Anrep effect

The Anrep effect describes the rapid increase in myocardial contractility in response to the sudden rise in afterload, the pressure the heart must work against to eject blood. This adaptive mechanism allows the heart to sustain stroke volume and cardiac output despite increased resistance. It operates through homeometric autoregulation, meaning that contractility adjustments occur independently of preload (the initial stretch of the heart muscle) or heart rate. The Anrep effect is characterized by a two-step elevation in myocardial contractility, in response to elevated afterload, involving two distinct mechanistic phases: a primary, rapid rise in contractility driven by sarcomeric strain sensing, and a secondary, sustained phase of contraction mediated by post-translational modifications of contractile proteins. First described by Gleb von Anrep in 1912 and further elaborated in the 1960s by Sarnoff et al., the Anrep effect represents a distinct cardiac regulation mechanism, differing fundamentally from the Frank-Starling mechanism, the slow force response, and the Gregg effect. While traditionally considered a short-term adaptation, recent studies suggest that the Anrep effect may also occur in chronic conditions involving persistent afterload elevation, such as hypertrophic obstructive cardiomyopathy.

The Anrep effect describes the rapid increase in myocardial contractility in response to the sudden rise in afterload, the pressure the heart must work against to eject blood. This adaptive mechanism allows the heart to sustain stroke volume and cardiac output despite increased resistance. It operates through homeometric autoregulation, meaning that contractility adjustments occur independently of preload (the initial stretch of the heart muscle) or heart rate. The Anrep effect is characterized by a two-step elevation in myocardial contractility, in response to elevated afterload, involving two distinct mechanistic phases: a primary, rapid rise in contractility driven by sarcomeric strain sensing, and a secondary, sustained phase of contraction mediated by post-translational modifications of contractile proteins. First described by Gleb von Anrep in 1912 and further elaborated in the 1960s by Sarnoff et al., the Anrep effect represents a distinct cardiac regulation mechanism, differing fundamentally from the Frank-Starling mechanism, the slow force response, and the Gregg effect. While traditionally considered a short-term adaptation, recent studies suggest that the Anrep effect may also occur in chronic conditions involving persistent afterload elevation, such as hypertrophic obstructive cardiomyopathy. ===Frank-Starling mechanism=== The Frank-Starling mechanism describes how increased preload (ventricular filling) stretches cardiac muscle fibers, enhancing stroke work through length-dependent activation of the myofilaments. This process aligns actin and myosin filaments for efficient cross-bridge formation while also recruiting myosin heads from dormant states into contraction-ready configurations. Additionally, stretching the sarcomeres sensitizes the thin (actin) filaments to calcium, promoting stronger and more sustained contractions. By contrast, the Anrep effect occurs at constant preload, triggered solely by afterload. It is characterized by increased contractility (steeper end-systolic pressure-volume relationship) and higher stroke work, without changes in stroke volume or end-diastolic volume. ===Slow force response=== This stretch-related (preload) response involves a gradual rise in contractility over several minutes (from 2 to 15 minutes, depending on species and experimental conditions) due to stretch-activated ion channels and G-protein-coupled receptors. It is mediated by angiotensin II and endothelin-1, which increase intracellular sodium and calcium concentrations through sodium-calcium exchangers. In contrast, the afterload-dependent response of the Anrep effect is initiated in milliseconds and concludes within 10 seconds, bypassing extracellular calcium regulation through the slow force response. Additionally, streptomycin, an inhibitor of stretch-activated ion channels, blocks the slow force response but does not affect the Anrep effect, reinforcing that the two mechanisms operate through distinct pathways. ===Gregg Effect=== This effect describes increased contractility due to improved coronary perfusion. It originates from changes in microvascular volume that trigger stretch-activated ion channels, resulting in increased intracellular calcium transient. The Gregg phenomenon generally begins to affect contractility approximately 5 seconds after onset, reaching peak force development within 40 seconds of sustained perfusion. However, the Anrep effect persists even in denervated, isolated hearts with constant coronary flow, eliminating perfusion-based explanations. Like the slow force response, the Gregg effect is sensitive to streptomycin, while the Anrep effect remains unaffected. ===Immediate (rapid) phase: myofilament strain-sensitive activation=== An acute rise in afterload increases ventricular wall tension, activating resting myosin heads due to afterload-dependent mechanosensing. This transition involves super-relaxed state myosin shifting into the contraction-ready state (i.e, disorder-relaxed), allowing myosin to bind to actin and form force-generating cross-bridges. ===Sustained phase: post-translational modifications=== Persistent afterload triggers oxidative stress, activating calcium/calmodulin-dependent protein kinase II. Myosin light chain 2 and cardiac myosin-binding protein C are phosphorylated, strengthening myosin-actin cross-bridge formation and prolonging systolic ejection time to maintain stroke volume. ==References==

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