility alone generally do not occur. Other examples: * An increase in sympathetic stimulation to the heart increases contractility and heart rate. * An increase in contractility tends to increase stroke volume and thus a secondary increase in preload. * An increase in preload results in an increased force of contraction by Starling's law of the heart; this does not require a change in contractility. * An increase in afterload will increase contractility (through the Anrep effect). * An increase in heart rate will increase contractility (through the Bowditch effect).

References

{{Cardiovascular physiology Cardiovascular physiology