Starling Mechanism Research Articles

The contractile adaption to preload depends on the amount of afterload.

AimsThe Frank–Starling mechanism (rapid response (RR)) and the secondary slow response (SR) are known to contribute to increases contractile performance. The contractility of the heart muscle is influenced by pre‐load and after‐load. Because of the effect of pre‐load vs. after‐load on these mechanisms in not completely understood, we studied the effect in isolated muscle strips.Methods and resultsProgressive stretch lead to an increase in shortening/force development under isotonic (only pre‐load) and isometric conditions (pre‐ and after‐load). Muscle length with maximal function was reached earlier under isotonic (L max‐isotonic) compared with isometric conditions (L max‐isometric) in nonfailing rabbit, in human atrial and in failing ventricular muscles. Also, SR after stretch from slack to L max‐isotonic was comparable under isotonic and isometric conditions (human: isotonic 10 ± 4%, isometric 10 ± 4%). Moreover, a switch from isotonic to isometric conditions at L max‐isometric showed no SR proving independence of after‐load. To further analyse the degree of SR on the total contractile performance at higher pre‐load muscles were stretched from slack to 98% L max‐isometric under isotonic conditions. Thereby, the SR was 60 ± 9% in rabbit and 51 ± 14% in human muscle strips.ConclusionsThis work shows that the acute contractile response largely depends on the degree and type of mechanical load. Increased filling of the heart elevates pre‐load and prolongs the isotonic part of contraction. The reduction in shortening at higher levels of pre‐load is thereby partially compensated by the pre‐load‐induced SR. After‐load shifts the contractile curve to a better ‘myofilament function’ by probably influencing thin fibers and calcium sensitivity, but has no effect on the SR.

Percutaneous left atrial appendage closure improves left atrial mechanical function through Frank–Starling mechanism

Modifications in left atrial (LA) flow velocities after left atrial appendage (LAA) exclusion have been shown in animal and exvivo models. In a substudy of PROTECT AF (Percutaneous Closure of the Left Atrial Appendage Versus Warfarin Therapy for Prevention of Stroke in Patients With Atrial Fibrillation), an objective improvement in quality of life was observed after LAA closure. The purpose of this study was to investigate the impact of LAA closure on LA transport function. Comprehensive transthoracic echocardiography evaluation (2-dimensional [2D]/3-dimensional [3D], 2D speckle tracking) was prospectively performed before and after LAA closure (at discharge and 45 days after procedure) in 33 patients. LAA closure was associated with a significant improvement in LA reservoir function at discharge and 45 days after the procedure with (1) increased maximum LA volume index, (2) increased 2D-LA reservoir volume and expansion index, and (3) increased 2D speckle tracking-derived peak atrial longitudinal strain (PALS) (27.9 ± 14 and 26 ± 12.6 vs 21.7 ± 10.7%, P <.0001). LAA closure was also associated with a significant improvement in LA contractile function with (1) increased LA ejection fraction and (2) increased speckle tracking-derived peak atrial contraction strain (PACS) in sinus rhythm patients (19.1 ± 6.8 and 18.1 ± 5.4 vs 14.4 ± 6.4%, P= .0006). Conversely, the slope of the relation between PACS and PALS remained unchanged (0.5 ± 0.27 and 0.53 ± 0.3 vs 0.5 ± 0.25, P = .99), thus arguing for an improvement in LA contractile function secondary to a Frank-Starling effect rather than a modification in its intrinsic contractility. LAA closure was associated with an improvement in LA mechanical function. These changes appeared to be related to a modification in loading conditions, that is, a Frank-Starling effect.

Clinical Implications of Physiologic Flow Adjustment in Continuous-Flow Left Ventricular Assist Devices.

There is increasing evidence for successful management of end-stage heart failure with continuous-flow left ventricular assist device (CF-LVAD) technology. However, passive flow adjustment at fixed CF-LVAD speed is susceptible to flow balancing issues as well as adverse hemodynamic effects relating to the diminished arterial pulse pressure and flow. With current therapy, flow cannot be adjusted with changes in venous return, which can vary significantly with volume status. This limits the performance and safety of CF-LVAD. Active flow adjustment strategies have been proposed to improve the synchrony between the pump and the native cardiovascular system, mimicking the Frank-Starling mechanism of the heart. These flow adjustment strategies include modulation by CF-LVAD pump speed by synchrony and maintenance of constant flow or constant pressure head, or a combination of these variables. However, none of these adjustment strategies have evolved sufficiently to gain widespread attention. Herein we review the current challenges and future directions of CF-LVAD therapy and sensor technology focusing on the development of a physiologic, long-term active flow adjustment strategy for CF-LVADs.

β-Arrestin mediates the Frank–Starling mechanism of cardiac contractility

The Frank-Starling law of the heart is a physiological phenomenon that describes an intrinsic property of heart muscle in which increased cardiac filling leads to enhanced cardiac contractility. Identified more than a century ago, the Frank-Starling relationship is currently known to involve length-dependent enhancement of cardiac myofilament Ca2+ sensitivity. However, the upstream molecular events that link cellular stretch to the length-dependent myofilament Ca2+ sensitivity are poorly understood. Because the angiotensin II type 1 receptor (AT1R) and the multifunctional transducer protein β-arrestin have been shown to mediate mechanosensitive cellular signaling, we tested the hypothesis that these two proteins are involved in the Frank-Starling mechanism of the heart. Using invasive hemodynamics, we found that mice lacking β-arrestin 1, β-arrestin 2, or AT1R were unable to generate a Frank-Starling force in response to changes in cardiac volume. Although wild-type mice pretreated with the conventional AT1R blocker losartan were unable to enhance cardiac contractility with volume loading, treatment with a β-arrestin-biased AT1R ligand to selectively activate β-arrestin signaling preserved the Frank-Starling relationship. Importantly, in skinned muscle fiber preparations, we found markedly impaired length-dependent myofilament Ca2+ sensitivity in β-arrestin 1, β-arrestin 2, and AT1R knockout mice. Our data reveal β-arrestin 1, β-arrestin 2, and AT1R as key regulatory molecules in the Frank-Starling mechanism, which potentially can be targeted therapeutically with β-arrestin-biased AT1R ligands.

Disulfide-activated protein kinase G Iα regulates cardiac diastolic relaxation and fine-tunes the Frank-Starling response.

The Frank–Starling mechanism allows the amount of blood entering the heart from the veins to be precisely matched with the amount pumped out to the arterial circulation. As the heart fills with blood during diastole, the myocardium is stretched and oxidants are produced. Here we show that protein kinase G Iα (PKGIα) is oxidant-activated during stretch and this form of the kinase selectively phosphorylates cardiac phospholamban Ser16—a site important for diastolic relaxation. We find that hearts of Cys42Ser PKGIα knock-in (KI) mice, which are resistant to PKGIα oxidation, have diastolic dysfunction and a diminished ability to couple ventricular filling with cardiac output on a beat-to-beat basis. Intracellular calcium dynamics of ventricular myocytes isolated from KI hearts are altered in a manner consistent with impaired relaxation and contractile function. We conclude that oxidation of PKGIα during myocardial stretch is crucial for diastolic relaxation and fine-tunes the Frank–Starling response.

A mutation in the glutamate-rich region of RNA-binding motif protein 20 causes dilated cardiomyopathy through missplicing of titin and impaired Frank-Starling mechanism.

Mutations in the RS-domain of RNA-binding motif protein 20 (RBM20) have recently been identified to segregate with aggressive forms of familial dilated cardiomyopathy (DCM). Loss of RBM20 in rats results in missplicing of the sarcomeric gene titin (TTN). The functional and physiological consequences of RBM20 mutations outside the mutational hotspot of RBM20 have not been explored to date. In this study, we investigated the pathomechanism of DCM caused by a novel RBM20 mutation in human cardiomyocytes. We identified a family with DCM carrying a mutation (RBM20(E913K/+)) in a glutamate-rich region of RBM20. Western blot analysis of endogenous RBM20 protein revealed strongly reduced protein levels in the heart of an RBM20(E913K/+ )carrier. RNA deep-sequencing demonstrated massive inclusion of exons coding for the spring region of titin in the RBM20(E913K/+ )carrier. Titin isoform analysis revealed a dramatic shift from the less compliant N2B towards the highly compliant N2BA isoforms in RBM20(E913K/+ )heart. Moreover, an increased sarcomere resting-length was observed in single cardiomyocytes and isometric force measurements revealed an attenuated Frank-Starling mechanism (FSM), which was rescued by protein kinase A treatment. A mutation outside the mutational hotspot of RBM20 results in haploinsufficiency of RBM20. This leads to disturbed alternative splicing of TTN, resulting in a dramatic shift to highly compliant titin isoforms and an impaired FSM. These effects may contribute to the early onset, and malignant course of DCM caused by RBM20 mutations. Altogether, our results demonstrate that heterozygous loss of RBM20 suffices to profoundly impair myocyte biomechanics by its disturbance of TTN splicing.

An anesthetic management of negative pressure pulmonary edema

Negative pressure pulmonary edema (NPPE) is one of the common complications of upper airway obstruction seen by anesthesiologist during either in induction or emergence sometimes both. Patients who have experienced NPPE are generally healthy without comorbidities. NPPE is a result of marked decrease in intrathoracic pressure caused by ventilator efforts against a closed glottis resulting in disruption of normal intravascular Starling mechanism, leading to transudation of intravascular protein, and fluids into the pulmonary interstitium. The onset of NPPE is usually rapid and without prompt recognition and intervention, the outcome can be fatal. This case report is of a 40-year-old female adult, who underwent right-sided percutaneous nephrolithotomy for stone in the right kidney otherwise uncomplicated surgical procedure.

Abstract 15031: Defining a Coupling Ratio: Tissue Doppler Velocities in Systole and Diastole Predict Greater Diastolic Than Systolic Impairment in Hypertrophic Cardiomyopathy

Introduction: Mitral annular velocities, E’ and S’, (cm/s) measured by Tissue Doppler Imaging, are used in clinical cardiology and in research as important descriptors of systolic and diastolic function. We hypothesized that the relationship between these velocities in normal individuals would be linear, and would define a ventricular systolic/diastolic coupling ratio. We further hypothesized that departure from this normal coupling ratio might provide insight into certain disease states. Methods: We reviewed consecutive ECHOs performed at our institution to define the coupling ratio in normal individuals (n=57) and in 2 disease states dilated cardiomyopathy (DCM) (n=20), and hypertrophic cardiomyopathy (HCM) (n=19). The average of peak septal and lateral velocities, obtained in systole and diastole were compared (ANOVA). Results: Baseline characters as depicted in the table. In normals, there is a linear relationship between S’ and E’. While the S’ and E’ velocities are lower in absolute terms in DCM, the coupling ratio is similar to that seen in Normals. HCM, by contrast shows a relatively lower coupling ratio than Normals or DCM (Figure attached). Conclusion: We have defined a simple coupling ratio of systolic and diastolic velocities in normal and cardiomyopathy. The greater relative impairment on diastolic velocities in HCM appears to reflect a greater relative impairment in diastolic filling than in systolic shortening. In other words, for a given degree of systolic fiber shortening, fiber lengthening/stretch and perhaps utilization of the Frank Starling mechanism would be disproportionately impaired.

Cardiohemodynamics and Efficiency of Frank - Starling Mechanism in Aging

In adult and old Wistar rats, we studied cardio-hemodynamics and efficiency of Frank − Starling mechanisms. To evaluate systolic and diastolic function of the heart in vivo, the left-ventricular pressure and volume with visualization of the curves during the cardiac cycle were recorded using ultra miniature catheter 2F (Millar Instruments, USA). We have revealed left ventricular diastolic dysfunction and impaired relaxation of the heart in old rats, which is evidenced by the reduction in the maximum rate of left ventricular pressure generation under an increase in both end-diastolic stiffness and active relaxation constant. The results indicate that the efficiency of Frank-Starling mechanism (ΔSV/ΔEDV) decreases in old rats. At the same time, oxygen consumption in old rats increases by 70%, and stroke work of the heart increases 1.7 times. It means that despite the similar stroke volume, the heart of old rats perform more stroke work, with greater oxygen consumption, indicating that the heart of old rats works less effectively.

System overview of the fully implantable destination therapy--ReinHeart-total artificial heart.

Owing to the lack of suitable allografts, the demand for long-term mechanical circulatory support in patients with biventricular end-stage heart failure is rising. Currently available Total Artificial Heart (TAH) systems consist of pump units with only limited durability, percutaneous tubes and bulky external equipment that limit the quality of life. Therefore we are focusing on the development of a fully implantable, highly durable destination therapy total artificial heart. The ReinHeart-TAH system consists of a passively filling pump unit driven by a low-wear linear drive between two artificial ventricles, an implantable control unit and a compliance chamber. The TAH is powered by a transcutaneous energy transmission system. The flow distribution inside the ventricles was analysed by fluid structure interaction simulation and particle image velocimetry measurements. Along with durability tests, the hydrodynamic performance and flow balance capability were evaluated in a mock circulation loop. Animal trials are ongoing. Based on fluid structure interaction simulation and particle image velocimetry, blood stagnation areas have been significantly reduced. In the mock circulation loop the ReinHeart-TAH generated a cardiac output of 5 l/min at an operating frequency of 120 bpm and an aortic pressure of 120/80 mmHg. The highly effective preload sensitivity of the passively filling ventricles allowed the sensorless integration of the Frank Starling mechanism. The ReinHeart-TAH effectively replaced the native heart's function in animals for up to 2 days. In vitro and in vivo testing showed a safe and effective function of the ReinHeart-TAH system. This has the potential to become an alternative to transplantation. However, before a first-in-man implant, chronic animal trials still have to be completed.

The Physiology of Volume Resuscitation

Currently static and dynamic indices of fluid responsiveness are used to guide fluid resuscitation. The physiological effect of a fluid bolus is considered in terms of the Frank–Starling mechanism. This is, however, an incomplete concept and ignores the effect of the venous system. This paper reviews the interplay between the stressed and unstressed volume, the mean circulatory filling pressure and the central venous pressure in determining organ and microcirculatory flow. We also review method for determining fluid responsiveness.

Pushing the limits: exercise stress in the healthy human heart.

Pushing the limits: exercise stress in the healthy human heart.

Cardiac thin filament regulation and the Frank-Starling mechanism.

The heart has an intrinsic ability to increase systolic force in response to a rise in ventricular filling (the Frank–Starling law of the heart). It is widely accepted that the length dependence of myocardial activation underlies the Frank–Starling law of the heart. Recent advances in muscle physiology have enabled the identification of the factors involved in length-dependent activation, viz., titin (connectin)-based interfilament lattice spacing reduction and thin filament “on–off” regulation, with the former triggering length-dependent activation and the latter determining the number of myosin molecules recruited to thin filaments. Patients with a failing heart have demonstrated reduced exercise tolerance at least in part via depression of the Frank–Starling mechanism. Recent studies revealed that various mutations occur in the thin filament regulatory proteins, such as troponin, in the ventricular muscle of failing hearts, which consequently alter the Frank–Starling mechanism. In this article, we review the molecular mechanisms of length-dependent activation, and the influence of troponin mutations on the phenomenon.

My paper 20years later: Effect of positive end-expiratory pressure on right ventricular function in humans.

In 1992, we published a report on the effect of positive end-expiratory pressure (PEEP) on right ventricular (RV) function in humans. We measured RV volumes and pressures and pericardial pressure (Ppc) as PEEP was increased from zero to 15 cm H20 in 12 patients after thoracotomy, using a pulmonary arterial catheter equipped with a rapid responding thermistor that allowed measurement of RV ejection fraction (RVef), while Ppc was measured via a pericardial balloon catheter. RV end-diastolic volume (EDV) was estimated as the ratio of stroke volume (SV) to RVef, whereas RV end-systolic volume (ESV) were estimated as RV EDV-SV. PEEP increased Ppc and Pra, but RVef unaltered. There was no relation between either RV filling pressure (Pra-Ppc) and EDV or the change in RV filling pressure and EDV, although EDV varied significantly with PEEP (p<0.05). The relations between EDV and both SV and RVef were weak (r=0.54 and 0.55, respectively). RVef varied inversely with ESV (r=-0.77), although it showed no relation to transmural peak pulmonary artery pressure (r=0.28). However, both absolute and relative changes in EDV corresponded closely with respective ESV values (r=0.94). We concluded that EDV varies independently of changes in filling pressure and that changes in ESV occur independently of changes in ejection pressure. These data can be explained by assuming that the RV shape changes can dissociate changes in RV EDV from changes in RV wall stress (preload). Thus, changes in RV EDV may or may not alter SV but should proportionately change ESV to a degree dependent on election pressure and contractility. Subsequent studies confirmed our findings which can be summarized as 1) RV filling is independent of Pra; thus central venous pressure cannot be used to estimate RV preload; and 2) for cardiac output to increase by the Starling mechanism the RV must dilate increasing RV ESV. Since the pericardium limits absolute biventricular volume, there is a finite limit to which cardiac output can increase by the Starling mechanism defined not by left ventricular contractility but by RV function. And 3) if fluid loading causes Pra to increase without increasing cardiac output, then resuscitation should stop as the patient is going into acute cor pulmonale. These truths help bedside clinicians understand the echocardiographic and hemodynamic signatures of both RV failure and volume responsiveness.

Dimensional reductions of a cardiac model for effective validation and calibration

Complex 3D beating heart models are now available, but their complexity makes calibration and validation very difficult tasks. We thus propose a systematic approach of deriving simplified reduced-dimensional models, in "0D"-typically, to represent a cardiac cavity, or several coupled cavities-and in "1D"-to model elongated structures such as muscle samples or myocytes. We apply this approach with an earlier-proposed 3D cardiac model designed to capture length-dependence effects in contraction, which we here complement by an additional modeling component devised to represent length-dependent relaxation. We then present experimental data produced with rat papillary muscle samples when varying preload and afterload conditions, and we achieve some detailed validations of the 1D model with these data, including for the length-dependence effects that are accurately captured. Finally, when running simulations of the 0D model pre-calibrated with the 1D model parameters, we obtain pressure-volume indicators of the left ventricle in good agreement with some important features of cardiac physiology, including the so-called Frank-Starling mechanism, the End-Systolic Pressure-Volume Relationship, as well as varying elastance properties. This integrated multi-dimensional modeling approach thus sheds new light on the relations between the phenomena observed at different scales and at the local versus organ levels.

Depressed Frank–Starling mechanism in the left ventricular muscle of the knock-in mouse model of dilated cardiomyopathy with troponin T deletion mutation ΔK210

It has been reported that the Frank–Starling mechanism is coordinately regulated in cardiac muscle via thin filament “on–off” equilibrium and titin-based lattice spacing changes. In the present study, we tested the hypothesis that the deletion mutation ΔK210 in the cardiac troponin T gene shifts the equilibrium toward the “off” state and accordingly attenuate the sarcomere length (SL) dependence of active force production, via reduced cross-bridge formation. Confocal imaging in isolated hearts revealed that the cardiomyocytes were enlarged, especially in the longitudinal direction, in ΔK210 hearts, with striation patterns similar to those in wild type (WT) hearts, suggesting that the number of sarcomeres is increased in cardiomyocytes but the sarcomere length remains unaltered. For analysis of the SL dependence of active force, skinned muscle preparations were obtained from the left ventricle of WT and knock-in (ΔK210) mice. An increase in SL from 1.90 to 2.20μm shifted the mid-point (pCa50) of the force-pCa curve leftward by ~0.21pCa units in WT preparations. In ΔK210 muscles, Ca2+ sensitivity was lower by ~0.37pCa units, and the SL-dependent shift of pCa50, i.e., ΔpCa50, was less pronounced (~0.11pCa units), with and without protein kinase A treatment. The rate of active force redevelopment was lower in ΔK210 preparations than in WT preparations, showing blunted thin filament cooperative activation. An increase in thin filament cooperative activation upon an increase in the fraction of strongly bound cross-bridges by MgADP increased ΔpCa50 to ~0.21pCa units. The depressed Frank–Starling mechanism in ΔK210 hearts is the result of a reduction in thin filament cooperative activation.

The effects of angiotensin II signaling pathway in the systolic response to acute stretch in the normal and ischemic myocardium

Acute myocardial stretch elicits a biphasic increase in contractility: an immediate increase, known as Frank–Starling mechanism (FSM), followed by a progressive increase, regarded as slow force response (SFR). In this study, we characterized the contractile response to acute stretch from 92 to 100% Lmax in rabbit papillary muscles (n=86) under normoxic and ischemic conditions, and its modulation by angiotensin II signaling pathway. Under normoxia, the FSM was independent of Na+/H+-exchanger, reverse mode of Na+/Ca2+-exchanger (r-NCX), AT1 receptor, AT2 receptor and PKC. Regarding the SFR, it was mediated by AT1 receptor activation and its downstream effectors PKC, Na+/H+-exchanger and r-NCX. Ischemia negatively impacted on the FSM and abolished the SFR, with the muscles exhibiting a time-dependent decline in contractility. Under ischemic conditions, FSM was not influenced by AT1 and AT2 receptors or PKC activation. AT1 receptor antagonism rescued the progressive deterioration in contractility, an effect partially dependent on AT2 receptor activation.

Pharmacological management of chronic heart failure: old drugs, new drugs and new indications

British Journal of Hospital Medicine, February 2013, Vol 74, No 2 Heart failure affects 1–2% of the population in the developed world and remains an important cause of hospitalization. After stabilizing patients through the acute phase of their illness, their outcome in terms of mortality, morbidity and further hospitalization can be altered by treatment with appropriate disease-modifying drugs. The hospital doctor plays a crucial role in restarting medications that were stopped during admission, titrating appropriate doses and initiating drugs with long-term benefits. This article gives an overview of the current drug options and the evidence to support their use in practice (Figure 1). The pharmacological therapy discussed focuses on evidence derived in patients with heart failure with impaired systolic function. Old drugs The neurohumoral axis: angiotensin-converting enzyme inhibitors and angiotensin receptor blockers The pathogenesis of heart failure is driven by two neurohumoral systems: the renin– angiotensin–aldosterone system and the sympathetic nervous system. A decrease in cardiac output stimulates activation of these two systems in an attempt to compensate for cardiac dysfunction via vasoconstriction and augmenting cardiac output using the Frank–Starling mechanism. However, left unchecked, the increase pre-load and after-load causes worsening of cardiac function through adverse remodelling of the myocardium and vasculature. Current treatments counteract these adverse remodelling processes. Pharmacological management of chronic heart failure: old drugs, new drugs and new indications

Effect of muscle length on cross-bridge kinetics in intact cardiac trabeculae at body temperature

Dynamic force generation in cardiac muscle, which determines cardiac pumping activity, depends on both the number of sarcomeric cross-bridges and on their cycling kinetics. The Frank–Starling mechanism dictates that cardiac force development increases with increasing cardiac muscle length (corresponding to increased ventricular volume). It is, however, unclear to what extent this increase in cardiac muscle length affects the rate of cross-bridge cycling. Previous studies using permeabilized cardiac preparations, sub-physiological temperatures, or both have obtained conflicting results. Here, we developed a protocol that allowed us to reliably and reproducibly measure the rate of tension redevelopment (ktr; which depends on the rate of cross-bridge cycling) in intact trabeculae at body temperature. Using K+ contractures to induce a tonic level of force, we showed the ktr was slower in rabbit muscle (which contains predominantly β myosin) than in rat muscle (which contains predominantly α myosin). Analyses of ktr in rat muscle at optimal length (Lopt) and 90% of optimal length (L90) revealed that ktr was significantly slower at Lopt (27.7 ± 3.3 and 27.8 ± 3.0 s−1 in duplicate analyses) than at L90 (45.1 ± 7.6 and 47.5 ± 9.2 s−1). We therefore show that ktr can be measured in intact rat and rabbit cardiac trabeculae, and that the ktr decreases when muscles are stretched to their optimal length under near-physiological conditions, indicating that the Frank–Starling mechanism not only increases force but also affects cross-bridge cycling kinetics.

Effects of temperature on the nitric oxide-dependent modulation of the Frank–Starling mechanism: the fish heart as a case study

The Frank–Starling law is a fundamental property of the vertebrate myocardium which allows, when the end-diastolic volume increases, that the consequent stretch of the myocardial fibers generates a more forceful contraction. It has been shown that in the eel (Anguilla anguilla) heart, nitric oxide (NO) exerts a direct myocardial relaxant effect, increasing the sensitivity of the Frank–Starling response (Garofalo et al., 2009). With the use of isolated working heart preparations, this study investigated the relationship between NO modulation of Frank–Starling response and temperature challenges in the eel. The results showed that while, in long-term acclimated fish (spring animals perfused at 20°C and winter animals perfused at 10°C) the inhibition of NO production by L-N5 (1-iminoethyl)ornithine (L-NIO) significantly reduced the Frank–Starling response, under thermal shock conditions (spring animals perfused at 10 or 15°C and winter animals perfused at 15 or 20°C) L-NIO treatment resulted without effect. Western blotting analysis revealed a decrease of peNOS and pAkt expressions in samples subjected to thermal shock. Moreover, an increase in Hsp90 protein levels was observed under heat thermal stress. Together, these data suggest that the NO synthase/NO-dependent modulation of the Frank–Starling mechanism in fish is sensitive to thermal stress.