Contractile proteins in myocardial cells are regulated by factor(s) released by blood vessels

Modulation of myocardial contraction by endocardial and coronary vascular endothelium

Endothelial cells in the heart, both endocardial endothelium and coronary vascular endothelium, influence myocardial contraction in isolated tissue and pump function in intact hearts by releasing diffusible agents that affect subjacent myocardium. Endocardial endothelium releases both nitric oxide (NO) and an unidentified "contraction-prolonging substance" ("endocardin") that respectively decrease and increase the duration of twitch contraction, probably by altering myofibrillar calcium sensitivity. These agents modulate the duration of ejection and the timing of relaxation, but without significantly altering early systolic behavior. Coronary vascular endothelium also releases NO, with similar effects on contraction, and in addition probably releases several other agents. Current work is aimed at identifying all of the agents involved in these novel endothelial influences and studying their potential physiologic and pathophysiologic roles in cardiac contractile and other functions.

The dynamic role of cardiac myosin binding protein-C during ischemia

Cardiac myosin binding protein C (cMyBP-C) is a myofibrillar protein important for normal myocardial contractility and stability. In mutated form it can cause cardiomyopathy and heart failure. cMyBP-C appears to have separate regions for different functions. Three phosphorylation sites near the N terminus modulate contractility by their effect on both the kinetics of contraction and the binding site of the N-terminus. The C terminal region binds to myosin rods and stabilizes thick filament structure. The aim of the study reported here was to test whether cMyBPC is important in producing the structural and functional changes that result from ischemia/reperfusion. In this study the sequential changes in cMyBP-C, contractility, and thick filament structure following dephosphorylation of cMyBP-C associated with ischemia and reperfusion have been studied in biopsied specimens from chronically instrumented dogs. One and two dimensional electrophoresis, electron microscopy and immunocytochemistry with multiple antibodies generated against different domains in cMyBP-C have been used to follow structural changes in cMyBP-C. Ischemia produced dephosphorylation of cMyBP-C. Subsequent reperfusion released the dephosphorylated cMyBP-C from myofibrils and activated proteolysis of the cytoplasmic cMyBP-C. This in turn leads to increased vulnerability of cMyBP-C to proteolysis and increased degradation of thick filaments. The state of cMyBP-C appears to be closely related to phosphorylation and dephosphorylation of serine 282. In the absence of the stabilizing action of cMyBP-C either as a consequence of genetic mutation or dephosphorylation, premature degradation of thick filaments occurs and is accompanied by persistent contractile dysfunction.

Inhibition of crossbridge function in the normal human heart by hypoxic endothelial superfusate

Endothelial cells release diffusible substances which modulate myocardial function. Oxygen pressure is one important factor for stimulation and modulation of endothelial function. Here we investigated the effects of a superfusate obtained from hypoxic (pO(2) 40-50 mmHg) porcine endothelial cell culture on human myocardial crossbridge cycling rate. Isometric force development and the rate constant for tension development of demembranated multicellular fibers from the left myocardium of a normal human heart were determined from the low-tension rigor by photolytic release of ATP from caged-ATP. Incubation with hypoxic or normoxic superfusates did not change maximal isometric force development. However, rate constant of tension development of the normal human heart fibers significantly decreased to 43.3% upon incubation with the hypoxic but not normoxic endothelial cell superfusate.

Ca2+-independent inhibition of myocardial contraction by coronary effluent of hypoxic rat hearts

Endothelial cells release agents that influence cardiac contraction. We recently reported that cultured hypoxic endothelial cells release an unidentified factor(s) that inhibits myocardial contraction. In this study, we investigated the effects of coronary effluent of isolated hypoxic rat hearts on isolated rat ventricular myocyte contraction. Coronary effluent collected during brief moderate hypoxia significantly depressed myocyte twitch shortening and decreased diastolic length, with only minor reduction in intracellular Ca2+ transients. These effects were similar to those of hypoxic rat coronary microvascular endothelial cell superfusates and were reversed by reoxygenation of hearts. "Hypoxic" coronary effluent exerted essentially Ca2+-independent effects on myofilament interaction in intact myocytes, as assessed by 1) peak Ca2+-shortening relations, 2) phase-plane analysis of instantaneous Ca2+-cell length relations, and 3) "steady-state" myofilament responses in tetanized, sarcoplasmic reticulum-disabled cells. Thus an unidentified substance(s) that inhibits myocyte shortening predominantly via effects on the myofilaments is reversibly released during acute moderate hypoxia of isolated hearts, presumably from coronary endothelial cells. Release of such an agent may be relevant to the cardiac contractile response to hypoxia.

Vascular endothelial cell-cardiac myocyte crosstalk in achieving a balance between energy supply and energy use

In isolated perfused hearts, endothelial cells in the coronary arterial vascular system release substances that can alter the contractility of the cardiac myocytes. There are at least two different substances, one that increases and another that decreases the contractility of cardiac myocytes. The rate of release of these endothelial-derived cardioactive substances depends on the oxygen tension in the immediate vicinity of the cardiac myocytes. As the local oxygen tension increases the contractility changes in the same direction. The oxygen sensor in this regulatory system is the cardiac myocyte, which then releases substances that regulate the secretion of endothelin and a relaxant by endothelial cells. The result is a loop involving cross talk between coronary endothelial cells and cardiac myocytes to modulate cardiac contractility in accordance with the oxygen supply to the cardiac myocytes. Preliminary data suggest that the change in contractility is related to a change in structure and position of the cross bridge due to phosphorylation of a protein in the thick filament.

Paracrine regulation of cardiac myocytes in normal and septic heart

A paracrine pathway for the regulation of cardiac contractile function by nonmuscle cells is documented in the heart. Coronary and endocardial endothelium release several diffusible agents, such as prostaglandins, endothelin-1, and nitric oxide, with an action on cardiac myocyte function. Cardiac diseases involving an immune or inflammatory mechanism, such as endotoxic shock, are now seen as conditions in which cross-talk between different cell types in the heart is clearly implicated. The potential biological relevance of inducible nitric oxide synthase in the myocardium, and the subsequent production of nitric oxide has been proposed as a mechanism of the cardiac depression observed in septic shock. In addition to cardiac myocytes, activated microvascular endothelial cells and cardiac endothelial cells may contribute to nitric oxide generation and, ultimately, to the depression of myocardial contractile activity during sepsis. This article reviews the local intercellular communication between cardiac myocytes and endothelial cells in the normal heart and discusses some of the mechanisms potentially claimed to depress heart function in sepsis.

Cardiodepressive mediators are released after ischemia from an isolated heart: role of coronary endothelial cells

OBJECTIVES

This study was designed to ascertain whether cardiodepressive mediators released after ischemia originate from coronary endothelial cells.

BACKGROUND

Endothelial cells modulate myocardial contractility under physiologic conditions. Few data are available describing the role of coronary endothelial cells on myocardial function after ischemia.

METHODS

Using a model of sequential perfusion of two isolated rat hearts, the effect of the reoxygenated coronary effluent of heart I was investigated on myocardial contractility of heart II. After 40 min of separate perfusion at constant flow (10 ml/min), the two hearts were perfused sequentially with (group I) or without (control group) preceding ischemia (10 min) of heart I. In groups II and III, the coronary endothelium of heart I was functionally removed by Triton X-100 or hyperkalemic infusion before global ischemia. Endothelial damage was confirmed by functional tests and electron microscopy.

RESULTS

Under control conditions no changes were observed in heart II during sequential perfusion. In contrast, after 10 min of ischemia in heart I, a marked reversible decrease in left ventricular pressure, left ventricular dP/dtmax and left ventricular dP/dtmin (-55%, -66% and -70%, respectively) was observed in heart II. Heart rate and coronary perfusion pressure did not change significantly. Selective endothelial damage of heart I before ischemia did not modify the negative inotropic effect observed in heart II.

CONCLUSIONS

Cardiodepressive mediators are released after ischemia during reperfusion from an isolated heart and induce a reversible negative inotropic effect in a sequentially perfused heart. It is unlikely that these agents are derived from the coronary endothelium.

Endothelial cell regulation of contractility of the heart

Endocardial and coronary vascular endothelial cells release substances that modify the contraction of cardiac myocytes. The major and possibly the sole up-regulating substance is endothelin. Several down-regulating substances are secreted, but none has yet been specifically identified. The relative amounts of up- and down-regulating substances are related to tissue oxygen tension. As pO2 rises, the concentration of up- and down-regulating substances, respectively, increases and decreases. Endothelin increases isometric force and decreases actomyosin ATPase activity thus increasing the economy of conversion of chemical to hydrodynamic energy. Beta-adrenergic agonists increase ATPase activity through an endothelial cell-dependent mechanism, leading to decreased economy. Therefore, two endothelial cell-dependent systems exist for regulating contractile efficiency: One involving endothelin appears to optimize the contraction for efficiency; the other, the beta-adrenergic-mediated system, optimizes for power.

The cardiac endothelium: cardioactive mediators

Endothelial cells within the heart release a number of substances that modulate myocardial contractile function. These agents include nitric oxide, endothelin, prostanoids, adenylpurines, and other substances that have so far been characterized only in bioassay studies. A notable feature of many of these agents is that they influence contractile behavior predominantly by modifying cardiac myofilament properties rather than altering cytosolic Ca2+ transients. A consequence of this subcellular action is often a disproportionate effect on myocardial relaxation and diastolic tone. The paracrine modulation of cardiac myocyte function by endothelial cell factors is likely to be an important mechanism contributing to the overall regulation of cardiac contractile function, both physiologically and in pathological states.

Influence of endothelin 1 on human atrial myocardium--myocardial function and subcellular pathways

The influence of endothelin 1 on isometrically contracting human atrial muscle strip preparations was investigated under physiological conditions (37 degrees C, 1 Hz, Ca2+ 2.5 mM). Endothelin dose-dependently increased isometric tension from 3 x 10(-10) M to 1 x 10(-7) M. At 1 x 10(-7) M the inotropic effect of endothelin was maximum with isometric tension being increased by 32 +/- 6% (n = 11, p < 0.05). At 1 x 10(-7) M endothelin the positive inotropic effect was preceded by a transient negative inotropic effect with a decline in tension by -5 +/- 1% (n = 11, p < 0.05). Endothelin prolonged time from peak tension to 50% relaxation (RT50) by 29 +/- 5%. With BQ123 a competitive antagonist of the ETA receptor positive inotropic effect and the prolongation of relaxation was significantly reduced and initial negative a inotropic effect was abolished, indicating a ETA receptor mediated effect. Preincubation with phorbolmyristateacetate (10(-5) M) to downregulate proteinkinase C (PKC) eliminated the positive inotropic effect of endothelin. Similarly, N-5,5-dimethylamiloride (10(-5) M) which inhibits Na+/H(+)-exchanger activity, abolished the positive inotropic effect of ET. However, with either PMA or DMA the initial transient negative inotropic effect was still present (-13 +/- 7%, n = 9, p < 0.05 and -3 +/- 1%, n = 6, p < 0.05). Furthermore, both substances did not abolish the prolongation of twitch time parameters observed under endothelin. After preincubation with PMA, endothelin prolonged RT50 by 18 +/- 6% and with DMA by 11 +/- 2%. Using the photoprotein aequorin as an indicator for intracellular calcium concentrations showed that the positive inotropic effect was mainly mediated by an increase of systolic intracellular calcium concentrations. Thus, the present data indicate that the positive inotropic effect of endothelin in human atrial myocardium results from activation of PKC with a subsequent activation of the Na+/H(+)-exchanger. However, the initial negative inotropic effects as well as the prolongation of relaxation seem to result from a different intracellular mechanism of endothelin.

The influence of endothelium-derived nitric oxide on myocardial contractile function

Nitric oxide released by cardiac endothelial cells modulates myocardial contractile function through elevation of intracellular 3',5'-cyclic guanosine monophosphate (cGMP). In the absence of agonist stimulation, nitric oxide typically enhances myocardial relaxation and reduces diastolic tone, without significantly altering the rate of force or pressure development. This pattern of effect is observed with nitric oxide or with cGMP analogues in isolated rat cardiac myocytes, isolated ferret papillary muscle preparations, and isolated ejecting guinea-pig hearts. In human subjects studied at cardiac catheterisation, low-dose bicoronary infusions of sodium nitroprusside or of substance P induce similar effects on left ventricular systolic and diastolic function. These changes may benefit from cardiac filling and coronary perfusion by increasing the diastolic interval, reducing extravascular compressive forces and increasing the driving pressure for filling, e.g., during exercise. Nitric oxide may also modulate inotropic and chronotropic responses to beta-adrenergic stimulation. Under pathological conditions, overproduction of nitric oxide by an inducible nitric oxide synthase may be detrimental for contractile function. Dysfunction of the constitutive nitric oxide pathway could also contribute to pathophysiology, e.g., in conditions characterised by diastolic dysfunction. The paracrine nitric oxide pathway is likely to be an important regulator of cardiac contractile function, acting in concert and interacting with other regulatory pathways.

Physiologic and pharmacologic factors that affect myocardial relaxation

Evaluation of the myocardial relaxation has become important in the last years. An impaired relaxation may precede contractile dysfunctions and even cause heart failure. To treat this impaired lusitropism it is necessary to properly assess the lusitropic state of the heart and understand how drugs affect the cellular mechanisms underlying myocardial relaxation (sarcoplasmic reticulum function, Ca2+ fluxes through the sarcolemma and myofilament Ca2+ sensitivity). Current information regarding these issues is provided in this review. The relative usefulness of the mechanical parameters used to evaluate the lusitropic state of the heart in experimental models applied in pharmacology will also be discussed.

Potassium channel openers and other regulators of KATP channels

1. Interest in ATP-sensitive K (KATP) channels first arose when it was shown that hypoglycaemic sulphonylureas, such as glibenclamide, closed these channels in pancreatic beta-cells to cause insulin release. The demonstration that certain smooth muscle relaxants (K channel openers) may exert their actions through opening a similar channel in vascular smooth muscle fueled further investigation of these channels and their physiological role in a variety of tissue types, including various types of smooth muscle, cardiac and skeletal muscle and neural and endocrine organ function. 2. The K channel openers have a variety of potential therapeutic applications, including disorders of smooth muscle hyperreactivity, such as hypertension, and a great deal of research has focused on this field. More recently, attention has turned to the cardiac actions of these compounds and this area is discussed in detail. One of the current problems is the lack of selectivity of KATP channel regulators. However, there have been a number of recent encouraging reports suggesting that, under certain pathophysiological conditions, the action of the K channel openers may be enhanced, conferring upon them some degree of selectivity. 3. A number of endogenous regulators of these channels have been identified, particularly in the category of endogenous openers of these channels. At present though, the physiological role of these channels and the endogenous regulators identified, is unclear. 4. It is evident that, although advances have been made, much work is still required to increase our understanding and ultimately to allow selective pharmacological manipulation of these channels to become a therapeutic reality.

Endothelial control of vascular and myocardial function in heart failure

The effect of vascular endothelium, endocardium, and coronary endothelium on vascular tone and myocardial contraction-relaxation sequence in heart failure is discussed. Vascular endothelium affects underlying vascular smooth muscle through paracrine secretion of relaxing and constricting factors. In heart failure, systemic vasoconstriction results not only from neuroendocrine activation, but also from disturbed local endothelial control of vascular tone because of impaired endothelial-dependent vasodilation and because of increased plasma concentration of endothelin. Experimental evidence obtained in isolated cardiac muscle strips established the influence of endocardial endothelium on the duration of myocardial contraction and on the onset of myocardial relaxation. By analogy to vascular endothelium, both diffusible agents that abbreviate (endothelial-derived relaxation factor-like substance) and those that prolong (endocardin) myocardial contraction have been shown to be released from the endocardium. Similar agents are released from the coronary endothelium and, because of the close proximity of capillaries and myocytes, could exert a major effect on myocardial performance. Endothelial dysfunction and concomitant lack of release of myocardial relaxant factors could explain left ventricular relaxation abnormalities observed in the cardiac allograft or in arterial hypertension. Since endothelial-derived relaxation factor or nitric oxide mediates the coronary reactive hyperemic response, a negative inotropic action of nitric oxide could contribute to left ventricular failure when left ventricular wall stress is elevated, as occurs after myocardial infarction in the noninfarcted zone and during left ventricular volume or pressure overload in the absence of adequate hypertrophy.

Role of endothelial cells in cardiovascular function

Since the first description of vascular endothelium-dependent relaxation in response to acetylcholine, the role of endothelial cells in the regulation of cardiovascular function has been increasingly studied. The identification of endothelial releasing factors such as nitric oxide and endothelin has enabled us to better understand the mechanisms involved in autoregulation. It has also been shown that both vascular and endocardial endothelium can modify the contractile characteristics of their adjacent myocardium. In the heart, these modulating effects of endothelial cells are more widespread than previously thought and, can be the result of the direct effects of endocardial and vascular endothelial cells and their indirect effects, via modulation of the myocardial response to inotropic agents.

Diastolic failure: pathophysiology and therapeutic implications

Primary diastolic dysfunction or failure is a distinct pathophysiologic entity. It results from increased resistance to ventricular filling, which leads to an inappropriate upward shift of the diastolic pressure-volume relation, particularly during exercise (exercise intolerance). The causes of diastolic failure are inappropriate tachycardia, decreased diastolic compliance and impaired systolic relaxation. Impaired (incomplete or slowed) systolic relaxation must be conceptually distinguished from compensatory prolonged systolic contraction (delayed or retarded relaxation). Optimal therapy will depend on the type of disease, the phase during the course of a given disease and the coexistence and relative contribution of various (de)compensatory processes. Treatment may consist of bradycardic, remodeling and lusitropic drugs.

Cell-cell signaling between adult rat ventricular myocytes and cardiac microvascular endothelial cells in heterotypic primary culture

It is unclear whether signaling between endothelial cells and muscle cells within ventricular myocardium, known to be important during cardiac development, remains physiologically relevant in the adult heart. Also, the mechanisms regulating the synthesis and activation of locally acting autacoids such as endothelins, cytokines known to have potent effects on contractile function and gene expression in cardiac myocytes, are unknown, as their cells of origin within ventricular muscle. Microvascular endothelial cells isolated from ventricular tissue of adult rats do not express endothelins constitutively. However, the appearance of preproendothelin mRNA can be increased in cardiac microvascular endothelial cells by heterotypic primary culture with adult rat ventricular myocytes. Cell-cell contact, or at least close apposition, appears to be necessary to increase preproendothelin mRNA, as medium conditioned by ventricular myocytes alone was ineffective when applied to monocultures of microvascular endothelial cells. The level of TGF beta precursor mRNA is also markedly increased in microvascular endothelial cells in coculture and precedes the appearance of endothelin precursor transcripts. In coculture, TGF beta acts as an autocrine cytokine, increasing endothelin precursor mRNA and inhibiting the rate of microvascular endothelial cell proliferation. This regulation of endothelial cell phenotype in heterotypic primary cultures suggests that dynamic, reciprocal cell-cell signaling may also be occurring between microvascular endothelium and ventricular myocytes in vivo.

Endothelial cells are required for the cAMP regulation of cardiac contractile proteins

The contractile proteins in mammalian cardiac muscle are regulated by a cAMP-dependent reaction that alters the activity of the actomyosin ATPase. The ATPase activity of cardiac actomyosin has also been shown to depend on factors released by small arteries in the myocardial tissue. Endothelial cells have been implicated in the regulation of the contractile force developed by isolated cardiac tissue. To determine whether endothelial cells are required for the cAMP-dependent regulation of the contractile proteins, the effect of cAMP on the actomyosin ATPase activity was measured in cryostatic sections of isolated, quickly frozen rat ventricular trabeculae. In half of the trabeculae, the endocardial endothelial cells had been damaged by a 1-sec exposure to 0.5% Triton X-100. In trabeculae with intact endothelial cells, cAMP increased actomyosin ATPase activity toward an apparently maximum value. In trabeculae with damaged endothelial cells, cAMP did not change actomyosin ATPase activity. The coronary venous effluent from an isolated heart has already been shown to modify the maximum isometric force developed by an isolated trabecula. The extent to which the force of the isolated trabecula is changed by the coronary venous effluent is closely related to the degree to which cAMP has up-regulated the actomyosin ATPase activity in the isolated heart donating the coronary effluent: the greater the degree of up-regulation of ATPase activity, the greater the increase in force produced by the effluent. These results indicate that endothelial cells are required for the cAMP-dependent regulation of cardiac contractile proteins to function, and these results further suggest that the myocardium autoregulates by modulating the cAMP regulation of contractile proteins with endothelial-derived factors.

Evidence for the existence of endothelial factors regulating contractility in rat heart

Force developed by isolated papillary muscle decreases as the cross-sectional area increases. The basis for this decline in force is not clear in as much as theoretical considerations and experimental data have indicated that the rate of diffusion of oxygen into thin bundles should not be limiting. Decline of maximum Ca-activated force with increasing cross-sectional area of detergent skinned papillary muscle can be attributed to the accumulation of inorganic phosphate in the center of the bundle. In both cases, the bundle of intact cells with a possible limitation of diffusion of oxygen into the bundle and of skinned cells with a limitation of diffusion of P(i) outward, the lowest level of activity should be in the center of the bundle. We have used quantitative histochemistry for measuring Ca- and actin-activated myosin ATPase activity in cryostatic sections of rapidly frozen isolated trabeculae. The technique is very sensitive and has sufficient spatial resolution to resolve individual myofibrils. At different times after dissection, ventricular trabeculae were quickly frozen, transversely sectioned and Ca- and actin-activated myosin ATPase, measured in serial sections both without and with 1 microM cAMP in the assay solution. In none of over 40 trabeculae studied was there an inward gradient of actin-activated ATPase activity of myosin. The most superficial cells had very low enzymatic activity. Cyclic AMP decreased the gradient by raising the enzymatic activity of the less active cells more that the more active cells. Ca-activated myosin ATPase was always uniform across the transverse section.(ABSTRACT TRUNCATED AT 250 WORDS)