Cardiol Res

Cardiology Research, ISSN 1923-2829 print, 1923-2837 online, Open Access

Article copyright, the authors; Journal compilation copyright, Cardiol Res and Elmer Press Inc

Journal website https://www.cardiologyres.org

Review

Volume 13, Number 4, August 2022, pages 190-205

The Metabolic Pathway of Cardiac Troponins Release: Mechanisms and Diagnostic Role

**Table 1.** Early Diagnostic Algorithms for Exclusion/Confirmation of Non-ST-Elevation Myocardial Infarction (NSTEMI) (0 - 1 H Algorithm and 0 - 2 H Algorithm)
1-h NSTEMI diagnostic algorithm
High-sensitivity cardiac troponin T (Elecsys; Roche)	< 5	< 12	< 3	≥ 52	≥ 5
High-sensitivity cardiac troponin I (Architect; Abbott)	< 4	< 5	< 2	≥ 64	≥ 6
High-sensitivity cardiac troponin I (Centaur; Siemens)	< 3	< 6	< 3	≥ 120	≥ 12
High-sensitivity cardiac troponin I (Access; Beckman Coulter)	< 4	< 5	< 4	≥ 50	≥ 15
NSTEMI: non-ST-segment elevation myocardial infarction; hs-cTn: high-sensitivity cardiac troponin.
hs-сTnT (Elecsys; Roche)	< 5	< 14	< 4	≥ 52	≥ 10
hs-сTnI (Architect; Abbott)	< 4	< 6	< 2	≥ 64	≥ 15
hs-сTnI (Centaur; Siemens)	< 3	< 8	< 7	≥ 120	≥ 20
hs-сTnI (Access; Beckman Coulter)	< 4	< 5	< 5	≥ 50	≥ 20

Table 1. Early Diagnostic Algorithms for Exclusion/Confirmation of Non-ST-Elevation Myocardial Infarction (NSTEMI) (0 - 1 H Algorithm and 0 - 2 H Algorithm)

1-h NSTEMI diagnostic algorithm

Troponin immunoassay, company (manufacturer)

Biomarker concentration that indicates an extremely low probability of an NSTEMI diagnosis, ng/L

Biomarker concentration that indicates a low probability of an NSTEMI diagnosis, ng/L

Changes in biomarker concentration after 1 h at which a diagnosis of NSTEMI should be excluded, ng/L

Biomarker concentration that indicates a high probability of an NSTEMI diagnosis, ng/L

Changes in biomarker concentration after 1 h at which a diagnosis of NSTEMI should be confirmed, ng/L

High-sensitivity cardiac troponin T (Elecsys; Roche)

< 5

< 12

< 3

≥ 52

≥ 5

High-sensitivity cardiac troponin I (Architect; Abbott)

< 4

< 5

< 2

≥ 64

≥ 6

High-sensitivity cardiac troponin I (Centaur; Siemens)

< 3

< 6

< 3

≥ 120

≥ 12

High-sensitivity cardiac troponin I (Access; Beckman Coulter)

< 4

< 5

< 4

≥ 50

≥ 15

2-h NSTEMI diagnostic algorithm

Troponin immunoassay, company (manufacturer)

Biomarker concentration that indicates an extremely low probability of an NSTEMI diagnosis, ng/L

Biomarker concentration that indicates a low probability of an NSTEMI diagnosis, ng/L

Changes in biomarker concentration after 2 h at which a diagnosis of NSTEMI should be excluded, ng/L

Biomarker concentration that indicates a high probability of an NSTEMI diagnosis, ng/L

Changes in biomarker concentration after 2 h at which a diagnosis of NSTEMI should be confirmed, ng/L

NSTEMI: non-ST-segment elevation myocardial infarction; hs-cTn: high-sensitivity cardiac troponin.

hs-сTnT (Elecsys; Roche)

< 5

< 14

< 4

≥ 52

≥ 10

hs-сTnI (Architect; Abbott)

< 4

< 6

< 2

≥ 64

≥ 15

hs-сTnI (Centaur; Siemens)

< 3

< 8

< 7

≥ 120

≥ 20

hs-сTnI (Access; Beckman Coulter)

< 4

< 5

≥ 50

≥ 20

**Table 2.** Metabolic Pathway of Cardiac Troponins
Main stages of metabolic pathway of cardiac troponins	Brief description of the stage and factors that affect metabolic pathway of cardiac troponins	Main clinical and diagnostic significance of metabolic pathway of cardiac troponins
AMI: acute myocardial infarction.
The stage of biosynthesis and release of cardiac troponin molecules from cardiac myocytes into the bloodstream	Cardiac troponin molecules are mainly synthesized in cardiomyocytes and can be released into blood serum both under physiological conditions (physical and psychoemotional stress) and during pathological processes (e.g., myocarditis, sepsis, hypertensive crisis, pulmonary embolism, Takotsubo syndrome and a number of others). The main mechanisms of release of cardiac troponin molecules into the bloodstream are: 1) cardiomyocyte necrosis; 2) cardiomyocyte apoptosis; 3) cardiac myocyte regeneration and renewal processes; 4) increased cell membrane permeability; 5) release of troponins via vesicular transport; 6) increased proteolytic degradation of troponin molecules within cardiac myocytes and release of reduced fragments even through the intact cell membrane. The first one is the key mechanism which is characteristic of AMI, while the others may be characteristic of physiological conditions and of reversible and irreversible cardiomyocyte damage in nonischemic and systemic pathologies (e.g., myocarditis, sepsis, arterial hypertension, and others). Using high-sensitivity immunoassays, it has also been found that the degree of release of troponin molecules from cells into the bloodstream depends on several biological factors: 1) gender (males have a higher degree of release); 2) circadian (more molecules are released from myocytes in the morning than in the evening and night); and 3) age-related (older patients have more molecules released from myocardial cells than younger patients).	Troponin concentration in blood serum has a direct correlation with the degree of release of troponin molecules from myocardium into the bloodstream. The extent of release of troponin molecules from the myocardium may depend on the type and severity of the pathological process or physical load (under physiological conditions). Gender peculiarities have an important clinical significance in modern diagnostic algorithms which are used in some algorithms of early diagnosis of AMI. Circadian and age-specific features are not yet reflected in the current clinical guidelines and diagnostic algorithms due to little study. However, as further research and new data become available, there is a probability of validating their clinical significance and subsequent introduction into practical medicine.
The stage of cardiac troponin molecules circulation in blood serum	The molecules circulating in blood serum can be affected by a number of enzymes belonging to the groups of proteases, kinases (phosphorylases), phosphatases, and oxidases. The activity of these enzymes may change under physiological and pathological conditions as well as medication. The study of specific factors influencing enzyme activity is a very interesting and extensive research area.	Concentration of cardiac troponins in blood serum may depend on the activity of a number of enzymes (proteases, kinases (phosphorylases), phosphatases and oxidases) that cleave and modify cardiac troponin molecules, leading to changes in the antigen-antibody interaction in immunochemical assays. The clinical significance of this step in troponin metabolism lies in the potential to obtain. Further studies are needed to identify specific enzymes and their specific influence on troponin molecules.
The stage of cardiac troponin molecules elimination from blood serum	Elimination of cardiac troponin molecules from blood serum can be accomplished by the following mechanisms: 1) elimination of molecules through hematotissue barriers (glomerular, heme-salivary, heme-placental, heme-encephalic, and others) into other body fluids (urine, oral fluid, amniotic fluid, pericardial fluid, cerebrospinal fluid); 2) uptake of cardiac troponin molecules by cells of reticuloendothelial system (mononuclear phagocyte system) (macrophages) and intracellular cleavage into amino acids within these cells; 3) cleavage of troponin molecules in bloodstream as a result of proteolytic enzymes (for example, thrombin enzyme).	Troponin concentration in blood serum has inverse dependence on elimination rate. Thus, when the rate/extent of elimination of cardiac troponin molecules from the bloodstream decreases, there will be an increase levels of cardiac troponins in blood serum. A very typical clinical example is the accumulation of cardiac troponin molecules and their increased blood serum concentrations in patients with chronic kidney failure without obvious signs of cardiovascular disease. Besides, depressed kidney function and decreased glomerular filtration rate may be observed in a number of other severe systemic diseases, which are often accompanied by a marked decrease in blood pressure (e.g., severe sepsis) or pathologies that cause damage to the renal vessels and renal glomeruli (e.g., diabetes mellitus). The clinical role of the other mechanisms of cardiac troponin elimination needs further research and clarification.

Table 2. Metabolic Pathway of Cardiac Troponins

Main stages of metabolic pathway of cardiac troponins

Brief description of the stage and factors that affect metabolic pathway of cardiac troponins

Main clinical and diagnostic significance of metabolic pathway of cardiac troponins

AMI: acute myocardial infarction.

The stage of biosynthesis and release of cardiac troponin molecules from cardiac myocytes into the bloodstream

Cardiac troponin molecules are mainly synthesized in cardiomyocytes and can be released into blood serum both under physiological conditions (physical and psychoemotional stress) and during pathological processes (e.g., myocarditis, sepsis, hypertensive crisis, pulmonary embolism, Takotsubo syndrome and a number of others). The main mechanisms of release of cardiac troponin molecules into the bloodstream are: 1) cardiomyocyte necrosis; 2) cardiomyocyte apoptosis; 3) cardiac myocyte regeneration and renewal processes; 4) increased cell membrane permeability; 5) release of troponins via vesicular transport; 6) increased proteolytic degradation of troponin molecules within cardiac myocytes and release of reduced fragments even through the intact cell membrane. The first one is the key mechanism which is characteristic of AMI, while the others may be characteristic of physiological conditions and of reversible and irreversible cardiomyocyte damage in nonischemic and systemic pathologies (e.g., myocarditis, sepsis, arterial hypertension, and others). Using high-sensitivity immunoassays, it has also been found that the degree of release of troponin molecules from cells into the bloodstream depends on several biological factors: 1) gender (males have a higher degree of release); 2) circadian (more molecules are released from myocytes in the morning than in the evening and night); and 3) age-related (older patients have more molecules released from myocardial cells than younger patients).

Troponin concentration in blood serum has a direct correlation with the degree of release of troponin molecules from myocardium into the bloodstream. The extent of release of troponin molecules from the myocardium may depend on the type and severity of the pathological process or physical load (under physiological conditions). Gender peculiarities have an important clinical significance in modern diagnostic algorithms which are used in some algorithms of early diagnosis of AMI. Circadian and age-specific features are not yet reflected in the current clinical guidelines and diagnostic algorithms due to little study. However, as further research and new data become available, there is a probability of validating their clinical significance and subsequent introduction into practical medicine.

The stage of cardiac troponin molecules circulation in blood serum

The molecules circulating in blood serum can be affected by a number of enzymes belonging to the groups of proteases, kinases (phosphorylases), phosphatases, and oxidases. The activity of these enzymes may change under physiological and pathological conditions as well as medication. The study of specific factors influencing enzyme activity is a very interesting and extensive research area.

Concentration of cardiac troponins in blood serum may depend on the activity of a number of enzymes (proteases, kinases (phosphorylases), phosphatases and oxidases) that cleave and modify cardiac troponin molecules, leading to changes in the antigen-antibody interaction in immunochemical assays. The clinical significance of this step in troponin metabolism lies in the potential to obtain. Further studies are needed to identify specific enzymes and their specific influence on troponin molecules.

The stage of cardiac troponin molecules elimination from blood serum

Elimination of cardiac troponin molecules from blood serum can be accomplished by the following mechanisms: 1) elimination of molecules through hematotissue barriers (glomerular, heme-salivary, heme-placental, heme-encephalic, and others) into other body fluids (urine, oral fluid, amniotic fluid, pericardial fluid, cerebrospinal fluid); 2) uptake of cardiac troponin molecules by cells of reticuloendothelial system (mononuclear phagocyte system) (macrophages) and intracellular cleavage into amino acids within these cells; 3) cleavage of troponin molecules in bloodstream as a result of proteolytic enzymes (for example, thrombin enzyme).

Troponin concentration in blood serum has inverse dependence on elimination rate. Thus, when the rate/extent of elimination of cardiac troponin molecules from the bloodstream decreases, there will be an increase levels of cardiac troponins in blood serum. A very typical clinical example is the accumulation of cardiac troponin molecules and their increased blood serum concentrations in patients with chronic kidney failure without obvious signs of cardiovascular disease. Besides, depressed kidney function and decreased glomerular filtration rate may be observed in a number of other severe systemic diseases, which are often accompanied by a marked decrease in blood pressure (e.g., severe sepsis) or pathologies that cause damage to the renal vessels and renal glomeruli (e.g., diabetes mellitus). The clinical role of the other mechanisms of cardiac troponin elimination needs further research and clarification.

**Table 3.** Possible Mechanisms of Cardiac Troponin Release From Myocytes
Mechanism of cardiac troponin release	Brief description of the mechanism	Literature source
Cardiomyocyte necrosis	Cell necrosis is accompanied by destruction of the cell membrane. This will contribute to the release of all cytoplasmic components from the cell into the blood serum	[4, 15]
Myocardial cell apoptosis	Cardiomyocyte apoptosis develops as a result of several factors (short-term myocardial ischemia, myocardial distension, increased activity of neurohumoral (sympathoadrenal) system) and may be accompanied by a significant increase in cardiac troponin levels. Small-scale apoptotic processes are possible in healthy people as a result of excessive activity of several factors (physical exercise, psycho-emotional stresses)	[95, 96, 108-111]
Myocardial cell regeneration and renewal	According to some researchers, a small part of cardiomyocytes can be renewed (replaced). Gradual death of senescent cardiomyocytes may result in the release of small amounts of cardiac troponin molecules into blood serum. This explains the normal levels of high-sensitivity troponins (less than the 99th percentile) in all healthy individuals.	[96, 122-124]
Increased permeability of the cell membrane of cardiac myocytes	The extent of cell membrane permeability is an important factor that determines whether intracellular molecules can be released from the cell to the outside. Cell membrane permeability is influenced by the following factors: 1) Myocardial ischemia which causes activation of proteolytic enzymes that can damage the plasma membrane; 2) Proteolytic enzyme activity which depends on the severity of the pathological process or the nature of the physiological factor; 3) Increased loading and distension of cardiac muscle tissue.	[10, 132-135]
Troponin release from cardiomyocytes by vesicular transport	According to this mechanism, cardiac troponin molecules can escape outside the cells as part of membrane vesicles. The activity of vesicular transport depends on the degree of ischemia: vesicular transport of troponins increases with ischemia induction. Only the cytoplasmic fraction of troponins can be released as part of vesicles in physiological conditions. This explains the small extent of troponins increase in blood serum.	[101]
Troponin molecules proteolytic degradation processes	The size of the molecule is considered as a factor influencing its capability to be released through the cell membrane; smaller molecules are released earlier and faster compared to larger molecules. A number of proteolytic enzymes (calpain, matrix metalloproteinases) can be activated under certain physiological and pathological conditions and catalyze the degradation of cardiac troponin molecules into small fragments that will contribute to their passage through the plasma membrane. This mechanism may be combined with a mechanism of increasing membrane permeability, especially when the degree and severity of the damaging factor (e.g., ischemia, myocardial stress) is significant.	[102]
Extracardiac expression of cardiac troponin molecules and troponin release from skeletal muscle tissues	This mechanism is quite controversial and requires further research for validation. According to several authors, cardiac troponin molecules can be expressed in skeletal muscle of patients under certain conditions (kidney failure, inherited myopathies) and then released from skeletal muscle, causing increased cardiac troponin levels in blood serum.	[149-152]

Table 3. Possible Mechanisms of Cardiac Troponin Release From Myocytes

Mechanism of cardiac troponin release

Brief description of the mechanism

Literature source

Cardiomyocyte necrosis

Cell necrosis is accompanied by destruction of the cell membrane. This will contribute to the release of all cytoplasmic components from the cell into the blood serum

[4, 15]

Myocardial cell apoptosis

Cardiomyocyte apoptosis develops as a result of several factors (short-term myocardial ischemia, myocardial distension, increased activity of neurohumoral (sympathoadrenal) system) and may be accompanied by a significant increase in cardiac troponin levels. Small-scale apoptotic processes are possible in healthy people as a result of excessive activity of several factors (physical exercise, psycho-emotional stresses)

[95, 96, 108-111]

Myocardial cell regeneration and renewal

According to some researchers, a small part of cardiomyocytes can be renewed (replaced). Gradual death of senescent cardiomyocytes may result in the release of small amounts of cardiac troponin molecules into blood serum. This explains the normal levels of high-sensitivity troponins (less than the 99th percentile) in all healthy individuals.

[96, 122-124]

Increased permeability of the cell membrane of cardiac myocytes

The extent of cell membrane permeability is an important factor that determines whether intracellular molecules can be released from the cell to the outside. Cell membrane permeability is influenced by the following factors: 1) Myocardial ischemia which causes activation of proteolytic enzymes that can damage the plasma membrane; 2) Proteolytic enzyme activity which depends on the severity of the pathological process or the nature of the physiological factor; 3) Increased loading and distension of cardiac muscle tissue.

[10, 132-135]

Troponin release from cardiomyocytes by vesicular transport

According to this mechanism, cardiac troponin molecules can escape outside the cells as part of membrane vesicles. The activity of vesicular transport depends on the degree of ischemia: vesicular transport of troponins increases with ischemia induction. Only the cytoplasmic fraction of troponins can be released as part of vesicles in physiological conditions. This explains the small extent of troponins increase in blood serum.

[101]

Troponin molecules proteolytic degradation processes

The size of the molecule is considered as a factor influencing its capability to be released through the cell membrane; smaller molecules are released earlier and faster compared to larger molecules. A number of proteolytic enzymes (calpain, matrix metalloproteinases) can be activated under certain physiological and pathological conditions and catalyze the degradation of cardiac troponin molecules into small fragments that will contribute to their passage through the plasma membrane. This mechanism may be combined with a mechanism of increasing membrane permeability, especially when the degree and severity of the damaging factor (e.g., ischemia, myocardial stress) is significant.

[102]

Extracardiac expression of cardiac troponin molecules and troponin release from skeletal muscle tissues

This mechanism is quite controversial and requires further research for validation. According to several authors, cardiac troponin molecules can be expressed in skeletal muscle of patients under certain conditions (kidney failure, inherited myopathies) and then released from skeletal muscle, causing increased cardiac troponin levels in blood serum.

[149-152]