Victor Claes

Onset of Mechanical Activation of Mammalian Heart Muscle in Calcium- and Strontium-Containing Solutions

The time course of the relationship between force, shortening velocity, and length during shortening was examined in mammalian heart muscle in calcium (Ca)-containing solutions and solutions with strontium (Sr) substituted for Ca by imposing load clamps of various amplitudes at various times during external shortening. The onset of this force-velocity-length relationship was measured by abruptly unloading the muscle at different times to nearly zero external load—by zero load clamping the muscle—under conditions of optimal damping; the subsequent unloaded active muscle shortening velocity was then analyzed with respect to time and length. In Ca- containing solution, the unloaded velocity in the length range around 90% of Lmax rose within 16% of the time required to reach peak isometric force to a maximum level appropriate to a given contractile state. Subsequently, the force-velocity-length relationship was independent of time at any load over a well-determined portion of external shortening. In Sr-containing solution, unloaded active velocity rose much slower to reach a steady state after 30–35% of the time to peak force, and the time independence of the force-velocity-length relationship was delayed until after this time. In Na-free Sr-containing solution, maximum unloaded shortening velocity and the time-independent portion of the force-velocity-length relationship were achieved again within 20% of the time to peak force. The effects of substitution of Sr for Ca thus resemble those of caffeine on mammalian heart muscle. These findings suggest a close relationship between the time course of the force-velocity-length relationship and the degree of activation.

Velocity of Shortening of Unloaded Heart Muscle and the Length-Tension Relation

A new technique permits removal of the resting force from the muscle during the course of shortening and study of the maximum velocity of shortening with almost zero load at physiological initial muscle lengths. This unloading has been performed as a sudden unloading to zero total load (zero load clamping), or according to the resting length-tension relations, or according to an arbitrary unloading intermediate between both former modes of unloading. Using these various modes of unloading to zero load during shortening, it has been demonstrated that the maximum velocity of shortening Vmax remains constant between maximum length (lmax) and a length 12.5% shorter than lmax. This unique force (zero load)-velocity (Vmax)-length (up to 12.5% below lmax) relation is independent of die initial muscle length over this range of lengths, independent of the time after the stimulus over the largest portion of the shortening phase, and independent of the sequence of length change and mode of unloading through which it arrived at zero load within this range of lengths.

Effect of Calcium on Force-Velocity-Length Relations of Heart Muscle of the Cat

The influence of calcium concentration in the bathing solution on the force-velocity relations of the isolated, electrically excited cat papillary muscle was examined. Shortening velocity was directly measured at minimum loads by unloading or load clamping the muscle from the preload (at the length, Lmax, where maximum active tensio was developed) to 0 (zero-load clamp), 2, 5, 10, and 15% of the total tension, P0. All measurements were made in the physiological length range between Lmax and 12.5% below Lmax. Increasing the calcium concentration augmented the measured maximum unloaded shortening velocity and shifted the normalized force-velocity relation when force was expressed as a fraction of P0. These observations suggest that the activating calcium has an action more complex than the binding to troponin alone.

Circadian rhythms, Wnt/beta-catenin pathway and PPAR alpha/gamma profiles in diseases with primary or secondary cardiac dysfunction.

Circadian clock mechanisms are far-from-equilibrium dissipative structures. Peroxisome proliferator-activated receptors (PPAR alpha, beta/delta, and gamma) play a key role in metabolic regulatory processes, particularly in heart muscle. Links between circadian rhythms (CRs) and PPARs have been established. Mammalian CRs involve at least two critical transcription factors, CLOCK and BMAL1 (Gekakis et al., 1998; Hogenesch et al., 1998). PPAR gamma plays a major role in both glucose and lipid metabolisms and presents circadian properties which coordinate the interplay between metabolism and CRs. PPAR gamma is a major component of the vascular clock. Vascular PPAR gamma is a peripheral regulator of cardiovascular rhythms controlling circadian variations in blood pressure and heart rate through BMAL1. We focused our review on diseases with abnormalities of CRs and with primary or secondary cardiac dysfunction. Moreover, these diseases presented changes in the Wnt/beta-catenin pathway and PPARs, according to two opposed profiles. Profile 1 was defined as follows: inactivation of the Wnt/beta-catenin pathway with increased expression of PPAR gamma. Profile 2 was defined as follows: activation of the Wnt/beta-catenin pathway with decreased expression of PPAR gamma. A typical profile 1 disease is arrhythmogenic right ventricular cardiomyopathy, a genetic cardiac disease which presents mutations of the desmosomal proteins and is mainly characterized by fatty acid accumulation in adult cardiomyocytes mainly in the right ventricle. The link between PPAR gamma dysfunction and desmosomal genetic mutations occurs via inactivation of the Wnt/beta-catenin pathway presenting oscillatory properties. A typical profile 2 disease is type 2 diabetes, with activation of the Wnt/beta-catenin pathway and decreased expression of PPAR gamma. CRs abnormalities are present in numerous pathologies such as cardiovascular diseases, sympathetic/parasympathetic dysfunction, hypertension, diabetes, neurodegenerative diseases, cancer which are often closely inter-related.

End-systolic pressure-volume relation estimated from physiologically loaded cat papillary muscle contractions.

Physiological loads were imposed on contracting isolated cat papillary muscles. The interaction of a hypothetical cylindrical ventricle with a three-element vascular impedance model dictated these physiological loads. The length-tension relation of the physiologically loaded muscle and the pressure-volume relation of the hypothetical ventricle were simultaneously analyzed while the resistive and capacitive components of the vascular impedance were varied widely. Both the end-systolic muscle length-tension relation and the end-systolic ventricular pressure-volume relation were constructed using stepwise increments in either peripheral vascular capacitance or peripheral vascular resistance. The slope of the relation line connecting the end-systolic pressure-volume points under stepwise increases in resistive load was smaller (p < 0.0005) than the slope of this line under stepwise increases in capacitive load. Therefore, the end-systolic pressure-volume relation behaves differently with respect to capacitive and resistive loads. The different loading pattern within the same beat under these varying loading conditions and the coincidence of end-systole with end-ejection in these naturally ejecting contractions are responsible for the shifts in slope of the end-systolic pressure-volume relation. Neither the slope nor the volume intercept of the end-systolic pressure-volume relation was changed when initial muscle length was decreased from 1.0 to 0.95 Imax When the Ca2+ concentration in the bathing solution was increased from 2.5 to 7.5 mM, the slope of the end-systolic pressure-volume relation increased (P < 0.0005), and the volume intercept of the curve decreased (P < 0.025). These results are similar to data reported for conscious animals and to data obtained from catheterization of the human left ventricle. Circ Res 46: 20-26, 1980

Thermodynamics in cancers: opposing interactions between PPAR gamma and the canonical WNT/beta-catenin pathway

Cancer cells are the site of numerous metabolic and thermodynamic abnormalities. We focus this review on the interactions between the canonical WNT/beta-catenin pathway and peroxisome proliferator-activated receptor gamma (PPAR gamma) in cancers and their implications from an energetic and metabolic point of view. In numerous tissues, PPAR gamma activation induces inhibition of beta-catenin pathway, while the activation of the canonical WNT/beta-catenin pathway inactivates PPAR gamma. In most cancers but not all, PPAR gamma is downregulated while the WNT/beta-catenin pathway is upregulated. In cancer cells, upregulation of the WNT/beta-catenin signaling induces dramatic changes in key metabolic enzymes that modify their thermodynamic behavior. This leads to activation of pyruvate dehydrogenase kinase1 (PDK-1) and monocarboxylate lactate transporter. Consequently, phosphorylation of PDK-1 inhibits the pyruvate dehydrogenase complex (PDH). Thus, a large part of pyruvate cannot be converted into acetyl-coenzyme A (acetyl-CoA) in mitochondria and only a part of acetyl-CoA can enter the tricarboxylic acid cycle. This leads to aerobic glycolysis in spite of the availability of oxygen. This phenomenon is referred to as the Warburg effect. Cytoplasmic pyruvate is converted into lactate. The WNT/beta-catenin pathway induces the transcription of genes involved in cell proliferation, i.e., MYC and CYCLIN D1. This ultimately promotes the nucleotide, protein and lipid synthesis necessary for cell growth and multiplication. In cancer, activation of the PI3K-AKT pathway induces an increase of the aerobic glycolysis. Moreover, prostaglandin E2 by activating the canonical WNT pathway plays also a role in cancer. In addition in many cancer cells, PPAR gamma is downregulated. Moreover, PPAR gamma contributes to regulate some key circadian genes. In cancers, abnormalities in the regulation of circadian rhythms (CRs) are observed. CRs are dissipative structures which play a key-role in far-from-equilibrium thermodynamics. In cancers, metabolism, thermodynamics and CRs are intimately interrelated.

Interactions between PPAR Gamma and the Canonical Wnt/Beta-Catenin Pathway in Type 2 Diabetes and Colon Cancer

In both colon cancer and type 2 diabetes, metabolic changes induced by upregulation of the Wnt/beta-catenin signaling and downregulation of peroxisome proliferator-activated receptor gamma (PPAR gamma) may help account for the frequent association of these two diseases. In both diseases, PPAR gamma is downregulated while the canonical Wnt/beta-catenin pathway is upregulated. In colon cancer, upregulation of the canonical Wnt system induces activation of pyruvate dehydrogenase kinase and deactivation of the pyruvate dehydrogenase complex. As a result, a large part of cytosolic pyruvate is converted into lactate through activation of lactate dehydrogenase. Lactate is extruded out of the cell by means of activation of monocarboxylate lactate transporter-1. This phenomenon is called Warburg effect. PPAR gamma agonists induce beta-catenin inhibition, while inhibition of the canonical Wnt/beta-catenin pathway activates PPAR gamma.

PPARs, Cardiovascular Metabolism, and Function: Near- or Far-from-Equilibrium Pathways

Peroxisome proliferator-activated receptors (PPAR α, β/δ and γ) play a key role in metabolic regulatory processes and gene regulation of cellular metabolism, particularly in the cardiovascular system. Moreover, PPARs have various extra metabolic roles, in circadian rhythms, inflammation and oxidative stress. In this review, we focus mainly on the effects of PPARs on some thermodynamic processes, which can behave either near equilibrium, or far-from-equilibrium. New functions of PPARs are reported in the arrhythmogenic right ventricular cardiomyopathy, a human genetic heart disease. It is now possible to link the genetic desmosomal abnormalitiy to the presence of fat in the right ventricle, partly due to an overexpression of PPARγ. Moreover, PPARs are directly or indirectly involved in cellular oscillatory processes such as the Wnt-b-catenin pathway, circadian rhythms of arterial blood pressure and cardiac frequency and glycolysis metabolic pathway. Dysfunction of clock genes and PPARγ may lead to hyperphagia, obesity, metabolic syndrome, myocardial infarction and sudden cardiac death, In pathological conditions, regulatory processes of the cardiovascular system may bifurcate towards new states, such as those encountered in hypertension, type 2 diabetes, and heart failure. Numerous of these oscillatory mechanisms, organized in time and space, behave far from equilibrium and are “dissipative structures”.

Influence of Caffeine and Other Inotropic Interventions on the Onset of Unloaded Shortening Velocity in Mammalian Heart Muscle: Time Course of Activation

The onset of maximum unloaded shortening velocity was measured in cat papillary muscles by analyzing velocity with respect to time and length after zero load clamps imposed at different times. Unloaded shortening velocity rose rapidly to a constant level by 20% of the time to peak force under all conditions studied except with application of 10 mM caffeine, which markedly slowed its onset. Load-clamping experiments showed that the force-velocity-length interrelationship which characterizes shortening remained constant throughout most of shortening except with caffeine, which delayed the onset of steady state. It is suggested that the time course of the force-velocity-length interrelationship usefully describes velocity active state, the onset of which can be measured independently of force generation at zero load. In afterloaded contractions, manifestation of velocity active state may be delayed by the slower time course of force generation which precedes shortening. It is concluded that velocity active state normally rises rapidly to a constant level which describes the contractile state. Caffeine slows the onset of velocity active state, and valid time-independent force-peak velocity curves cannot be obtained in the presence of this drug.

Uniform sarcomere behaviour during twitch of intact single cardiac cells

Behaviour of sarcomere length was analysed in different regions of single cardiac cells (n = 249) of the ventricle, both at rest (n = 144) and during twitch contractions (n = 57). At rest, regional distribution of sarcomere length proved to be uniform. In the leaky cell (n = 48), resting sarcomere length was not affected over longer periods of time (up to 2 h), nor by lowering the ATP concentration (from 5 mM to 2.5 mM and 500 microM), nor by increasing free calcium within subactivating ranges (5, 20, 60 microM). No statistical differences could be detected between resting cell dimensions and sarcomere length between cells isolated from left and right ventricle (n = 64), nor between cells from epicardial or endocardial layers (n = 80). During twitch contraction in the intact unloaded cardiac cell (n = 32), sarcomere lengths in different regions were analysed every 20 ms and behaved synchronously, presenting arguments for uniformity during the myocardial contraction-relaxation cycle in the free-lying intact cardiac cell.