Lusitropy is the rate of
myocardial relaxation. The increase in
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 ...
ic
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
of
cardiomyocyte
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle ...
s via increased uptake leads to increased myocardial contractility (positive
inotropic
An inotrope is an agent that alters the force or energy of muscular contractions. Negatively inotropic agents weaken the force of muscular contractions. Positively inotropic agents increase the strength of muscular contraction.
The term ''inotro ...
effect), but the myocardial relaxation, or lusitropy, decreases. This should not be confused, however, with catecholamine-induced calcium uptake into the
sarcoplasmic reticulum
The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion levels are ke ...
, which increases lusitropy.
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Positive
Increased catecholamine levels promote positive lusitropy, enabling the heart to relax more rapidly. This effect is mediated by the phosphorylation of
phospholamban and
troponin I
Troponin I is a cardiac and skeletal muscle protein family. It is a part of the troponin protein complex, where it binds to actin in thin myofilaments to hold the actin-tropomyosin complex in place. Troponin I prevents myosin from binding to ac ...
via a
cAMP-dependent pathway. Catecholamine-induced calcium influx into the sarcoplasmic reticulum increases both inotropy and lusitropy. In other words, a quicker reduction in cytosolic calcium levels (because the calcium enters the sarcoplasmic reticulum) causes an increased rate of relaxation (+ lusitropy), however that also enables a greater degree of calcium efflux, back into the cytosol, when the next action potential arrives, thereby increasing inotropy as well. Do not confuse this calcium mechanism with calcium uptake from the extracellular fluid. Increased calcium uptake from the extracellular fluid into the cytoplasm decreases lusitropy in the absence of catecholamine stimulation, but increased calcium uptake into the sarcoplasmic reticulum, via catecholamines, increases lusitropy and inotropy.
Negative
Relaxation of the heart is negatively impacted by the following factors:
# Calcium overload – too much intracellular calcium
# Reduced rate of calcium removal from myocyte through pumps if calcium is not removed from the cell quickly enough.
#: a. Plasma membrane
Calcium ATPase (Ca ATPase) this primary active transporter pumps calcium out of the myocyte between beats
#: b.
Sodium-Calcium (Na/Ca) exchanger this secondary active transporter pumps calcium out of cell between beats
# Impaired Sarco-Endoplasmic Reticulum Calcium ATPase (
SERCA) this primary active transporter pumps calcium from the cytoplasm of the myocyte into its sarco-endoplasmic reticulum.
Therefore, any impairment of the transporters in (2) and (3) would have a negative lusitropic effect.
In contrast, enhancement of these same transporters would have a positive inotropic effect
References
* https://www.hu.liu.se/lakarprogr/t2/t2-filer/1.59634/Lusitropy.pdf
{{Cardiovascular physiology
Cardiovascular physiology