Growth hormone reverses age-related cardiac myofilament dysfunction in rats.

We tested the hypotheses that aging is associated with a reduction in overall cardiac contractility and myofilament force generation that could be reversed with growth hormone (GH) replacement. Three groups of male Brown-Norway rats were studied: young (Y(SAL): 8 mo old, n = 13), old (O(SAL): 28 mo old, n = 13), and old GH-treated (O(GH): 28 mo old, n = 12; 300 microg bovine GH, twice a day for 30 days). The left ventricular (LV) pressure-volume relation was derived in isolated hearts, after which isolated trabecular muscles from these hearts were permeabilized and maximal myofilament force generation (Fmax) was measured. LV developed pressures at a LV volume of 0.3 ml were significantly depressed with age: 84 +/- 6 vs. 71 +/- 6 mmHg (Y(SAL) vs. O(SAL), respectively, P = 0.001) and not restored by GH (69 +/- 4 mmHg). Fmax was reduced in the aged hearts: 47.5 +/- 3.12 vs. 35.9 +/- 3.03 mN/mm2 (Y(SAL) vs. O(SAL), respectively, P = 0.014) but was restored with GH replacement to 46.7 +/- 3.12 mN/mm2 (O(SAL) vs. O(GH), P = 0.021). Our results suggest that cellular myofilament contractility is reduced with aging and restored with GH replacement.

Localization of tissue transglutaminase in human carotid and coronary artery atherosclerosis: implications for plaque stability and progression.

Although atherosclerosis progresses in an indolent state for decades, the rupture of plaques creates acute ischemic syndromes that may culminate in myocardial infarction and stroke. Mechanical forces and matrix metalloproteinase activity initiate plaque rupture, whereas tissue inhibitors of metalloproteinases have an important (albeit indirect) role in plaque stabilization. In this paper, an enzyme that could directly stabilize the plaque is described. Tissue transglutaminase (TG) catalyzes the formation of epsilon(gamma-glutamyl)lysine isopeptide bonds that are resistant to enzymatic, mechanical, and chemical degradation. We performed immunohistochemistry for TG in atherosclerotic human coronary and carotid arteries. TG was most prominent along the luminal endothelium and in the medium of the vessels with a distribution mirroring that of smooth muscle cells. Variable, often prominent, immunoreactivity for TG was also seen in the intima, especially in regions with significant neovascularization. Additionally, TG was detected in fibrous caps and near the "shoulder regions" of some plaques. A monoclonal antibody to the transglutaminase product epsilon(gamma-glutamyl)lysine isopeptide demonstrated co-localization with TG antigen. Transglutaminase activity was found in 6 of 14 coronary artery atherectomy samples. Cross-linking of TG substrates such as fibrinogen, fibronectin, vitronectin, collagen type I, and protease inhibitors stabilized the plaque. Furthermore, the activation of transforming growth factor-beta-1 by TG might be an additional mechanism for the promotion of plaque stabilization and progression by increasing the synthesis of extracellular matrix components.

Cross-bridge kinetics in rat myocardium: effect of sarcomere length and calcium activation.

We tested the hypotheses that Ca(2+) concentration ([Ca(2+)]) and sarcomere length (SL) modulate force development via graded effects on cross-bridge kinetics in chemically permeabilized rat cardiac trabeculae. Using sinusoidal length perturbations, we derived the transfer functions of stiffness over a range of [Ca(2+)] at a constant SL of 2.1 micrometer (n = 8) and at SL of 2.0, 2.1, and 2.2 micrometer (n = 4). We found that changes in SL affected only the magnitude of stiffness, whereas [Ca(2+)] affected the magnitude and phase-frequency relations. The data were fit to complex functions of two exponential processes. The characteristic frequencies (b and c) of these processes are indexes of cross-bridge kinetics, with b relating to cross-bridge attachment to and c to detachment from certain non-force-generating states. Both were significantly affected by [Ca(2+)], with an increase in b and c of 140 and 44%, respectively, over the range of [Ca(2+)] studied (P < 0.01). In contrast, SL had no effect on the characteristic frequencies (P > 0.6). We conclude that Ca(2+) activation modulates force development in rat myocardium, at least in part, via a graded effect on cross-bridge kinetics, whereas SL effects are mediated mainly by recruitment of cross bridges.

Are efforts at expanding the donor pool misdirected?

BACKGROUND: The number of patients potentially benefiting from heart transplantation far exceeds the number of hearts available. This has led to an increasing interest in use of hearts from previously unacceptable donors. However, the long-term outcome of such hearts is largely unknown. Research on other parts of the donor process may provide a greater number of additional hearts of high quality. METHODS: Journal reviews were conducted to identify proposed methods for use of previously unacceptable donor hearts, as well as research regarding the consent process. Data from the United Network for Organ Sharing were obtained to identify the reason consented heart referrals were not recovered. Data from the Association of Organ Procurement Organizations were obtained for consent rates in the United States. Calculated estimates were made for the number of excess hearts potentially available by use of extended donor criteria versus increasing the consent rate. RESULTS: More than 40 articles proposing extended donor criteria were identified versus only 12 articles about consent. Of the 2199 consented heart referrals not recovered in 1995, more than 1300 fit into a category amenable to a proposed strategy for use of extended donors. If these strategies were used aggressively (50% of the time), 701 additional hearts would have been available. Alternatively, if the consent rate were increased a comparable amount, 1260 excess hearts of high quality would have been available. CONCLUSION: Although research into extended donor criteria is probably justified, transplantation programs should direct research efforts into the consent process as a potential source of additional hearts.

The Frank-Starling mechanism is not mediated by changes in rate of cross-bridge detachment.

We tested the hypothesis that the Frank-Starling relationship is mediated by changes in the rate of cross-bridge detachment in cardiac muscle. We simultaneously measured isometric force development and the rate of ATP consumption at various levels of Ca2+ activation in skinned rat cardiac trabecular muscles at three sarcomere lengths (2.0, 2.1, and 2.2 microns). The maximum rate of ATP consumption was 1.5 nmol.s-1.microliter fiber vol-1, which represents an estimated adenosinetriphosphatase (ATPase) rate of approximately 10 s-1 per myosin head at 24 degrees C. The rate of ATP consumption was tightly and linearly coupled to the level of isometric force development, and changes in sarcomere length had no effect on the slope of the force-ATPase relationships. The average slope of the force-ATPase relationships was 15.5 pmol.mN-1.mm-1. These results suggest that the mechanisms that underlie the Frank-Starling relationship in cardiac muscle do not involve changes in the kinetics of the apparent detachment step in the cross-bridge cycle.

Decreased myocyte tension development and calcium responsiveness in rat right ventricular pressure overload.

BACKGROUND: The contractile dysfunction observed in end-stage myocardial hypertrophy has at its base an abnormality in myocyte function. However, whether depressed contractile function is related to an alteration in contractile protein function is presently unknown. METHODS AND RESULTS: Contractile force, tension, and calcium responsiveness were measured in single-skinned myocytes isolated from rats with right ventricular hypertrophy (RVH) and control rats. RVH was induced by pulmonary artery constriction for 36 weeks and was associated with significant myocyte hypertrophy. Myocytes were attached to micropipettes that extended from a force transducer and motor. Isometric force was measured over a wide range of calcium concentrations at two sarcomere lengths (SLs). Maximal force was increased in the RVH group: 1.20 +/- 0.10 versus 1.62 +/- 0.13 mg at SL = 2.0 microns and 1.33 +/- 0.10 versus 1.84 +/- 0.15 mg at SL = 2.3 microns (P < .05). Maximal tension, however, was reduced in the RVH group: 24.3 +/- 1.91 versus 37.5 +/- 2.92 mN/mm2 at SL = 2.0 microns and 27.4 +/- 1.78 versus 41.8 +/- 3.19 mN/mm2 at SL = 2.3 microns (P < .01). The concentration of calcium ions required for half-maximal activation was increased in the RVH group: 2.64 +/- 0.13 versus 3.47 +/- 0.22 mumol/L at SL = 2.0 microns and 2.23 +/- 0.15 versus 2.86 +/- 0.18 mumol/L at SL = 2.3 microns (P < .01). The slope of the force-calcium relationship (Hill coefficient) was decreased in the RVH group at SL = 2.0 microns (4.3 +/- 0.4 versus 3.1 +/- 0.2, P = .04) but not at SL = 2.3 microns (3.8 +/- 0.2 versus 3.6 +/- 0.2, P = NS). CONCLUSIONS: These results suggest that the depressed cardiac function of end-stage myocardial hypertrophy may be due, in part, to altered contractile protein function.

Right ventricular contractile protein function in rats with left ventricular myocardial infarction.

We studied contractile function in cardiac trabeculae isolated from the right ventricles (RV) of rats with experimental heart failure (HF) induced by left ventricular (LV) myocardial infarction (24 wk post-MI; n = 6) and from sham-operated rats (n = 7). Sarcomere length (SL) was measured by laser diffraction techniques, and force (F) was measured by silicon strain gauge. SL was kept constant at all times by computer feedback control. HF was associated with marked LV dilation and pulmonary congestion. In intact, RV twitching trabeculae, HF was associated with a depression of the F-SL relation at extracellular Ca2+ concentration ([Ca2+]o) = 1.5 mM and a depression of the F-[Ca2+]o relation at SL = 2.0 microns. HF was also associated with a significant depression of the F-intracellular [Ca2+] relation at SL = 2.0 microns measured after chemical permeabilization of these RV trabeculae (skinned fibers). Our results suggest that reduced force development in this model of HF is due, in part, to depressed function of the contractile filaments.

Effect of adenosine on contractile state and oxygen consumption in isolated rat hearts.

We studied the effects of adenosine on oxygen consumption and contractile state in 17 isolated, crystalloid-perfused, isovolumically contracting rat heart preparations at constant coronary flow. In 10 experiments we determined adenosine-contractile state dose-response relationships in three groups of hearts using two different perfusates and in the presence and absence of adrenergic blockade. Adenosine consistently reduced contractile state in a dose-dependent fashion, reducing the ventricular pressure developed at a constant ventricular volume by 24% on average at its maximal effect. An adenosine concentration of 111 microM on average produced 50% of the maximal effect. In seven experiments we determined the end-systolic pressure-volume and oxygen consumption-pressure-volume area (MVO2-PVA) relationships at two calcium concentrations (1.5 and 0.75 mM) and with adenosine 400 microM (1.5 mM Ca2+). Contractile state was indexed by the developed pressure at a ventricular volume of 0.3 ml (P0.3). Compared with 1.5 mM Ca2+, mean P0.3 was reduced by 38% with 0.75 mM Ca2+ and by 18% with adenosine. Whereas the MVO2-PVA slopes did not change, the mean MVO2 intercept was reduced by 22% with 0.75 mM Ca2+ and by 13% with adenosine. The MVO2 intercept, which represents the oxygen consumed by the unloaded heart, was directly related to P0.3. This relationship, which represents the oxygen cost of contractility, was not affected by adenosine. We conclude that at constant coronary flow and perfusion pressure adenosine reduces myocardial contractility and the oxygen consumed for excitation-contraction coupling. However, adenosine does not affect the slope of the MVO2-PVA relation or the oxygen cost of contractility.(ABSTRACT TRUNCATED AT 250 WORDS)

End-systolic pressure-volume and MVO2-pressure-volume area relations of isolated rat hearts.

We tested the utility of a standard isolated, crystalloid-perfused, isovolumic rat heart preparation for studying ventricular metabolism in terms of the myocardial oxygen consumption-pressure-volume area (MVO2-PVA) relations. The end-systolic pressure-volume relations (ESPVRs) determined between volumes of 0.15 and 0.65 ml were fit equally well by linear and nonlinear regression analysis within the data range but predicted widely differing volume-intercept (Vo) values. Linear regression analysis of the ESPVRs provided a mean slope (Ees) of 419 +/- 186 mmHg.g.ml-1 and Vo of 0 +/- 0.12 ml, respectively (n = 6). The MVO2-PVA relations were linear with a slope and MVO2 intercept of 1.30 +/- 0.31 x 10(-5) ml O2.mmHg-1.ml-1 and 0.38 +/- 0.09 x 10(-3) ml O2-beat-1.g-1, respectively. These MVO2-PVA parameters were not significantly different from those obtained when nonlinear regression analysis was applied to the ESPVR. Decreasing perfusate calcium concentration ([Ca2+]) (n = 7) resulted in a downward shift in the ESPVR, a decrease in the MVO2-PVA intercept (0.52 +/- 0.26 vs. 0.34 +/- 0.20 x 10(-3) ml O2.beat-1.g-1, P less than 0.01), and no significant change in the MVO2-PVA slope (1.33 +/- 0.47 vs. 1.57 +/- 0.69 x 10(-5) ml O2.mmHg-1.ml-1, NS). We conclude that this preparation may be a useful alternative to more expensive preparations for selected experiments in cardiac energetics.

Influence of metabolic substrate on rat heart function and metabolism at different coronary flows.

The influence of metabolic substrate on contractile strength, myocardial oxygen consumption (MVO2), high- and low-energy phosphate levels, and intracellular pH were determined in isovolumically contracting isolated rat hearts perfused with solutions containing either glucose or hexanoate at both high and low coronary perfusion pressures (CPP). Contractile strength was not significantly influenced by substrate at a CPP of 80 mmHg. As coronary flow was decreased, developed pressure measured at a fixed left ventricular volume (LVV) was lower during hexanoate than glucose perfusion. The relationship between MVO2 and mechanical work determined at a CPP of 80 mmHg over a range of LVVs was shifted upward in a parallel manner when substrate was switched from glucose to hexanoate. The MVO2-work relationship measured at a fixed LVV but over a range of coronary flows (7-20 ml/min) was also parallel shifted upward on switching from glucose to hexanoate. Basal MVO2 was greater during hexanoate than glucose perfusion by an amount that accounted for two-thirds the total increase in MVO2 observed between the substrates under unloaded beating conditions. The remainder of the difference was attributed to increased energy requirements for excitation-contraction coupling. Inorganic phosphate concentrations increased more and phosphocreatine concentrations decreased more during low-flow conditions (3 ml/min) when hearts were perfused with hexanoate compared with glucose. Thus hexanoate decreases myocardial efficiency compared with glucose in large part by increasing non-work-related oxygen demands. This inefficiency impacts adversely on contractile strength and high-energy phosphate concentrations at low coronary flows.