Contractility
Encyclopedia
Myocardial contractility is the intrinsic ability of the heart to contract independent of preload and afterload. Changes in the ability to produce force during contraction result from different degrees of binding between myosin
(thick) and actin
(thin) filaments. The degree of binding that occurs depends on concentration of calcium ions in the cell; in an intact heart, it is usually the action of the sympathetic nervous system (through catecholamine
s) that determines the concentration of calcium ions in the cytosol
of cardiac muscle cells. All factors that cause an increase in contractility work by causing an increase in intracellular [Ca++] during contraction.
and epinephrine
) that enhance contractility are considered to have a positive inotropic effect.
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:
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
(thick) and actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
(thin) filaments. The degree of binding that occurs depends on concentration of calcium ions in the cell; in an intact heart, it is usually the action of the sympathetic nervous system (through catecholamine
Catecholamine
Catecholamines are molecules that have a catechol nucleus consisting of benzene with two hydroxyl side groups and a side-chain amine. They include dopamine, as well as the "fight-or-flight" hormones adrenaline and noradrenaline released by the adrenal medulla of the adrenal glands in response to...
s) that determines the concentration of calcium ions in the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
of cardiac muscle cells. All factors that cause an increase in contractility work by causing an increase in intracellular [Ca++] during contraction.
Inotropy
Contractility is synonymous with inotropy and may be altered through the administration of inotropic agents. Drugs such as catecholamines (norepinephrineNorepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
and epinephrine
Epinephrine
Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...
) that enhance contractility are considered to have a positive inotropic effect.
Model as a contributing factor
Under one existing model , the five factors of myocardial performance are considered to be- Heart rateHeart rateHeart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....
- Conduction velocity
- Preload
- AfterloadAfterloadAfterload is the tension or stress developed in the wall of the left ventricle during ejection. Following Laplace's law, the tension upon the muscle fibers in the heart wall is the product of the pressure within the ventricle, multiplied by the volume within the ventricle, divided by the wall...
- 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 effectAnrep effectThe Anrep effect is an autoregulation method in which myocardial contractility increases with afterload. It was experimentally determined that increasing afterload caused a proportional linear increase in ventricular inotropy....
). - An increase in heart rate will increase contractility (through the Bowditch effectBowditch effectThe Bowditch Effect is an autoregulation method by which myocardial contractility increases with an increase in heart rate. Also known as the Treppe phenomenon or Treppe effect....
).