Logan Steele
Cyclic adenosine 3',5'-monophosphate (cAMP) is a key regulator of cardiac function, controlling heart rate, contraction strength, and relaxation. cAMP is generated in response to the activation of Gs protein-coupled receptors, which stimulates adenylyl cyclase (AC) and increases intracellular cAMP levels. This process is particularly important in cardiac myocytes, where it acts as the main second messenger for β-adrenergic receptors, influencing contractile function and heart rate. The β-adrenoceptor stimulation leads to phosphorylation of proteins involved in contraction, such as troponin I, and increases intracellular Ca2+ levels, which are essential for heart contractility. The balance between β1AR and β2AR receptors also plays a role in modulating contractile properties, with chronic β2AR stimulation promoting cardiac hypertrophy and dysfunction. Understanding the spatial control of cAMP signaling is crucial for developing novel therapeutic strategies and regulating cardiac contraction.
| Characteristics | Values |
|---|---|
| Definition | 3′-5′-cyclic adenosine monophosphate (cAMP) is a pleiotropic intracellular second messenger |
| Function | Regulates numerous cardiac functions, including heart rate, contraction strength, and heartbeat relaxation |
| Synthesis | Synthesized in response to activation of Gs protein-coupled receptors or membrane-bound ACs (adenylyl cyclases) |
| Role | Mediates catecholaminergic control on heart rate and contractility |
| Effects | Positive chronotropic, inotropic, and lusitropic effects on β-adrenergic receptors |
| β-Adrenoceptors | Stimulation leads to PKA-mediated phosphorylation, accelerating troponin C-Ca2+ off-rate and enhancing contraction and relaxation |
| Phosphorylation Targets | Troponin I (TnI), phospholamban (PLB), LTCC, RyR, myosin binding protein C |
| Spatial Organization | Plays a role in regulating cAMP outcomes in cardiac physiology |
| Compartmentalization | MRP4 expression may contribute to local MRP4-modulated contraction of cardiac myocytes induced by β-adrenoceptor activation |
| Therapeutic Potential | Exogenous cAMP has shown potential protective effects against cardiac hypertrophy and fibrosis in mice |
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What You'll Learn
cAMP's role as a second messenger
Cyclic adenosine 3',5'-monophosphate (cAMP) is a pleiotropic intracellular second messenger that plays a key role in regulating cardiac function. It is generated in response to the activation of Gs protein-coupled receptors, which stimulate adenylyl cyclase (AC) to produce cAMP. cAMP, in turn, mediates the effects of catecholamines on heart rate, contraction force, and relaxation.
CAMP is involved in modulating the strength of cardiac muscle contraction by regulating the intracellular concentration of Ca2+ ions. An increase in cAMP levels leads to the phosphorylation of the LTCC, RyR, and other proteins, resulting in an increased influx of Ca2+ ions through voltage-dependent L-type Ca2+ channels (LTCC). This triggers the release of intracellular Ca2+ stores from the sarcoplasmic reticulum, activating myofilaments and leading to muscle contraction.
The β-adrenergic receptor (βAR) is a critical component in this process. When catecholamines, such as norepinephrine, are released under stress conditions, they activate βAR, a subfamily of G-protein coupled receptors (GPCRs). This activation initiates a signalling cascade that stimulates AC to produce cAMP. cAMP then mediates the positive chronotropic, inotropic, and lusitropic effects of β-adrenergic stimulation, enhancing heart rate, contraction force, and relaxation.
The role of cAMP as a second messenger in cardiac muscle contraction is further highlighted by its involvement in β-adrenoceptor stimulation. β-adrenoceptor activation leads to PKA-mediated phosphorylation of troponin I (TnI), accelerating the troponin C-Ca2+ off-rate. This allows for faster force development and shortening during systole, and faster force relaxation and re-lengthening during diastole. Additionally, β-adrenoceptor stimulation induces PKA-mediated phosphorylation of phospholamban (PLB), a negative regulator of the sarcoplasmic reticulum Ca2+ ATPase (SERCA). This results in increased Ca2+ reuptake into the sarcoplasmic reticulum, further enhancing muscle relaxation.
Moreover, cAMP's role extends beyond β-adrenoceptors, as cardiac myocytes express a variety of other proteins that are influenced by cAMP and PKA-mediated phosphorylation. This includes metabolic enzymes and transcription factors, indicating the diverse and essential role of cAMP in regulating cardiac muscle contraction.
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β-adrenoceptor stimulation
Β-adrenoceptors are G-protein coupled receptors associated with the Gs heterotrimeric G-protein. β1-AR is the most prominent subtype, and it is mainly responsible for the positive chronotropic and inotropic effects of catecholamines. β2-AR also increases cardiac function, but its ability to activate non-classical signalling pathways suggests a function distinct from β1-AR. β3-AR stimulation has been shown to have a negative inotropic effect.
The β-AR signalling pathway serves as a primary component of the interface between the sympathetic nervous system and the cardiovascular system. In the heart, cAMP mediates the catecholaminergic control of heart rate. cAMP is a pleiotropic intracellular second messenger generated in response to activation of Gs protein-coupled receptors. cAMP directly binds to and opens the HCN channels, increasing their open probability, which increases chronotropy.
In cardiac myocytes, MRP4 has been shown to enhance cAMP formation, contractility, and cardiac hypertrophy. The compartmentalisation of MRPs expression may also play an important role in the intra- and extracellular cAMP signalling processes. For example, caveolin-rich membrane MRP4 localisation could explain the local MRP4-modulated contraction of cardiac myocytes induced by activation of β-adrenoceptor.
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Phosphorylation of troponin I
The phosphorylation of TnI modulates the sensitivity of this calcium switch, thereby regulating the positioning of troponin for optimal control of cardiac muscle contraction. This process is mediated by protein kinase A (PKA), which is activated by β-adrenergic stimulation. PKA specifically phosphorylates TnI, leading to a decrease in the Ca2+ sensitivity of muscle contraction. This results in an accelerated rate of muscle relaxation, allowing faster force development during systole and faster force relaxation during diastole.
In addition to PKA, TnI can also be phosphorylated by protein kinase C (PKC), a family of serine/threonine kinases activated within the heart muscle by various agonists. The specific phosphorylation of TnI by PKC has been associated with the inhibition of myofibrillar actomyosin MgATPase. Furthermore, studies have shown that the phosphorylation of TnI by PKA and PKC plays a crucial role in controlling cardiac twitch dynamics.
The process of TnI phosphorylation has been extensively studied using various techniques, including NMR spectroscopy, EPR interspin distance measurements, and paramagnetic spin labeling. These approaches have provided valuable insights into the structural dynamics of the Tn complex and the role of TnI phosphorylation in modulating its function. By understanding the mechanisms underlying TnI phosphorylation, researchers can develop novel therapeutic strategies for cardiac diseases and explore the potential of TnI phosphorylation in regulating cardiac muscle contraction.
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cAMP and cardiac contractile function
CAMP, or 3′-5′-cyclic adenosine monophosphate, is a pleiotropic intracellular second messenger that plays a key role in regulating cardiac function. It is generated in response to the activation of Gs protein-coupled receptors, which stimulate adenylyl cyclase (AC) and catalyse the formation of cAMP. cAMP is involved in modulating heart rate and the strength and relaxation of each heartbeat.
In cardiac myocytes, cAMP constitutes the main second messenger for β-adrenergic receptors' signalling to fulfil positive chronotropic, inotropic, and lusitropic effects. β-adrenoceptor stimulation leads to PKA-mediated phosphorylation of troponin I (TnI), accelerating troponin C-Ca2+ off-rate and allowing faster force development and shortening during systole. This results in quicker force relaxation and re-lengthening during diastole. Additionally, catecholamines induce PKA-mediated phosphorylation of phospholamban (PLB), a negative regulator of the sarcoplasmic reticulum Ca2+ ATPase (SERCA). This increases Ca2+ re-uptake in the sarcoplasmic reticulum and myofilament relaxation.
The β-adrenergic receptor signalling also induces phosphorylation of the LTCC and the ryanodine receptor, increasing the amount of intracellular Ca2+ necessary for heart contractility. cAMP is also involved in the response to various hormones and neurotransmitters, with its synthesis stimulated by different GsPCRs and ACs exhibiting distinct, and even opposite, effects on cardiomyocyte viability. For example, β1AR activation leads to phosphorylation of LTCC, PLB, RyR, TnI, and myosin binding protein C, causing positive inotropic and lusitropic effects. On the other hand, β2AR activation leads to a more restricted effect, selectively phosphorylating LTCC and resulting in a lesser positive inotropic effect without any lusitropic effect.
The role of cAMP in cardiac contractile function is further highlighted by its involvement in the activation of PKD, which regulates cardiac contractile function through phosphorylation of cardiac TnI. Chronic infusion of exogenous cAMP has been shown to attenuate cardiac hypertrophy and fibrosis, implying potential protective effects in different tissues. Additionally, cAMP signalling is regulated by its efflux into the extracellular space through multidrug resistance proteins (MRP), which may also influence intra- and extracellular cAMP signalling processes and local MRP4-modulated contraction of cardiac myocytes induced by β-adrenoceptor activation.
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cAMP and catecholamines
Cyclic adenosine 3',5'-monophosphate, or cAMP, is a second messenger, or cellular signal occurring within cells, that is important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and is used for intracellular signal transduction in many different organisms, conveying the cAMP-dependent pathway. In the heart, cAMP mediates the catecholaminergic control of heart rate.
Catecholamines are a type of neurotransmitter that is released from the adrenal glands into the bloodstream during stressful situations, invoking a "fight or flight" response. Catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (If) and by promoting inward Na+/Ca2+ exchanger current (INCX) by a "Ca2+ clock" mechanism in sinoatrial nodal cells (SANCs). The multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII) is required for physiological heart rate responses to β-adrenergic receptor (β-AR) stimulation.
In addition, β-adrenoceptor stimulation leads to PKA-mediated phosphorylation of troponin I (TnI), accelerating troponin C-Ca2+ off-rate and allowing faster force development and shortening during systole and faster force relaxation and re-lengthening during diastole. Catecholamines also induce PKA-mediated phosphorylation of phospholamban (PLB), a negative regulator of the sarcoplasmic reticulum Ca2+ ATPase (SERCA), resulting in increased Ca2+ re-uptake in the sarcoplasmic reticulum and myofilament relaxation (lusitropic effect).
The depolarizing current through LTCC (ICa) contributes to the plateau phase of the cardiac action potential as well as to pacemaker activity in nodal cells. This influx of Ca2+ triggers the release of intracellular stores of Ca2+ from the sarcoplasmic reticulum via the Ryanodine receptor (RyR), which results in activation of myofilaments contraction. Alterations in density or function of LTCC have been implicated in a variety of cardiovascular diseases, including atrial fibrillation or heart failure.
In summary, cAMP mediates the catecholaminergic control of heart rate, and catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel.
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Frequently asked questions
cAMP is short for 3′-5′-cyclic adenosine monophosphate, a pleiotropic intracellular second messenger. It is generated in response to the activation of Gs protein-coupled receptors and plays a key role in regulating cardiac function. cAMP modulates heart rate and the strength of contraction and ease of relaxation of each heartbeat.
Catecholamines are released under stress conditions and modulate cardiac activity via G-protein-coupled receptor (GPCR) signalling. They activate β adrenergic receptors (βAR), a subfamily of GPCRs, leading to the synthesis of cAMP. This, in turn, results in an increase in intracellular Ca2+ ions, which is necessary for heart contractility.
β1-AR activation leads to the phosphorylation of several proteins, resulting in positive inotropic and lusitropic effects on heart function. β2-AR activation, on the other hand, has a more restricted effect, leading only to the phosphorylation of LTCC and causing a lesser positive inotropic effect without any lusitropic effect.
